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With the national "East to West", "South Power North" and other projects to promote, High-voltage DC cable will be used as the main force. High-voltage direct current cable is needed for the construction of the new energy large-scale access and the cable of the city transmission. High Voltage DC Cable is the only means of transmission of offshore wind power and Sea island.
High voltage and ultra-high voltage cable has become an indispensable material in modern urban power grid project, but the field of High-voltage EHV cable has started late in China, and materials, equipment and manufacturing technology have relied mainly on introduction and imitation for a long time. Below we say, the high-voltage cable for DC voltage withstand test of the main problems in what aspects?
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 alternating voltage depends on the dielectric constant of each medium, especially the distribution of the DC electric field intensity in cable accessories such as the terminal Head and the junction box, and the distribution of AC electric field intensity And the mechanism of insulation aging under DC voltage and the aging mechanism under AC voltage are different. Therefore, the DC voltage withstand test can not simulate the operating conditions of high-voltage cable.
2. High-voltage cable under the DC voltage will produce a "memory" effect, storage accumulation of single polar residual charge. Once the "memory" caused by the DC voltage withstand test is available, it will take a long time to release the DC bias pressure. If the cable is put into operation before the DC residual charge is completely released, the DC bias will be superimposed on the peak frequency voltage, so that the voltage on the cable is far more than its rated voltage, which may lead to cable insulation breakdown.
3. DC voltage withstand test, there will be electrons injected into the polymer medium, the formation of space charges, so that the area of the electric field strength, which is difficult to penetrate. The space charge is easily generated by the semiconductor bulge and the pollution point of the high-voltage cable. However, if there is a surface flashover or cable attachment breakdown at the end of the test, will cause the cable core line generated wave oscillation, in the place where the space charge has accumulated, because the oscillation voltage polarity rapidly change polarity, so that the field strength of the site significantly increased, may damage insulation, resulting in a number of breakdown.
4. High-voltage cable fatal weakness is easy to produce water branches in the insulation, once the formation of water branches, in the DC voltage will be quickly converted into electrical branches, and the formation of discharge, accelerated insulation deterioration, so that after the operation of the power frequency voltage under the action of the formation of breakdown. But the pure water branch in the alternating current work voltage also can maintain the considerable withstand pressure value, and can maintain for a period of time.
Life security above all, high pressure and low pressure refractory cable for its unique advantages in high-rise buildings, underground railway, underground street, large power plants and other important industrial and mining enterprises and fire safety and fire-fighting life-related areas have been widely used. We believe that there will be fewer accidents caused by cable problems.