Talking about the problem of DC withstand voltage test for high voltage cable

by:AAA     2020-03-18
With the advancement of national projects such as 'west-to-east power transmission' and 'South-to-North power transmission', HVDC cables will become the main force to be applied. The large-scale access of new energy and the cable construction of urban transmission all need to apply HVDC cables. HVDC cable is the only transmission means for offshore wind power and offshore isolated island. High-voltage and ultra-high-voltage cables have become indispensable materials in modern urban power grid projects, but the field of high-voltage and ultra-high-voltage cables in China started late, and materials, equipment and manufacturing technology have long relied mainly on introduction and imitation. Next, let's talk about what are the main aspects of DC withstand voltage test for high voltage cables? 1. Under DC voltage, the electric field distribution of cable insulation depends on the volume resistivity of the material, while the electric field distribution under AC voltage depends on the dielectric constant of each medium, in particular, the distribution of DC electric field strength in cable accessories such as cable terminals and connector boxes is completely different from that of AC electric field strength, moreover, the mechanism of insulation aging under DC voltage is different from that under AC voltage. Therefore, DC withstand voltage test cannot simulate the operating conditions of high voltage cables. 2. High Voltage Cables will produce 'memory' effect under DC voltage to store and accumulate unipolar residual charges. Once there is a 'memory' caused by 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 will be superimposed on the peak value of the power frequency voltage, making the voltage value on the cable far exceed its rated voltage, this may lead to cable insulation breakdown. 3. During the DC withstand voltage test, electrons will be injected into the polymer medium to form space charge, which will reduce the electric field strength there and make it difficult to break down. Space charge is easily generated at the semiconductor projection source and pollution point of high voltage cable. However, if the surface flashover or cable accessories breakdown occurs on the cable terminal during the test, it will cause wave oscillation on the cable core. At the place where space charge has accumulated, as the polarity of the oscillating voltage rapidly changes to a different polarity, the strength of the electric field here increases significantly, which may damage the insulation and cause multi-point breakdown. 4. A fatal weakness of high-voltage cable is that it is easy to produce water branches in insulation. Once water branches are generated, they will be quickly converted into electric branches under DC voltage and form discharge, accelerating insulation deterioration, so that after operation, breakdown is formed under the action of power frequency voltage. However, pure water branches can still maintain a considerable withstand voltage value under ac working voltage and for a period of time. Life Safety is above everything else, due to its unique advantages, high-voltage and low-voltage fire-resistant cables are widely used in high-rise buildings, subways, underground streets, large power stations and important industrial and mining enterprises and other places related to fire safety and fire fighting and lifesaving. We believe that accidents caused by cable problems will be less and less.
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