1. It is not suitable to adopt AC voltage withstand test, but DC voltage withstand test
High voltage electrical equipment is generally tested by AC withstand voltage test for its main insulation withstand voltage strength, while power cable is often limited by the capacity of test equipment due to its large capacitance, so it is difficult to carry out power frequency AC withstand voltage test. In addition, the AC withstand voltage test may produce free discharge in the hole of oil paper insulated cable and damage the cable. The insulation strength of the cable damaged by the same high AC voltage is much greater than that of DC voltage. Therefore, DC withstand voltage test has become a common method to check the insulation performance of cables. DC withstand voltage test, small equipment capacity, high voltage. Under the action of DC voltage, the voltage in the insulation of the power cable is distributed according to the resistance. When the power cable has defects, the voltage will be mainly applied to the parts related to the defects to make the defects more easily exposed, which can not be achieved by AC withstand voltage test.
2. During DC voltage withstand test, negative polarity connection must be adopted
Generally, when conducting 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 ACSR Cable is related to the voltage polarity. If the cable core is connected to the positive pole, under the action of electric field, the moisture in the cable insulation layer will penetrate and move to the lead skin with weak electric field. As a result, the defects are not easy to find, and the breakdown voltage is 10% higher than that when the cable core is connected to the negative pole. Therefore, the negative connection should be used for DC withstand voltage test of power cable.
3. Influence of temperature on DC withstand voltage test
The insulation resistance of cable decreases with the increase of temperature and increases with the decrease of temperature, just like other high-voltage apparatus; The leakage current increases with the increase of temperature and decreases with the decrease of temperature. It can be seen that temperature has a great influence on the test data. It is very important to convert the test data according to the recorded temperature. If the power cable is cut off for a long time, the actual temperature of the cable should be recorded during the insulation test. The cable test is usually done after several hours of power failure. At this time, the temperature of the cable core is close to the soil temperature. Because the test time is relatively fixed every year, the soil temperature generally has no great difference, but the test data can not be converted according to the recorded outdoor temperature, but should be calculated according to the soil temperature. The temperature of different places is also different. The outdoor temperature of the cable placed in the open air is subject to the outdoor temperature, and the water temperature of the cable placed in the water is subject to the recorded water temperature. For the cable just cut off, the core temperature of the cable 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 voltage shared by the insulation near the cable core is higher than that near the lead sheath; If the temperature difference is large, the insulation resistance near the cable core will decrease due to the increase of temperature, and the voltage shared by the insulation resistance near the cable core will decrease, which may be smaller than that near the lead skin. Therefore, it is easy to find the insulation defects near the cable core in the cold state, and it is easy to find the insulation defects near the lead skin in the hot state.
4. During DC withstand voltage test, the cable must be fully discharged
The capacitance of power cable is very large. After DC withstand voltage test, the energy of residual charge is still relatively large, which directly affects the measurement of insulation resistance and absorption ratio. If the discharge time of the cable is short after the first DC withstand voltage test and the residual charge is not discharged completely, the charging current and absorption current will decrease compared with the first time, so the phenomenon of false increase of insulation resistance and decrease of absorption ratio will appear.
In addition, insulation resistance test immediately after DC withstand voltage test will produce false phenomenon of insulation resistance decrease and absorption ratio increase. This is mainly caused by the opposite polarity of the connection voltage of the megohmmeter measuring the insulation resistance and the DC withstand voltage. In the DC withstand voltage test of cable, if the discharge is not sufficient, measure the insulation resistance immediately, then the insulation resistance meter needs to output a lot of charge to neutralize the residual charge in the cable, resulting in false reduction of insulation resistance. Because the DC withstand voltage test time is generally 5min, the discharge time of cable after DC withstand voltage test should be more than 5min. The longer the cable is, the longer the discharge time is. After insulation resistance test, the discharge time is longer than the charging time.
5. During DC withstand voltage test, it must be shielded
When DC withstand voltage and DC leakage tests are carried out on power cables, the stray current caused by equipment has a great influence on the test results because the test voltage is higher and the leakage current of cables with good insulation is smaller. In order to eliminate the influence of stray current on the test results, the microammeter is connected to the high voltage side, and the high voltage lead and the microammeter are shielded. Because the micro ammeter is connected to the high-voltage circuit and the high-voltage lead and micro ammeter are shielded, the influence of the corona of the high-voltage lead and the stray current of the test equipment on the test results can be eliminated, and the accuracy of the test results is high. This kind of wiring can be used for the cable sheath with or without insulation to the ground.
Tuopu electric, a professional manufacturer of DC high-voltage generator, thinks that under harsh environmental conditions, the leakage current on the cable surface is large, so that the test data can not reflect the real situation of insulation. This method can completely eliminate the influence of the surface leakage at both ends of the cable, and can measure the real leakage current data of the cable insulation.