10kV High-Voltage Cable DC Withstand Voltage Test Standard
1. Under DC voltage, the electric field distribution in cable insulation depends on the volume resistivity of the material, while under AC voltage, it depends on the dielectric constant of each medium. Especially in cable accessories such as cable terminations and joint boxes, the DC electric field strength distribution is completely different from the AC electric field strength distribution. Moreover, the aging mechanism of insulation under DC voltage is different from that under AC voltage. Therefore, the DC withstand voltage test cannot simulate the operating conditions of high-voltage cables.
2. High-voltage cables under DC voltage will produce a "memory" effect, storing and accumulating unipolar residual charges. Once the "memory" caused by the DC withstand voltage test occurs, it takes a long time to release this DC bias. If the cable is put into operation before the DC residual charges are completely released, the DC bias will be superimposed on the peak value of the power frequency voltage, causing the voltage on the cable to far exceed its rated voltage, which may lead to insulation breakdown of the cable.
3. During the DC withstand voltage test, electrons will be injected into the polymer dielectric, forming space charges, which reduce the electric field strength at that location, making breakdown difficult to occur. Space charges are prone to form at semiconductor protrusions and contamination points of high-voltage cables. However, if surface flashover occurs at the cable termination or breakdown of cable accessories during the test, it will cause wave oscillations on the cable core. At locations where space charges have accumulated, due to the rapid polarity change of the oscillation voltage to opposite polarity, the electric field strength at that location significantly increases, potentially damaging the insulation and causing multi-point breakdown.
4. A fatal weakness of high-voltage cables is the easy formation of water trees in the insulation. Once water trees are formed, under DC voltage they will quickly transform into electrical trees and cause discharge, accelerating insulation degradation, leading to breakdown under power frequency voltage after operation. However, pure water trees can still maintain a considerable withstand voltage value under AC working voltage and can last for a period of time.
