Baoying's Analysis of the Technical Problems to Be Paid Attention to in Using DC Withstand Voltage Test in Insulation Test of Power Cable

1, it is best not to use AC withstand voltage test, to use DC voltage test high-voltage electrical equipment is generally through the AC withstand voltage test to test the main insulation compressive strength, and the power cable because of its large capacity, often It will be limited by the capacity of the test equipment, and it is difficult to conduct AC voltage withstand test. In addition, the AC voltage withstand test may cause free discharge in the oil-paper insulated cable cavity and damage the cable. The same high AC voltage damages the cable insulation strength far greater than the DC voltage. Therefore, the DC withstand voltage test will become a common method to check the insulation performance of the cable. DC withstand voltage test, equipment capacity is small, high voltage. The power cable is under the DC voltage. The voltage in the insulation is distributed according to the resistance. When the power cable is defective, the voltage will be mainly added on the defect-related part, which will make the defect easier to expose. This is the AC voltage withstand test. Unable to do it.
2. DC voltage withstand test must use negative polarity connection. Generally, when performing DC voltage withstand test, only need to pay attention to whether the wiring is correct and ignore the problem of voltage polarity. The power cable dc breakdown strength is related to the voltage polarity. If the cable core is connected to the positive pole, under the action of the electric field, the moisture in the cable insulation will infiltrate and move to the weaker lead field. As a result, the defect is hard to find and the breakdown voltage is high. Increases the cable core by 10% compared to the negative cable connection. Therefore, a negative polarity connection should be used for DC withstand voltage tests on power cables.
3. Effect of temperature on test during DC voltage withstand test The cable insulation resistance, like other high-voltage electrical appliances, will decrease as the temperature rises and increase as the temperature decreases; the leakage current will increase as the temperature rises. As the temperature decreases, it will decrease. It can be seen that the temperature has a great influence on the test data. It is important to scale the test data according to the recording temperature. If the power cable has a long power outage, the actual temperature of the cable should be recorded during the insulation test. The cable test is generally done after a power outage for several hours. At this time, the temperature of the cable core is closer to the soil temperature.
Because the annual test time is relatively constant, the soil temperature is generally not much different, but the test data can not be converted according to the recorded outdoor temperature, but should be converted according to the soil temperature. The temperature of different places to be placed will also be different. The cable placed in the open air is based on the outdoor temperature. The cable placed in the water is based on the recorded water temperature. The cable core temperature of the cable needs to be tested for the cable that just lost power. The voltage distribution between core and lead depends on the insulation resistance, so the core and lead temperature have a great influence on the voltage distribution. When the temperature difference is not great, the voltage near the insulation of the cable core is higher than the voltage near the lead skin; if the temperature difference is large, the insulation resistance near the cable core is relatively reduced due to the increased temperature, and the insulation resistance near the cable core is shared by the cable core. 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 the thermal defect is easily found near the lead skin.
4. DC withstand voltage test, the cable must be fully discharged power cable capacity is large, after the DC withstand voltage test, the residual charge energy is still relatively large, directly affect the insulation resistance and absorption ratio measurement. If the cable is short after the first DC voltage withstand test and the residual charge is not exhausted, the charge current and the sink current will be reduced for the first time. This will cause a false increase in the insulation resistance. There is also a phenomenon that the absorption ratio decreases.
In addition, the insulation resistance test immediately after the DC withstand voltage test results in a false phenomenon that the insulation resistance decreases and the absorption ratio increases. This is mainly caused by the fact that the polarity of the megohmmeter wiring voltage, which measures the insulation resistance, is opposite to the polarity of the DC voltage voltage. In the DC withstand voltage test of the cable, if the discharge is not sufficient, immediately measure the insulation resistance, then the insulation resistance meter needs to output a lot of charge to neutralize the residual charge in the cable, resulting in a false reduction of the insulation resistance. Since the DC voltage withstand test time is generally 5 minutes, after the DC voltage withstand test of the cable, the discharge time is more than 5 minutes. The longer the cable is, the longer the discharge time is. After the insulation resistance test, the discharge time is greater than the charge time.
5, DC withstand voltage test, must be shielded When the power cable DC withstand voltage and DC leakage test, due to higher test voltage, good insulation of the cable leakage flow is small, so the device caused by the stray current on The impact of the test results will be great. In order to eliminate the effect of stray currents on the test results, micro-ampere-meters are used to connect the high-voltage side, high-voltage leads, and micro-ampere metering shields. This test wiring, due to the use of micro-ampere meter connected to the high-pressure circuit, and the high-voltage lead and micro-ammeter plus shielding, it can eliminate the impact of high voltage lead corona and stray current test equipment on the test results, the test The accuracy of the results is high. This type of wiring can be used for insulation or non-insulation of cable sheaths.
Specializing in the production of DC high voltage generator manufacturers Baoying Tongwei Electric believes that under harsh environmental conditions, the cable surface leakage current is large, so that the test data can not reflect the real situation of insulation. The use of two ends of the cable plus shielding to eliminate the surface leakage current, this method can completely eliminate the effects of leakage on the surface of both ends of the cable, and can measure the true leakage current data of the cable insulation.
Baoying Tongwei Electric Equipment Factory is a professional manufacturer of loop resistance tester, DC resistance tester, DC high voltage generator, dielectric loss tester, automatic ratio change tester, zinc oxide arrester tester, high voltage switch mechanical characteristic tester , And other power testing equipment, instrumentation of the high-tech units, the majority of users are welcome to negotiate.

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Posted on August 25, 2019