Study On Transient Behavior Of Space Charge In Polyethylene Materials For High Voltage DC Cables

With the accelerated adjustment of the energy consumption structure,electricity consumption has shown a trend of continuous growth in recent years,therefore the long-distance transportation of electric energy has become an inevitable demand for the development of power systems.As a carrier of large-capacity transmission,high-voltage DC cables have been widely used in transmission and distribution systems.Currently,the main insulating material of high-voltage DC cables is mainly made of polyethylene material.Under the DC electric field,it is easy to generate space charge inside the material.The accumulation of space charge affects the internal electric field distortion and even leads to breakdown.Therefore,studying the space charge behavior of polyethylene for high-voltage DC cables is of significance for the safe operation of power system.Based on the principle of pulsed electro-acoustic(PEA)method,the rapid dynamic measurement of space charge was realized in solid insulation.In order to study the space charge behavior of imported DC cable's XLPE material at high temperatures,undegassed and degassed XLPE samples were prepared for the tests,including DC conduction,dielectric and space charge during steady and transient processes at high temperatures.The results show that both the conduction current and dielectric loss of undegassed XLPE are larger than those of degassed XLPE.Negative charge accumulation is observed in the bulk of de-gassed and undegassed XLPE under-30 kV/mm,and the distortion ratio of internal electric fields is below 25% at 30 ? and 70 ?.A positive space charge packet appears and subsequently moves quickly within the first 2 s polarization under-100 kV/mm at 70 ?.However,the injection and migration of a negative space charge packet could only be observed when the applied field raises to-150 kV/mm.Here,both polarities of space charge packets follow negative differential mobility.In this paper,low density polyethylene samples of different crystal forms were obtained by controlling the cooling rate after melting.Polarized microscopic observation,X-ray diffraction analysis,differential scanning calorimetry analysis,electrical conductivity test,DC breakdown test were performed on LDPE samples with different cooling rates.A high repetition rate pulsed electro-acoustic system was developed to observe space charge evolution in heat-treated low-density polyethylene films at prebreakdown at 70 ?.The results show that the slower the cooling rate,the larger the spherulite size,the clearer the crystal profile and the higher the crystallinity.Before prebreakdown,ice-water cooling LDPE films appeared multiple positive space charge packets and alternately migrated towards the cathode,while air cooling and slow cooling LDPE films existed many negative charge injection and accumulation.Due to the space charge effect,the maximum electric field inside the sample is distorted: the ice water cooling sample distortion ratio is 65.1%,the air cooling is 59.5%,and the slow cooling is 46.6%.