Space Charge Characteristics at the XLPE/EPDM Interface Under DC Voltage Superimposed by a Repetitive Impulse Voltage

In a dc transmission system, cable lines are vulnerable to switching or lightning overvoltage, which leads to the superposition of an impulse voltage on the dc voltage. Moreover, because of the inductive–capacitive effect of electrical equipment insulation, electromagnetic energy conversion between components causes oscillation of the cable system, inducing high-amplitude impulse waves with frequencies of hundreds or even thousands of hertz. These waves can seriously threaten the long-term operation reliability of power equipment insulation. Based on the existing pulsed electroacoustic (PEA) space charge measurement system for dc superimposed impulse voltage in our laboratory, the space charge characteristics of cross linked polyethylene (XLPE)/ethylene propylene diene monomer (EPDM) composite dielectrics under dc voltage, impulse voltage, and dc superimposed impulse voltage are measured. The experimental results show that much more heterocharges formed in XLPE much easily cross the interface and migrate into the EPDM bulk under dc voltage. The lower impulse voltage has little effect on the XLPE/EPDM samples, while the higher impulse voltage up to ±33 kV/mm and above can induce the interface charges to drop or increase drastically. When the dc voltage is superimposed with impulse voltage with the same polarity, the interface charges decrease first and then increase reversely due to the multiplication of equivalent impulse voltage. Conversely, when a dc voltage is superimposed with impulse voltage with opposite polarity, the interface charges reduce because the decrease in the equivalent opposite-polarity impulse voltage, with a low amplitude, affects little on the charge accumulation.