1 Background

Prior to the adoption of polymer-based insulation products, the dominant cable insulation material used in power cables was an impregnated fluid Kraft paper composite insulation encased in a lead sheath. Although this cable design provided excellent in-service life, it is slowly being replaced by polymers for the following reasons:

  • Environmental concerns with the lead sheaths used on fluid impregnated paper insulated cables,

  • Reduced maintenance costs for polymer-insulated cables dielectric insulation,

  • Loss of expertise required for installing fluid-impregnated paper-insulated cables,

  • Reduced installation costs for polymeric-insulated cables,

  • No fluid leaks to locate and repair,

  • Weight reduction (no lead sheath required), allowing for the installation of longer cable lengths,

  • Reduced risk of fire during earthquakes,

  • Reduced dielectric losses [1].

From an historical perspective, the following timeline details the introduction of polymers as a power cable insulation material.

1812:

First power cables used to detonate mineral ores in Russia.

1880:

DC cables insulated with jute in “Street Pipes”—Thomas Edison (USA).

1882:

Thomas Edison used rubber-insulated cables on Pearl Street in New York.

1890:

Ferranti developed the concentric construction for cables.

1900:

Cables insulated with natural rubber.

1917:

First screened cables.

1925:

The first pressurized paper cables.

1925:

Rubber-insulated cables could support voltages up to 7500 Volts.

1903:

PVC first used in Germany.

1933:

A natural rubber alternative was discovered in Germany by IG Farben.

1937:

Polyethylene developed.

1942:

First use of Polyethylene (PE) in cables.

1944:

Butyl rubber was commercialized by Standard Oil.

1954:

First DC power transmission cable—Gotland (Sweden) subsea connection.

1955:

Ethylene propylene rubber (EPR) was developed.

1962:

EPR insulated power cables commercially available.

1963:

Invention of cross-linked polyethylene—XLPE.

1967:

Use of HMWPE insulation on UG cables in the US (Unjacketed with tape shields).

1968:

First use of XLPE cables for MV (mostly unjacketed, tape shields).

1972:

Problems associated with water trees (MV) and contaminants (HV) first identified in unjacketed HMWPE and XLPE cables.

1972:

Introduction of extruded semi-conductive screens.

1973:

Superclean materials used for 84 kV Aland (Sweden–Finland) XLPE subsea cable.

1978:

Widespread use of polymeric jackets in North America.

1982:

Water Tree Retarding insulating materials introduced for MV in USA and Germany.

1988:

First use of XLPE cables (without joints) at 500 kV within a pump storage scheme in Japan.

1989:

Supersmooth conductor shields introduced for MV cable in North America.

1990:

Widespread use of water tree retarding insulating materials in Belgium, Canada, Germany, Switzerland, and USA.

1993 and 1997:

Long-term 400 kV qualification tests at CESI Italy for BEWAG Berlin (Germany) and Copenhagen (Denmark) projects.

1999:

World’s first commercial DC cable using XLPE—Gotland (Sweden) 80 kV.

2000:

First 500 kV long distance XLPE cable with joints installed in Tokyo (Japan).

2001:

World’s highest voltage ac XLPE cable – Dachaoshan Power Stn. (China); 525 kV.

2002:

Murraylink (Australia) ±150 kV HVDC 171 km

2018:

First HVDC XLPE submarine at 400 kV HVDC—Nemo, Belgium to UK, 130 km.

A detailed overview of the early developments is provided by RM Black in “The History of Electric Wires and Cables” [2].

Today power cables use primarily solid-extruded insulation compounds of polyethylene in its cross-linked form. Thermoplastic polyethylene was first used, but quickly evolved into longer lasting cross-linked materials. In North America, high molecular weight polyethylene (HMWPE) was also used with poor results. For medium voltage cables, either cross-linked polyethylene (XLPE), ethylene propylene rubber (EPR), or polypropylene (PP) are used as insulation for medium voltage cables. For high and extra high voltage cables, mainly XLPE is used.

This chapter describes the commercially available XLPE compounds and the differences that exist in these compounds.

Most of the XLPE compounds that are used today are based on high pressure tubular reactor low density polyethylene (LDPE). This process maximizes the chance of producing clean compounds, one of the many requirements for power cable insulation compounds. On the other hand, EPR is produced in a different way and then compounded as cable insulation. EPR is mainly used for medium voltage cables and HV cables up to a maximum of 150 kV due to higher dielectric losses.

At the beginning of this century, polypropylene (PP) was introduced as an insulation compound for medium voltage cables and has recently seen equal market share with XLPE in several countries. With PP, the final formulation is produced during the cable production process. Further investigations are currently underway using other polymers such as polyolefin (POE) or polybutylene (PB-1) for medium voltage applications. PP is currently used up to 150 kV for AC cables and has passed Type and Prequalification (PQ) testing for 600 kV DC cables.

This chapter presents the differences between cable compounds and examines the specific features of individual XLPE compounds that are currently available on the market. In addition, details will be presented on the application of different polymers that are currently used for the semi-conductive layers as part of the cable insulation system.

After the discovery of water trees in the 1970s, more stringent requirements for the insulation were introduced, for example, cleanliness of the insulation must be at a certain level to avoid initiation of water trees. Furthermore, it should be highlighted that some of the properties of polyethylene may vary within commercially available materials. Also, some reflections on the inherent variability of a material that is produced in production campaigns of many thousands of tonnes will be discussed. This chapter will also go into more detail on commonly used cross-linked polyethylene (XLPE), which is the main material used for power cable applications [3]. Moreover, it should be noted that cables insulated with EPR can have water trees, only the detection is more difficult than for XLPE-insulated cables.

The differences that exist will be described in commercially produced polyethylene, specifically high-pressure, low density polyethylene (LDPE), since this is most commonly used in power cable applications as the base resin. The basic “surprise” is that LDPE, even if it is called hydrocarbon, contains not just these two basic atoms, hydrogen and carbon. A basic knowledge of the production process reveals that oxygen might be added into the hydrocarbon chain. Indeed oxygen may have a significant influence on the electrical properties and in this case not just “oxidized particles” which are generated later during the production process. This oxygen might come from the chain transfer agent or the initiators that are used when producing LDPE in the reactor.

Insulations for cables, unlike almost all other polymer components are required to have a very long operational life (>50 years) and are installed in complex locations which make their replacement very problematic. Thus, as a consequence of the need for longevity, the different antioxidants used will also influence the overall properties of the cable insulation. They will additionally influence the decomposition of the peroxide that is used for cross-linking of polyethylene. The chemistry of the peroxide needs to be examined further.

2 Production of XLPE

XLPE is a compound where peroxide, antioxidant and maybe other additives are added to LDPE. To completely understand the different performances of XLPE it is important to understand the production process for LDPE and the variations that can occur between different manufacturing processes and producers.

2.1 Production of LDPE

First of all, normally only low density polyethylene (LDPE) is used for the insulation of power cables. In some rare cases, medium density polyethylene might be used, but most products are produced by a high pressure process. The catalytic low pressure process is not used to produce polyethylene that is the base material for XLPE. The structure of low density polyethylene is very simple, just an endless combination of C2H4-groups, Fig. 1.

Fig. 1
figure 1

Theoretical picture of LDPE

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Already, this is not correct, as LDPE is in fact a branched polymer with long-chain and short-chain branching, plus the long chains might be the backbone of another chain (Fig. 2).

Fig. 2
figure 2

Branched LDPE

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The first production of polyethylene by Pechmann only worked because the autoclave was leaking. Oxygen was unknowingly introduced into the process during the production of LDPE where oxygen is needed in addition to carbon and hydrogen. Today, oxygen is introduced either directly or via peroxides. The type of peroxide might vary from producer to producer and with the product produced. This oxygen is incorporated with a low amount into the polyethylene chain. The amount of oxygen in the chain has an influence on the electrical properties such as loss factor and resistance. Simultaneously, a chain transfer agent is added into the reactor. This agent regulates the chain length of the polyethylene and is added into the chain of LDPE [4]. The reactor conditions are up to 2500 bar at 400 ℃. LDPE can be produced in a tubular reactor or an autoclave reactor. The first reactor is the preferred method to produce LDPE for XLPE [5].

A low density polyethylene is characterized by the following parameters:

  • Melt index at 2.16 kg, 190 ℃

  • Melt index at 21.6 kg, 190 ℃

  • Molecular weight distribution

  • Melting point

  • Short length branching

  • Long length branching

  • Terminal vinyl

  • Vinyliden content.

These parameters are not exclusive.

2.1.1 Influence of the Chain Transfer Agent

In the polymerization reactor, organic peroxides dissociate homolytically to generate free radicals. Polymerization of ethylene proceeds by a chain reaction. Initiation is achieved by addition of a free radical to ethylene. Propagation proceeds by repeated additions of monomer. Termination may occur by combination (coupling) of radicals or disproportionation reactions. Chain transfer takes place primarily by abstraction of a proton from a monomer or solvent by a macro radical. A low molecular weight hydrocarbon, such as butane, may be used as chain transfer agent to lower molecular weight. Termination reactions illustrated how that the end groups in LDPE are most commonly a vinyl group or an ethyl group. In addition to the use of chain transfer agents, molecular weight may also be varied by adjusting pressure and temperature. Higher pressures lead to higher molecular weight. Branching tends to increase at higher temperatures. Reactivity ratios are important in determining reactor “feed” composition of ethylene and co-monomer required to produce a copolymer with the target co-monomer content. Because the relative proportion of co-monomer changes as polymerization proceeds, adjustment of co-monomer feed with time may be necessary [6].

During the polymerization of ethylene, different types of chain transfer agents (CTA) like alkanes, α-Olefinens, or ketones are used. Figures 3 and 4 show the influence of commercially used CTAs on the loss factor of cross-linked polyethylene with a common antioxidant (AO 1) for this process. The loss factor has been measured at a low stress level with a Schering Bridge using a low electrical field and a pressed plaque as the test specimen. On a tested cable, the loss factor can be significantly higher depending on an increased electrical field. The influence of antioxidants on the electrical properties will be explained.

Fig. 3
figure 3

Loss factor depending on the type of CTA used [1]

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Fig. 4
figure 4

Loss factor depending on the temperature and amount of chain transfer-agent CTA 2 [7]

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Depending on the process and the producer, the amount of CTA could vary. Figure 3 shows the influence of the amount of CTA on the loss factor. As mentioned earlier, this influence could increase with a high electrical field. Certain types of CTA will create a polyethylene that is not suitable for cable applications.

In a recent study by Fothergill et al., the influence of the CTA on the electrical resistivity was demonstrated [7]. It was mentioned in this study that decomposition products of peroxide will increase the differences between these two polymers. Additionally, if the use of different types of peroxides is taken into the equation, only variation estimates can be achieved from this test.

During the last years, dienes have also been used as CTA [7, 8] to increase the reactivity of the polyethylene. The use of these dienes has much increased the reactivity of the polyethylene with peroxide. The total influence of the CTA on the properties of XLPE has not completely been studied and care should be taken if we transfer older knowledge to this cross-linking process since it is not a first order chemical reaction anymore. The dependence of conductivity on the type of CTA is shown in Fig. 5.

Fig. 5
figure 5

Conductivity depending on the type of CTA [7]

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2.2 Cross-Linked Polyethylene

2.2.1 Cross-Linking Process

After this short introduction on polyethylene, it is now important to look more into the cross-linked polyethylene (XLPE) which is commonly used as an insulation material in the power cable industry. The different processes of cross-linking will not be discussed since this would go too far. Instead the focus will be on the peroxide cross-linking process since this is the predominant process in the industry. The chemical structure picture of cross-linked polyethylene normally given in the literature is very simplistic. However, recent studies by Smedberg et al. have shown that this picture is not correct and XLPE looks more like a spider web with different types of cross-linking points [10]. It was demonstrated in this research that besides chemical cross-linking, there are also physical cross-links that are nearly equally as strong and both influence the structure of the cross-linked polyethylene. A knowledge of the amorphous and crystalline structures will help to understand what is happening with these structures during the cross-linking process and manufacture of the cable.

2.2.2 Influence of Peroxide and Antioxidant

Polyethylene and especially cross-linked polyethylene are never used without peroxide and antioxidants. The choice of these additives will influence the morphology of the insulation. So far there have been limited studies done evaluating the influence of different peroxides. This is maybe due to the fact that dicumyl-peroxide is normally used as a cross-linking agent. However, several studies have been done evaluating the morphology using different antioxidants. The crystallization temperature (Tcr) of a pure LDPE was found to be 91.9 ℃. This temperature can be influenced by additives like antioxidants. Several combinations of antioxidants were mixed with polyethylene such as:

  • 1,1’-Thiobis-(2-methyl-4-hydroxy-5-tert-butyl-methylphenol (AO1)

  • Pentaerythiritol-tetrakis-(3-3,5-di-tert-butyl-4-hydroxyphenyl)-propionate (AO2)

  • Octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate (AO3)

  • 4,4-Thio-bis(2-tert-butyl-5-methyl-phenol) (AO4)

  • Dioctadecyl 3,3’-thiopropionate (AO5)

There are other antioxidants on the market and in recent years, suppliers have come up with liquid antioxidants. The highest increase of Tcr was noted for the samples mixed with AO5 of 97 ℃—a noteworthy increase in crystallization [8]. It should be noted that with a classical peroxide cross-linking process, AO1 is grafted to the polymer back-chain during the cross-linking process and therefore does not migrate and stays stable inside the cable during the material life time.

2.2.2.1 The Influence of Antioxidants

Several different mixtures of antioxidants are used in commercial XLPE formulations. The antioxidants are chosen both for their long-term protection (since cables are designed for a life-time of fifty plus years) and also for the interference or lack of interference in the cross-linking process. Several antioxidants are also used in combination. The influence of these antioxidants on the properties of XLPE have been evaluated in many studies with some highlighted here. Campus et al. demonstrated that a LDPE with methyl-ethyl-ketone (MEK) or Butanon-1 as chain transfer agent has large fully rounded spherulites which are present only in LDPE base resins without additives. Antioxidants like AO1 will influence these structures [9]. The advantage of this antioxidant is that it is grafted during the cross-linking process at the polyethylene backbone and therefore cannot be removed by solvents or other physical processes [10]. In a study by Fukunaga [11] the influence of different antioxidants on the crystallinity and conductivity of polyethylene was evaluated. The polyethylene used was produced in a tube reactor with propane as the CTA. This used AO5 together with a phenolic antioxidant AO2 and showed that the conductivity was dependent on the concentration and type of antioxidants.

The choice of a good antioxidant is very important since it not only influences the ageing behaviour of the cable but also the electrical properties in dry and wet conditions. The antioxidant should also stay within the cable over the total life time of 50 years (plus) and not exudate.

Campus, et al., confirmed [10] that the cross-linked segments of XLPE are situated in the amorphous phase and therefore do not modify the crystalline structure on the level of the elementary cells. They also confirmed that with (AO4), the crystallinity decreases from 48% to 45% for this particular LDPE [12]. The electrical properties like tree inception voltage were studied by Kjellqvist et al. [13]. He used as a base polymer a polyethylene that had been produced on a tubular reactor. More details about the polymer were not revealed in this study. However, he mixed several different antioxidants and combinations and their influence on the tree inception voltage compared to a virgin polymer. It is worthwhile to mention that this, as previously expressed, only one of the parameters for how to choose the additives.

2.2.2.2 The Influence of Peroxide and Cross-Linking Parameters

For low and medium voltage peroxide cross-linkable materials, silane cross-linking is utilized as well as radiation and UV cross-linking for low voltage materials. Dicumyl-peroxide is commonly used for insulation compounds, as this is a solid peroxide with a melting point of around 40 ℃. It is melted and added via a so-called soaking process to the insulation compound (Figs. 6, 7, and 8).

Fig. 6
figure 6

Tree inception voltage depending on antioxidant type

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Fig. 7
figure 7

Structure of dicumyl-peroxide

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Fig. 8
figure 8

Structure of tert Butyl-benzyl-peroxide

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Also in certain cases, liquid peroxide can be used at room temperature.

This peroxide has a melting point of 10 ℃ and can therefore be handled by pumps without prior melting. However, it also exudes faster and is therefore not recommended for compounds that need to be stored or transported. Another peroxide is used for semi-conductive compounds; Bis(1-tert-butyldioxy-1-methylethyl) benzene. This peroxide exists in a meta and a para-form and is also solid at room temperature. It reacts slower than dicumyl-peroxide and is therefore preferred for these compounds to prevent scorch (Fig. 9).

Fig. 9
figure 9

Structure of 1,4-Bis(1-tert-butyldioxy-1-methylethyl) benzene

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The decomposition products of peroxide and the way they are created are described in several publications [14] and are mainly acetophenone (AP), α-methyl-styrene (MS), cumylalcohol (CA), methane and water. Methane should be removed from the cable for safety reasons [15]. However, the other cross-linking by-products will influence the electrical properties under AC and DC conditions. The creation of acetophenone and cumylalcohol is a competing reaction and, depending on the cross-linking temperature, one or the other is preferred. Cumylalcohol can further react to form α-methyl-styrene and water [12]. In this study, the amount of chemically created water is dependent on the curing time and temperature. It clearly demonstrates that today’s thinking of faster and faster curing times might not be optimal for a good cable, but unfortunately economics are forcing the cable maker to go that way. This study was done on a polymer that was produced on a tabular reactor with CTA 1 and AO1. The amount of water created was calculated by the amount of α-methyl-styrene, since each time there is also a small amount of “other” water inside the polymer.

It is known that acetophenone increases the AC water tree inception voltage, decreases the insulation resistance and increases the space charge after a DC voltage application. In addition, it has recently been pointed out that the dielectric loss factor of PE increases considerably at high temperatures and in high electrical stress regions because of conductivity enhancement, in spite of PE being a non-polar polymer. This study was carried out on a polyethylene with a melt index of 3, produced in a tubular reactor. The authors soaked acetophenone into the polymer and also measured impurities of the added acetophenone. However, it is not clear if any acetophenone created inside the polymer would have had the same effect [14]. This leads to another subject on the influence of curing by-products, which were studied by Aida et al. [16]. This study was performed on an unknown polyethylene and discussed the influence of the by-products. It was demonstrated that during the time when these by-products evaporate, the electrical losses and the electrical resistivity will change. This only measured by-products created when using dicumyl-peroxide.

Hayami et al. evaluated the influence of these by-products on the leakage current under DC stress [17]. He recommended to remove all by-products to reduce the leakage current, but under practical conditions this will not be possible. One might reduce these products to an acceptable level determined by the manufacture and customer. It should be noted that in a real cable a semi-conductive layer will also be present, and in most of the commercial compounds supplied, at least for higher voltages, these compounds have a different peroxide as a cross-linking agent, Bis-(Tert-butyldioxyisopropyl) benzene. This peroxide is a bis-peroxide and has, besides methane, completely different reactive by-products to dicumyl-peroxide, which are also far more difficult to remove. These by-products present in lower amounts, will also migrate into the insulation during cross-linking and with further processing of the cable, influence the electrical parameters. In addition, the low molecular weight polar components of the semi-conductive compounds will migrate into the insulation changing the properties of the latter. This was confirmed by a study of Diego, et al. on a medium voltage cable with a strip-able screen [18]. The migration of low molecular weight from the outer and inner semi-conductive layers into the insulation is dependent firstly on the chemical structure of these layers and their molecular weight distribution. The temperature in the cross-linking tube and the time will influence the migration from the outer layer. Additionally, the type of cross-linking tube, whether Vertical Continuous Vulcanizing line (VCV), Catenary Continuous Vulcanizing line (CCV) or Mitsubishi, Dainichi Continuous Vulcanizing line (MDCV or Long Land Die) has an influence on the stress introduced into the cable and this factor will also influence the electrical properties of a cable.

Choo et al. studied the space charge accumulation under temperature and electric field gradients on a commercial medium voltage cable [19]. Taking into account that the construction of these cables was relatively unknown and that interpreting results for a commercially produced DC cable application, and since the insulation and semi-conductive materials were specially selected for this application, it is recommended to use a non-polar screen with acetylene black for both the inner and outer semi-conductive screens. This was previously described in a patent from the former Alcatel Cable.

Generally, the compounder uses dicumyl-peroxide, which decomposes into several by-products (Table 1). The influence of these by-products themselves is evaluated under several conditions. The ratio of these by-products during the cable production nevertheless depends on the chemical composition of the XLPE itself, the production parameters and the degassing process. It should be noted that these are the by-products only for the peroxide used in the insulation. Generally, in Europe, different peroxides are used in the semi-conductive compounds.

Table 1 List of peroxide by-products
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In addition to the above by-products, additional decomposition products may be present from the other peroxides and antioxidants that have been used inside the compounds for power cable insulation.

2.3 Compounds for Medium Voltage Cables

Resistance to water tree growth is very important for medium voltage cables since many of these cables are now produced without water barriers.

2.3.1 Water Treeing

During the early 1970s medium voltage cables were failing at a far too early stage. Intensive research discovered that tree-like dendritic structures were the reason for these failures. Since they were the result of water accumulation, these structures were called water trees. Water trees are small tree-like or dendritic growths that appear in the polymeric insulations of medium voltage power cables. They frequently look like bushes or trees and improved optical microscopy techniques concluded that every water tree is made up of chains of water filled voids, which are alignments of microcavities. Water trees grow during the service life of power cables from defects where the electric field is amplified. The longer the water tree, the more dangerous it becomes, because the insulation breakdown voltage decreases with the increase of the length of water trees. Or in other words, the dielectric strength in the immediate area of a water tree decreases. In Fig. 10, a dielectric failure occurred through a streamer water tree where the dielectric strength was reduced by the presence of the streamer water tree. The failure occurred during an AC withstand test on a full-size MV cable.

Fig. 10
figure 10

Electrical failure through a water tree during AC withstand testing on full-sized MV cable

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Another very important phenomenon in power cables affected by water trees is the conduction process. This is also of great interest for both users and manufacturers of cables because electrical conduction is one of the factors that reveals the level of degradation within the cable. Concerning polymer conductivity, the study of the mechanisms of thermo-electrical degradation of the polyethylene insulation is essential to develop improved assessment strategies.

The creation and growth of water trees can be traced to the following reasons:

  1. 1.

    Dirty compounds, physical and chemical contaminants.

  2. 2.

    Unclean handling of the compounds.

  3. 3.

    Voids or bubbles in the insulation.

  4. 4.

    Graphitised insulation screens.

  5. 5.

    Poor interface between screens and the insulation.

  6. 6.

    One plus two or two plus one extruder cross heads.

  7. 7.

    Steam curing and the resultant orange peel effect as well as an internal water halo.

  8. 8.

    No protection against water migration into the cable core.

  9. 9.

    Pure polyethylene has poor resistance to the growth of water trees.

It should be noted that a water tree alone will not lead to cable breakdown, but it reduces the dielectric strength of the insulation in the immediate area of the water tree. Eventually an electrical tree will initiate and thus a breakdown will occur (Fig. 11). An electrical tree is initiated at a water tree by over-voltages such as switching surges and/or lightning. Water trees in cable insulation can be present for many years without cable failure, but as soon as electrical tree forms, failure can be very quick by comparison. Also, of note, there is no partial discharge when water trees are present and growing, but partial discharge initiates as soon as an electrical tree appears.

Fig. 11
figure 11

Electrical tree (above) and an electrical tree initiated from water tree (below)

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Water tree structures are identified as vented trees, which start from the screens, and bow-tie trees which grow inside the insulation (Fig. 12). The reason for vented trees is normally an irregularity inside the screen or the interface of the insulation. For this reason, interfacial smoothness requirements are written into cable specifications along with chemical cleanliness of the screens to avoid ion migration into the insulation. Bow tie trees will grow from a void or contamination inside the insulation. But for water trees to grow, a certain concentration of water is required of 70 phr. Below this level, it will be a “dry” cable. There is always some ppm of water inside a cable, since water is created during the cross-linking process as described earlier.

Fig. 12
figure 12

Vented trees in XLPE [3]

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Water trees in fact consist of an accumulation of water. If cable core wafers are left exposed to air, the water within the water tree will evaporate and the water tree structure will visibly disappear. To make a permanent record of a water tree, a staining method is widely used that often contains methylene blue die. Hence, the blue colour of most examples of water trees. It is very well documented for XLPE and most of their tree retardant variations, but for EPR or polypropylene, other staining agents need to be used, if not an erroneous assumption will be made that there are no water trees present.

2.3.1.1 Testing Compounds for the Resistance to Water Tree Growth

Since water trees in field aged cables can only be observed after several years in service, tests were developed to accelerate the growth of water trees. This test program should replicate as close as possible real cable life, but yet achieve this objective in a reasonable time. The most common tests last between one year and two years. There are several tests used in the industry to evaluate compounds, but only two tests on cables are recognized around the world to evaluate the cable performance in wet conditions. It should be emphasised that these tests accelerate the growth of water trees, but yet by changing one of the parameters will give different results. In IEEE Guide 1407, the different parameters are highlighted. The worldwide accepted tests are either according to CENELEC HD 605 protocol or according to ICEA S-97-682. Both tests will give different results. The CENELEC HD 605 protocol utilize two different frequencies, 500 Hz for 3000 h, and 50 Hz for two years.

Recently, CIGRE introduced an additional requirement for wet high voltage cables used in array systems [20]. In this program, the water, that is be used, should have a salt content of between 3.0 and 6.0% plus, the test-program should be carried out following specific requirements. There are also other local tests carried out that should be handled with care. Most of the tests just evaluate the insulation material with the influence of processing and the semi-conductive screens and therefore can only give a restrictive picture of cable performance. Additionally, if the test requirements are too remote from life-time requirements, the real performance of a cable may not be assessed. It has been shown in many tests that an Arrhenius approach, by doubling the frequency or increasing the temperature, or by adding salt content to the test water will only work up to a certain level. Above a certain accelerating factor threshold level the test will evaluate a different behaviour of the materials under a completely different set of conditions. It should be also noted that the staining effect is not as easy as it seems (Table 2).

Table 2 Conditions for water tree tests
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2.3.2 Cleanliness

As already mentioned, early XLPE compounds were quite dirty, as at the time, there was no appreciation for the need of cleanliness, since water trees were not yet discovered, and contamination issues had not arisen. Moreover, recently, new processes have been used where other peroxides are used making the decomposition products different, so their influence needs to be investigated.

Additionally, tests have been performed using the same compound, once before the material handling was upgraded to a Class 1000 clean room and once again afterwards. Also, the influence of cleanliness on the long-term performance has been demonstrated.

In Fig. 13, a two-year wet ageing test was carried out with a current XLPE compound that does not fulfil the requirements in terms of cleanliness for medium voltage application. Three of the six cables failed during the test at 3 Uo (Fig. 14).

Fig. 13
figure 13

Influence of cleanliness on the results of a two-year test according to CENELEC HD605

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Fig. 14
figure 14

Influence of cleanliness in material handling on the relative results of a two-year test according to HD605

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2.3.3 Material Handling

Furthermore, the handling of these compounds, for the program presented in Fig. 13, was done in a non-clean way. For medium voltage applications, it may not be necessary to use a clean room, but clean handling should be one of the recommendations that includes a closed feeding system to avoid the introduction of outside contaminations, like dust and insects, for example.

2.3.4 Influence of Extrusion

As mentioned earlier, historically, the first cables were produced either with separate extrusion heads or with a so-called 1 + 2 or 2 + 1 extrusion heads. Here, the inner semi-conductive screen was extruded through a separate head and only the two other layers were extruded with some distance in between. In the worst-case scenario, the hot inner screen was transported through plain air. In Fig. 12, the two-year results of two cables are compared; in one case, the cable was produced using a 2 + 1 extrusion head. First the conductor screen and the insulation screen were extruded together and then after roughly 50 cm distance the insulation screen was extruded. This resulted in a 50% drop in AC breakdown strength after a two-year ageing test program. The ageing process as outlined in the shape or β factor did not change. The reference cable was produced with a triple head, as it is the case for nearly all cables today and also required by most of the utilities (Fig. 15).

Fig. 15
figure 15

Influence of triple extrusion

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2.3.5 Water Tree Retardant Compounds

Cross-linked polyethylene proved to be unreliable for medium voltage power cable applications. So compound producers introduced special compounds that are marketed under the name of TRXLPE that is tree retardant XLPE. It should be noted that this name is not properly defined, and the requirements are not standardized. In fact, the IEEE ICC working group which had the task to define tree retardant cross-linked polyethylene (TRXLPE), could not come to a successful conclusion.

For these compounds two different approaches were taken:

  1. 1.

    Polymer-modified XLPE

  2. 2.

    Additive-modified XLPE

Both compounds have a slightly higher loss factor than XLPE without additives. But for medium voltage cables, this is not an issue since the electrical loss is below 1 mW/m at normal operation up to 130 ℃.

For polymer-modified XLPE, the compound producer introduced a second polymer that needs to be compounded into the LDPE. The second polymer appears as small islands inside the LDPE base resin and acts as a retardant to water tree initiation and growth. The second polymer can be either a

  1. 1.

    polar compound

    1. a.

      Ethyl-acrylate

    2. b.

      Butyl-acrylate

  2. 2.

    or non-polar

    1. a.

      LLDPE

    2. b.

      SEBS

    3. c.

      α-Polyolefines.

Both possibilities are commercially available and can be processed similarly to XLPE. The non-polar compounds have the advantage that they can also be used for higher voltages, since their loss factor is in the same low range as for XLPE and not dependent on electrical stress. In beginning, the 1990s trial versions, one of these compounds contained a 12 nm filler of fumed silica. Even if the laboratory test results like Ashcraft and mini-cable tests were promising, the end results on the final cable evaluation did not show any improvement over the commercially available compounds (Fig. 16).

Fig. 16
figure 16

VDE 0273 results of a fumed-silica filled polymer modified XLPE

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Therefore, further production utilizing this product as well as another additive of a liquid silane compound was stopped. There were also other problems in the performance of these cables that are beyond the scope of this chapter.

2.3.6 Additive Modified XLPE

In this case, a special additive is added to the XLPE which retards the growth of water trees in the insulation. Here, special additives that will retard the growth of water trees inside the insulation are added during the production of the compound. The dispersion of these additives is very important, as a non-uniformly dispersed additive might act as a contamination or hot spot. These additives are relative neutral to the electrical strength of the compound, but will retard the growth of water trees in an efficient manner such that this is not a problem anymore. They also work very well with strippable insulation screens, since they are nonpolar (Fig. 17).

Fig. 17
figure 17

Bowtie water tree (300 μm) in TR XLPE insulation after 17 years in service [3, p. 138]

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Today’s cables using present day compounds with up-to-date production technology will give the utility a cable that last at least 50 plus years. Figure 18 shows the improvement in recorded failure rates between first- and second-generation insulation compounds (Fig. 18).

Fig. 18
figure 18

Cable failures at a utility in Germany over the years [21]

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2.4 High Voltage and Extra High Voltage Compounds

Generally, these compounds are the same as medium voltage compounds and differ only in the requirements for cleanliness. Some special compounds are used for MDCV lines or cables with a small conductor. These compounds are called low sag as they have a lower melt index than the XLPE compounds commonly sold for this application. As mentioned above the TRXLPE compounds have a slightly higher dielectric loss factor and are therefore not considered. This is also the reason why EPR is limited to 150 kV. So for these compounds the main focus is cleanliness.

As shown in Fig. 19, water trees might be a problem for high voltage cables. However, these small vented trees observed after 40 years in service were not the reason for replacement of the cable. It was just that the conductor was too small to carry the future load. However, the use of water tree retardant additives, that are non-polar, and thus not negatively influencing the electrical loss factor, might provide an advantage for future development.

Fig. 19
figure 19

Water trees in a high voltage cable, 110 kV, 40 years in service

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Already wet designed 66 kV array cables for offshore wind farms are the standard, while future work might extend to even higher voltages to reduce the weight of these cables which normally have outer lead sheaths as water barriers. Figures 20 and 21 show the loss factor on newly developed materials compared to older products that are already on the market.

Fig. 20
figure 20

Dielectric loss factor versus temperature on plaques

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Fig. 21
figure 21

Dielectric loss factor versus voltage at 100 ℃ on plaques

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The ease of processing a compound is a normal requirement for extruder operation as an over reactive compound will create pre-cross-linked particles, called scorch. These particles can reduce the electric breakdown strength and thus reduce the performance of a cable. For cleanliness measurements different and diverse tests at the compounder are performed. Thus, it is difficult to compare cleanliness specifications without a background knowledge of each test. Also, the sample size plays a role since the compound supplier does not normally perform a 100% check of the compound supplied.

It is generally agreed that for high voltage cables (IEC 60840), particles bigger than 100 μm can create problems and for extra high voltage cables (IEC 62087), particles bigger than 70 μm can create problems. Also compound handling by the cable manufacturer, unloading, conveying etc., influences the contamination level inside the cable. Normally, all cable manufacturers use filters to reduce the amount of dust and hence contaminants inside the cable.

The choice of antioxidant for this application is also important since some of these have a polar characteristic or due to the influence of the pH-level of the compound can influence the creation of water during the cross-linking process. Both will negatively influence the dielectric loss factor.

Recently, compounds came on the market that reduce the degassing time; a process that needs to be carried out for XLPE cross-linked cable to remove the methane that is produced during the cross-linking process [22]. CIGRE has come up with recommendations on how to test for the amount of methane content in a cable [15]. Four laboratory methods were evaluated: gas chromatography (GC), gas pressure measurement, Raman spectroscopy, and conductivity method. The GC method proved to the most popular as well as the most accurate.

2.5 Compounds for DC Cables

Extruded DC XLPE cables have been installed around the world since the 1990s. At first, the development of these cables was challenging, since at the beginning, space charges were created during a longer use of these cables which resulted in a pre-mature breakdown at very low levels. With understanding and the introduction of different convertor technology, extruded DC cables received a welcome boost. The first extruded DC cable was a ±80 kV, 50 MW cable in Gotland, Sweden, which has been in operation since 1999. This cable was installed with voltage source convertors and was designed for a conductor temperature of 70 ℃. The compounds used for these cables and for many more installations around the world was an unfilled, specially designed XLPE compound for DC application that included special semi-conductive screens. Today, cables up to ±320 kV have been installed and are running without any major problems.

It should be noted that two other commercial extruded systems have been installed, one in Japan and one in Europe, where the conductor design temperature was 90 ℃. The one in Japan, Honchu-Hokkaido link, is a ±250 kV, 300 MW link and operates with line-commuting convertors without any problems. The link in Europe is currently the highest voltage with ±400 kV DC, 1000 MW, currently installed (2019). The XLPE insulation compounds used for these cables have an inorganic filler to improve the volume resistivity of the insulation. There are projects in China and Germany at ±525 kV DC. For these cable projects, new compounds, all unfilled XLPE, have been introduced, some cable makers have now approved HVDC cables for 90 ℃ conductor temperature.

2.6 Compounding of XLPE

The design of a compounding line has changed very little in the last sixty years. It has not changed for medium and high voltage compounds as well as for unfilled DC compounds. For medium voltage compounds, the peroxide can be directly compounded into the insulation, but for high voltage and extra high voltage compounds due to cleanliness requirements, the peroxide is added by a physical absorption process, called “soaking”. An XLPE compounding plant is depicted in Fig. 22.

Fig. 22
figure 22

XLPE compounding plant

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First a suitable LDPE resin at a higher temperature is mixed together with other polymers and the antioxidant or additives in a molten stage until completely homogenized. During this process the temperature profile is very important since the melt should not be too hot so as to degrade either the antioxidant or the polymer. Afterwards the melt is filtered according to the required melt filtration level making sure that the filter is not too tight as this may degenerate the polymer due to a high melt stress and thus create gels which can negatively influence the properties of the cable insulation. At this point, the polymer strands are cut into pellets and premixed with molten and filtered dicumyl-peroxide in a tumble mixer. This mixture is conveyed into a silo and kept for a certain time, some hours, at a certain temperature. During this time, the peroxide is absorbed into the granules and then after a defined cooling process packed for later transportation. During all of this handling, the properties of the product, especially the cleanliness has to be checked. The temperature during this process needs to be carefully monitored, since the compound has the tendency to exude. Polyethylene cannot tolerate a high amount of filler and can reject these fillers over time. This process is known to be an exudation process. Unfortunately, peroxides and antioxidants are also fillers for polyethylene and thus have the tendency to exude. The antioxidant at a temperature of around 50 ℃ and the peroxide at lower temperature below 5 ℃. Therefore, it is important to keep the compound at a certain temperature. The exudation of peroxide in particular can create a so-called peroxide nest inside the conveying system and the resultant accumulation of material or lumps will fall into the extruder. This in turn can create a slipping screw with cable core diameter variations and/or a higher amount of peroxide within the cable insulation and thus a weak spot within the cable, which can reduce both the withstand and impulse levels of the cable insulation.

Existing compounding plant designs originated at the beginning of the 1960s and since then, have been gradually optimized to produce consistent and reliable compounds, known worldwide as XLPE. XLPE applications span over a wide range of voltages from 5 to 600 kV and beyond. However, the concept remains the same, continuous compounding where different polymers are mixed along with additives such as antioxidants, scorch retarder, cross-linking enhancer, etc., followed by a batch peroxide soaking process with controlled heating, cooling and residence time sequences. Ideally, the soaking is conducted in an enclosed soaking tower (up to 50 m. high) where the chain of operations including polymer pre-weighing to packaging of the finished product is conducted. Both, the continuous and the batch processes are preferably harmonized to produce a regular output.

Here, polymers and additives, respectively, mean LDPE, copolymers, and combinations thereof. Additives mean antioxidant packages, plus optionally voltage stabilizers, cross-linking booster, scorch retarders, etc.

2.6.1 New Method to Produce XLPE

This new method is the same as for the conventional XLPE compounding line except that two types of equipment have been inserted, one after the filter and one, right before packaging (Fig. 23).

Fig. 23
figure 23

New process to produce XLPE

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These two additional pieces of equipment, in light green colour, are first Post Extrusion Peroxide Addition (P.E.P.A) system and second, a large capacity pellet sorter. The pellet sorter removes contaminants above 60 μ.

PEPA is a cooler/mixer that takes the polymer’s relatively high temperature (200 ℃) necessary due to the fine filtration, down to 115–125 ℃, well below the peroxide decomposition temperature. Reaching this relatively low temperature, the peroxide is then injected in filtrated liquid form.

2.6.2 Pros and Cons

Pros and cons are detailed in Table 3 below that is not an exhaustive list. Many more positive criteria can be found in favour of the LSHC (Linear Short Hyper Clean) process.

Table 3 Pros and Cons
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The new process’s main saving is contributing to carbon footprint reduction by the exclusion of the 50 m soaking tower infrastructure. Process engineers have compared kWh (energy) needed to produce one kilogram of final XLPE compound produced with the conventional process to that of the new one. Not surprisingly a reduction of some 40% has been demonstrated in favour of the new process. For example, it is 0.35 kWh/kg of XLPE for the old process versus less than 0.20 kWh/kg of XLPE with the new one.

(*) This is taking into consideration the huge tower infrastructure reduction including mirror polished, vessels, massive heating and cooling system, day bins, electrical valves, demineralized and deionized transportation water, air-compressor, air treatment, elevators, manpower, maintenance, etc.

2.7 Production of Cable Cores

As mentioned earlier, it is not only the compound and its chemical composition that influences the properties of a cable. It is also the production process, that is, how the cable core is produced has an influence on the properties of the cable. A VCV line is in principle a tower, where the cable is extruded down to the ground and cooled. Here, the handling of conductors and cables is of utmost importance. The stress added to the cable is high since the whole weight of the cable needs to be supported from the top.

For a CCV line, one needs to distinguish between lines used for medium voltage and high/extra high voltage cables. Medium voltage lines have a small angle and are designed for high speed processing. The conductor size is usually a maximum of 800 mm2. For high voltage and extra voltage cables, the angle of the line is much steeper and very close to a VCV line. Here, the biggest problem is the small conductor and thick insulation layer, since the insulation has the tendency to droop, the so-called pear drop effect. This can be overcome by a special configuration of the line or by using so-called low-sag compounds.

In these lines, the material is cross linked with heated nitrogen gas at around 10 bar and then cooled down to a conductor temperature of around 80 ℃, still under pressure of 10 bar either in water, for medium voltage cables or an inert gas like nitrogen. The latter is less efficient for cooling, but mostly required by utilities in their specification.

The third-line type, an MDCV line which is in principle a long die which forms the cable. Here, the cable is gently pushed through a die, which has the same diameter as the cable core and heated via an oil. The surface temperature is much lower than in the other processes. Also, here the core is under pressure to prevent foaming of the polyethylene due to the gases like methane that are created during the cross-linking process. One concern is that a large conductor might sag inside the insulation and not be centred.

All three processes have their advantages and disadvantages and are used to produce high quality cables up to 600 kV. A CCV line is the preferred line for medium voltage cables while a MDCV line is not used for this application due to the slow line speed.