Cable Insulation Testing Methods

Precautions for Insulation Testing of Power Cables 

１  Avoid AC withstand voltage tests; opt for DC withstand voltage tests.

    High-voltage electrical equipment typically undergoes AC withstand voltage tests to assess the main insulation's strength. However, due to their larger capacitance, power cables often face limitations in the capacity of testing equipment, making it difficult to conduct AC withstand voltage tests at power frequency. Moreover, AC withstand voltage tests may cause arc discharges in the voids of oil-paper insulated cables, potentially damaging the cables. The damage to cable insulation strength from high AC voltages is significantly greater than from DC voltages. Therefore, DC withstand voltage tests have become a common method for inspecting cable insulation performance. DC withstand voltage tests feature smaller equipment capacity and higher voltages. Under DC voltage, the voltage in the insulation of power cables is distributed according to resistance. When a power cable has defects, the voltage is mainly applied to the areas related to the defects, making it easier to expose the defects—a capability not achievable with AC withstand voltage tests.

    ２ During DC withstand voltage testing, a negative polarity connection must be used.

    During DC withstand voltage tests, attention is often solely focused on the correct wiring, while the issue of voltage polarity is overlooked. The DC breakdown strength of power cables is related to the voltage polarity. For instance, if the cable core is connected to the positive pole, under the influence of the electric field, the moisture in the cable insulation will渗透 towards the lead sheath with a weaker electric field, making it difficult to detect defects. As a result, the breakdown voltage is 10% higher compared to when the cable core is connected to the negative pole. Therefore, it is essential to use a negative polarity connection when conducting DC withstand voltage tests on power cables.３ The Impact of Temperature on High-Voltage Direct Current (DC) Insulation Test

Insulation resistance of cables decreases with rising temperatures and increases with decreasing temperatures, just like other high-voltage electrical equipment. Leakage current increases with rising temperatures and decreases with decreasing temperatures. It is evident that temperature has a significant impact on test data. It is crucial to convert test data based on recorded temperatures. For power cables that are off for an extended period, attention should be given to recording the actual cable temperature during insulation testing. Cable tests are generally conducted after several hours of power outage, at which point the cable core temperature is close to the soil temperature. Since the testing period is relatively fixed each year, there is usually little variation in soil temperature. However, test data should not be converted based on recorded outdoor temperatures but rather on soil temperatures. The temperature varies depending on the placement location; outdoor temperature is used for cables placed outdoors, while recorded water temperature is used for cables submerged in water. For cables that have just been powered off, the cable core temperature should be tested. 

    The voltage distribution between the cable core and sheath depends on the insulation resistance, so the temperature difference between the cable core and sheath significantly affects the voltage distribution. When the temperature difference is small, the insulation near the cable core bears a higher voltage than that near the sheath; however, when the temperature difference is large, the increased temperature reduces the insulation resistance near the cable core, resulting in a decrease in the voltage shared by the insulation near the core, which may even be less than that near the sheath. Therefore, insulation defects near the cable core are more easily detected during cold-state DC withstand tests, while those near the sheath are more readily identified during hot-state tests.

 ４ During DC withstand testing, the cable must be fully discharged.

    The capacitance of power cables is significant, and after undergoing a DC withstand test, the remaining charge still holds a considerable amount of energy, which directly impacts the measurement of insulation resistance and absorption ratio. If the cable isThe.No content provided for translation.Following the DC withstand test, the discharge time was short, and the remaining charge was not fully released before the insulation resistance test was conducted. The charging current and absorbed current would be...No Chinese content provided..No content provided for translation.The reduction in size leads to a false increase in insulation resistance and a decrease in the absorption ratio.  

    Additionally, conducting an insulation resistance test immediately after a DC withstand test can produce false phenomena of reduced insulation resistance and increased absorption ratio. This is primarily caused by the opposite polarity of the megohmmeter's connection voltage and the DC withstand voltage. During the DC withstand test, if the discharge is not sufficient, and the insulation resistance is measured immediately, the insulation resistance meter needs to output a large amount of charge to neutralize the remaining charge in the cable, resulting in a false decrease in insulation resistance. Because the DC withstand test typically takes 5After the DC withstand voltage test of the cable, the discharge time must be greater than 5 min.The longer the cable, the longer the discharge time. After the insulation resistance test, the discharge time is longer than the charging time.５ During DC withstand voltage tests, shielding is mandatory.

    During DC withstand voltage and DC leakage tests on power cables, due to the high test voltage, cables with good insulation have a smaller leakage current, thus the stray current induced by the equipment significantly affects the test results. To eliminate the impact of stray current on the test results, a microammeter is connected to the high-voltage side, with the high-voltage leads and microammeter shielded. This test wiring, by connecting the microammeter in the high-voltage circuit and adding shielding to the high-voltage leads and microammeter, can eliminate the effects of corona discharge from the high-voltage leads and stray currents from the test equipment on the test results, resulting in higher accuracy. This wiring method is applicable to both insulated and uninsulated cable sheath-to-ground conditions.

    Under adverse environmental conditions, the cable surface leakage current is significant, causing the test data to not reflect the true insulation situation. By adding shielding to both ends of the cable to eliminate surface leakage current, this method can completely remove the impact of surface leakage at both cable ends, allowing for the measurement of the cable insulation's true leakage current data.