Uprating and Upgrading

Abstract

Many countries with major electrical infrastructure are frequently confronted with four coinciding critical issues,

This chapter is based on Cigré TB 353 (2008) and the author and reviewers would like to acknowledge the original TB authors and contributors. In addition, TB 353 has 59 case studies and examples which are commended to the reader as an additional source of knowledge and understanding of uprating and upgrading overhead lines.

Originally published by Cigré, 2014, under the ISBN 978-2-85873-284-5.

Republished by Springer International Publishing Switzerland with kind permission

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

18.1 Introduction and Definitions

Many countries with major electrical infrastructure are frequently confronted with four coinciding critical issues,

• much of this infrastructure was constructed in the fifties and the sixties, which result in the age of the assets being over 50 years;

• the design life of much of the infrastructure is in many cases about 50 years and has matured beyond the engineering serviceability and or economic life and requires some form of life extension;

• the need to increase capacity of the existing infrastructure places extraordinary demands on utilities to establish strategies to uprate the existing infrastructure; and

• approvals for the construction of new overhead lines are often difficult to obtain and in many cases result in critical delays to meet network capacity needs.

The following definitions will be used throughout the chapter.

• increased utilization: increased utilization is one or any combination of uprating, upgrading, life extension and refurbishment of an overhead line

• asset renewal: increasing the reliability and or availability of an overhead line by any combination of uprating, upgrading, life extension and refurbishment.

• life extension: extensive renovation or repair of an item without restoring their original design working life Cigré TB 175 (Cigré TB 175 2000). Life extension results in a decrease of the probability of failure and no change to the consequence of failure.

• refurbishment: extensive renovation or repair of an item to restore their intended design working life. Refurbishment results in a decrease of the probability of failure and no change to the consequence of failure.

• upgrading: increasing the original structural strength of an item due to, for example, a requirement for higher meteorological actions or increasing the original electrical performance of an overhead line. Upgrading results in a decrease in the probability of failure and no change in the consequences of failure.

• uprating: increasing the electrical characteristics of a line due to, for example, a requirement for higher electrical capacity, or larger electrical clearances. Uprating will increase the electrical capacity of the line and therefore potentially increasing the consequences of a failure.

18.2 Purpose

The purpose of this Chapter is to provide a general overview of the economic and technical considerations in order to facilitate considerations for up-rating and upgrading of overhead lines. The outline of the chapter is described in Figure 18.1.

Figure 18.1

Outline of Uprating and Upgrading Overhead Lines.

18.3 General Economic and Technical Considerations

To increase the utilization of existing overhead lines, a number of economic and technical factors need to be considered. Some of these factors are influenced by the basic need to increase system capacity. The decision to increase the utilization of an existing overhead line will be influenced by the asset life, the load growth forecast, the planning horizon, the value of capital, a cost benefit analysis, economic optimization and consideration of other constraints. This Section will discuss each of these factors. Table 18.1 provides a summary of the various asset renewal options, risk management considerations, drivers, propositions and the effects on an overhead line.

Table 18.1 Overhead Line Increased Utilization Options

18.3.1 Increasing System Capacity

The growth in demand for energy, the new developing energy markets and that electricity is fundamental to a modern society are all placing a greater demands on existing overhead lines. Growth in demand may be driven by normal demographic growth demands and or growth demands triggered by changes of technology and standards of living such as the affordability of domestic air conditioning. In more recent times, the growth of alternative and renewable energies is also placing new demands on the existing network of overhead lines.

The fundamental objective of an network asset owner is to continuously assess the capacity of the electrical network to determine the most economic and technical viable options to meet the increasing demands for electricity to ensure that the reliability of the electricity supply to customers is not compromised and the increasing demand for electricity associated with existing and future residential, commercial and industrial development is met.

Decisions to increase the utilization of an overhead asset in a timely manner are influenced by technical and economic factors, including the age of the asset.

18.3.2 Optimum Time for Renewal (Cigré TB 294 2006; Cigré TB 353 2008)

The economic end of life of a long lived asset has been defined where the cumulative net present value (NPV) of cost of the asset including depreciation, maintenance, losses and risk costs per years of service is a minimum.

This can best be illustrated by the cumulative NPV costs shown in Figure 18.2, in which after the initial capital costs (o to a) and the annual maintenance and risk costs are a minimum for many years (a to b). During this period of time, the long-run average cost of the asset is decreasing. At some point in time, the maintenance costs and the risk cost of failure start to increase (b to c) until the long-run average cost per year of service starts to increase (beyond c). With some assets, at this point in time, when the long-run average cost is at a minimum value or the tangent of the angle δ is a minimum, the asset would be replaced with a new asset as shown in Figure 18.3. The new asset would have new long-run average costs which would be less than the previous long-run average annual costs.

Figure 18.2

Example of Economic End of Life.

Figure 18.3

Example of Economic End of Life.

An everyday example of this principal is the purchase of an automobile. After the initial purchase, very little maintenance is required. After ten or fifteen years, more maintenance is required and there is a risk that the automobile will not perform the designed function to transport the owner to his destination as scheduled. At this point in time, a new automobile would be purchased. Over the time span of purchasing several automobiles in this manner, the owner would receive the most benefit, that is travelled the longest distance for the least long run average cost per kilometre.

Unfortunately, an overhead line asset cannot be replaced as easily as an automobile and this simple economic model does not fully apply to the concerns of network asset owners. The most important exception to this model is that historical initial and maintenance costs are of little concern to the asset owner and a more important concern is the future maintenance and risk costs from the current point in time, which is from “now” as illustrated in Figure 18.4.

Figure 18.4

Example of Optimum Time for Renewal.

Thus the definition is modified to exclude historical costs as most of these costs are likely to be unknown and may not be relevant in considering future investment. Therefore for purposes other than normal accounting procedures, the technical end of life and the optimum time for renewal of an overhead asset will be defined as follows:

• Technical end of life is when the overhead line fails to perform within the normal operating requirements without abnormal maintenance or when the overhead line is no longer fit for the original purpose; or

• Optimum time for renewal is when the cumulative NPV of the future annual costs of the overhead line including maintenance, losses and risk costs per years of service as illustrated in Figure 18.4 when δ₁ is equal to the minimum long run average costs of an asset renewal project as illustrated in Figure 18.5 as δ₂.

Figure 18.5

Example of Optimum Time for Renewal.

The need of the network asset owner is to determine the optimum time of renewal to uprate, upgrade or refurbish an overhead line in order to produce the minimum long-run average cost for the overhead line within a specified planning horizon as illustrated in Figure 18.5 to produce the minimum tan δ₂.

The optimum time for renewal and the technical end of life are intrinsically linked as the optimum time for renewal is affected by a large number of technical factors, including

• original design of the asset;

• original construction materials and workmanship;

• operating environment that considers corrosion, ultra violet radiation, extreme ambient temperature, lightning, wind and ice exposure;

• level of accumulated damage through overloading and faults;

• maintenance standard and quality of material and workmanship;

• technological advances such as revised safety standards, design codes and legislation changes; and

• operational or environmental constraints on increased utilization for continued use.

The renewal option for an overhead line asset may include corrective, responsive and or preventive measures from any one or a combination of the following actions:

• repair at each failure;

• operate until first failure and then initiate responsive action;

• uprate to increase the electrical characteristics;

• upgrade to increase the mechanical strength;

• refurbish to extensively renovate or repair to restore the intended design working life;

• life extension to extensively renovate or repair without restoring the intended design life; or

• dismantle high risk obsolete assets with or without replacement.

All future management decisions should be based on a comparison of various options with the “do nothing” option. Therefore, assuming an example where the age of the asset in Figure 18.4 is forty years, so that “now” is year 40. Also assume a strategic view or the planning horizon is the next twenty years, reflecting future costs into today’s values utilizing the NPV of future costs. An examination of the cumulative NPV of all the future costs associated with a renewal project as illustrated in Figure 18.5, it is clear that the minimum long-run average cost associated with this asset renewal project expressed in cumulative NPV cost per years of service is the slope of tan δ₂.

Cigré TB 353 (Cigré TB 294 2006) discusses several case studies and also explains in detail the concept of the optimum time for project renewal.

18.3.3 Planning Horizon and Net Present Value

The planning horizon selected for evaluation should cover all significant cost and benefit items throughout the life cycle for the overhead line for the renewal options considered. For overhead lines consisting of major components with differing life expectancies, the planning horizon should be chosen to synchronise replacement of different components to optimise the replacement costs. For example, insulators may have a life expectancy of 17 years and fittings may have a life expectancy of 32 years. For economic reasons, the common multiple periods for the insulators would be 15, 30, 45 and 60 years and for fittings 30 and 60 years.

Financial return for capital investments for a long planning horizon is achieved by recognising the time dependent value of capital, the entitlement to interest earnings and the NPV of capital. Consideration of the level of data accuracy, load seasonal demand variation, organisational budget control and long term budget variability suggests that annual aggregation of costs is a practical approximation. This approximation also applies when considering the effect of inflationary trends and NPV determination. A sensitivity analysis of the variation of the data assists in determining this approximation.

18.3.4 Cost-Benefit

Cost-benefit assessment based on NPV is widely accepted because influential factors will allow examination of a range of trade-off options. The future cost of construction such as a new overhead line, upgrading, component replacement, operation, energy losses, risk costs and the maintenance of the overhead line are estimated and calculated for each year within the planning horizon and converted to the NPV by applying a discount rate. The improvement of network availability should also be captured in the cost-benefit assessment by including an equivalent financial value to represent the community benefit.

The optimization focus is to minimise “costs minus benefits”. A project option can deliver savings when the net present value of all cost-benefit summed over the assessment horizon is lower than that of the base case. The input data, intermediate processes and results of such assessment form an essential part of the analysis for justifying network renewal projects.

The economic and financial analysis of the renewal should take into account the following factors within the assessment horizon for the base case asset configuration and any renewal options under consideration;

• weighted average cost of capital;

• optimum time for renewal of an overhead asset;

• operating expenditure, including inspection and maintenance;

• statistical and probabilistic analysis of risk cost including failure rates and consequences that include supply interruption costs; and

• energy losses.

18.3.5 Optimization

The cost-benefit analysis for the project options should provide project economic viability and associated ranking. The net present cost of the do nothing option would provide the base case for comparison and any option with net present cost lower than the base case would be considered economical and viable.

Under most circumstances, renewal expenditure may be scheduled with considerable degree of flexibility and offer considerable opportunities for optimisation. In this regard, a proposed renewal option may be uneconomical in the current year if the capital expenditure cannot be adequately compensated for by the reduction of other costs. With the exception of capital expenditure component, all other cost items for a renewal option are likely to increase with the degree of “wear-out” accumulated on the existing overhead line targeted for renewal. Hence, an uneconomical option in the current year may become economical in subsequent years as all other costs gradually increase over time. However, the planning horizon should always take into account the time required for permitting, planning and executing any major renewal project.

The timing of a renewal project should be optimised by selecting the year of implementation to minimise the NPV of the cost-benefit. For a given project option there could be three types of possible results where the net present cost is

• minimal for renewal in the current year indicating that renewal is overdue;

• minimal for renewal at a future year within the planning horizon indicating the optimal time for renewal; or

• decreasing with delaying renewal throughout the planning horizon and indicating that renewal should be deferred beyond the planning horizon.

If there is more than one viable renewal option, the option with the lowest net present cost at its optimal time should be selected as the preferred option.

18.3.6 Constraints

Ideally the preferred option of all economically viable renewal projects should be implemented at the optimal time and thus the scheduled network asset renewal capital expenditure will deliver the most favourable cost benefit outcome to the asset owner. However, there are many practical constraints, which may prevent the timely execution of the optimum options. Typical constraints may be

• financial constraints due to limited expenditure budget or restrictions on raising revenue;

• labour resource constraints from either or both internal employees and external contractors;

• supply security constraints prohibiting coincidental outages of parts of the network;

• technical constraints due to testing and approving emerging technology;

• timing constraints due to obtaining approval from government authorities and or public consultation processes; and or

• sequential dependence among other network projects.

Some of the above resource constraints can be pooled. When a pooled resource becomes binding, the conflict has to be resolved by re-scheduling projects from the pool. Taking the original optimal timing as the ideal case, each rescheduling action whether to advance or defer can be assessed for its costs by calculating the corresponding increase in net present cost. Due to the difference in the sensitivity of net present cost to the timing for each project, the project with the smallest increase in costs should be selected for rescheduling.

In addition, there are a number of factors affecting the project renewal which should be considered when evaluating options and include.

18.3.6.1 Environmental Protection Measures and Permits

Any construction activities, including work on existing overhead lines, affect environment. Strict environmental regulations in many countries require that network asset operators must secure necessary work permits prior to carrying out field work, especially in environmentally sensitive areas. In some cases, the regulatory process might be quite lengthy and it may include review of an appropriate Environmental Protection Plan documents prepared by a power utility. Regulatory bodies may also impose very strict restrictions on types of construction activities and schedules. These may include: acceptable noise levels, use of specific materials and design configurations, protection of endangered wildlife and plant species, protection of religious/sacred sites and many others. This process can result in extensions to a project schedule.

18.3.6.2 Overhead Line Outage Availability

In current electricity markets, many utilities are operating their power lines close to their capacity. Under such conditions it is becoming increasingly difficult to remove overhead lines from service. Those utilities which use live-line work procedures may perform work on energized lines by deploying qualified personnel. Strict safety measures and proper work techniques must be used to complete live-line work. This often results in a lower productivity, higher number of staff involved and higher labour wages. However, these higher premiums are often offset by revenues realized from network charges.

As an alternative, a temporary line by-pass can be used to isolate an overhead line sections or an individual structure from the energized circuit. Such installation allows line crews to have access to the isolated and de-energized line section for a limited period of time. Since such installations are of temporary nature, lower safety factors are often used in design.

18.3.6.3 Working on Structures under Structural Loads

Safety of field staff is a primary concern while working on an overhead line under structural load. It is essential to carry out a comprehensive structural analysis of the overhead line system considered for upgrading prior to any field work. Existing conductor tensions, component dead weight and resulting loads transferred onto structural supports must be carefully examined and taken into account when developing work procedures and selecting required equipment.

18.3.6.4 Use of Heavy Equipment

Overhead line components are often under heavy loads and may require use of heavy construction equipment. Proper safety procedures must be adhered to and only qualified personnel must be allowed to work with heavy machinery.

18.3.7 Terminal Equipment Considerations

All overhead line source and end point connections involve terminal equipment. The primary terminal equipment generally consists of insulators, overhead connections to disconnectors, disconnectors, current and voltage transformers, circuit breakers and in some cases power line communication coupling equipment. Consideration for the uprating of an overhead lines must include a review of the rating of the terminal equipment to ensure compatibility with the line rating.

18.3.8 Electric and Magnetic Fields

The voltage of an overhead line produces an electric field and the current flowing in an overhead line produces a magnetic field. Considerations of uprating of an overhead line by increasing the thermal capacity or increasing the voltage rating or changing the geometry of the conductor configuration may result in changes to the electric and magnetic fields. The overhead line uprating design should consider the potential changes to the electric and magnetic fields limited by statutory and safety regulations.

18.4 Overhead Line Uprating

The definition of uprating is increasing the electrical characteristics of an overhead line due to a requirement for higher electrical capacity or larger electrical clearances. Uprating will increase the electrical capacity of the line therefore based on risk, potentially increasing the consequences of a failure.

Overhead lines are electrically modelled as either long or short. A long overhead line in general is a major interconnector between load centres or load centres and generation centres and voltage regulation is one of the principal operating criteria which results in determining the level of power transfer capacity. On the other hand a short overhead line in general is an interconnector around or within load centres and power transfer capacity is determined by thermal capacity and is the principal operating criteria.

The operation of long overhead lines is strongly influenced by the voltage regulation and the need to ensure that the receiving end voltage is within defined tolerances. The decision to uprate a long overhead line is therefore linked to improving the voltage regulation.

Short overhead lines are influenced by the thermal capacity of the conductors and the need to ensure that the electrical safety clearances are not breached. Therefore the decision to uprate a short overhead line is linked to improving the thermal capacity.

This Section is limited to discussions of increasing system needs to considerations of uprating either long or short overhead line elements and it is acknowledged that a number of other methods may be used to increase overhead system capacity such as Flexible AC Overhead Systems. Further, an emerging strategic network uprating option is the conversion of existing AC overhead lines to DC operation and this is discussed in Section 18.4.3.

The variety and number of methods and techniques adopted and implemented to uprate overhead lines is directly influenced by the considerable variation of overhead line designs and construction methods employed. Nevertheless, uprating of overhead lines generally fall into either increasing voltage capacity or increasing thermal capacity. In some circumstances, the opportunity is taken to simultaneously increase the voltage and the thermal capacity of an overhead line. Table 18.2 provides a list of the most common voltage and thermal capacity uprating mechanisms, the associated methods, techniques and solutions. In this Section will be discussed each of the methods as well as the conversion of AC lines to DC lines.

Table 18.2 Overhead Line Uprating Mechanisms

18.4.1 Increasing Thermal Rating (Cigré TB 353 2008)

The practicality of increasing the thermal rating of an overhead lines is a function of a number of variables;

• the terrain – overhead lines in hilly and mountainous terrain, where structure location is dictated by terrain rather than load, may have many structures that are not structurally loaded to design limits;

• the meteorological conditions and corridor characteristics;

• the condition of the conductor, insulation and structures;

• the cost of losses, which is a function of the planned length of time to operate at high electrical loads;

• the line to ground clearances regulations;

• the structure loadings regulations;

• the overhead line length; and

• the original structural design capacity of structures.

Increasing the thermal rating of overhead lines may be accomplished by one or a combination of the following:

• increasing the conductor area by either adding conductors to existing conductors or conductor bundles or replacing existing conductors with new conductors of different size or construction;

• increasing the conductor rating by changing the thermal rating criteria based on a statistically-based meteorological study and an evaluation of the characteristics of the line corridor;

• installation of special conductors intended for high temperature and or low sag operation;

• increasing the conductor operating temperature limit, which will require one or more of the following to maintain adequate ground clearance:

  • increasing conductor tension which may require associated structure reinforcement;

  • modifying structures and or insulation to increase ground clearance;

  • insertion of new structures in critical spans;

  • installing negative sag devices; and or

  • excavation at key locations to increase ground clearance;

• modifying the overhead line to achieve a higher surge impedance load;

• installation of active line rating systems (this may not be considered a method to increase line ratings as the method does not change the maximum capacity of the line but rather allows a better utilization of the existing capacity);

• increasing the rating at very nominal cost by a very precise survey of the conductor clearances against standards as the survey results may show that in hilly terrain there may be a very small number of structures to raise or ground profile to modify to achieve a significant increase in rating and or suspension insulator offsets may be used to remove sag from critical spans, although at the expense of a sag increase in adjacent spans.

These methods will be discussed in more detail in the following Sections.

18.4.1.1 Increasing Conductor Rating Increasing Conductor Area (Cigré TB 175 2000; Cigré TB 178 2001)

Increasing conductor cross sectional area (csa) will increase the thermal capacity of an overhead line. However, the following factors should be considered,

• in general, the higher the csa of the conductor, the higher the conductor weight. This may be mitigated to some degree by changing material or conductor type. Increased conductor weight will generally result in increased tensions, which will most likely require reinforcement of termination and angle structures. Higher csa may increase the ice and wind load on the conductor, increasing the vertical load on all structures. (Note: there are conductors with trapezoid wires where the csa of the conductor increases without an increase of the diameter and thus may increase the capacity of the line by about 10?%, with lower the impact on structural loading than an equivalent increase in csa using standard round stranding); and

• the larger the diameter of the conductor the larger the wind and ice loads which results in increases in the horizontal loads on the structures.

There are two basic solutions for increasing conductor area of an overhead line either with or without substantial reinforcement of the structures and foundations. Beyond the network requirements arising from expected electric loads in the future, there are structural factors to be considered before any decision about reinforcement. Some of these factors are

• it is important to make a distinction between the design status and the actual status of the structures. It is easy to evaluate the structural capacity of the structures in design status, however, the actual status of the structures need thorough investigation and evaluation;

• the structural capacity of the overhead line may be understated and additional loading may be applied without the need for substantial reinforcement to the structures;

• some structures may need only partial reinforcement on the body or the crossarms therefore may be strengthened relatively easily and economically;

• no additional load is advisable without substantial reinforcement on overhead lines where frequent mechanical failures have occurred in the past;

• overhead lines where structure location is governed by terrain rather than structure load limits, ie through very rough terrain, may have less than full utilization of structural design load capacity for a significant portion of structures and these structures may have increased loads with little or no reinforcement; and

• new conductors may operate on the same temperature or at higher temperatures than the old conductors therefore sagging and or conductor clearance is a key issue for consideration.

It is quite common to replace aluminium conductor steel reinforced (ACSR) with all aluminium alloy conductor (AAAC) with the same or slightly higher diameter as the ACSR. If the diameter of AAAC conductor is the same as the diameter of the ACSR then its weight is less, and the current carrying capacity of the line will increase up to 40?% (Cigré TB 141 1999) depending on the clearances and the conductor tension. Even higher capacities can be achieved if the diameter of the conductor is marginally higher. Changing ACSR to similar diameter AAAC does not need substantial reinforcements to the structures and foundations. Using AAAC with the same diameter as the ACSR but with trapezoidal sections will further increase the csa and the thermal capacity, with little or no structural loading impact. Also, replacement of standard ACSR with trapezoidal stranded ACSR with the same diameter will cause some increase in tensions and vertical load but should not change the wind and ice loading.

Conductors on overhead lines can be replaced with conductors of substantially larger diameter or conductors can be added to existing bundles or single conductors can be bundled. In addition to substantially increasing the thermal capacity, the addition of a bundled conductor configuration changes the surge impedance loading (SIL) of the overhead line, by reducing the reactance and the voltage drop by 10?% to about 15?%. The transfer capacity may be doubled if a new bundle is created in each phase doubling the number of the conductor. However, these methods generally require substantial reinforcement to the structures and foundations. Also line to structure clearances will require evaluation and perhaps some structure reconfiguration.

In order to minimize the effects of higher structural loads on the structures, special wires can be used. As mentioned previously, formed wires will have higher aluminium content in the same csa which results in less wind load and ice loads.

High Temperature Conductors (Cigré TB 244 2004)

Cigré TB 244 illustrates how an overhead line rating increases as a function of design temp for a same size conductor and there is a range of conductors that have been developed for higher than traditional temperature operation and some of these conductors are:

• aluminium-zirconium alloy conductors - TAL, ZTAL, XTAL;

• aluminium-zirconium alloy conductors steel reinforced - TACSR;

• aluminium-zirconium alloy conductors invar steel reinforced which are low thermal coefficient conductors - TACIR, ZTACIR, XTACIR;

• gap-type aluminium conductors steel reinforced - GTACSR, GZTACSR;

• aluminium conductor steel supported – ACSS; and

• aluminium conductor composite reinforced – ACCC.

The common features of these conductors are

• increase the design temperature of the line without any substantial increase of the conductor sags;

• the application of low thermal coefficient materials are used for the core material;

• depending on the conductor type, maximum operating temperature up to 230 °C;

• high losses during the peak hours and if the duration of the peak period is long then the costs of losses may be substantial;

• current carrying capacity of the overhead line may increase by up to 100?% without any substantial reinforcement to the suspension structures however for some conductor types the tensions are higher requiring reinforcement of terminal and angle structures.

Meteorological Studies

The traditional means for establishing thermal conductor ratings has been to perform the determination based a commonly used set of weather parameters deemed to be conservative for most situations. Cigré TB 299 (2006) suggests effective wind speed of 0.6 m.s⁻¹, 40 ° C ambient temperature and solar radiation of 1000 w.m⁻¹. Too often, less conservative criteria have later been selected based on quasi-scientific evidence as an easy way to increase these ratings. This is consider a significant risk and can lead to an unacceptable level of conductor clearance infringements. However, thorough scientific study of the meteorological variables can produce a less restrictive criteria that would still provide the level of risk management warranted. This approach is viable only if there is sufficient meteorological data to do so and a statistical study is conducted by competent professionals who consider the local effects of terrain.

18.4.1.2 Increasing Conductor Temperature & Maintaining Ground Clearance (Cigré TB 175 2000; Cigré TB 207 2002; Cigré TB 353 2008)

Knowledge of the material behaviour and limits of conductors when subjected to various heating conditions is essential when designing and operating overhead lines. Two areas are of particular importance to increasing the rating of an overhead line by increasing the operating temperatures of conductors is the loss of tensile strength by annealing and the longer term permanent conductor elongation through metallurgical creep. The additional effects of increasing conductor temperature on the galvanized steel core, current carrying connectors and joints (Cigré TB 216 2002), conductor hardware and the protective properties of grease are also key considerations and detailed in Cigré TB 353 (2008).

Conductor Annealing (Cigré TB 353 2008)

The design maximum operating temperature of a conductor is partly a function of the acceptable level of permanent loss of tensile strength or annealing of the conductor. Annealing is caused by the heating of a material generally followed by a cooling period. During the annealing process, the material experiences a change in its microstructure and for metals, this not only results in a loss in tensile strength but also an increase in conductivity. In general, changes in conductivity will be insignificant compared with the changes of tensile strength.

Isothermal annealing curves illustrate the permanent loss of tensile strength when a conductor operates at elevated temperatures. It is appropriate to establish the maximum design temperature at which a conductor can operate while maintaining acceptable levels of degradation of tensile properties.

More recent research indicates that the annealing characteristics of a conductor depend not only on temperature and time of exposure but also on the diameter of the wires in the conductor. Smallest wire size suffering the greatest loss in strength and the largest size the least.

The temperature limit for normal operation of AAC, AAAC and ACSR of 100 °C results in an approximate loss of strength of 3?% of the original tensile strength after 1000 hours operation at this temperature.

For ratings for short time conditions (eg. when one circuit has to carry more than normal current for a short time), both the maximum temperature and the duration of the emergency load should be taken into account in determining the annealing of the aluminium wires. The annealing effect is cumulative. For example, if a conductor is heated to 150 °C under emergency conditions for 24 hours a year for 30 years it is much the same as heating the conductor continuously at that temperature for 720 hours. For this example the loss of ultimate strength in AAC would be approximately 15?%. For 30/7 ACSR the ultimate tensile strength would be reduced approximately 7?%. (The loss of strength with 30/7 ACSR may be less than 7?% since the increased max elongation of the partially annealed aluminium may be offset by the increased utilization of the steel core tensile strength beyond 1?% elongation) The effect is less significant for ACSR where an increase in temperature results in a load transfer from the aluminium to the steel wires. The steel provides a substantial component of the strength of the conductor and is essentially unaffected by the temperature up to 200 °C. Above this temperature for normal steel there is a gradual fall off in strength until at a temperature of 400 °C where the strength of the wire is only half of that of the room temperature strength.

If ratings for emergency conditions are to be applied then the combined effects of elevated temperature and increased tensile loading on the steel core due to compression loading in the aluminium wires on the sag of the line should be taken into account. Practically, the tension in a line reduces with increasing temperature so the effect is less severe (Cigré TB 244 2004).

Conductor Permanent Elongation (Cigré TB 353 2008)

Conductor permanent elongation is non-recoverable for inelastic material deformation that is a logarithmic function of conductor stress, conductor temperature and exposure duration. Permanent elongation begins at the instant of applied axial tensile load and continues at a decreasing rate providing tension and temperature remain constant. Permanent elongation consists of, in the short term, primarily wire radial and tangential movement during the early loading period (settlement & short-time, high-tension creep elongation) and in the longer term, primary metallurgical logarithmic creep (long-time, moderate-tension creep elongation).

Conductors operating at elevated temperatures will experience elevated conductor creep. In changing from low temperature conductor creep to high temperature conductor creep, it is necessary to convert the equivalent low temperature elongation equivalent time to an equivalent high temperature elongation equivalent time and project the longer term reduction in conductor tension and increase in conductor sag. At higher than everyday temperatures, the tension in the aluminium strands reduces. This reduction tends to offset the increase in creep at higher temperatures. In particular, for ACSR this creep is not higher than at normal temperatures.

Increasing the conductor operating temperature of an overhead line within the previously mentioned material limits of the conductor and maintaining ground clearance will increase the thermal rating of the overhead line. A typical relationship of the thermal rating of non- homogenous and homogenous conductors is illustrated in Figure 18.6. This relationship is essentially based on the fact that increasing the current will increase the thermal elongation which results in a reduction of conductor tension and a corresponding increase in conductor sag.

Figure 18.6

Typical conductor thermal capacity to conductor sag relationship for a given span.

For example, increasing the operating temperature from 85 °C to 100 °C of a 207 mm² ACSR non homogenous conductor results in an increase of the conductor thermal capacity from 565 A to 655 A or about 15?% which will result in a conductor sag increase of about 0.57 meters in 400 meter span or about 5?%. Similarly, for the same 400 meter span, for a 506 mm² ACSR homogenous conductor increasing the operating temperature from 85 °C to 100 °C results in an increase of the conductor thermal capacity from 880 A to 1030 A or about 17?% which will result in a conductor sag increase of about 0.62 meters or about 4?%.

To ensure ground clearance is maintained, mechanisms are required to compensate for this increased sag. Increasing the conductor operating temperature of an overhead line and maintaining ground clearance may be achieved by either increasing conductor tension, application of negative sag devices and or increasing the conductor attachment height. Increasing conductor attachment height is carried out by structure body extensions and or insulator crossarms and is discussed in Sections 18.4.2.

Increasing Conductor Tension (Cigré TB 373 2005)

Increasing conductor tension is one of the most common forms of increasing the rating of an overhead line. One of the most significant considerations in increasing conductor tension is the increased likelihood of aeolian vibration and associated increase in probability of longer term conductor permanent damage and even failure by metal fatigue. Notwithstanding this when consideration is given to increasing conductor tension it would be normal that the line would have been in service for a considerable period of time which would suggest that the aeolian vibration performance of the line is well known and the consequences of increasing conductor tension would be well understood.

Other design verification considerations would include

• increased conductor tension loads on angle and tension structures and even suspension structures under broken conductor loading conditions or other imbalanced longitudinal loading condition;

• increased foundation loads for angle and tension structures;

• increased conductor tension loads on tension insulators and associated fittings;

• for spans with differing conductor attachment relative levels, the changes in the weight span and resultant changes in suspension structure clearances for suspension insulators or changes in load factors for V string insulators

• increased conductor loads on all conductor joints under tension; and

• aluminium strands of the conductor will start creeping again if the tension is increased after more than 10 years.

The technical limits of increasing conductor tension is normally determined by

• the capacity of tension and termination structures and any associated cost benefit of increasing this capacity; and or

• aeolian vibration and or the fatigue limits of the conductors. The fatigue limits of conductors and the safe design tension has been the subject of widespread international research. An example of the publication of this research Cigré TB 373 (2005) and associated recommendation for conductor safe design tensions at average temperatures of the coldest month as a function of terrain category for homogeneous and non-homogeneous conductors and is given in Figure 18.7. Any proposed increase in conductor tension should be considered within the context of conductor safe design tension criteria.

Figure 18.7

Cigré SCB2.11.04 Conductor Safe Design Tensions Recommendation (where: L?=?actual span length (m), D?=?conductor diameter (cm), m?=?mass of the conductor per unit kg/m, length H?=?horizontal tension in the conductor and w?=?weight of the conductor per unit length).

A typical relationship of conductor tension parameter (H/w) and conductor sag for non-homogenous and homogenous conductors is shown in Figure 18.8 and illustrates that decreasing the conductor sag will require a corresponding increase in the conductor tension.

Figure 18.8

Typical Relationship of Conductor Tension Parameter (H/w) and Sag.

18.4.1.3 Negative Sag Devices

Negative sag device remain relatively new overhead line hardware and are based on a reaction to increasing conductor temperature by decreasing the effective length of conductor in the span thus mitigating thermal expansion experienced by the conductor during high temperature operations.

The negative sag device is activated by the same temperature changes that cause the conductor to sag. As temperature rises, conductor lengthens and the conductor sag increases. Under same circumstances, the negative sag device changes the device’s geometry to decrease span length. As the conductor temperature returns to normal and sag is no longer excessive then the negative sag device returns to the original shape.

18.4.1.4 Increasing Conductor Attachment Height

Increasing conductor attachment height to compensate for increased conductor thermal ratings is a natural consideration to compensate for increased conductor sag. Increasing conductor attachment height may be carried out by either the insertion of structure body extensions into existing structures and or application of insulator crossarms to existing steel crossarms.

Structure Extensions (Cigré TB 178 2001)

General experience indicates that given the magnitude of the required structural works it is not economical to increase the heights of all structures in an overhead line. Nevertheless it has been found that in many cases an increase in maximum operating temperature may be achieved by selectively increasing the height of approximately 10?% of the structures. Selective inclusion of structure extensions is normally limited to the suspension structures given the inherent structural and practical difficulty of including a body extension into a tension or terminal structure.

Design verification of increasing conductor attachment heights by the inclusion of a body extension would include

• structural capacity of the existing structure to determine the availability of any marginal capacity;

• design of the structure extension which may in general be limited to the structure where the structure geometry would permit the inclusion of a structure extension; and

• an assessment of the foundation capacity and any increased overturning moments.

The technical limits (Cigré TB 308 2006) of the inclusion of a body extension would normally be determined by the structure and foundations ultimate load factor for the defined loading conditions.

Increasing Structure Height

Increments in the structure height have generally been achieved by inserting a new steel panel into the lower portion of the structure as illustrated in Figure 18.9. Usually a 2 to 3 metre new extension is enough to achieve the new desired clearance.

Figure 18.9

Increasing Structure Height.

The new panel is designed to provide compatible interface to the upper and lower parts of the structure. As the original base width of the structure is concurrently not changed this causes increases on the lower body stresses and this may require either a completely new reinforced lower part (this is not very common) or reinforcements on the existing lower body.

The reinforcements on the existing lower body will generally consist of duplication of the main members as illustrated in Figure 18.10 or complete substitution with members of larger cross sectional area. Replacement of cross bracing members may also be required.

Figure 18.10

Duplication of Leg Members.

Foundations (Cigré TB 141 1999)

In any uprating or upgrading consideration, current practice is not to change the structure base width with the aim to reuse the existing foundation. This causes increments on the foundation loading since they are now subjected to greater loads, due to higher conductor attachment points and the associated larger overturning moments. These moments will increase the structure uplift and compressive loads.

Increased uplift reactions can be counteracted by adding additional concrete within the soil frustum or in critical cases by the installation of injected micro-piles plus a new concrete block connected to the existing foundation. Compressive resistance is normally not a problem and if it is the proposed solution can be achieved with the same injected micro-piles technology.

Erection Techniques

The current erection technique consists of either using an external support structure or using an internal central mast. The later technique provides the advantages of standardization in the process, ease of transport, reduction in installation time and lower cost. Depending on the conditions, raising of the structure height is an erection process that can be done with the overhead lines in service without any outage. This can be attained with sophisticated raising erection equipment and a team with extensive knowledge and experience.

Insulator Crossarms or Insulator Modifications (Cigré TB 353 2008)

Similar to increasing conductor tension, the replacement of existing insulators with some form of insulating crossarm or modifying the existing insulator arrangement are some of the most common and cost effective means of increasing the rating of an overhead line. The relative conductor attachment level is raised permitting the conductor sag to increase. Composite insulator provide a useful mechanism to design hybrid insulator sets to fulfil the geometrical, electrical and mechanical requirements of a new insulator system, with in most cases, enhanced performance.

One of the most important considerations in replacing existing crossarms or insulators is an understanding of the pollution levels and the performance of the existing insulation design of the overhead line. Notwithstanding this, when consideration is given to modifying the existing overhead line insulation it would be normal that the line would have been in service for a considerable period of time which would suggest that the insulation performance of the line is well known and the consequences of changing the insulation design would be well understood. Other design verification considerations would include changes in the,

• mechanical loads of the insulator arrangements;

• coupling point of the applied insulator loads on the structure;

• electrical clearance envelope caused by changes in the insulator swing;

• changes in the conductor attachment fittings and associated loads; and

• structure longitudinal load and restrained insulator movement.

The technical limits of insulator crossarms or insulator modifications are normally determined by required electrical clearance window, required coupling and creepage length of the insulators and flexibility of changing the insulator loading structure coupling points.

18.4.1.5 Use of Interspaced Structures

The use of interspaced structures is another way to increase conductor ground clearance. The selected spans and locations would normally be where the greatest amount of sag occurs or at the point on the ground profile where a clearance problem exists at an increased conductor operating temperature. It is desirable to install the structures at the mid-point of the span in order to minimize the amount of inline tension that would be affecting the structures tangential strength requirements. Essentially the structure should be capable of supporting the conductor weight and wind span and any ice loads.

Factors influencing the design and application of this solution may include items such as ownership of the right of way or easement restrictions as well as aesthetic considerations in built up areas.

18.4.1.6 Increasing Thermal Rating by Active Real Time Line Rating Systems (Cigré TB 299 2006; Cigré TB 353 2008)

Most utilities base the overhead line “book” ratings based on deterministic assumptions of a low wind speed and direction, high ambient temperature and full solar radiation. The most significant of these variables is the assumed effective wind speed. Cigré TB 299 suggests certain default weather conditions that are suitably conservative but suggests the possibility of performing field studies to determine regional specific weather assumptions or the use of real-time monitors to calculate dynamic line ratings. If properly selected, the weather assumptions used in line rating calculations result in a small risk of the conductor exceeding the design temperature when line current equals full rated load. This is illustrated in Figure 18.11.

Figure 18.11

Relationship of Overhead Line Load Factor and Static & Real Time Ratings.

The objective of using real time monitoring is to increase the line rating weather conditions as favourable compared to the assumptions used in static line ratings. This is illustrated in the green area in Figure 18.11 and simultaneously avoiding the rare conditions when the actual rating conditions are unfavourable as illustrated in the red area. During these periods, because the thermal state of the conductors changes rather slowly, with a time constant of about 10 to 20 minutes, the network operators may have sufficient time to reduce power flow and eliminate any clearance infringement risk.

The capacity increase and benefits of uprating achieved by real time monitoring depends on the static rating weather assumptions and the line design temperature. The benefits also depend on the economic and regulatory criteria used in each country.

Based on current experience, the rating gains vary between 5 to 15?% and higher gains may be achieved in special cases. For example, application at wind farms has shown realized gains of 30 to 50?%. The increase of line rating near wind farms is the result of prevailing winds. In addition real time monitoring can help to avoid uneconomic system dispatch when electricity costs are at peak.

Furthermore, a Cigré survey (Cigré TB 353 2008) indicated that under most circumstances overhead line thermal limits are caused by clearance limits and not material annealing limits. This has supported the consideration of adopting real time ratings of overhead lines to enable lines to operate closer to the clearance limits more often by utilizing the frequent occasions when the prevailing climatic conditions would permit higher ratings than those that would have been assumed by conservative deterministic static ratings. The difficulty in implementing dynamic line ratings concerns their lack of predictability. In most lines, it is not possible to predict line ratings beyond 1 to 4 hours and this prevents their use in determining transmission capacity for generation contracts which are usually determined a day ahead.

In summary, the objective of real time monitoring of an overhead line is based on

• the thermal rating of an overhead line is the maximum current that the circuit can carry without exceeding its temperature limit;

• the current required to enable the conductor to reach a given temperature can be far higher when the cooling is greatest than when the cooling is low which implies higher ratings at times of high wind speeds, low ambient temperatures or combinations of these parameters and vice versa; and hence

• real time monitoring is the monitoring of parameters such that the conductor position above the ground may be determined in real time at a current instant and the permissible thermal limits are then calculated to optimize the power flow of the overhead line.

Three common methods are employed to provide real time ratings and are the determination of,

• line clearances based on either real time conductor tension or sag measurements (direct method);

• conductor distributed temperature measurements using phase conductor embedded sensors (direct method) and direct measurement of conductor temperature; and

• prevailing climatic conditions by the installation of weather stations and the application of deterministic methods in rating (indirect method).

Two specific methods dominate the practical utility applications, tension and sag monitoring methods and weather methods used in different ways in several countries.

Line Tension and Sag Monitors

The tension or sag monitors are mounted on selected tension structures along the overhead line. At each location, the conductor’s mechanical tension or sag is measured by a tension sensor or sag sensor.

When using a tension monitor, conductor tension is measured by an electronic load cell and communicated to the utilities’ control room where an algorithm determines the real time conductor temperatures, real time line ratings and provides alarms of possible clearance violations. A sag monitor performs the same function by using a video sensor and target hung on the conductor to directly measure the actual sag and clearance in the monitored span.

In general, the monitored line tension or sag follows the average temperature of the line section between the adjacent tension structures and gives a representation of the rating conditions of a long section of the line. Typically, two monitoring locations, each monitoring two adjacent line sections, are required at line lengths of up to 25 to 35 km. The rating of the overhead line is then determined based on the lowest rating or highest temperature of the monitored sections. For longer lines, additional monitoring locations are required.

Conductor Temperature Sensing (Cigré TB 498 2012)

Cigré TB 498 provides a complete guide to the application of direct real time monitoring systems applicable to overhead line which includes temperature sensing.

Weather Stations and Application to Deterministic Rating Methods (Cigré TB 299 2006)

Weather monitors can be used to calculate conductor temperature and ratings using various methods such as the Cigré and IEEE methods. The Cigré and IEEE methods are applicable to all dynamic rating methods to calculate line ratings. The weather stations typically monitor wind speed, ambient temperature and solar radiation. Although wind direction can also be monitored most rating calculations default to using a wind direction at a small angle to the conductor because of the high variability of wind direction.

Because wind conditions are highly dependent on the terrain and sheltering of the line, weather monitors must be mounted in the actual overhead corridor to be monitored. Use of weather data at airports or other remote locations away from the overhead line may have little or no correlation with the weather conditions at overhead line corridors.

Weather stations report data from a single point of the line and line ratings calculated from weather data will in general be influenced by the number of weather stations installed to monitor the overhead line. Weather based ratings can be least accurate when the wind speed is low which is the most critical rating condition.

The cost of weather monitoring stations are relatively low but the associated maintenance costs can be high.

18.4.1.7 Probabilistic Rating of Overhead Lines (Cigré TB 207 2002)

The ampacity of a conductor is defined as the “current which will meet the design, security and safety criteria of a particular line on which the conductor is used”. Thus the thermal capacity of an overhead line is a function of the conductor integrity and the safety of the public. As such the thermal rating of an overhead line is depending on the following factors,

• ambient conditions;

• current;

• conductor type, rating and any bundle configuration;

• design temperature;

• exposure of the overhead line to the public;

• likelihood of above factors occurring simultaneously; and

• probability of flashover if the above conditions occurred simultaneously.

Probabilistic rating of overhead lines allows one to use the actual prevailing weather data to determine the risk associated with a particular current and design temperature. Caution is expressed when considering a change to probabilistic ratings. If probabilistic ratings are to be used, they should be based on a thorough scientific study of both the meteorological conditions of the area and the characteristics of the line corridors and it is important that the study be conducted by competent professional. Such a study will allow determination of the level of risk associated with a given deterministic rating method and provide guidance for establishing an appropriate risk level for future operations. The resulting probabilistic ratings are often less conservative than the deterministic method resulting in higher current ratings.

There are two major probabilistic methods of determining the thermal rating of conductors, the absolute method and the exceedence method.

The absolute method determines the current for a specific level of risk or probability of an unsafe condition arising. The primary benefit of using probabilistic rating is that it permits a utility to fully manage the risk inherent in overhead line operations. When desired, this method permits conductor ratings to be varied across geography, terrain, season and/or diurnal period while maintaining a constant level of operational risk. This risk can be expressed in terms of 1?×?10⁻⁶, for example, allowing the comparison of overhead line safety to that of a nuclear power plant or other structures.

The exceedence method determines the amount of time the conductor will exceed the design temperature at full rated load. The method is normally used assuming a flat load profile that is assuming that the overhead line carries full load at all times.

For uprating of lines, it is possible to make use of these methods taking into account the local weather conditions as well as the actual load profile on the line to determine possible increases in the line rating. Caution must be exercised relating to possible low wind conditions on the line route as the location of the weather stations are not likely to be within the overhead line right of way.

The use of this method of rating should not be used to revise the “book” rating on a permanent basis. It is a method to use for a short term and may also be used to determine the effectiveness of real time monitoring systems.

The following Figure 18.12 shows the difference in rating between a flat load profile (blue line) and the load profile of the overhead line (red line). For example for a 10?% exceedence for Hare conductor (105-Al/Si A-6/1) the rating can be increased from 290A to approximately 390A.

Figure 18.12

Variation of Conductor Thermal Capacity with Load Profile.

The method used in the rerating of lines is to determine the present exceedence limit used with the present “book” rating method or flat load profile. Then keeping this exceedence level constant, determine the new rating with the actual load profile. That is, if the Hare conductor “book” rating was 290A (blue line), the exceedence would be 10?%. With the actual load profile, and the local weather conditions, the rating could be increased to 390A (red line).

In addition, to this rating, care should be taken to inspect the line to ensure that the actual line design temperature complies with the original specifications. The line must be inspected for hot joints and broken conductor strands. With the new rating, an engineering assessment needs to be carried out to determine the possible maximum conductor temperature to ensure that there is no danger of annealing. (see Section 18.4.1.2)

18.4.1.8 High Surge Impedance Loading Lines (HSILL) (Cigré TB 353 2008)

HSILL is a technology that was initially developed in order to increase the capacity of overhead lines by maximising and equalising the electromagnetic field distribution on the conductors. The technique eventually evolved into an overhead line optimisation concept.

HSILL technology strives at a total optimisation of all significant electrical and geometrical parameters of an overhead line. In comparison, conventional overhead lines are designed on a step by step procedure, changing one parameter at a time and keeping some other parameters fixed such as the conductor cross section, number of sub-conductors per phase, phase conductors and bundle sub-conductors spacing and so on. Whereas the HSILL concept is a total optimisation process in order to reach more efficient and economical solutions.

HSILL concept represents a considerable change in the usual procedures such as the use of asymmetrical and or large conductor bundle configurations. The technology was initially developed for the overhead transmission of electrical energy from large generating units over long distances, to maximise overhead line capacity by operating the line at “the natural power” or the surge impedance loading (SIL). In recent times the technology has evolved to being an electrical design optimisation technique, known as expanded bundle technology (EXB), suitable for

• design of new overhead lines of high overhead capacity;

• uprating of existing overhead lines, to increase the overhead capacity; and

• modification of overhead lines electromagnetic parameters aiming to optimise the power flow distribution in overhead systems.

In the design of new overhead lines the optimisation of electromagnetic parameters leads to higher SIL and consequently a higher overhead capacity compared to conventional overhead lines. This is obtained by means of an optimised electromagnetic field distribution. Essentially increasing conductor bundle diameter results in increased shunt capacitance, reduced series inductance, reduced surge impedance to Z_s?=?√ (L/C), and increased surge impedance load resulting in increased power transfer.

In the uprating of existing lines, this technology explores different possibilities according to design characteristics of the original line. Either by the rearrangement of existing conductors or by addition of one or more conductors (not necessarily of the same type as the existing ones) the obtained result is an increased overhead capacity. Typical 230 kV in service overhead lines have increased the overhead line overhead capacity by 38?% in one case and 60?% in another case. In both cases the cost benefit ratio was 18?% and 25?% respectively compared to the total cost of a new overhead line. Typical HSILL overhead line capacity increases are illustrated in Table 18.3.

Table 18.3 Typical HSILL Overhead Line Capacities

In the optimisation of overhead lines, a variation of the technique is used when desired values of overhead line electrical parameters (mainly the reactance) are selected instead of deigning for the maximum possible overhead capacity of a single line. Obtaining the desired parameters results in a better power flow distribution among the overhead lines in a given corridor or network. This possibility could be one of the most promising applications of the HSILL/EXB technology, since the ability to vary overhead line parameters may provide considerable gains with reduced investments.

HSILL and or EXB techniques are therefore well suited either to design new lines or to refurbish and or uprate existing overhead lines. In cases where overhead capacity limits are associated with voltage limits, SIL optimisation of line parameters may provide a solution by reducing series reactance and offering an economical and technical alternative to series compensation. In those cases where overhead capacity is limited by thermal ratings, the use of EXB techniques may allow for a better distribution of current flows on the overhead system, postponing or even eliminating the need for replacement conductors.

Another advantage of HSILL and or EXB technology is associated with systems having significant thermal generation, where sub-synchronous resonance may become a problem if the needed series compensation levels are high. Here again, the adequate choice of overhead line parameters may eliminate the problem.

Finally, it has become quite noticeable in the last decade that a technique allowing for greater overhead line capacity in the same right of way has an economic contribution which extends far beyond the simple overhead line construction costs, since the environmental issues are quite favourably met by HSILL and or EXB overhead lines as they provide means for higher power density flowing on a given corridor. Examples of HSILL and or EXB overhead lines are illustrated in Figures 18.13 and 18.14, respectively.

Figure 18.13

230 kV Experimental HSILL Overhead Line.

Figure 18.14

500 kV EXB Overhead Line 744 km Long.

In summary, HSILL technology enables the design of overhead line configurations that optimises electric and magnetic field distributions and consequently the electrical parameters and the power overhead capacity of an overhead line, as well as the current sharing among different lines in the system.

18.4.2 Increasing Voltage Rating (Cigré TB 353 2008)

Increasing the voltage of an overhead network is generally the most effective strategic way of providing a quantum step change in overhead capacity. In general for each nominal voltage level the natural capacity of the network is increased by about 4 to 5 times. An example which is not complicated by Corona, radio interference and audible noise considerations is the simple case of increasing the voltage of an existing 33 kV overhead line to 132 kV with the existing conductor configuration by which the capacity of an overhead line will quadruple with relatively small marginal cost of line reconfiguration works. Another relatively simple example of uprating a 110 kV line to 275 kV is shown in Figure 18.15.

Figure 18.15

Overhead Line Structure Uprated from 110 kV to 275 kV.

This Section will discuss the basic electrical design requirements and strategies that may be implemented for existing overhead lines to increase the voltage rating. Given the enormous world wide variety of overhead line designs the discussion will be limited to the basic principles. An example of a typical overhead line voltage uprating study is shown in Table 18.4.

Table 18.4 Typical Overhead Line Voltage Uprating Comparisons

18.4.2.1 Requirements

The basic design considerations are

• clearances to ground, to support structures, to over crossings of other power lines, roads and railway lines and clearances to adjacent structures and vegetation;

• conductor motion and phase to phase electrical clearance between conductors;

• clearance between earth wires and conductors;

• insulation requirements for power frequency, switching and lightning surges;

• clearance for live line maintenance;

• conductor surface voltage gradient, Corona onset voltage and radio interference voltages which are influenced by conductor diameter and conductor bundle diameter; and

• audible noise.

Clearances

One of the main criteria for an overhead line is to provide sufficient vertical clearance to the ground, over crossings, objects, supporting structures and vegetation; horizontal electrical clearance to adjacent structures, objects and vegetation; and clearances between phase conductors and earth wires and conductors. This criteria must comply with statutory regulations and or industry codes. Consideration of uprating an overhead line from the present voltage to a higher voltage will require the application of the higher voltage criteria to the uprated overhead line.

Internal clearance to supporting structures is a further primary insulation criteria consideration. Critical flashover values for air gaps are required to be assessed for power frequency voltage and switching & lightning overvoltages and applied to the insulator and conductor configuration for the uprated structure. Consideration of critical flash-over voltages for insulator arrangements that are subject to horizontal swing movement are also required to be assessed to ensure satisfactory air clearance performance.

Earth wire to phase conductor clearance is a further important point to analyse if the line to be uprated was not originally provided with an earth wire. Structure extensions will probably be required to achieve adequate clearances and the body structure may need to be reinforced.

Phase to phase clearance and corresponding differential conductor motion is also a primary air clearance criterion for the uprated structure. Conductor motion may be induced by wind gusting, galloping, ice load shedding and fault currents.

In consideration of uprating an existing overhead line to a higher voltage rating, the design may result in the reduction of phase to phase clearances which will require special analysis of the phase to phase switching surge withstand. This analysis would include an examination of the maximum insulation stress and minimum insulation strength for higher voltage overhead line and also involve a probabilistic analysis of switching surge magnitudes and wave shapes with varying prevailing climatic variables such as air density, humidity, temperature and ice depositions.

In addition, internal clearance from conductor to structure is required to be provided for coinciding basic insulation level requirements and overhead line maintenance requirements.

Lack of adequate clearance could eliminate live-line maintenance operations or require more sophisticated and expensive maintenance procedures.

Insulation

For the insulation design of an uprated overhead line, an understanding of the critical flashover voltage transients and power frequency voltages and the corresponding insulation withstand is required.

Power frequency insulation design criteria is based either on the suggested geometric insulator creepage distance or the insulator deposited density of salt index for given pollution level. In consideration of uprating an existing overhead line the historical pollution performance of the line in general, would be well known and the opportunities to develop a design to meet the required new design voltage should be well understood. In addition, many insulator manufacturers are well placed to provide custom insulator designs to meet particular power frequency design criteria.

Transient voltages may be either lightning or switching surges. Switching surges determine the insulation design for higher voltage lines or lines that have low earth resistance or where lines operate in regions of low keraunic levels. Lightning determines the insulation design for lower voltage lines or lines that have high earth resistance or where lines operate in regions of high keraunic levels.

The insulation design will be determined from the primary variables of air clearances, the insulator geometry such as V-string, I-string, tension, post and or horizontal-V and the structure geometry. Secondary variables are size of conductor or possible bundle conductor configuration, phase to phase clearances, number of insulators and the application of Corona shields. Influencing climatic factors include air density, humidity, precipitation, temperature and pollution and ice depositions.

The outcome of the insulation design for an overhead line subject to voltage uprating is increased insulator surface creepage length and increased critical flashover transient voltage distances to meet the new voltage rating of the overhead line.

Audible Noise, Interference Voltage and Corona (Cigré TB 244 2004)

Conductor surface voltage gradient is an important factor to be considered when uprating the overhead line voltage. For voltage gradients above a critical level the conductor will commence the Corona phenomenon resulting in the production of noise and power frequency energy loss. The effect of Corona is visible light, audio noise and radio & television frequency interference.

The magnitude of conductor surface voltage gradient is dependent on operating voltage, conductor diameter, phase conductor spacing and in the case of bundled conductors the bundle diameter, bundle configuration and the number of sub-conductors in the bundle. Changing the conductor geometry results in a number of changes to the electric and magnetic fields, the radio interference voltage and the audible noise and these the effect on the changes of conductor geometry is illustrated in Table 18.5. The fields, Corona and other phenomena, their impacts and mitigation are covered in the Chapter 6.4.

Table 18.5 Influence of Electrical Parameters with Changes of Conductor Geometry

For voltage uprating projects involving significant changes in operating voltage, overhead lines with single conductor present significant difficulties to achieve satisfactory economical outcomes without the consideration of reconductoring with larger conductors and or the installation of bundled conductors. In these cases, the additional wind, weight and ice loads created by the new larger conductors and or conductor bundles may result in overloaded structures and or structure upgrading which is not economical. For example, a 132 kV overhead line with 3750 mm horizontal phase spacing with a conductor diameter of 25 mm would have a surface voltage gradient of about 12 kV.cm⁻¹. This line is under consideration for uprating to 220 kV. In this case the solution would require the installation of additional sub conductors to form a conductor bundle to meet a reasonable voltage gradient criteria and practical phase spacing. The installation of additional sub conductors may result in the structures being structurally overloaded making the uprating proposal unviable.

Notwithstanding this, there are many examples in the world where overhead line designs have been implemented in such a way that at some future point in time the line may be reconfigured to allow an increase in the voltage rating. An example of this is the operation of a double circuit 230 kV overhead line with suitable conductor diameter to meet the voltage gradient criteria with a horizontal conductor formation. At some time in the future, the line can be given over to a single circuit 500 kV quad bundle overhead line by aggregating the six twin bundle phase conductors into three phases with four conductors per phase bundles. The initial and final conductor separation is designed to meet both the single circuit 500 kV and the double circuit 230 kV configurations.

The concern of conductor surface voltage gradient is normally limited to overhead lines with small conductors, insufficient phase to phase distances and or operating voltage over 220 kV as the designed conductor spacing and conductor diameter for lower voltage overhead lines generally result in gradients that are below the criteria threshold. This is illustrated in Table 18.6 for two voltage gradient levels, 12 kV/cm and 18 kV/cm.

Table 18.6 Conductor Surface Voltage Gradient Versus Phase Spacing for a Single Conductor

The table indicates that in the case of a voltage uprating of a 132 kV line with a 30 mm diameter conductor to a 220 kV line, would require a phase spacing of or 3220 mm with a conductor voltage gradient of 18 kV/cm. If a conductor voltage gradient of 12 kV/cm were required, then a practical phase spacing would require a conductor in excess of 40 mm. It should be noted that conductor motion criteria would dictate that the 132 kV phase spacing should more likely to be about 2100 mm. Therefore, voltage uprating for single conductor lines is very dependent on the maximum allowed conductor voltage gradient, conductor diameter and phase spacing. Practical application of voltage uprating may require a larger conductor or bundled conductors.

18.4.2.2 Increasing Clearances

The first and most elementary consideration for increasing the voltage of an overhead line is to ascertain the phase to earth clearance of the line at the proposed higher voltage of the uprated overhead line. This will determine whether opportunities exist to modify the existing overhead conductor attachment height to accommodate the new voltage rating within the context of a cost effective technical solution.

For example take a 132 kV overhead line with a design ground clearance of 7.5 m and a 1 900 mm long cap and pin insulator arrangement suspended from a crossarm with conductors in a flat formation and the crossarm supported by two poles. Considerations is being given to increasing the voltage rating of this line to 330 kV which requires 9.0 meters ground clearance with an insulator string length of 2900 mm. In this case, the conductor attachment height is required to increase 1500 mm to compensate for the required new ground clearance and an additional 1000 mm to compensate for the additional insulator string length. The aggregation of the increase in the conductor attachment height is 2500 mm and the existing structure and insulator arrangement will be required to be redimensioned to accommodate this design.

A design option would be to consider a combination of insulated crossarms and the insertion of pole extensions into the existing structures or simply the insertion of pole extensions into the structure to raise the conductor by the required 2 500 mm. If this was required for every structure on the overhead line then this may not be considered a suitable technical and economical solution for the particular overhead line.

Notwithstanding, there are a number of cases where utilities have successfully increased the voltage rating of existing overhead lines by examining in detail, the line sections and selectively applying uprating options to existing structures.

Hence, one of the most significant considerations in increasing voltage rating of an existing overhead line is the capacity of the existing line design and supporting structure design to accommodate the required increased ground clearance within the economic constraints of the existing structure geometry. Other coinciding design verification considerations have been mentioned in Section 18.4.2.1.

18.4.2.3 Insulating Crossarms

The replacement of existing insulators with some form of insulating crossarm or modifying the existing insulator arrangement is the most common form of increasing the voltage rating of an overhead line by increasing the relative conductor attachment level to provide greater ground clearance and at the same time increasing the insulator creepage distance and transient voltage flashover distances. Section 18.4.2.1 details insulator crossarm and associated insulator modifications. Design verification considerations are also mentioned in Section 18.4.1.4 and in addition increasing the conductor attachment height of the structure requires an assessment of the foundation capacity and any increased overturning moments of the structure.

18.4.2.4 Structure Extensions

General experience indicates that given the magnitude of the required structural works it is not economical to increase the heights of all structures in an overhead line for thermal capacity uprating projects (see Section 18.4.1.). In the case of voltage uprating projects which tend to be traditionally more strategic long term network development projects, it may be considered a viable option to invest in the uprating of the line structures by using structure extensions, which could be either or both body extension or leg extension. The inclusion of a structure extension is more common to suspension structures given the inherent structural and practical difficulty of including an extension into a tension or terminal structure.

Design verification of increasing conductor attachment heights by the inclusion of an extension are mentioned in Section 18.4.1.4.

The technical limits of the inclusion of an extension would normally be determined by the structure and foundations ultimate load factor for the defined loading conditions.

18.4.2.5 Mid Span Insulators

During the design considerations of mid span clearances and corresponding conductor motion, in some cases sufficient clearances cannot be maintained without limiting mid span conductor movement. A mid span insulator arrangement is illustrated in Figure 18.6. The availability of light weight composite insulators provides an excellent way to limit and control mid span conductor movement. In these cases, the application of phase to phase conductor spacers allows greater utilisation of existing conductor geometry at existing structures and permits voltage uprating that would otherwise have been excluded from economic and or technical consideration (Figure 18.16).

Figure 18.16

Uprating from 245 kV to 420 kV by Changing Insulator Sets and Using In-span Insulator Sets.

18.4.3 AC to DC Overhead Line Conversion (Cigré TB 583 2014)

An overall network strategic of increasing the capacity of existing AC major overhead lines is by the conversion to DC operation and especially applicable for stability limited AC lines. Conversion to DC provides additional system benefits such as improved control of power flow and enhanced stability & reliability of the surrounding AC system. The DC voltage can often be higher than the existing AC phase to ground voltage without major interventions to structures or conductors as the existing structure air clearances can be utilized more efficiently in the absence of high switching overvoltages coupled with the Corona effects are less severe under DC operation. The major technical obstacle to conversion is the required insulator length in polluted conditions, while the major economic hurdle is the cost for the converter stations. This Section will discuss the possibilities and constraints associated with conversion of AC lines to DC and details a number of design considerations.

18.4.3.1 Requirements

The basic designs considerations are

• fundamental difference between AC and DC lines;

• DC line geometric configurations;

• Corona and field effects including audible noise;

• Insulation co-ordination; and

• economic including terminal stations.

18.4.3.2 Fundamental Differences between AC and DC Lines

Above about 245 kV, AC overhead lines are largely designed with respect to switching overvoltages and Corona effects, which are both related to the peak of the operating voltage, while the power capacity is determined by the rms voltage. DC lines, on the other hand, benefit from lower slow-front overvoltages and less severe Corona effects due to the influence of space charges, suggesting that some existing AC lines may be better utilized in terms of higher power transfer capability by application of a comparatively high DC voltage. However, insulator pollution is more crucial under DC and may be an important obstacle to conversion in polluted areas by limiting the attainable DC voltage level coupled with the existing insulators may not be suitable for DC operation due to corrosion effects. Both issues may generally be overcome by replacing the existing insulators with composite longrod insulators.

DC Line Geometric Configurations

Conversion of an AC line to DC can be done by various configurations, imposing different limitations related to the utilization of existing conductors as follows and illustrated in Figure 18.17:

Figure 18.17

Possible configurations for AC lines converted to DC.

• Monopole configurations utilize all three conductors for power transfer, but require current return through the earth which may not be allowed for several reasons;

• Bipole configurations utilize only two conductors for power transfer in normal operation, while the third conductor is used for metallic return under contingencies. Different conductor rearrangements may enhance the utilization of conductors in bipole configurations;

• Tripole configurations utilize all three conductors for power transfer to a certain extent by the use of an additional bi-directional converter; and

• Hybrid configurations comprise AC and DC circuits running in parallel and require special attention with regard to electrostatic and electromagnetic coupling between the circuits.

18.4.3.3 Corona and Field Effects

An important aspect of the Corona effects is that audible noise and radio interference from DC lines which decrease in wet conditions due to the influence of space charges. This is in contrast to AC lines where Corona effects in wet conditions is decisive for the design. As a consequence, the Corona effects in dry weather are among the parameters which determine the attainable DC voltage of a converted line. As an example, proposed limits for the audible noise level of DC lines are often about 10 dBA lower than for AC lines due to the longer duration of dry weather conditions. It should be noted that the highest audible noise level is produced by the positive DC conductor.

The mutual influences on the surface voltage gradient of the conductors when calculating the Corona effects of hybrid lines should be considered as a static charge will be induced on the AC conductors by the DC electric field, while time-varying charges will be induced on the DC conductors by the AC electric field. Hence, the voltage gradient on the surface of the AC conductors will include a DC component, while the gradient at the surface of the DC conductors will include an AC component. These electrostatic effects will influence the Corona activity on the AC as well as the DC conductors.

The field effects at ground level are also different for AC and DC lines. While AC electric fields are independent of Corona effects, the DC electric fields at ground are significantly influenced by Corona on the conductors and the corresponding generation of space charges. While the space charges limit the electric field at the surface of the conductors, the field strength is enhanced at the ground level. The resulting electric field in combination with the space charges may cause annoying perceptions for humans under the line, and is therefore another important parameter for determination of the attainable DC voltage level. Regarding possible health effects associated with the fields, the most important difference between AC and DC lines is that the electric and magnetic fields from DC lines are static, meaning that no induction effects are caused in the human body.

No internationally recommended limit has yet been proposed for static electric fields and Cigré TB 388 (2009) proposes 25 kV/m as limit in fair weather and 40 kV/m as 5?% exceedance level for inclement weather based on perception threshold. For hybrid configurations, the human sensitivity to the electric field is further enhanced due to the simultaneous presence of both AC and DC fields.

18.4.3.4 Insulation Coordination

When lightning strikes a DC line, the fast-front overvoltages appearing between the DC conductors and the tower depend on the magnitude and polarity of the lightning current as well as the polarity of the DC conductor. The DC voltage after conversion may be higher than the instantaneous voltage under AC, suggesting that the composite overvoltages occurring between conductor and tower on DC lines may be somewhat higher than on AC lines.

Slow-front overvoltage levels are rather low in DC systems, and the insulation design is often dominated by requirements on the pollution performance. Considering the space available on the towers, it is necessary to apply a detailed design approach for the DC insulators as described in Cigré TB 518 (2012). It is recommended to use the statistical approach to achieve an optimal dimensioning of the DC insulators for polluted conditions.

In many jurisdictions National regulations for the required safety clearance to ground are often expressed in terms of the AC system voltage and therefore not directly applicable to DC. However, if it is conservatively assumed that both fast-front and slow-front overvoltages are limited by flash-overs across the line insulators, the required safety clearances may be expressed in relation to the insulator striking distance by applying the appropriate gap factors for the respective air gaps and overvoltage types. Since slow-front overvoltage levels of DC lines are often low enough to prevent insulator flashovers, only fast-front overvoltages need to be considered for determination of the safety clearances.

18.4.3.5 Economic Considerations

The relation between capacity gain and costs for different uprating alternatives from AC to DC are discussed in general terms in Cigré TB 425 (2010). Regarding the specific costs for line uprating, many of the economic considerations are discussed in Section 18.3 also apply to the conversion of an overhead line from AC to DC with one important exception: the cost for terminal equipment will dominate in terms of the cost for the converter stations. In this respect, it is important to appreciate that the benefit of conversion is limited to incremental transmission capability while the cost for converter stations is governed by total capability. The effective cost multiplier as function of the capacity gain by conversion is illustrated in Figure 18.18. As an example, if the capability of a line increases from 1000 MW to 2000 MW by conversion to DC, the gain is 1000 MW while the converter stations must have a capability of 2000 MW.

Figure 18.18

Effective cost multiplier as function of the capacity gain by AC to DC conversion.

18.4.3.6 Feasibility Study of Hybrid Lines

Paper B2-105 to Cigré Session 2014 (Sander et al. 2014) describes a pilot project of a hybrid line in Germany. The related feasibility study is focused on finding the most favorable DC polarity configuration when one circuit of 380 kV double-circuit lines (equipped with twin conductor bundles) is converted from AC to DC. Main tower dimensions are shown in Figure 18.19.

Figure 18.19

Main dimensions of the two 380 kV double-circuit tower types.

Composite insulators are proposed to replace the ceramic longrod insulators in order to utilize the available space in the most efficient way. The dimension of the composite insulators is determined by statistical calculation for varying pollution levels in terms of 2?% Equivalent Salt Deposit Density (ESDD) level. Using laboratory pollution test data in combination with an acceptable pollution flashover rate and an estimated frequency of pollution events, the required insulator length at ±400 kV DC varies from 3.4 m to 4.8 m for ESDD levels from 0.02 to 0.06 mg/cm². The length of the existing AC insulators is 3.8 m.

Using the required DC composite insulator lengths, the maximum allowable conductor sag is determined for the ruling span while respecting the minimum conductor clearance to ground as well as the maximum conductor temperature. Since the required DC insulator length exceeds the existing AC insulator length at higher pollution levels, the maximum conductor sag is limited either by the minimum conductor clearance to ground, or by the maximum conductor temperature. The maximum conductor temperatures are used to calculate the maximum current ratings at varying ambient temperature utilizing the Cigré basic rating method (Cigré TB 207 2002). The corresponding thermal power rating is calculated for 380 kV AC and ±400 kV DC as shown in Figure 18.20. Since the transmission capacity of an AC line is often limited by stability constraints, the surge impedance loading (SIL) is indicated as well.

Figure 18.20

Power capacity of AC and DC circuits at varying ambient temperatures for different pollution levels and corresponding DC insulator lengths.

18.4.3.7 Experimental Studies of Hybrid Lines

Paper B2-105 (Sander et al. 2014) also presents results from comprehensive experimental studies of Corona and field effects as well as insulation coordination aspects of hybrid lines. The line configuration under study is shown in Figure 18.21. Since the bipolar DC circuit is positioned at the same side of the tower as one of the AC circuits, and since the line is equipped with quadruple conductor bundles, the Corona and field effects cannot be directly compared with the effects of the twin conductor bundle line used for the feasibility study.

Figure 18.21

Hybrid line configuration under study.

The basic dielectric design is based on the insulation coordination procedure given in IEC 60071–2 (1996). For determination of the required air clearances to ground, to the neutral conductor and between AC phase and DC pole conductors, laboratory tests on were carried out model arrangements as shown in Figure 18.22.

Figure 18.22

Model arrangements for dielectric tests on hybrid line.

The slow front overvoltages occurring during earth fault are decisive for the air clearances on DC lines. Therefore, switching impulse tests were carried out to find the gap factor K of the configuration in question. Based on these results, the minimum air clearances to the crossarms as well as to the earthed return conductor were established for different DC voltages assuming an earth fault overvoltage level of 1.7 p.u.

For determination of air clearances between AC phase and DC pole conductors, a combined voltage stress has to be applied. Therefore the AC conductor was subjected to switching impulse voltage and the DC conductor to DC voltage, and vice versa. For combined voltage stress a gap factor K has to be taken into account which is not only depending on the arrangement but also on a factor α which describes the relation of the negative component to the total component (sum of negative and positive components). By means of these factors the required air clearance between phases can be obtained for different DC voltages assuming an earth fault overvoltage level of 1.7 p.u. for the DC conductor and a switching overvoltage level of 2.3 p.u. for the AC conductor.

External air clearances have to be considered with regard to safety, in particular lightning over-voltages; the overvoltage shall not cause a flashover between conductor and earthed objects, but shall lead to flashover across the insulator. As the DC insulators will be longer compared to the AC insulators, the flashover voltage will also be higher; thus the required external air clearance has to be adapted. Laboratory tests were carried out to find out the lightning withstand voltage of the DC insulators in order to determine the required air clearance between conductor and earthed objects for a gap factor of 1.3.

For investigation of the DC ground-level electric field and ion current density a similar model arrangement as for the dielectric studies was applied. The field strength was recorded by a field mill and the ion current by a plane electrode. The results are presented in Figure 18.23 for conductor heights of 10 and 15 m. It has to be noted that the ion current is strongly dependent on the ambient conditions, in particular the wind speed, consequently a noticeable scatter is observed.

Figure 18.23

Measured DC electric field and ion current density at ground level.

Audible noise measurements were conducted in one span of a 2.5 km test line. The measuring arrangement is shown in Figure 18.24a. The line has a bipolar configuration similar to type 2 in Figure 18.19 with the positive DC conductor in the top position. The minimum conductor clearance to ground is 17 m. To simulate the capacitive coupling with the adjacent AC circuit, a DC voltage of 450 kV and 490 kV was applied. The results are presented in Figure 18.24b. The investigation demonstrates that at the edge of the ROW (Right of way), which is about 60…80 m, the AN is already decreased so far that the level admitted in Germany during night time is fulfilled, even if a DC voltage of 490 kV is assumed.

Figure 18.24

Arrangement and results of audible noise measurements.

Due to Corona discharges on the DC conductors, a DC current is injected into the AC conductors on a hybrid line. The DC current may increase the magnetizing current of power transformers and inductive voltage transformers, possibly leading to saturation effects. Depending on line configuration and ambient conditions, the ion current may amount up to 15 mA/km. Consequently, remedial measures are of interest for long hybrid line, in particular for lengths of more than 100 km. To study the compensating effects of line transpositions, measurements of the ohmic coupling were carried out on the test line. The test configuration and the results are shown in Figure 18.25. The measurement results reveal that the ion currents injected into the adjacent AC conductors differ despite the symmetrical arrangement and nearly equal DC Corona currents; this can be explained by ion current drift in the wind direction. Thus, complete compensation cannot be expected in practice; however, the current in conductor 3, which is arranged in the middle between the negative and positive conductor demonstrates that a significant compensation effect can be achieved.

Figure 18.25

Test configuration and results of ohmic coupling measurements.

18.5 Overhead Line Upgrading

The definition of upgrading is increasing the original structural strength of an overhead line element and or component due to a requirement for higher meteorological actions and or electrical performance. Upgrading will decrease the probability of failure. Upgrading is often associated with uprating.

A prime example of upgrading is the meteorological data collected for many years may show that there could be higher ice and wind loads on overhead lines compared to the loads the lines were originally designed to withstand. Thus overhead lines may need to be upgraded to maintain reliability standards. In addition, new overhead line modelling techniques and risk based probability design approach allow more detailed analyses to assess adequacy and risks associated with various overhead line components. Upgrading of an overhead lines depends on a number of factors and some of these factored are detailed in Table 18.7.

Table 18.7 Factors Effecting Life and Performance of Overhead Line Elements

Generally the upgrading concerns the whole overhead line or some part of it, but there are some exceptional cases, such as the upgrading of only one structure due to installation of telecommunication equipment. Unanticipated rapid deterioration of an overhead line component may also require upgrading.

A comprehensive engineering study is required to consider all available upgrade options. Since overhead lines have many different design configurations and material choices, subsequently choices of upgrade solutions are often numerous. An engineering study should consider all choices available and assess their reliability levels, financial costs and practicability (availability of manpower, materials, outage requirements, etc.). When assessing financial cost of a given option, a life-cycle cost benefit analysis should be used to select the most cost effective solution and the associated project implementation timing. The engineering study should take into account further conditions of the maintenance of the line and whether live line maintenance is required. Such a decision could influence the design of some components such as structures, fittings and insulator strings.

18.5.1 Structures

18.5.1.1 Background

The overhead line structure is a key component and provides support to other components such as conductors and/or insulators. The structure has the most physical and aesthetic impact on the general public and private property. Overhead line structures are expected to perform satisfactorily for a very long time and often for 50 years or more.

During the course of an overhead line lifespan, the structure may be expected to carry additional functions or loads in excess of those specified in the original design. This is quite common as many utilities where network operators and system planners demand greater utilization from the existing overhead lines and thus strategically, resources are dedicated on planned overhead line structure upgrades.

Upgrading an overhead line structure might also be as a result of in service failure due to unexpected weather events and or human made causes. In many cases, asset owners will only inventory higher strength suspension structures of a particular design class, for emergency spare structures. Typically, it is the lower strength suspension structures that fail and the structures will automatically be replaced with the higher strength equivalent structures, thus upgrading the line at the failed locations. If the failed overhead line is redundant or has low priority in the network, an upgrade option might be chosen instead of an emergency situation restoration. In this case the entire line, not just the failed portion, might be considered for upgrading.

18.5.1.2 Verification Considerations

Successful upgrading of overhead line structures depends on a number of factors.

Upgrade Studies

It is possible that the original structure capacity was not fully utilized during installation for various reasons such as unusual terrain conditions or perhaps site specific restrictions of availability of materials and therefore has reserve strength capacity. In such cases, structure upgrade can be achieved with minimum effort. In these cases it is critical that the original design assumptions be re-examined to confirm the basis of the design.

Availability of Original Design Information

It is essential to know the assumptions used in the design of the original structure such as strength and the parameters of the materials used and the design loads. This enables a designer to determine the existing structural capacity of the structure and to perform studies necessary to increase it.

Lattice overhead line structures are often of an old vintage. Some very old structure designs might have not been well documented. In some cases the properties of the steel material are unknown and member properties not documented making it very difficult for an engineer to model such designs. Material testing and careful engineering assessment may be required in these cases.

Assessment of Field Conditions

A field inspection is necessary to determine the condition of the existing structure which has been exposed to both the environmental elements and the overhead line loads. Chapter 6.3 lists the generic techniques that are currently available and can be used to find these defects in-situ. Such inspection might reveal reduction of capacity of individual structural members which might lead to a lower overall capacity of the structure. It may also trigger a need to replace these members in order to restore the original strength. Steel structures located in high industrial environment might be showing reduction in strength of the members due to corrosion of the steel.

Wood poles in particular should be treated with a detailed engineering assessment since they are products of nature and their physical properties are somewhat unpredictable. Furthermore, these properties change with time depending on environmental conditions. It is thus necessary to determine physical dimensions of the wood pole and condition of the wood to determine its strength. Wood pole deterioration may occur at higher rates in parts of the wood pole with drilled holes where there is potential for water and humidity to penetrate the wood.

Chapter 13.8 covered the detail review investigation of foundation assessment so that decisions can be made as to whoether to refurbish, upgrade or accept the current condition of installed foundations.

18.5.1.3 Technical or Practical Limitations

Overhead line structures offer designers multiple choices and large flexibility for performing upgrades. However structure upgrades have also technical and practical limitations, most of which were listed in Section 18.3.6. Additional limitations in upgrading structures include,

• variability in wood pole sizes - unlike other man-made materials, wood poles are natural product, come in variable sizes and their supply depends on harvesting patterns used by wood pole producers which may lead to difficulties matching required size and strength.

• supply of wood poles - availability of wood poles, especially in lengths exceeding 30 metres, may be limited.

• availability of steel angles sizes -in many cases, especially when original overhead line structures are of a very old, it might be difficult to find matching lattice steel members. Reasons for discontinuation of certain sizes may be new design codes, conversion of measure system (ie British to Metric) or different supply sources. This may lead to adjusting the detailing of joints to fit different size members.

Wood Pole Structures

Wood is a common material for building overhead line structures in many countries. Wood poles offer flexibility in designing custom structures whether it is required to match site specific conditions or to provide fast design to address emergency needs. Wood poles are easy to handle and assemble in the field. Upgrading wood structures may be involve,

• replacement of wood poles with higher timber class or of larger diameter;

• replacement of wood poles with poles of stronger species;

• replacement of wood poles with engineered laminated wood products;

• the addition and or replacement of braces;

• replacement of wood cross arms by steel arms or use of reinforcing metal channels to provide higher bending moment capacity or to increase structure height; and or

• use of stay (guy) wires to improve structural horizontal capacity.

Steel Lattice Structures

Steel lattice towers have been used successfully throughout the world as a design choice for overhead lines for almost a century now and are often considered as good candidates for upgrading considerations. A number of upgrade options exist for steel lattice towers and include,

• doubling or duplication of angle sections (often leg members);

• replacement of angle sections with larger section members;

• addition of redundant and or diagonal members;

• reconfiguration of low strength sections;

• addition of guy (stay) wires;

• upgrade of bolts to higher grade;

• use of tower extensions to raise conductor height; and

• use of longer length cross arms.

18.5.2 Foundations (Cigré TB 141 1999; Cigré TB 308 2006)

Foundations are located for the most part under ground level and the foundations are critical to the security and reliability of an overhead line. In general, a foundation failure will have significant consequences for the structure and may result in the failure of the structure of an overhead line. Failure of a foundation will result in long restoration times.

18.5.2.1 Background

Upgrading foundations generally arises from changing environment and load requirements from unexpected and extreme weather events that produce load excursion. Some of the load conditions that must be considered are wind loadings, snow and or ice loadings, landslides loads and or snow creep loads.

After a failure event, design loads are often recalculated. Generally this will lead to an upgrading of the structure design and often an upgrading of the foundations. Upgrading can be applied to the entire overhead line or for a limited number of structures and foundation in an existing overhead line.

New Standards and Legislation

In particular, catastrophic events can cause changes in Standards and or Legislation. In general, Standards will apply to new structures and foundations, however in some jurisdictions may also apply to existing overhead lines. New understanding of structure loadings can also lead to changes of existing Standards. In this case, for existing overhead lines, an assessment will be carried out where the changes in the Standard will be followed. Changes in Legislation for example in the case of electromagnetic fields (EMF), can lead to changes in overhead line conductors and thus changes in foundations. In some cases these changes can lead to adaptation of other structure configurations and this will have an influence on the foundations.

Changing the Function of a Structure

Changes of an existing overhead line by for example additional connections and or the conversion of structures to terminate cables can change the function of the structure such as suspension structure may be changed to an angle or a termination structure, and thus the new loads on a structure, may require the foundation to be upgraded.

Changes in Conditions Near Foundations

Frequently as a result of construction or mining in the vicinity of a structure, the soil characteristics around the foundation change and the design conditions are no longer valid. As a result these foundations may require to be upgraded of or reinforced.

Increasing Structure Height

There are a number of causes for which existing structures must be raised. Increase wind loads on the conductors of the overhead line will influence the foundations. Raising structures will lead to a redetermination of the structure foundation which may require the foundation to be upgraded. Causes for increasing structure height can be:

• larger sag of conductors as result of higher operation temperature mostly within the framework of uprating;

• new infrastructure under overhead lines such as roads, railways and other overhead lines; and or

• placing buildings or other infrastructures under overhead lines.

Uprating

Uprating options mentioned in Section 18.4 often require upgrading of the foundation.

18.5.2.2 Verification Considerations

Successful upgrading of overhead line foundations depends on a number of factors that must be considered.

Upgrade Studies

It is possible that the original foundation capacity was not utilized during installation for various reasons such as unusual terrain conditions, site specific restrictions of availability of materials and therefore has sufficient unused capacity. In such cases, foundation upgrade may be achieved with minimum effort. In these case it is critical that the original design assumptions be re-examined to confirm the basis of the design.

Availability of Original Design Information

It is essential to know the assumptions used in design of the original foundation such as strength and parameters of materials used and design loads. This enables a designer to determine the existing load capacity of the foundation and to perform studies necessary to increase it.

Foundation designs of some very old structure designs might have not been well documented. In some cases there may be deviation in the construction practice deviates from the design drawings. Material assessment and testing and careful engineering reviews may be required in these cases.

Assessment of Field Conditions

A field inspection is necessary to determine condition of the existing structure and foundation exposed to both the environmental elements and the overhead line loads. It may be necessary to excavate near the foundation to inspect the below ground condition of the foundation. Such inspection may reveal reduction of capacity of the foundation.

18.5.2.3 Technical or Practical Limitations

Beside the general constraints which are mentioned in Section 18.3.6 there will be for the upgrading of foundations several technical and practical limitations. These limitations may lead to restriction in the working methods, use of equipment, material and tools, and also in the design of upgrading of the foundation. It is possible that the technical and practical limitations make upgrading of the foundation impossible and an entirely new foundation must be chosen. Some examples of the limitations for foundation upgrading are:

Outage Availability

For upgrading foundations heavy equipment will normally be required. In general, the larger the upgrading of foundations, the heavier the equipment will be. Depending on the size of equipment used for upgrading it may be necessary to de-energize the overhead line. When it is not possible to de-energize the overhead line or the outage time is limited then this may require a redesign of the upgrading employing smaller equipment. Accordingly, the time necessary for upgrading a foundation and the associated outage availability is a key design and planning consideration.

Construction of Existing Foundation

There are several types of foundations for overhead lines. The use of any particular category of foundation will depend to a degree on both the support type and the geotechnical conditions present. Typical foundations are spread footing, shaft and anchor and are illustrated in Figures 18.26, 18.27 and 18.28 respectively.

Figure 18.26

Spread Footings Foundations.

Figure 18.27

Shaft Foundations.

Figure 18.28

Anchor Foundations.

Depending on the type of foundation used for the existing structure the solutions for upgrading may be limited. In some cases, it may be necessary to use different type of foundations to upgrade an existing foundation. For example, a ground anchor foundation illustrated in Figure 18.28 may be used in combination with an existing pad and chimney foundation illustrated in Figure 18.26 as an effective solution.

Permission from Land Owners

For privately owned land, upgrading an existing foundation may require the consent of the land owner. It may be a constraint when the upgrading of the foundation could limit the land owner for using the ground near to the structure.

Safety Issues

Working on existing foundations will have an effect on the stability of the structure. This is especially when the upgrading activities may be executed in windy areas or in windy periods and thus the use of temporary constructions may be required. The type of temporary construction may have effect on the outage time and also on the consent of the land owner. The use of temporary construction will have significant effect on the costs of upgrading the foundation.

18.5.3 Insulator Strings

Insulator strings are a key component of an overhead line providing the mechanical connection between conductors and support structures and at the same time electrical insulation between live line parts and earthed parts. They are covered extensively in Chapter 10 of this book. Depending on the environmental conditions, new insulator strings may be expected to be in service for over 40 years.

18.5.3.1 Background

Upgrading of an insulator string is achieved either by increasing the mechanical strength or by improving the electrical insulation performance. The methods of increasing the mechanical strength are limited such as strengthening of the weakest component in the string or multiple strings, the options to improve the electrical performance vary considerably and generally, the major objective is to improve the insulation pollution performance.

A pollution flashover of an overhead line is initiated by deposits of airborne contaminant particles on the line insulators surfaces. Under dry conditions, these deposits are harmless however, when they are wetted by light rain, fog or high humidity events, the salts in the contaminants dissolve, forming a conducting film on the surface of the insulators. This film reduces the leakage distance across the insulator surface and compromises the insulators withstand power frequency voltage capacity. If the withstand capability falls below the designed stress level, a pollution flashover will occur. The contaminant particles may be of natural origin or they may be generated as a result of industrial, agricultural, or construction activities. Overhead line flashovers occur due to a range of containments such as sea salts, road salts, cement dust, fly ash, potash, limestone and gypsum and may arise from a changing environment where the overhead line is located.

Depending on the frequency and the amount of rain, a varying amount of the contaminants could be collected on an insulator before they are cleaned by a natural rain washing event. During the period between the natural washings, if the insulator contamination level reaches the critical value, the moisture, the light rain or high humidity event could cause a flashover event.

A contaminated insulator flashover is more damaging to an overhead line reliability than a lightning or a switching flashover. Generally, an overhead line can be successfully reclosed after a lightning or a switching flashover, but a contaminated insulator flashover is frequently followed by additional flashovers if the atmospheric moisture producing condition persists and may not be able to be re-energized until the pollution is removed from the insulator.

The overhead line pollution performance is a function of the pollution level of the environment which may range from light to very heavy and determines the minimum nominal specific creepage distance for the insulator. The insulator pollution performance is therefore a function of the creepage distance and also the shape of the insulator.

Upgrading a overhead line pollution performance may consist of replacing the insulators with increased creepage distances and or introducing insulators with deep ribs such as a fog shape designs. In some arid countries an aerodynamic flat profile shape has been found to be satisfactory to minimize pollution flashover events. Upgrading of insulator strings is also required in cold climates where freezing rain could cause icicles and salt to form between various insulator units thus bridging the gap and reducing leakage distance.

Other mitigation techniques consist of the application of hyperphobic silicone greases, recurrent insulator washing, the use of semiconducting glazed insulators and or installation of polymeric or composite insulators. Other methods of insulation performance improvement are,

• increasing the arc distance by increasing the number of insulators or the length of insulator;

• increasing the leakage distance by increasing the number of insulators or providing a different shape of insulator; and or

• changing the pollution characteristics of the insulator using different shaped insulators for fog or desert environments or different insulation materials such as non-ceramic materials.

Choosing one or a combination of the mentioned methods depends on the overhead line geometry, progress in new material investigation, expenditures and the service experience of the utility.

18.5.3.2 Verification Consideration

Although new analysis methods and latest investigation applications assist designers to specify all possible loads and stresses on components, experience from in service operation remains one of basic sources of data when considering upgrading of some components. Designers should take into account,

• environmental conditions stated in the original design;

• parameters such as the mechanical and electrical strength of original components;

• operation experience and the records of failures and other events from extreme and or critical loading incidents;

• causes of any future deterioration such as vandalism, birds, industrial pollution, et al;

• availability of alternative designs and material technology from the market;

• traditional solutions for a particular place or country; and

• standard inventory considerations.

Upgrading of insulator strings affects other parts of an overhead line such as structures and conductors, consideration should also include the impact on all line components and the associated condition.

Verification of Parameters

Many utilities have established engineering standards which according to coordination of insulation levels specify parameters of the components of an insulator strings. Such standards assist designers to choose the proper components in accordance with the specified standards.

Mechanical Strength

When crossing any highway, railway or waterway, multiple suspension insulator strings or increased the strength class of the insulator is often used to reduce the probability of failure. In addition, deterioration of certain associated fittings should be considered as part of any upgrading option.

Pollution Performance

Improving the pollution performance of insulation is improving the ability of an insulator to resist contamination in ambient air. In some arid countries an aerodynamic flat profile shape has been found to be satisfactory to minimize pollution flashover events. The mitigation techniques may also consist of the application of hydrophobic silicone greases, recurrent insulator washing or the use of semi conducting glazed and polymeric or synthetic insulators with good hydrophobic properties.

Radio and TV Interference Resistance

Loose, poorly fitting and unearthed hardware can cause radio and TV interference. Minimising the number of connections or the application of hold down weights on lightly loaded insulator strings may decrease possible sources of radio and TV interference.

Verification of Used Insulation Material

Glass and ceramic insulators have a long history of satisfactory experience. The main advantage of glass insulators is that after glass failure the mechanical connection is maintained thus avoiding a dropped conductor. The ceramic insulators are produced from stable ceramic porcelain material and have excellent service performance over many decades. Both types of insulators are easy to add to or replace using live line techniques. Ceramic and glass insulators are both heavy and fragile. The composite insulators are a relatively newer type of insulator and an overhead line upgrading application is growing as the insulators,

• have a low weight strength ratio;

• are less sensitiveness to the mechanical impact;

• have high bending strength and have excellent application in rigid geometries; and

• have exceptional resistance to flashover in contaminated areas due to the hydrophobic characteristics of some composite materials.

18.5.3.3 Technical and Practical Limitations

New types of insulators allow a variety of modifications for upgrading and applications such as V and T strings and coupled with the variety of possible insulation materials such as glass, ceramic or composite allows flexibility to meet different requirements for shape and strength.

Continued development of new composite insulation materials has resulted in limited service experience which hinders an understanding of the materials longer term performance experience. Accelerated laboratory and field tests are necessary for performance and lifetime estimation of these types of insulators.

In addition, demands for minimizing of operation outages are practical limitations that effect planning of upgrading of an overhead line. Live line maintenance technique is one of the solutions for upgrading insulation without taking a line outage.

18.5.4 Upgrading or Improving Electrical Characteristics

The electrical parameters that provide opportunities to upgrade an overhead line are improvements in lightning performance or outage rate; improvements in insulator pollution performance; improvements in Corona, radio & television interference and audible noise; reductions in earth potential rise; reductions in electric and magnetic field (EMF) levels and reductions in induction in adjacent long parallel metallic infrastructure such as pipelines and or metallic telecommunications. These upgrading strategies are discussed in detail in the following Sections.

Reducing the levels of EMF of an overhead line may not be considered as upgrading the electrical characteristics of the line as the outcomes do not reduce the probability of failure. However these changes will reduce the environmental impact of an overhead line. Notwithstanding this, the opportunity will be taken to briefly discuss in the following Section strategies to reduce EMF.

18.5.4.1 Lightning Performance

The determination of the lightning performance of an overhead line requires a mathematical study based on probability involving nonlinear complex electromagnetic behaviour of the interactions of the lightning, conductors, insulators, the structure and the earthing. Minimizing the probability of outages due to lightning is a critical aspect of overhead line design and is normally undertaken at the conceptual stage so the structure configuration, insulation levels and earthing may be coordinated to achieve the desired level of lightning protection. The major factors that affect the lightning performance of an overhead line are,

• the keraunic level or ground flash density;

• the stroke current magnitude and wave shape;

• the structure height;

• the presence of an overhead earth or shielding wires and the associated geometry of the overhead earthwire relative to the phase conductors;

• the conductor phase to structure clearance;

• the midspan clearance between conductors and overhead earthwires;

• the insulator arcing distance;

• whether the structure is conductive or non-conductive and for nonconductive structures such as wood whether the fittings and crossarms are bonded and or earthed or unearthed; and

• for earthed structures the structure earth resistance.

With the exception of the intrinsic keraunic level, stroke current magnitude and wave shape, upgrading the lightning performance of an overhead line may be achieved by modifying one or a number of the mentioned factors. Opportunities to improve the lightning performance of overhead line will focus on the three principal lightning flashover mechanisms as follows:

• insulator and or structure flashover for overhead lines without overhead earthwires;

• shielding failures for overhead lines with overhead earthwires; and

• back flashovers.

Firstly, insulator and or conductor to structure flashover arises from either a direct lightning strike on the conductor and or for lines generally above 275 kV switching transient over voltage flashovers. In either case, the transient over voltage performance of an overhead line may be improved by installing insulators with longer insulator arcing distance and or increasing the conductor to structure clearance. In most cases, the original conceptual design of conductor, structure, insulator geometry has been optimized and any transient voltage upgrading opportunities for overhead lines without overhead earthwires will most likely require a complete reconfiguration of the insulator and crossarm arrangement as additional clearances are not likely to be practicable or possible. In some cases, it may be effective to add additional weight at the bottom of the insulator I strings to reduce insulator swing and increase conductor to structure clearances. For overhead lines without overhead earthwires, improvements in transient voltage performance for at least lightning strikes may be achieved by improving the shielding of the conductors by the installation of overhead earthwires or the installation of surge diverters. This will be discussed in detail later. In the meantime, a typical suspension insulator string illustrating, maintenance approach distance, lightning withstand clearance and power frequency clearance is shown in Figure 18.29.

Figure 18.29

Examples of Swing Conditions for Clearance and Wind Criteria.

Secondly, a common outage cause for overhead lines with overhead earthwires is a lightning shielding failure where the lightning strikes the conductor directly without being intercepted by the overhead earthwires. Typical shielding arrangements with single and double earthwires are illustrated in Figure 18.30. For overhead lines with existing overheads earthwires the lightning performance will be improved by reducing the shielding angle. This would normally be achieved by redesigning the overhead earthwire and conductor geometry and this may include changing the structural attachment point of the conductor or earthwire or changing the insulator design. Differing shielding angles by differing overhead earthwire attachments resulting in superior lightning performance are illustrated in Figure 18.31.

Figure 18.30

Typical Shielding Arrangements for Single and Double Earthwires.

Figure 18.31

Typical Shielding Failure Rates for Varying Shielding Angles (Cigré TB 353 2008).

Similarly, installation of overhead earthwires for overhead lines without overhead earthwires, will result in a significantly improved lightning performance. The number of overhead earthwires is a function of the required outage performance and the available marginal structural capacity. Double circuit overhead lines and overhead lines with flat conductor configuration without overhead earthwires will in general require the installation of two earthwires to achieve a satisfactory outage performance. The installation of one or two overhead earthwires to reduce shielding failures will contribute additional wind structural loads to suspension, tension and termination structure and additional tension loads to tension and or termination structures. In this regard, structures may require substantial modification and structural capacity assessments to accommodate the additional overhead earthwires. Additional foundation loads is also a fundamental consideration.

It is also important to mention that any changes in clearances may eliminate opportunities to carry out live line maintenance.

The installation of surge diverters for overhead lines with or without overhead earthwires is also an effective technical mechanism to improve transient over voltage performance and is used widely in some countries. The installation of diverters for some overhead lines with optimized design may provide the only option to improve transient over voltage performance. This may also be the only viable solution for overhead lines with an unacceptable level of switching transient over voltage flashovers. The outage rate of the overhead line is a function of the number of diverters installed per kilometre and the earthing resistance of the diverter. For example a 66 kV overhead line without overhead earthwire, a structure earthing resistance of 5 ohms with an expected 35 strikes per 100 km.year⁻¹ and diverter installed with a separation of 200 m would result in an outage rate of 0.15 per 100 km.year⁻¹. Increasing the diverter separation to 500 m would result in an outage rate of 4.9 per 100 km.year⁻¹. Clearly this example illustrates that diverters are novel technical solution to improve the over voltage performance of an overhead line, nevertheless the overall effectiveness of the solution would be the subject of a cost benefit analysis.

The final mechanism discussed is the back flashover and is the predominant cause of lightning induced flashovers on overhead lines. The sequence of a back flashover event leading to an insulation failure is,

• a lightning strikes an earthwire at or near a structure;

• current flows in the overhead earthwire(s) towards the structures either side of the lightning strike;

• the current flows down one or more structures;

• the current in the overhead earthwire induces voltages in the phase conductors;

• the current which flows down the structures is reflected at the earth;

• the reflected current establishes a voltage on the structure;

• the current which flows to the other structures via the earthwire is reflected at each structure and earth;

• when the voltage across any part of the overhead line insulation exceeds its electrical insulation strength, there will be a back flash-over from structure to the conductor; and

• often there will be a number of simultaneous back flashover failures.

Reduction of the incidence of back flashover is achieved by one or all of the following strategies,

• increasing the insulator arcing distance and or increasing the conductor to structure clearance thereby reducing the probability of back flashover;

• reducing the structure earth resistance as illustrated in Figure 18.32 thus reducing the structure voltage to earth and the voltage across the insulators and reducing the probability of back flashover; and

• reducing the conductor earthwire separation which improves the coupling and increases the conductor voltage relative to the earth thereby reducing the voltage across the insulators and the probability of back flashover.

Figure 18.32

Typical Outage Rate as a Function of Structure Earth Resistance.

The most common and generally the most cost effective strategy is reducing the structure earth resistance.

18.5.4.2 Corona, Radio & Television Interference and Audio Noise Mitigation

In general, for overhead lines above 200 kV, Corona discharges form at the surface of conductor and or hardware when the electric field intensity on the surface exceeds the breakdown strength of air and the air ionizes. Corona results in electrical losses which are proportional to the length of the overhead line. Several conditions control the breakdown strength, the air pressure, the electrode material, incident photo-ionizations and the type of voltage.

The presence of small protrusions such as water droplets, snowflakes, contamination or protrusions or sharp points on the insulator and or hardware surfaces may produce elevated electric fields and thus air ionization. The ionization of air generates light, audible noise, radio noise and in some extreme cases conductor vibration.

Minimizing the likelihood of Corona is a critical aspect of overhead line design and is normally undertaken at the conceptual stage in the selection of conductors, the bundled diameter and configuration for multi-phase conductors and the insulators fittings such as grading rings.

Corona activity may result in radio and television interference. The primary concern for interference is for amplitude modulated signals. Frequency modulated and television broadcasting signals are much less affected by Corona.

Audible noise arises from Corona discharges and consists of humming, crackling, frying or hissing characteristics. The intensity of the Corona noise is influenced by the weather conditions and the noise increases during periods of rain, snow and fog. The noise level is inversely proportional to the square of the separation distance and as one moves away from the overhead line then the level of noise reduces significantly.

Cigré TB 147 (1999) indicates that from surveys carried out that Corona noise is not a major problem for existing overhead lines. Nevertheless opportunities to upgrade the performance of the overhead line may arise as Corona may occur on contaminated insulators, or as a result of looseness or protrusions in hardware, or on imperfect or damaged conductor surfaces or from inappropriate insulator fitting designs and grading rings.

18.5.4.3 Reductions in Structure Earth Potential Rise

Most higher voltage overhead lines are normally earthed at every structure location to provide a local low impedance circuit to the body earth for transient earth fault currents. Low impedance earthing is necessary to minimize the possibility of elevated touch and step voltages in the vicinity of the structure in the event of an earth fault. High touch and step voltages may result in unsafe structure voltage and a risk of electric shock and the possibility of electrocution. The structure earth potential arises from a fault current flowing to earth via the structure and hence the magnitude of the potential rise is a function of the fault current and the structure earth resistance.

Low impedance earthing also minimizes the risk of structure voltage rises exceeding the flash-over voltage of an insulator string causing and indirect lightning strike outage or back flash over as mentioned in Section 18.5.4.1.

Allowable step touch potentials are generally defined in various jurisdictions through standards, regulation and or codes and one such example is IEC 60479. IEC 60479 defines the physiological effects of body current as a function of duration of current flow.

The design of the overhead line earthing system will depend on the structure earth resistance criteria and will vary with the soil resistivity along the route of the line. Two methods are commonly used to achieve an effective earthing system for an overhead line, the provision of a continuous earthwire and the placement of locally buried structure earth electrodes having a low earth resistance. The earthwire provides an alternative return current path to the source reducing the magnitude structure earth fault current and hence the structure earth potential rise. Typical earth fault structure current paths are illustrated in Figure 18.33.

Figure 18.33

Typical Overhead Line Earth Fault Structure Current Paths (Cigré TB 353 2008).

Upgrading to reduce structure potential rise may consist of installing additional structure earthing, installing buried grading rings at step locations to reduce step touch potential, installing larger overhead earthwires to reduce the current flowing in the structure, the application of insulating paints and or epoxies to structure touch locations and or the installation of high speed protection schemes to reduce electric shock duration.

There are basically three types of methods to upgrade structure earthing and are counterpoise, metal cladding and deep drilling. All the methods use either a galvanized steel or copper rod that is placed in a hole at various depths in the ground and lengths from the structure leg and each method will be discussed in more detail.

The aim of implementing structure footing improvements is to achieve a low resistance connection between the structure and an earth. The soil type and resistivity are basic determining factors and influence which method of earthing is economical and technically feasible. Counterpoise and metal clad methods can only be applied in normal ground soil conditions and the deep drilling is normally applied in rocky terrain. In areas where there is basically only rock alternative methods like line surge diverters may be considered as the only feasible option to improve lightning performance of the line.

Counterpoise is the method that is mostly used for the earthing of structures. The method consists of a metal strap or copper rods that is connected to the structure leg at the bottom and is buried in the ground in a trench at an average depth of 0.5 m. It is buried normally at an angle of 45⁰ and the distance is dependent on the soil resistivity and the required structure earthing.

Metal clad earthing consists of either copper rods or galvanized steel rods connected to the structure at the structure leg foundation interface. The rods are buried in a trench of about 0.5 metres and are then horizontally buried for about 7 meters long before it is buried in vertical holes.

In some circumstances soil resistivity is very high requiring special techniques such as deep drilling. Deep drilling method is the placing of copper or steel rods in a hole bored and varies in depth depending on the soil type and resistivity.

18.5.4.4 Reductions in Electric and Magnetic Field Levels

Electric and magnetic fields (EMF) from overhead lines and concerns over suggested health effects has emerged as a major public policy issue for utilities worldwide. Cigré has considered the subject and the associated debate and published in Cigré TB 147 (1999).

The strength of the EMF depends on both line voltage, line current and on the conductor geometrical parameters. The electric field decreases rapidly with lateral distance from the line and is further reduced by grounded objects like trees, lamp posts, buildings and other structures. The magnetic field also decreases rapidly with lateral distance from the line. Minimizing the EMF is a critical aspect of overhead line design and is normally undertaken at the conceptual stage in the selection of overhead line phase conductor geometry. For existing overhead lines existing conductor and structure geometry presents major technical constraints to retrospectively modifying the design to reduce the EMF.

Nevertheless, increasing the line height and or using a triangular phase configuration are the most effective ways of reducing the maximum electric fields at ground level. Opportunities to minimize magnetic fields also include phase reversal for double circuit overhead lines, screening conductors and employing split phasing. In some cases, the right of way width may be increased to allow lower EMF at the edge of the right of way.

18.5.4.5 Induction Mitigation

As human habitation development continues particularly in urban fringe areas and restrictions are placed on the flexibility to select linear routes for infrastructure, the likelihood and community pressure for cohabitation of linear assets will increase. As a result, it has become common for overhead lines, pipelines, conveyors, rail traction systems and metallic telecommunication networks to coexist in infrastructure corridors.

There are three electromagnetic interference mechanisms between an overhead line and coexisting long conductive infrastructure and are inductive, conductive and capacitive coupling. The induced voltage due to inductive and conductive coupling is directly proportional to the overhead line current. The induced voltage due to capacitive coupling is proportional to the operating voltage of the overhead line. The level of induced voltages is influenced by the overhead line and other infrastructure separation distance and the parallel length. Other factors include:

• earthing of the parallel infrastructure and the overhead line;

• soil resistivity;

• conductivity of the parallel infrastructure;

• cathodic protection equipment install on the parallel infrastructure;

• presence of shielding of the parallel infrastructure

• presence of isolating joints in the parallel infrastructure;

• level of insulation on the parallel infrastructure.

The opportunities for the implementation of mitigation strategies on existing overhead lines are limited. Some options to be considered are magnetic field shielding, relocation of structure earthing as the structure earthing should be separated from the adjacent infrastructure as far as practical, minimizing earth currents by introducing conductor transpositions and or the isolation of shielding earth wires and finally the reconfiguration of the overhead line phase conductor geometry.

18.6 Highlights

Uprating and upgrading overhead lines is a very complex planning, technical and economical process. There are many economic considerations and many technical options and the planning and implementation of overhead line uprating and or upgrading may involve considerable amount of time to formulate an appropriate cost and technically effective solution.

The Chapter describes in some detail the range of economic considerations coupled with an extensive range of uprating and upgrading options. Most importantly the Chapter highlights various design options and associated verification considerations to provide guidance on practical solutions to uprating and upgrading overhead.

18.7 Outlook

From about the early 90’s and onwards many countries with major electrical infrastructure were frequently confronted with four coinciding critical issues, much of this infrastructure was constructed in the 50’s and 60’s, which results in the age of the assets being over 50 years; the design life of much of the infrastructure was in many cases about 50 years and has matured beyond the engineering serviceability and or economic life and required some form of life extension; the need to increase capacity of the existing infrastructure places extraordinary demands on utilities to establish strategies to uprate the existing infrastructure; and the approvals for the construction of new overhead lines are often difficult to obtain and in many cases result in critical delays to meet network capacity needs.

Since about 2000, further consideration of renewal energy is also having a significant influence of network design and capacity needs. Some planning forecasts projected to 2050 even suggest energy storage will be economical for domestic energy consumers and “leaving the Grid” may be a viable option.

Accordingly, given the considerable uncertainty about “future network design and topography,” coupled with the cost of newly constructed overhead lines and the associated environmental impediments, is anticipated that there will be further and increased emphasis on increasing the utilisation of existing electrical infrastructure and this will included overhead lines. And thus upgrading and uprating of overhead lines will continue to be one of the major planning options considered.

The future will also see further development in insulation properties and conductor materials and construction techniques which will enhance the range of uprating and upgrading options currently available.