Keywords

1 Introduction

Gas insulated metal-enclosed transmission line (GIL) has the advantages of high reliability, small transmission loss, low failure rate, and is not affected by external factors such as climate change [1]. It is an advanced power transmission method with great development prospects, and has gradually become an important direction for the construction of future power transmission networks.

The temperature rise effect caused by conductor Joule heat, enclosure induced current and eddy current heat loss causes the temperature of GIL conductor, enclosure and insulating gas to rise [2], which leads to changes in the insulating properties, and endangers the operation safety of GIL. Therefore, the research on the temperature rise characteristics of GIL can provide a reference for the structural design of GIL equipment and provide guarantee for the safe operation of GIL equipment.

At present, numerical simulation methods are mainly used at home and abroad to study the temperature rise characteristics of GIL. Literature [3,4,5] established a two-dimensional model of GIL, and studied the transient temperature rise curve, the influence of different insulating gases and conductor diameters on the internal temperature distribution of GIL. Reference [2] carried out the research on the influence of external wind speed and solar radiation on the temperature field distribution of GIL in the 3D simulation model of GIL. Reference [6] carried out an experimental study on the temperature rise characteristics of GIL expansion joints, and compared with the simulation results. The temperature rise of GIL will cause the strain field and electric field of GIL to change. For multi-physics coupling, corresponding research has been carried out at home and abroad. Reference [7] used SF6-N2 mixed gas to carry out related research on the electric field distribution under the temperature rise of GIL. It can be seen from the research status that the temperature rise characteristics of three-post insulators are important factors affecting the safe operation of GIL. However, at present, there is still a lack of relevant studies on the influence of GIL temperature rise on the internal stress of three-post insulators and the thermal expansion characteristics of the envelope.

Therefore, this paper builds a three-dimensional simulation model of GIL, and studies the factors affecting the temperature rise of GIL for the tri-post insulator GIL, as well as the effect of temperature rise on the strain field of GIL. The goal of clarifying the temperature rise characteristics of GIL and clarifying the factors affecting the local temperature rise of GIL has been achieved.

2 GIL Multiphysics Simulation Model

This research uses the COMSOL finite element simulation software to create a three-dimensional GIL model, and conducts simulation research on the internal temperature field, strain field and electric field of the GIL.

2.1 Geometric Model

The research object of this paper is the 1100 kV tri-post insulator GIL. Its basic structure is shown in Fig. 1, which can be divided into a central conductor (1), a connecting cylinder (2), an epoxy resin insulator (3), and an aluminum alloy enclosure (4).

Fig. 1
A diagram of the tri-post insulator G I L. It is a cylindrical structure with a conductor and an insulator. The numbers from 1 to 4 are labeled.

Schematic diagram of tri-post insulator GIL, central conductor (1), connecting cylinder (2), an epoxy resin insulator (3), aluminum alloy enclosure (4)

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In order to prevent the asymmetry of the simulation results caused by meshing, the 3D geometric model of the GIL is simplified, as shown in Fig. 2.

Fig. 2
A cross-sectional diagram of the tri-post insulator G I L. It is a half-cylindrical structure with a conductor and an insulator.

GIL tri-post insulator simulation model

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2.2 Flow Field-Temperature Field Mathematical Model

At the initial ambient temperature of 20 °C, the physical parameters of insulating gas, enclosure and insulator are shown in Table 1.

Table 1 Physical parameters of insulating gas, enclosure and insulator
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Under the set rated working conditions, the rated current is 6000 A, the ambient temperature is 20 °C, and the insulating gas pressure is 0.4 MPa. The insulating gas inside the GIL is set to SF6. The parameters of the influencing factors of the temperature rise characteristics used in the simulation are shown in Table 2.

Table 2 GIL external influence factor settings
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2.3 Temperature-Strain Field Mathematical Model

On the basis of inputting the thermal physical properties of the material in the coupled flow-heat analysis, in the thermal stress analysis, the mechanical properties of the material should also be input. The mechanical properties of the material required for the simulation are shown in Table 3.

Table 3 Mechanical parameters of enclosure and insulator materials
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First, fixing the two ends of the enclosure and the conductor, and the thermal–mechanical coupling analysis of the GIL is carried out. Then, fixing one end of the enclosure, and symmetrical boundaries are imposed on the inner and outer sides of the enclosure to explore the axial thermal expansion characteristics of the GIL enclosure.

3 Results and Analysis

3.1 GIL Temperature Field

The temperature distribution is shown in Fig. 3, and the gas flow rate distribution is shown in Fig. 4. It can be seen from the figure that the temperature of the insulating gas inside the GIL presents a distribution law of high above and low below. The maximum gas velocity is about 0.09 m/s.

Fig. 3
An illustration of the G I L model representing temperature distribution. It is a hemispherical model with a maximum heat of 335 Kelvin at the center. Data are estimated.

GIL temperature distribution

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Fig. 4
An illustration of the G I L model representing flow velocity distribution. It is a hemispherical model with a maximum heat of 0.09 meters per second at the top, and the major portion has 0 meters per second. Data are estimated.

GIL gas flow velocity distribution

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The temperature distribution of the GIL tri-post insulator is shown in Fig. 5. It can be seen from the figure that the temperature of the insulator gradually decreases from the conductor to the enclosure. The internal temperature of the insulator is slightly lower than the surface temperature.

Fig. 5
An illustration of the G I L insulator model representing temperature distribution. It is a pipe-like L-shaped model with a maximum heat of 335 Kelvin at the junction. Data are estimated.

Temperature distribution of GIL insulator

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The temperature distribution of the GIL enclosure is shown in Fig. 6. The temperature of the upper surface of the enclosure is the highest and the temperature of the lower surface is the lowest. The temperature on both sides of the upper surface of the enclosure is higher than that of the middle part.

Fig. 6
An illustration of the G I L enclosure model representing temperature distribution. It is a hemispherical sheet model with a maximum heat of 335 Kelvin at the top and a minimum heat of 314 Kelvin at the bottom. Data are estimated.

Temperature distribution of GIL enclosure

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3.2 Effect of Different Factor on GIL Temperature Field

Figure 7a–c are GIL temperature rise curves obtained by changing load current, ambient temperature or gas pressure.

Fig. 7
A set of 3 multiple-line graphs of the temperature versus load current, ambient temperature, and gas pressure. The parameters are conductor and enclosure in the graphs and ambient in graph 2. All lines follow an increasing trend except the line conductor in graph 3.

Influence of load current a, ambient temperature b, and gas pressure on GIL temperature distribution

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It can be seen from Fig. 7a that the relationship between the temperature of the GIL conductor and the enclosure is a nonlinear positive correlation. Figure 7b shows that the GIL conductor and enclosure temperature are positively correlated with the ambient temperature. The temperature difference between the enclosure and the ambient temperature does not change with the change of the ambient temperature. Figure 7c shows that the GIL conductor temperature is more sensitive to the change of insulating gas pressure at lower air pressure, but remains basically unchanged when the air pressure reaches about 0.4 MPa.

3.3 Effect of Different Factor on GIL Thermal Strain

The simulation results of GIL enclosure stress (a), insulator stress (b) and GIL enclosure thermal expansion (c) are shown in Fig. 8. It can be seen from the figure that the thermal stress of the GIL enclosure is axisymmetric. The thermal stress at both ends is the largest and decreases in the middle of the pipeline. The maximum stress inside the insulator occurs at the junction of the conductor and the insulator, because the temperature of the insulator part close to the conductor is the highest. There is also a large stress distribution at the junction between the two ends and the enclosure.

Fig. 8
A set of 3 illustrations of the G I L model representing thermal strain simulation. It consists of G I L enclosure stress, thermal stress inside the insulator, and G I L enclosure thermal expansion. The maximum temperature is at the thermal expansion.

GIL thermal strain simulation results

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Under rated conditions, the GIL enclosure with a length of 1 m thermally expands axially, with an upper surface of 2.11 mm and a lower surface of 1.83 mm.

4 Conclusion

In this paper, the temperature rise of the tri-post insulator GIL is calculated by the method of finite element simulation, and some related factors affecting the temperature rise and the strain field of tri-post insulator GIL are quantitatively analyzed. Based on the temperature rise results, the conclusions are as follows:

  1. (1)

    The temperature of the insulating gas inside the GIL presents a distribution law of high above and low below, and the temperature above the enclosure is higher than the temperature below the enclosure. The relationship between the influencing factors and the temperature rise effect of GIL was quantitatively analyzed.

  2. (2)

    The thermal stress of the GIL enclosure is axisymmetric, and the maximum stress of the insulator occurs at the junction of the conductor and the insulator. The maximum stress of the insulator increases with increasing temperature. The thermal expansion of the enclosure increases linearly with temperature, and the difference between the upper and lower surfaces is basically unchanged.