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| Study on Space Charge and Field Distribution Characteristics of XLPE/SR under Low Electric Field Polarity Reversal Based on the M-W Model |
| Song Ke1, Qian Dingdong1, Bao Guodong1, Wang Wei1, Qi Jiale2 |
1. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources North China Electric Power University Beijing 102206 China;
2. Electric Power Research Institute of State Grid Beijing Electric Power Company Beijing 100096 China |
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Abstract As the key equipment of connecting the cable system, high voltage direct current (HVDC) cable accessory has become the weakest insulation point of the cable system due to the composite insulation composed of different dielectrics. Compared with cable body insulation of a single dielectric, the HVDC cable accessory of composite insulation is more easily to accumulate space charges at the interface due to the discontinuity of dielectric. In the line commutated converter high voltage direct current (LCC-HVDC) transmission system, the voltage polarity needs to be reversed when performing the power flow reversal adjustment, and cable accessories will withstand polarity reversal electric field. Although it is temporary, an extremely high electric field would be applied locally due to the cumulative effect of space charge, which threatens the safety of cable system.
Firstly, the conductivities of cross-linked polyethylene (XLPE) and silicone rubber (SR) under different electric fields were measured at room temperature. Then, fitting these experimental data, the conductivity model parameters of XLPE and SR could be obtained. Finally, the finite element method was used to study the space charge and electric field distribution characteristics of XLPE/SR under polarity reversal voltage based on the Maxwell-Wagner model. Furthermore, the effects of voltage reversal time and voltage amplitude on the interface charges and the maximal transient electric field were discussed.
The conductivity test results show that, the conductivity of SR is higher than XLPE within 14 kV/mm. As the field increases, the conductivity of XLPE shows an exponential growth trend and its change is obvious, while that of SR changes less and increases relatively slowly. Simulation results show that: Firstly, set the field to 5 kV/mm, voltage reversal time to 30 s and temperature to 25℃. When voltage reversal is completed, the residual negative charges at the interface enhance the field in the SR, which causes the maximal transient field reaches 8.06 kV/mm, and weaken the field in the XLPE, which reaches -1.14 kV/mm. The space charge and field distributions in the steady state before and after voltage inversion show a "mirror" distribution. Secondly, keep the temperature and field unchanged, when voltage reversal time is 5 s, 30 s, 90 s, the maximal transient field distortion rate in the SR is 63.82%, 61.30%, 55.18%, respectively. Thirdly, hold voltage reversal time and temperature unchanged, When voltage amplitude is 1.56 kV, 2.60 kV, 3.64 kV, the XLPE steady-state electric field distortion rate before voltage inversion is 125.55%, 110.38%, 90.31%, and that of SR is -85.40%, -74.28%, -59.56%,while the maximal transient field distortion rate in the SR is 71.78%, 61.29%, 46.59%, respectively.
The results show that: (1) The conductivity of SR is higher than XLPE. There is a clear difference in the conductivity properties of XLPE and SR as a function of electric field, which is one reason for the accumulation of interface charges. (2) Since interface charge polarity change lags voltage polarity change, the residual negative charge at the interface would cause the maximal transient field in the SR. (3) The shorter the reverse period, the more residual negative charge at the XLPE/SR interface, resulting in a higher maximal transient field in the SR. (4) As the applied voltage increases, The conductivity ratio of SR to XLPE decreases gradually, so the maximal transient field distortion rate in SR decreases with increasing electric filed.
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Received: 28 June 2022
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2023, Vol. 38 
