1. It is not advisable to use AC withstand voltage test, and DC withstand voltage test should be adopted.

High-voltage electrical equipment generally tests its main insulation withstand voltage through AC withstand voltage test. Because of its large capacity, power cables are often limited by the capacity of test equipment, making it difficult to conduct power frequency AC withstand voltage test. In addition, the AC withstand voltage test may cause a free discharge in the hole of the oil-paper insulated cable to damage the cable, and the same high AC voltage damages the cable insulation strength much more than the DC voltage. Therefore, the DC withstand voltage test is a common method for checking the insulation properties of cables. DC withstand voltage test, small equipment capacity and high voltage. Under the action of DC voltage, the voltage of the power cable is distributed according to the resistance. When the power cable is defective, the voltage will be mainly applied to the defect-related part, so that the defect is more easily exposed. This is the AC withstand voltage test cannot be done. of.

2. When the DC withstand voltage test is required, a negative polarity connection must be used.

Generally, when performing the DC withstand voltage test, only pay attention to whether the wiring is correct, and ignore the problem of voltage polarity. The DC breakdown strength of the power cable is related to the polarity of the voltage. For example, if the cable core is connected to the positive pole, under the action of the electric field, the moisture of the cable insulation layer will migrate to the lead skin with weaker electric field. As a result, the defect is not easily found, and the breakdown voltage ratio is The cable core is increased by 10% when it is connected to the negative pole. Therefore, the DC voltage withstand test of the power cable should use a negative polarity connection.

3. Influence of temperature on test during DC withstand voltage test

The cable insulation resistance, like other high-voltage electrical appliances, decreases with increasing temperature and increases with decreasing temperature; leakage current increases with temperature and decreases with decreasing temperature. It can be seen that the temperature has a great influence on the test data. It is important to convert the test data to the recorded temperature. If the power cable has a long power outage, pay attention to the actual temperature of the cable during the insulation test. The cable test is usually done after a few hours of power failure. At this time, the temperature of the cable core is close to the soil temperature. Because the annual test time is relatively fixed, the soil temperature is generally not much different, but the test data cannot be converted according to the recorded outdoor temperature. It should be converted according to soil temperature. The temperature of different places is different. The cable placed in the open air is subject to the outdoor temperature. The cable placed in the water is used to record the water temperature. The cable temperature of the cable is tested for the cable that has just been powered off. The voltage distribution between the core and the lead depends on the insulation resistance, so the temperature of the core and the lead has a great influence on the voltage distribution. When the temperature difference is not large, the voltage of the insulation near the cable core is higher than that near the lead; if the temperature difference is large, the insulation resistance close to the core is relatively lowered due to the increase in temperature, and the insulation resistance close to the core is shared. The voltage is reduced and may be less than near the lead. Therefore, in the cold state, the DC withstand voltage test is easy to find the insulation defect near the cable core, and in the hot state, it is easy to find the insulation defect near the lead.