Study On The Characteristics Of Space Charge Accumulation In AC XLPE Cable Under DC Voltage

Uprating renovation,converting the existing cable lines from AC to DC operation,can make full use of the existing transmission corridor and maximize the transmission capacity of the original AC transmission lines.But the problem of space charge accumulation was not considered under actual operation when the AC XLPE cable insulation was designed.The accumulation of space charge will be serious in the AC XLPE cable under DC voltage,which will cause the local electric field doubled.This will reduce the dielectric strength and accelerate the deterioration of the insulation.In order to investigate the characteristics of space charge accumulation and electric field distribution in the AC XLPE cable under DC voltage,COMSOL Multiphysics software was used to simulate the temperature distribution under temperature gradient in the cable insulation.The electric field distribution and the characteristics of space charge accumulation in the 66 kV AC XLPE cable,based on the mathematical model of conductivity,were also simulated to study the influence by the factors of temperature gradient,electric field,stressing time and polarity reversal.Temperature gradient in the cable has a great influence on the accumulation of space charge and the distribution of electric field.The electric field distribution of AC XLPE cable is mainly determined by the conductivity of insulation materials under DC voltage,and the phenomenon of electric field distribution reversion will happen under higher temperature gradient.The temperature gradient will not only make the space charge on the high temperature side easily escape but also accelerate the migration of the carrier to the low temperature side,resulting in the serious accumulation of the positive and negative space charge in the low temperature side.The maximum electric field will appear outside the insulation when stressing time is 8hours and space charge density is-0.15C/m~3 at temperature gradient of 45?.The maximum electric field will be 6.71 kV/mm,which is relatively safe for the actual operation of the AC cable.The transient electric field is determined by the dielectric constant and the conductivity of the insulation material,but mainly determined by the dielectric constant during the voltage reversal.The maximum transient electric field will occur close to the cable core and higher than the steady-state electric field.The maximum transient electric field during the voltage reversal process is closely related to the temperature gradient,the magnitude of the voltage,and the voltage polarity reversion time.The maximum transient electric field will be higher under higher temperature gradient and when the magnitude of the voltage is very high.The maximum transient electric field will increase and the location of the maximum electric field transient will move to the conductor shield as the voltage polarity inversion time decrease.The reason of the transient maximum electric field occur are that the directional migration of carriers and the entrapment,detachment,neutralization,dissipation of space charge will finally lead the total amount of the positive and negative space charge accumulation to reach the dynamic maximum.The study results will lay a foundation for determination of DC operating voltage of 66 kV cable line changed to DC operation and construction of DC power distribution network in the future.