- 1.Disconnect all electrical equipment connected to the tested cable.
- 2.Use a megger to test the insulation parameters of each phase of the cable, and carry out the ultra-low frequency withstand voltage test only after the test is qualified.
- 3.Set the test voltage value: Umax = 3uo, where uo is the rated phase voltage of the cable. Example 1: cable parameter: the rated line voltage is 10kV, and the rated phase voltage uo = 6kV, so the test voltage setting value is:
Umax = 3uo = 18kV
See Table 4 for the setting value of 0.1Hz ultra-low frequency test voltage of various rubber and plastic insulated power cables.
Table 4 0.1Hz ultra-low frequency test voltage and time of various rubber plastic insulated power cables.
| Rated voltage U0/US(kV) | Handover test | Preventive experiment | ||||
| multi ple | Test voltage（kV） | Test time（mins） | multip le | Test voltage（kV） | Test time（mins） | |
| 1.8/3 | 3U0 | 5 | 60 | 3U0 | 5 | 15 |
| 3.6/6 | 3U0 | 11 | 60 | 3U0 | 11 | 15 |
| 6/6 | 3U0 | 18 | 60 | 3U0 | 18 | 15 |
| 6/10 | 3U0 | 18 | 60 | 3U0 | 18 | 15 |
| 8.7/10 | 3U0 | 26 | 60 | 3U0 | 26 | 15 |
| 12/20 | 3U0 | 36 | 60 | 3U0 | 36 | 15 |
| 21/35 | 3U0 | 63 | 60 | 3U0 | 63 | 15 |
| 26/35 | 3U0 | 78 | 60 | 3U0 | 78 | 15 |
Note: UN is the rated voltage of the cable, and uo is the phase voltage of the cable.
4.Test time: 60 minutes for handover test and 15 minutes for preventive test.
5.Setting current value of overcurrent protection:
Estimation method of capacitive current (or leakage current) of ultra-low frequency withstand
voltage test article:
Io=2πfCU=2×3.14×0.1CU(mA)
C is the electric capacity of the cable to the ground, unit: UF; u is the effective value of the test voltage, unit: kV.
Example 2: a 10KV (UN = 10kV, uo = 8.7kv) cable is 4km long, with a single phase to ground capacitance of 0.21uf/km and 0.1Hz super
If the low frequency test voltage is 26kv (peak value), the leakage current is approximately:
Io=2πfCU=2×3.14×0.1CU=0.628×0.21×4×26/ 2=9.69（mA）
Setting current value of overcurrent protection: I=kIo