Influence of spacer insulator material temperature characteristics on its electric properties

1 Introduction

Spacer insulators are important components of GIS.They are insulators that connect up the central conductor and the shell of GIS and provide strong support for the central conductor.Sometimes,they also act as a safety barrier to isolate the air chambers [1-4].Many failures of GIS results from the deterioration of the spacer insulators' performance.The GIS insulation level is directly related to the insulation characteristics of spacer insulators [5-7]; its surface is gassolid interface,the central conductor and shell have different potential,and the distortion of the local electric field can easily lead to flashover along the insulation.There are large amounts of spacer insulators in GIS,so the improvement in the insulating property of the spacer insulator is an effective operation to reduce the GIS failure rate.

During the GIS operation period,the temperature of spacer insulators presents a gradient distribution.The central conductor of GIS will follow by operating current.According to Joule’s Law,the high current will produce a large amount of heat,the conductor temperature will rise rapidly and emit heat to the surroundings.The GIS confined space is not conducive to heat dissipation.The spacer insulator’s center is connected with the central conductor directly,and the temperature of central region of spacer insulator significantly increase; The spacer insulator’s edge is connected with the shell whose temperature is close to the environmental temperature.Further,spacer insulators are made of low thermal-conductivity materials,so the temperature of spacer insulator presents a gradient distribution.As the temperature increases,the flashover voltage of the material decreases,and the distribution of the spacer insulator electric field changes too,which may lead to flashover.

The analysis above shows that the heat produced by the operation current adversely affects spacer insulators.The temperature characteristics of the spacer insulator material’s performance are noteworthy.In order to understand the cause of the failure of the spacer insulator,it is necessary to test the temperature characteristics of the material.

Although domestic and foreign GIS manufacturers have mastered the manufacturing process of spacer insulators with various voltage levels,there are hardly any studies on the influence of the temperature characteristics of spacer insulators on their performance.In this study,the insulator’s temperature distribution is analyzed,and also the influence of material's temperature characteristics on the electrical properties of the insulator is analyzed.Experimental and theoretical data are provided for the design of the insulators.Our work provides important reference for the failure analysis of the spacer insulators.

2 Experimental

2.1 Spacer insulator temperature measurement experiment

Temperature rise under operation is an important indicator of high-voltage switchgears’performance,many parts of the device,including the spacer insulator,are affected by the temperature rise.We used thermocouples to measure the temperature distribution of a spacer insulator under working conditions.The temperature measurement of the spacer insulator was done in Shangdong Tai’an High Voltage Switchgear Co.Ltd.

Fig.1 Temperature test points on the spacer insulator

We selected 11 measuring points on the upper surface of the spacer insulator,along the vertical direction from top to bottom,as shown in Fig.1.

During the test,the current passing through the central conductor was 3.465 kA,and SF6 gas pressure in the sealing device was 0.4 MPa.

After the test reaches the stable state,which means the temperature change of the test points is less than 1 degree per hour,the temperature and temperature rise of the spacer insulator and the nearby measuring points are recorded.The temperature distribution curve along the spacer insulator vertical diameter is shown in Fig.2.

Fig.2 Spacer insulator temperature distribution

The temperature of the central conductor is the highest,reaching 92.9 °C at the stable state; the metal shell temperature is only 52.4 °C,and the temperature of the spacer insulator is between the temperatures of these two parts.Among the measuring points on the spacer insulator,the highest temperature is recorded at measuring point 5,which is the closest to the insert and located in the upper part of the central conductor.Points 1-5,and 7-11 are equally spaced; according to the distribution temperature curve,the parts near the central conductor exhibit a wider temperature difference,while the temperature variation of the parts near the shell is very small.

2.2 Temperature characteristics of spacer insulator material’s relative permittivity

When the discoid test sample is placed in the middle of electrode,it can be regarded as a dielectric capacitor composed of the insulating material,and its capacitance can be measured by using a high-voltage capacitor bridge.The temperature characteristics of the spacer insulator material's relative permittivity was measured by using a Schering bridge.The Schering Bridge is an AC bridge circuit,as shown in Fig.3.The Schering Bridge has the advantage that the balance equation is independent of frequency.When the bridge is balanced：

Fig.3 Schering Bridge

where h is the total thickness of the sample and A is the electrode area; we can calculate Cx from R3,R4,and Cn,and then,the relevant parameters can be substituted to calculate the relative permittivity.

Five identical test samples were set up for each set of tests to avoid measurement errors during the test.The relative dielectrics of the samples were measured at 50 ℃,75 ℃,90 ℃,100 ℃,110 ℃,125 ℃,150 ℃,respectively,and the samples were heated with the electrodes.The initial temperature was set to 25 ℃.

Fig.4 shows the relative permittivity of the samples when the temperature is gradually increased in the range of 25 to 150 ℃.

Fig.4 Change in relative permittivity with temperature

From the curve,it can be observed that the relative permittivity increases slowly below 90 ℃,and the curve becomes steeper from 90 ℃.The permittivity rise slows down after 125 ℃,and eventually,the permittivity does not increase.

The glass transition temperature of the epoxy resin material is generally between 90 ℃ and 120 ℃.The permittivity increases significantly at the glass transition temperature,which is enhanced by the thermal motion of molecular chain rotation and polarization greatly enhanced.

3 Electric field simulation of spacer insulator

3.1 Electric field simulation model and material parameter setting

COMSOL Multiphysics AC/DC module [8]contains a variety of electromagnetic field calculation modules,and can calculate electromagnetic field distribution under different conditions.In this study,the Electric field(ec)module in the AC/DC Module is selected to simulate the electric field of the spacer insulator,and the electric field distribution of the spacer insulator and its adjacent area was obtained.The spacer insulator is an axisymmetric structure,so the plane analysis method was used to model half of its shaft cross-section,as shown in Fig.5,and the 3D model can be obtained by rotating 360° of the model in Fig.5.

Fig.5 Spacer insulator calculation model

The red area in Fig.5 represents the spacer insulator; the gray area represents the central conductor,shield,shell and other components; and the remaining blue area represents the SF6 gas.

Scalar potentials are introduced to characterize the electric field.In an isotropic,linear,uniform medium,the potential satisfies the Laplace equation or the Poisson equation:

ρ is the charge density in C / m3.

The central conductor loading voltage φ0 is 1050 kV which is the lightning impulse voltage peak value,the shell is set to ground.

When there are two different permittivity values of the medium,at the junction of two dielectrics,the following equation should be satisfied:

ε1 and ε2 are the permittivitys of different materials,respectively,and is the external normal vectors of the interface.

The electric electric field is given by:

E is the electric field intensity vector in V / m.

3.2 Electric field analysis under room temperature

The potential and electric field distributions of spacer insulators were obtained by the finite element method.The overall potential and electric field distribution are shown in Fig.6.

Fig.6 shows that the equipotential lines in the vicinity of the central conductor and the shielding cover are denser and the electric field is higher than other region of spacer insulator; the distribution of equipotential lines near the shell and the flange is scatteredand the electric field is the lowest of spacer insulator.In the vicinity of the center insulator of the spacer insulator,the electrical field strength of the shielding surface is the strongest; the electric field between the central conductor and the insulator is weaker,and the electric field strength of border area of the central conductor,insulator and the SF6 gas is even weaker,because this area is influenced by the central conductor and the shield.

The surface electric field distribution curve of the spacer insulator is shown in Fig.7.The abscissa represents the horizontal distance of the point from the center conductor axis of symmetry.The center insert radius is 50 mm,so the abscissa shows a range of 50-180 mm.

Fig.6 Spacer Insulator overall potential and electric field distribution

Fig.7 Spacer insulator surface electric field distribution curve

With the increase in the radial distance,the surface electric field strength increases first and then decreases.The maximum electric field strength on the surface of the insulator is 10.71 kV/mm.The peak value of the concave electric field appears at a radial distance of about 19 mm from the central conductor edge.The peak value of the convex electric field is 9.00 kV/mm,which appears at a radial distance of about 60 mm from the central conductor edge.In the radial distance at 50-100 mm intervals,the concave electric field strength is higher than that on the convex surface.With the further increase in the distance,the concave electric field strength is lower than the convex surface strength.The concave electric field strength distribution is more uneven than the convex surface strength.

When there are two kinds of dielectric materials placed between the parallel plate electrodes and the interface of the dielectric is not perpendicular to the electrode surface,in the dielectric with the lower permittivity,on the side which the angle between dielectric interface and the electrode surface is less than 90°,the electric field is enhanced.Under AC voltage,the analysis of the central conductor,spacer insulator,and shell can be equivalent to a coaxial cylindrical capacitor,and the parallel plate electrode electric field distribution is similar to the coaxial cylindrical capacitor.At room temperature(25 ℃),the relative permittivity of SF6 is less than that of the epoxy resin material,on the spacer insulator concave side,the electric field stronger.

3.3 Electric field analysis under operation temperature

A three-dimensional model containing the spacer insulator and components directly connected to it was established according to the actual size of a spacer insulator,as shown in Fig.8.

Fig.8 Three-dimensional spacer insulator model

The heat source of GIS is mainly the eddy current heat of the conductor and the Joule heat inside the spacer insulator.3.465 kA was applied to the central conductor in the model.GIS has a large amount of gas(SF6)inside,so the gas flow must be considered.The temperature distribution of the spacer insulator was calculated by using the thermal calculation module of COMSOL Multiphysics.The results of the temperature simulation calculations are very close to the test results.

When the operation current flows though the central conductor,the spacer insulator temperature increases and the permittivity of the epoxy resin material increases with the temperature; the permittivity changes will affect the interface electric field distribution.In order to analyze the change in the electric field strength,the relative permittivity of epoxy resin material temperature characteristic is considered; the temperature range of the simulation between room temperature to 90 ℃,according to the test data,set the relative permittivity as:

T represents Kelvin temperature,is the coupling variables of electric field and temperature field.

As the concave surface undergoes a large temperature change,so we chose it as the research object.The relative permittivity distribution along the spacer insulator's surface is shown in Fig.9.

Fig.9 Relative permittivity distribution along the spacer insulator’s surface

The relative dielectric constant of the material at room temperature is 5.322,and the relative permittivity of the spacer insulator along the surface rises from 5.54 to 5.99 under the operating conditions.

The change in the relative permittivity results in a change in the electric field strength along the surface.The electric field distribution of the spacer insulator’s surface is shown in Fig.10.The blue line is the electric field strength distribution curve at room temperature,and the red line is electric field strength distribution along the surface when the working current flows in central conductor.

Fig.10 Spacer insulator surface electric field strength distribution curve

It can be seen that the distribution of the electric field along the surface is almost the same and the electric field change in different regions is different.The electric field strength decreases slightly along the part near the central conductor,while it increases slightly along the part near the shell.For the concave surface,the maximum electric field of the insulator surface decreases slightly,from 10.71 kV/mm down to 10.48 kV/mm.

The flashover phenomenon of the insulation material is a complex physical phenomenon that occurs under a highstrength electric field.To analyze the flashover process,the permittivity,resistivity,and other characteristics of the insulation material must be considered.Generally,to a certain extent,the flashover voltage of the polyimide material decreases when the temperature increases.This is because when the temperature increases,the energy of the electron around the insulator will increase,and the thermal motion of gas molecules also increases; this will accelerate collision ionization between electrons and gas molecules,the electronic collapses easier to form,results in the critical flashover voltage decrease [9-11].

According to the results of the flashover test of epoxy resin reported in the literature [12],when the temperature rises to 80 ℃ from 25 ℃,the surface flashover voltage of the epoxy resin will be reduced by 5.2% in the air.With the increase in the temperature,the permittivity of the spacer insulator material increases,but electric field strength of the spacer insulator remains almost unchanged.Further,the possibility of flashover increases,as the insulation performance of the spacer insulator deteriorates.

4 Conclusions

In this study,we performed relevant experiments to test the spacer insulator temperature distribution under operating conditions,and analyzed the temperature characteristics of the material electrical properties.Then,based on the results,we calculated the electric field distribution of the spacer insulator by using the finite element simulation.The main conclusions of this paper are as follows:

Under operating conditions,the temperature of the inserts reaching 92.9 °C; the metal shell temperature is only 52.4 °C,and the temperature of the spacer insulator is between those of these two parts.The parts near the insert exhibit a wider temperature difference.

The relative permittivity of the epoxy resin increases with the temperature.When the temperature is under the glass transition temperature,the permittivity increases slowly; when near the glass transition temperature,the permittivity increases rapidly.

We established a calculation model for the spacer insulator by using the finite element simulation software.Further,we calculated the temperature of the spacer insulator under the operating conditions,and analyzed the change in the relative permittivity along the surface and its influence on the electric field distribution.

When the temperature is increased to 92.9 °C,the flashover electrical field strength also decreases,but the extent of its decrease is much greater than that of the surface electric field.The insulating performance of the spacer insulator also decreases.

This study provides a new perspective for the failure analysis of the spacer insulators,that when temperature rises,the materials temperature characteristics will deteriorate the electrical performances of spacer insulator.The spacer insulator and its materials performances under electro-thermal coupling effect need further analysis.

Acknowledgements

This work was supported by the State Grid Science and Technology Project(Title:Research on technical characteristics and quality control of UHV GIL insulators,expansion joints and plug contacts,No.GYB11201801457).