305 / 2018-09-25 23:31:19

A Space Charge Measurement Method of XLPE Based on Polarization and Depolarization Current

high voltage cable,XLPE,space charge,direct measurement,trap density

After the high-voltage DC XLPE cables are put into operation, the cable insulation gradually ages under multiple factors such as electric field, mechanical stress, temperature and moisture. In the process of aging, the crosslinked polyethylene molecular chain gradually breaks, and the microscopic defects are continuously expanded. Under DC electric field, space charge is easy to accumulate in microscopic defects, causing partial electric field distortion which further accelerates insulation aging, and even causes electrical branches and ultimately causes breakdown of insulation. It is currently believed that the accumulation of space charge has a great influence on the aging of high voltage DC cables. Traditional space charge measurement methods are very complicated and difficult due to the need to test the distribution of space charge. When space charge distribution in XLPE needn’t to be known, the direct measurement of space charge is the preferred method.
The polarization and depolarization current (PDC) of the dielectric at low voltage contains the de-trapping current component. The integral and time constant of the de-trapping current directly reflect the energy level and density of space charge in dielectric. In this paper, space charge is injected into XLPE sheet specimen at high electric field strength. After removing the electric field for 600s, most of the dipoles in the sample are verified to complete the relaxation process and restored to the initial state. Then PDC test are applied to sample at low voltage. The discharge of space charge is basically completed during the polarization process. So when using polarization current minus the depolarization current, the sum of the conduction current and the de-trapping current of space charge is obtained. The de-trapping current is extracted from the attenuation component. After integration of the de-trapping current, the curve of the amount of charge decay with test time is obtained. Based on previous researches, the release time of trapped charge can be related to trap depth logarithmically while there is no electric field. But in this experiment trapped charge escape from traps under electric field. So, the relationship between release time of trapped charge and trap depth are amended according to Poole-Frankel effect. The effective mobility of carriers in the presence of an electric field is recalculated, so that the formula for calculating the charge density in dielectric is corrected. After fitting the decay curve of charge amount with time, time constant and coefficient A was calculated. Then, energy level and density of space charge in the sample can be obtained.
The space charge accumulation in XLPE was analyzed under different space charge injection voltage, and injection time. The results show that with injection time and injection voltage increasing, more space charge accumulate in traps, while trap depth remain constant. Besides, samples processed by different thermal aging time are inject space charge and test PDC under same conditions. The results show that with thermal aging time increase, time constant of fitting curve of the charge amount with time curve and trap depth increase. But charge amount reduce first and then increase as thermal aging aggravate which may because of the crystallization promotion effect of XLPE in the early stage of thermal aging.
Through the test results, the possibility of using PDC test as a direct measurement method of space charge is verified. And this method can exclude the effects of polarization and conduction currents in dielectric, the space charge de-trapping current component can be extracted directly. And after considering Poole-Frankel effect, the effective mobility of carriers in dielectric is recalculate and the calculation formula of the space charge energy level and density in the presence of an electric field is corrected. Then through de-trapping current, energy level and density of trapped charge can be calculate.