1. AC withstand voltage test is not suitable; DC withstand voltage test is preferred.
High-voltage electrical equipment is generally tested for its main insulation withstand voltage through AC withstand voltage tests. However, due to the large capacitance of power cables, they are often limited by the capacity of test equipment, making it difficult to perform power frequency AC withstand voltage tests. In addition, AC withstand voltage tests may cause free discharge in the cavities of oil-paper insulated cables, damaging the cable. The same high AC voltage damages the cable insulation strength far more than DC voltage. Therefore, DC withstand voltage testing has become a common method to check cable insulation performance. DC withstand voltage testing requires small equipment capacity but high voltage. Under DC voltage, the voltage distribution in the insulation of power cables is based on resistance. When there are defects in the power cable, the voltage will be mainly applied to the parts related to the defects, making the defects easier to expose, which AC withstand voltage testing cannot achieve.
2. Negative polarity connection must be used during DC withstand voltage testing.
Generally, during DC withstand voltage testing, only attention is paid to correct wiring, while ignoring the issue of voltage polarity. The DC breakdown strength of power cables is related to voltage polarity. If the cable core is connected to the positive pole, under the action of the electric field, moisture in the cable insulation layer will permeate and move towards the lead sheath where the electric field is weaker, making defects less likely to be detected, and the breakdown voltage is 10% higher than when the cable core is connected to the negative pole. Therefore, negative polarity connection should be used for DC withstand voltage testing of power cables.
3. The influence of temperature on the test during DC withstand voltage testing.
The insulation resistance of cables, like other high-voltage electrical equipment, decreases with rising temperature and increases with falling temperature; leakage current increases with rising temperature and decreases with falling temperature. It can be seen that temperature has a significant impact on test data. It is important to convert test data based on the recorded temperature. If the power cable has been out of service for a long time, attention should be paid to recording the actual temperature of the cable during insulation testing. Cable tests are usually performed after several hours of power outage. At this time, the temperature of the cable core is close to the soil temperature. Since the test time each year is relatively fixed, the soil temperature generally does not differ much, but the test data should not be converted based on the recorded outdoor temperature, but should be converted based on the soil temperature. The temperature also varies depending on the placement location. For cables placed in the open air, the outdoor temperature is used as the standard; for cables placed in water, the recorded water temperature is used as the standard; for cables that have just been powered off, the temperature of the cable core should be tested.
The voltage distribution between the cable core and the lead sheath depends on the insulation resistance, so the temperature of the cable core and the lead sheath has a great influence on the voltage distribution. When the temperature difference is small, the insulation near the cable core bears a higher voltage than that near the lead sheath; if the temperature difference is large, due to the increase in temperature, the insulation resistance near the cable core relatively decreases, and the voltage shared by the insulation resistance near the cable core decreases, possibly becoming less than that near the lead sheath. Therefore, performing DC withstand voltage tests in a cold state makes it easy to detect insulation defects near the cable core, while in a hot state, it is easy to detect insulation defects near the lead sheath.
