Cable Insulation Testing

Precautions for Insulation Testing of Power Cables 

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

    High-voltage electrical equipment typically undergoes AC withstand voltage tests to evaluate the main insulation's voltage resistance. However, due to their large electrical capacity, power cables are often limited by the capacity of the testing equipment and find it difficult to conduct AC withstand voltage tests. Moreover, AC withstand voltage tests may cause ionization discharge in the voids of oil-paper insulated cables, potentially damaging the cable. The damage to cable insulation strength from the same AC voltage is far greater than that from DC voltage. Therefore, DC withstand voltage tests have become a common method for inspecting cable insulation performance. DC withstand voltage tests have smaller equipment capacity and higher voltage. Under the influence of DC voltage, the voltage in the insulation of power cables is distributed according to resistance. When a power cable has a defect, the voltage is mainly applied to the area related to the defect, making it easier to expose the defect, which is something that AC withstand voltage tests cannot achieve.

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

    During DC withstand tests, attention is typically 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. If the cable core is connected to the positive terminal, under the influence of the electric field, the moisture in the cable's insulation layer will渗透 towards the lead sheath, which is weaker in the electric field, making defects difficult to detect. This results in a breakdown voltage that is 10% higher than when the cable core is connected to the negative terminal. Therefore, negative polarity connections should be used for DC withstand tests of power cables.

３ The Impact of Temperature on High-Voltage DC Insulation Resistance Testing

Insulation resistance of cables decreases with rising temperatures and increases with decreasing temperatures, just like other high-voltage electrical equipment. Leak 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, if the power outage duration is long, attention should be given to recording the actual cable temperature during insulation testing. Cable tests are typically performed after several hours of power outage, at which point the cable core temperature is close to soil temperature. Since the test time is generally fixed each year, there is usually little variation in soil temperature. However, test data should not be converted based on recorded outdoor temperatures; instead, they should be based on soil temperature. The temperature at different placement locations varies; outdoor temperature is used for cables placed outdoors, while recorded water temperature is used for cables submerged in water. For freshly power-off cables, the cable core temperature needs to 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 easily detected during DC withstand tests in the cold state, while defects near the sheath are more easily identified in the hot state.

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

    The capacitance of power cables is substantial, and after undergoing DC withstand tests, the remaining charge's energy is still considerable, which directly impacts the measurement of insulation resistance and absorption ratio. If the cable isNo Chinese content provided..No Chinese content provided.After 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 the absorbed current will be...No Chinese content provided..No content provided for translation.The reduction in size may lead to a false increase in insulation resistance and a decrease in the absorption ratio.  

    Moreover, conducting an insulation resistance test immediately after a DC withstand voltage test can produce a false phenomenon of reduced insulation resistance and increased absorption ratio. This is primarily caused by the opposite polarity of the megohmmeter's test voltage terminal and the DC withstand voltage terminal. During the DC withstand voltage 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 voltage test usually takes about 5The discharge time must be greater than 5 minutes after the DC withstand voltage test of the cable.The longer the cable, the longer the discharge time. After the insulation resistance test, the discharge time exceeds the charging time.

５ During DC withstand voltage tests, shielding must be applied.

    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. Consequently, the stray current caused by the equipment has a significant impact on the test results. To eliminate the influence of stray current on the test results, a microammeter is connected to the high-voltage side, with the high-voltage leads and microammeter being shielded. This type of test wiring, by connecting the microammeter to 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 current from the test equipment on the test results, resulting in higher accuracy. This wiring method can be used for both insulated and uninsulated cable sheaths.

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