1. Under DC voltage, the electric field distribution of cable insulation depends on the volume resistivity of the material, and the electric field distribution under AC voltage depends on the dielectric constant of each medium, especially in the cable accessories such as cable terminations and connector boxes. The distribution of field strength and the distribution of AC electric field strength are completely different, and the mechanism of insulation aging under DC voltage is different from that under AC voltage. Therefore, the DC withstand voltage test cannot simulate the operating conditions of the high voltage cable.
2. The high voltage cable will produce a "memory" effect under DC voltage, storing and accumulating unipolar residual charge. Once there is "memory" due to the DC withstand voltage test, it takes a long time to release this DC bias. If the cable is put into operation before the DC residual charge is completely released, the DC bias voltage will be superimposed on the peak of the power frequency voltage, so that the voltage value on the cable far exceeds its rated voltage, which may cause cable insulation breakdown.
3. During the DC withstand voltage test, electrons are injected into the interior of the polymer medium to form a space charge, which reduces the electric field strength at that place, making it difficult to break down. Space charge is easily generated at the semiconductor bumps and contamination points of the high voltage cable. However, if the surface flashover or cable accessory breakdown occurs at the cable termination head during the test, wave oscillation will occur on the cable core. At the location where the space charge has accumulated, the polarity of the oscillating voltage rapidly changes to an opposite polarity. The electric field strength is significantly increased, which may damage the insulation and cause multiple clicks.
4. A fatal point of the high-voltage cable is that water branches are easily generated in the insulation. Once the water branches are generated, they will be quickly converted into electric branches under DC voltage, and discharges will be formed, which will accelerate the insulation degradation, so that the voltage is applied after the operation. A breakdown is formed underneath. The simple water branch can maintain a considerable withstand voltage under the AC operating voltage and can be maintained for a period of time.