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Power Cable Fault Test Report

Field Basic Information

• Laying method: direct burial.
• Both ends: one end on a 35kV tower, the other at the booster station.
• Insulation test: tested at 5000V, insulation of every phase is normal.
• Intermediate joints: two intermediate joints.
• Site condition: the new cable was energized for the first time and tripped immediately after energization.

Instruments Used

XHGG502, 5/50, 40/6, XHDD503E, and a 5000V megohmmeter.

Fault Nature

Using an electronic megohmmeter at the 5kV range, the insulation resistance of A-ground, B-ground, C-ground, A-B, B-C and A-C was measured and all values were above GΩ.

Insulation testing was normal, but a subsequent DC withstand test found that phases B and C could not withstand 30kV.

The fault is therefore classified as a flashover high-resistance fault.

Cable Length

The marked total cable length is 1100 m, and the measured length is 1092.6 m.

Fault Distance

Coarse Location Methods

Low-voltage pulse test and high-voltage flashover test.

Waveforms

Waveform Analysis

From the low-voltage pulse waveform, only the full cable length could be observed; no short-circuit or open-circuit fault was seen.

From the high-voltage impulse flashover waveform analysis, the fault point is 544.5 m from the test end, very close to the second intermediate joint, so the second intermediate joint was suspected to be the faulty point.

Cable Route Tracing

The on-site cable route was clear.

Fault Pinpointing

Walking to the second cable joint pit, obvious discharge was heard inside the pit and the fault point was confirmed; when a high-voltage pulse was then applied to another phase, the fault point remained at the same location.

Fault Test Summary

• A megohmmeter reading is only a reference value; normal resistance does not guarantee the absence of a fault, and a withstand test is still required to confirm the cable condition.
• When two phases show problems at the same time, the fault is very likely at the same location.