Effect Of Temperature On Space Charge Distribution In Polymer Insulation

Polymer insulated power cable, with advantages of simpler structure, higher conductor temperature and environmental friendly, has been successfully used for HVAC power transmission. However, its application in HVDC power system is far behind. One of the most main reasons is believed to be the accumulation of space charge in polymer insulation when subjected to a DC field, which can accelerate insulation degradation through local field enhancement, thermionic emission, electromechanical energy storage and release. Besides, the interface between the insulation of cable and its accessory can behave as favored sites for charge build up, thus make the cable accessory weak points of the insulation system. Although many works have been done with regard to the space charge formation and transport in polymer dielectrics, most of them were conducted at room temperature, whereas in a practical cable system, the temperature of the insulation varies with the load condition. Hence, the space charge behavior in polymer insulation requires further study in relation to a broad temperature range.This paper presented the measurement results of high field conduction and space charge distributon in cross-linked polyethylene(XLPE), ethylene-propylene-diene monomer(EPDM) single layer sample, and space charge behavior in XLPE/EPDM double-layer sample, for a temperature range from 30°C to 90 °C. The mechanism of space charge formation and the effect of temperature on charge transport was discussed. Based on the experimental research, the numerical simulation of the space charge behavior in XLPE, EPDM single layer and XLPE/EPDM double layer sample was conducted at different temperature.According to the high field conduction and space charge distribution in XLPE and EPDM single layer, the process of bipolar charge injection and impurity dissociation was observed in both XLPE and EPDM. The threshold field for space charge accumulation apparently decreased with the temperature, probably associated with acceleration of charge injection and ionic dissociation. The apparent mobility of charges increased with the temperature, which indicates enhancement of charge detrapping and transfer rate at higher temperature. Comparison of the threshold characteristic indicates that the injection barrier hight in EPDM is lower wheares the dissociation barrier hight of the impurities in EPDM is higher.According to the space charge distribution in XLPE/EPDM double layer, with the increase of applied field, the interfacial charge at each temperature all showed positive polarity at first and then turned to negative, and the transition field decreased with the increase of temperature below 70 °C. At relatively low temperature and field(30 °C and 50 °C, 10 k V/mm), the formation of the positive interfical charge may caused by Maxwell-Wagner polarization; After the temperature or applied field increased(30 °C to 70 °C, 20 k V/mm), the decrease of positive interfacial charge and appreance of negative interfacial charge is believed to be associated with injected electrons; At relatively high temperature and field(90 °C, 30 k V/mm), the decrease of negative interfacial charge is probably due to acceleration of hole injection and transport.The bipolar charge and ion transport model was modified for space charge simulation in different temperature with different types of impurities: the effect of temperature on injection current density, dissociation coefficient of impurities, apparent mobility and detrapping coefficient was introduced; and the impurities were divided into type A(dissociated into holes and positive ions) and type B(dissociated into electrons and negative ions). The comparison of simulation and experimental results show that the impurities in XLPE are easily being dissociated into holes and negative ions, the space charge accumulation in XLPE is formed by electrons, holes and negative ions; wheases the impurities in EPDM may contain both type A and type B, the space charge accumulation in EPDM is formed by electrons, holes, positive ions and negative ions.The bipolar charge transport model was used to simulate the charge behavior in double layer sample. Different boundary conditions of idea interface, interfical barrier hight and interfical deep trap were introduced to simulate the effect of interface property on charge transport. The space charge behavior in XLPE/EPDM double-layer were simulated based on Maxwell-Wagner theory and bipolar charge transport model, respectivedly. The comparison of simulation and experimental results show that the Maxwell-Wagner theory can only represent the polarity of the interfiacal charge at relatively low temperature and field(30 °C and 50 °C, 10 k V/mm). The simulation results of the bipolar transport model reveals the transport of injected charge in the double-layer sample and its accumulation at the interface. With comparison of different boundary conditions, it is speculated that the accumulation of injected charge at the interface is mainly due to different electric filed and charge mobility at different layer, but has little relation with interfical barrier hight or interfical deep trap.