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10kV Underground Cable Fault Detection — Baqiao Substation Case Study

▎Background and Cable Configuration

A 10kV underground distribution network at Baqiao required urgent diagnosis on two cables between high-voltage switchgear and transformer isolation switches. Cable A was an 880-meter 8.7/10kV YJV 3×95mm² copper cable with one intermediate joint. Cable B was an 80-meter 8.7/10kV YJLV 3×50mm² aluminum cable without joints. Both cables suffered service interruptions from suspected high-resistance ground faults, demanding rapid fault location to minimize outage duration.

▎Diagnostic Process

Step 1: Insulation Resistance Measurement — The XHMR-5kV tester at 2500V DC confirmed single-phase faults on both cables. Cable A: C-phase to ground at 0.05 MΩ with healthy phases exceeding 40 GΩ. Cable B: A-phase to ground at 0.03 MΩ with healthy phases above 10 GΩ. These extremely low readings established the high-resistance nature of both faults.

Step 2: Path Identification — Cable A had an existing route record. Cable B required active tracing using the 507 path finder in direct connection mode, successfully mapping the 80-meter underground route within minutes.

Step 3: TDR Baseline and HV Flashover Pre-Location — The 502 TDR established baseline traces at 880m (Cable A) and 80m (Cable B). High-voltage energy was injected via operation transformer and 40/6 pulse capacitor discharging through the sphere gap into each faulty phase. Flashover waveform analysis identified Cable A's fault at 386 meters and Cable B's fault at 30 meters from their respective test terminals.

Step 4: Acoustic Pinpointing — The 503 acoustic-magnetic synchronous pinpointer confirmed both pre-locations with clear discharge signatures. Cable A exhibited maximum discharge sound with ground vibration at approximately 380 meters. Cable B showed peak acoustic discharge at approximately 30 meters, enabling precise excavation marking at both fault locations.

▎Results Summary

▎Key Takeaways

Integrated Workflow Efficiency — Both cables were diagnosed within a single field day using the XZH TEST suite. Seamless instrument compatibility between insulation measurement, path tracing, TDR pre-location, and acoustic pinpointing eliminated workflow interruptions.

HV Flashover Necessity — Faults at 0.03–0.05 MΩ required high-voltage energy injection for waveform capture; low-voltage TDR alone was insufficient, validating HV flashover for challenging high-resistance fault scenarios.

Preventive Maintenance Insight — Cable A's fault at 386m, beyond the intermediate joint, indicated joint deterioration as the root cause. This finding supports proactive replacement planning for similar-aged joints across the network.