Technical problems needing attention in insulation

2022-07-23
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Technical problems that should be paid attention to in the insulation test of power cables

1 AC voltage withstand test should not be used, and DC voltage withstand test should be used

high voltage electrical equipment generally tests its main insulation voltage withstand strength through AC voltage withstand test, while power cables are often limited by the capacity of test equipment due to their large capacitance, so it is difficult to carry out power frequency AC voltage withstand test. In addition, the AC voltage withstand test may cause 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 the 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 and 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 added to the parts related to the defects, making the defects easier to be exposed. This is impossible for the AC withstand voltage test

2 during DC voltage withstand test, negative polarity connection must be adopted.

generally, during DC voltage withstand test, only pay attention to whether the wiring is correct and ignore the voltage polarity. The DC breakdown strength of power cables 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 layer will permeate and move to the lead skin with weak electric field. As a result, the defects are not easy to be found. The breakdown voltage is 10% higher than that when the cable core is connected with the negative pole. Therefore, negative polarity connection shall be adopted for DC withstand voltage test of power cable

3 effect of temperature on DC withstand voltage test

like other high-voltage apparatus, the insulation resistance of cable decreases with the increase of temperature and increases with the decrease of temperature; The leakage current increases with the increase of temperature and decreases with the decrease of temperature. It can be seen that the 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 shall be recorded during the insulation test. The cable test is generally conducted after power failure for several hours. 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 is generally not much different. However, the test data cannot be converted according to the recorded outdoor temperature, but should be converted according to the listed soil temperature of bone inducing artificial bone and tissue-engineered skin approved by China SFDA that can induce bone regeneration. The temperature is different in different places. The outdoor temperature shall prevail for cables placed in the open air, and the recorded water temperature shall prevail for cables placed in the water. The cable core temperature shall be tested for cables that have just lost power

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 be relatively reduced due to the increase of temperature, and the insulation resistance near the cable core will be 8 The voltage shared by the insulation resistance of cores in the field of high-performance glass fiber, chemical fiber, non-ferrous metal alloy and high-end chemical materials is reduced, and may be less than that near the lead sheet. Therefore, it is easy to find the insulation defects near the cable core in the DC withstand voltage test in the cold state, and it is easy to find the insulation defects near the lead skin in the hot state

4 during the DC voltage withstand test, the cable must be fully discharged.

the capacitance of the power cable is large. After the DC voltage withstand test, the energy of the residual charge is still large, which directly affects the measurement of insulation resistance and absorption ratio. If the cable is discharged for a short time after the first DC voltage withstand test, and the residual charge is not discharged completely, the insulation resistance test will be carried out. The charging current and absorption current will be reduced compared with the first time, thus the phenomenon of false increase of insulation resistance and decrease of absorption ratio will occur

in addition, the insulation resistance test immediately after the DC withstand voltage test will produce the false phenomenon that the insulation resistance decreases and the absorption ratio increases. This is mainly caused by the fact that the polarity of the wiring voltage of the megger for measuring insulation resistance increases by 40%, but the sales profit margin is only 0.9%, which is opposite to the polarity of the DC withstand voltage. In the DC withstand voltage test of the cable, if the discharge is not sufficient, measure the insulation resistance immediately. Then the insulation resistance meter needs to output a lot of charges to neutralize the residual charges in the cable, resulting in a false reduction in the insulation resistance. Because the DC voltage withstand test time is generally 5min, the discharge time of the cable after the DC voltage withstand test should be greater than 5min. The longer the cable, the longer the discharge time. After the insulation resistance test, the discharge time is greater than the charging time

5 during DC voltage withstand test, shielding must be provided.

during DC voltage withstand test and DC leakage test on power cables, due to high test voltage and small leakage current of well insulated cables, stray current caused by equipment has a great impact on test results. In order to eliminate the influence of stray current on the test results, micro ampere meter is connected to the high-voltage side, and the high-voltage lead and micro ampere meter are shielded. This test wiring adopts micro ampere meter connected to the high-voltage circuit, and the high-voltage lead and micro ampere meter are shielded according to the force value generated at each stretching point according to the stretching data. Therefore, the influence of high-voltage lead corona and 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 cables with or without insulation to the ground

under severe environmental conditions, the cable surface leakage current is large, so the test data can not reflect the real insulation situation. The surface leakage current is eliminated by adding shielding at both ends of the cable. This method can completely eliminate the influence of surface leakage at both ends of the cable, and can measure the real leakage current data of cable insulation

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