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DC voltage withstand test of high voltage cable
With the rapid advancement of some projects such as the “West Power Transmission to the West†and “Southern Power Transmission to the North,†high-voltage DC cables will also become the main force used by us. Large-scale access to new energy sources and the construction of urban power transmission cables all need to be applied to this type of cable. This kind of cable is the only means of transmission of electricity for offshore wind power and islands on the sea. High-voltage and extra-high-voltage cables have become one of the indispensable materials in modern urban power grid project engineering. However, the field of high-voltage and ultra-high voltage cables in China started relatively late. Materials, equipment, and manufacturing technologies have long relied mainly on the introduction and imitation. Here we will talk about the problems that arise when the high-voltage cable is subjected to a DC voltage withstand test.
1. Under DC voltage, the electric field distribution of cable insulation is mainly determined by the volume and resistivity of the material, while the electric field distribution under AC voltage is dependent on the dielectric constant of various media, especially at the end of the cable. The distribution of the DC electric field strength and the distribution of the AC electric field strength in the cable accessories such as the head, the connector box, etc. are completely different, and the mechanism of the insulation aging under the DC voltage and the insulation aging mechanism under the AC voltage are not same. Therefore, the DC voltage withstand test of a high voltage DC cable cannot simulate the operation of a high voltage cable under normal operation.
2. The high-voltage cable generates a "memory" effect under the stimuli of the DC voltage, and stores and accumulates unipolar residual charges. Once the so-called "memory" caused by the DC withstand voltage test has taken place, it takes a long time to completely release the DC bias. If the cable is put into operation before the DC residual charge is fully discharged, the DC bias voltage will be superimposed on the peak of the power frequency voltage, making the voltage on the cable far exceed its own rated voltage, which may lead to cable insulation. Break down this consequence.
3. When a high-voltage DC cable is subjected to a DC voltage withstand test, electrons are injected into the interior of the polymer medium, so that space charge is formed, and the electric field intensity at the place is lowered, so that it is difficult to cause breakdown. Space charges are easily generated at the semiconductor bumps and contamination points of high-voltage cables. However, if a flashover of the cable terminal or a breakdown of the cable attachment occurs during the test, wave oscillations will occur on the cable core. In the space where the space charge has accumulated, the polarity of the voltage will be affected by the oscillation. This quickly changes to a different polarity, so that the electric field intensity at that location increases significantly, and insulation damage may occur, resulting in breakdown at multiple points.
4. One of the most deadly weaknesses of ordinary high-voltage cables is that water can easily be produced in the insulation. Once the water branch is generated, it will be rapidly converted into electrical branches under the action of the direct voltage, and the discharge will be formed, accelerating the deterioration of the insulation, so that after the operation, a breakdown occurs under the action of the commercial frequency voltage. The simple water branch can maintain a certain pressure value under the AC operating voltage, and can maintain for a long period of time, which is also a very important reason for us to choose high-voltage DC cables.
We all know that the safety of life and property is above all else. Because of its unique advantages, high-pressure, low-voltage, fire-resistant cables are associated with fire safety and firefighting in high-rise buildings, underground railways, underground pedestrian streets, large power stations, and important industrial and mining companies. The place has been widely used. Therefore, we also believe that fire accidents caused by cable problems will become less and less in the future.