11 kV Feeder Earth-Fault Trips After Rain
At a 66/11 kV substation, the 11 kV 'Bathinda Rural-3' feeder is 18 km long, supplies 42 distribution transformers, and peaked at 1.9 MVA last month. Its earth-fault relay is fed from a 50/1 A core-balance CT and is set at 0.4 A secondary (20 A primary), while the overcurrent relay is set at 180 A primary. In the last ten days the feeder tripped on earth fault four times, every time within an hour of rain. A lineman wants to raise the EF setting to 1.0 A secondary (50 A primary) to stop the trips. Field tests on a tail-end 100 kVA transformer show insulation resistance 0.8 MΩ at 500 V and earth resistance 14 Ω. One 11 kV pin insulator on a spur has a flashover scar. The 66/11 kV transformer is 5 MVA, Z=8.5%, and its neutral is earthed through a 50 mm² conductor. Peak load current is 105 A.
1. Rank the three most probable causes of the repeated earth-fault trips, using the given numbers. What single next measurement would you take, and why?
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Ranking:
- Moisture in the tail-end 100 kVA transformer insulation - IR is 0.8 MΩ at 500 V, below the usual field minimum of about 1 MΩ, and the trips cluster after rain. A wet winding leaks to earth at low current; the EF relay sees it before an overcurrent relay would.
- Flashover at the scarred 11 kV pin insulator - a wet polluted surface can track at 11 kV/√3 = 6.35 kV to earth, and the scar shows a prior puncture path. This is consistent with rain-triggered trips.
- High earth resistance, 14 Ω - above the usual ≤5 Ω target for a distribution transformer. It raises touch/step voltage and distorts the EF current path, but it is not the initiator: even with 14 Ω, a phase-to-earth source of 6.35 kV can drive roughly 6,350/(14+1) ≈ 420 A, far above the 20 A primary pickup. Next measurement: isolate the suspect transformer and perform a 1-minute IR test at 500 V, then a polarization index (10 min/1 min). If PI < 1.5, moisture is confirmed. Also megger the insulator and check the CBCT secondary circuit for moisture. IR alone is a snapshot; PI separates moisture from a clean but low-resistance path.
2. The lineman asks to raise the EF relay from 20 A to 50 A primary. Should you approve? Support your answer with a fault-current calculation and protection philosophy.
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No. With a 14 Ω transformer earth and an assumed 1 Ω substation earth, a solid phase-to-earth fault at that transformer drives I ≈ (11,000/√3)/(14+1) ≈ 423 A. That is about 21 times the existing 20 A pickup, so the relay is not tripping because the setting is too low; it is seeing a real earth path. Raising pickup to 50 A would still detect this particular fault but would reduce sensitivity to higher-resistance faults - dry soil, arcing, wet wood - which is exactly the class of fault that starts fires and kills livestock. The EF setting should be chosen for sensitivity and coordinated with upstream: keep 20 A primary if the CBCT and downstream earthing allow, and fix the insulation and electrode. If maloperation is proven by primary injection, correct the CBCT wiring or relay; do not use a setting as a substitute for insulation. A 50 A pickup also makes a 100 kVA transformer's LV earth fault less likely to be cleared from the 11 kV side.
3. After drying the transformer and replacing the insulator, what acceptance tests and values would you require before re-energising, and what would you do about the 14 Ω electrode?
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Acceptance: (i) IR at 500 V ≥ 5 MΩ for the LV winding and ≥ 100 MΩ for the 11 kV winding, with PI ≥ 1.5; (ii) earth resistance ≤ 5 Ω at the transformer, measured by fall-of-potential; (iii) winding resistance and ratio within 2% of nameplate or other phases; (iv) oil BDV ≥ 30 kV for the 11 kV class if oil-filled, plus a visual check for sludge; (v) CBCT secondary loop and relay injection test at 20 A primary to prove the trip. For the electrode, 14 Ω is too high: drive two additional 3 m rods at least 3 m apart, bond them with 25×6 mm GI strip, and use bentonite/backfill; retest. If ≤5 Ω cannot be reached, install a separate earth mat and record it; do not re-energise on the old electrode. Finally, energise on no-load and check for leakage current before loading.