380V High-impedance Single-phase Ground Fault Test

I. Pre-Test Preparation

II. Test Process

Step 1: Determine the nature of the fault The 500V insulation megger separately tests the insulation resistance of the three-phase cable to ground, and all three phases are zero to ground. The three phases were retested with a multimeter. The resistance values of A, B, and C to ground were all about 10kΩ, and the interphase test between A and B was 600Ω. It was initially judged that it was a short-circuit fault between A and B.

Step 2: Pre-positioning the fault 

First, use the WL20 wave reflector low-voltage pulse method to test the full length of the three phases of the cable. The length of the three phases is about 165m;

It was initially determined that it was a high-resistance fault between A and B phases. A voltage of 7kV was applied to the A and B phases. After the fault point was broken down, the pulse current waveform was collected as shown in the figure below. The waveform was obviously cyclical, and the fault distance was 162m.

Step 3: Find the cable path

Step 4: Accurately fix the point 1. The total length of the cable measured by the low-voltage pulse is 165 meters, and the pulse current waveform test length is 162 meters. The fault location is basically consistent with the total length of the cable, and the fault point is located at the end of the cable. 2. A 7kV high-voltage pulse was applied to the fault phase, and a fixed-point meter pp20 reached the end to set the point. Since the end of the cable was at the corner of the first and second floors of the residential building, the point was first set at the first floor. It was found that the fault point discharge sound was emitted near the corner of the first floor. The acoustic-magnetic time difference was the smallest, and the discharge sound in the earphone was the largest. The minimum acoustic-magnetic time difference was 2.9ms, and the point was successful.

III. Test Summary

1. In this fault test, the obtained waveform is relatively ideal. The full-length three-phase test waveform at the test end is basically the same as the inter-phase test fault waveform, and it can be confirmed as an end fault.
2. At the fixed point, the minimum acoustic-magnetic time difference is 2.9ms, the sound is obvious and the distance is consistent, and it is finally confirmed that this place is the fault point.