How to use insulation megger to measure insulation resistance of power cable

1. The performance of power cable insulation material decreases irreversibly with service time and environment, which is called insulation aging. The manifestations of insulation aging are as follows: breakdown strength decreases, dielectric loss tangent increases, local leakage and discharge increase, mechanical strength and other properties decrease.
2. In the aging process, the time required for the insulation performance to fall below the specified allowable range is usually called insulation life. Under normal service conditions, the service life of XLPE cable and oil immersed armored cable is 30 years and 40 years respectively. Under the action of power frequency AC voltage, partial discharge in insulation material is the main reason for insulation aging of power cable. In the oil immersed cable, the partial discharge leads to the decomposition of impregnant and paper fiber and the formation of waxy crystalline state. In XLPE cables, the insulation material is gradually damaged by partial discharge, forming micropores, cracks and dendritic buds, which gradually develop in depth in the insulation material until insulation breakdown.
3. The insulation aging degree of power cable can be measured by insulation resistance test and dielectric loss tangent tan δ Test, withstand voltage test and other electrical tests. Insulation resistance test shall be conducted for power cables below 6kV. 500 ~ 1000V grounding resistance tester (megohmmeter or megger) is used for cables with voltage below 1KV; 1000 ~ 2500V grounding resistance tester is used for 1kV and above cable.

The following items should be noted during the test.
① The method of measuring the insulation resistance is suitable for the cable which is not too long. Measure the insulation resistance and calculate the absorption ratio. Under the same test conditions, the better the insulation value is, the greater the absorption ratio is.
② Generally, the insulation resistance value of power cable is not specified, and the test value should be compared with the original record. Generally, the insulation resistance of 1 ~ 3KV cable should not be less than 200m Ω; 6 ~ 10kV cable should not be less than 400m Ω; 35kV Cable should not be less than 600m Ω; The unbalance coefficient of three-phase insulation shall not be greater than 2.5. During the test, the temperature, air humidity and other weather conditions shall be recorded for reference.
③ Before measurement, the cable shall be fully discharged, that is, the cable core and the metal sheath of the cable shall be grounded.
④ The surface of cable terminal bushing shall be wiped clean before testing. Use the other insulating core of the cable as the shielding circuit, connect the conductors at both ends of the insulated core to the bushing or insulation of the insulated core to be tested with flexible metal wires, wind them for several turns, and then connect them to the shielding terminal g of the grounding resistance tester.
⑤ After each test, the cable shall be grounded and discharged. The longer the cable line is, the better the insulation condition is, and the longer the grounding discharge time is, generally not less than 1min.