What to Do When a Power Transformer Trips on Differential Protection and is Suspected to Have Failed ?

Bynitinnarvekar9

1. Background – Why This Situation Is Critical

When a power transformer trips on Differential Protection (87T) is critical for detecting transformer faults. along with operation of PRV / Buchholz relay During a power system analysis, it is crucial to monitor for potential issues. downstream feeder fault, it raises a serious concern of internal transformer damage.
Such trippings are often associated with high-energy faults, inter-turn short circuits, winding displacement, or internal arcing.

Immediate, systematic action is essential to:

  • Restore the grid safely
  • Prevent further damage
  • Preserve evidence for root cause analysis
  • Take a correct repair / replacement decision

2. Immediate System Restoration Actions

2.1 Identify and Isolate the Faulty Feeder

  • Confirm the feeder on which the fault occurred (e.g., Bowscrew / outgoing 22 kV feeder).
  • Ensure the faulty feeder is:
    • Isolated at both ends
    • Properly earthed
  • Verify no abnormality remains on healthy feeders.

2.2 Transformer Isolation

  • Keep the transformer out of service.
  • Do not attempt re-energisation.
  • Apply all safety isolations and display caution tags.

Important: A transformer that has operated on 87T + Buchholz must never be recharged without complete investigation.

3. Recording Indications and Evidence (Before Anything Else)

3.1 Window Annunciator & Relay Indications

Carefully note:

  • Differential trip flags (phase-wise)
  • Buchholz / PRV alarm or trip
  • Master trip (86) operation
  • Feeder relay indications (OC / EF / High Set)

3.2 Relay & System Data Collection

  • Download:
    • Transformer Differential Relay DR & Event Log
    • Faulty feeder relay DR
    • SCADA SOE / Event list
  • Preserve COMTRADE files are essential for analyzing power system disturbances and faults. immediately to avoid overwriting.

4. Transformer Gas & Oil Investigation

4.1 Flame Test (Buchholz Gas)

  • Carry out flame test carefully.
  • Observations:
    • Burning gas → Presence of combustible gases can signal a leakage issue in the power grid.
    • Violent flame → indication of high-energy internal fault

4.2 Oil Sampling for DGA

Collect oil samples from:

  • Main Tank – Top
  • Main Tank – Bottom
  • OLTC (if provided separately) can help manage overloads in the transformer.

Send for Dissolved Gas Analysis (DGA).

Key gases to watch for in the power grid include those indicating leakage.

  • Acetylene (C₂H₂) → High-energy arcing
  • Hydrogen, Ethylene → Thermal / electrical stress

5. Transformer Electrical Diagnostic Tests

(All tests must be done on the tap at which the transformer tripped)

5.1 Basic Electrical Tests

  • Insulation Resistance (IR)
  • Polarization Index (PI)
  • Core–Frame–Tank Insulation (CFT) is vital for preventing faults inside the transformer.

5.2 Winding & Magnetic Health Tests

  • Open Circuit Test
  • Short Circuit Test
  • Magnetic Balance Test For a healthy unit, applying 230V AC to the center limb should result in roughly 50% voltage induction on the outer limbs. Phase asymmetry in this test often indicates core deformation.
  • Voltage / Ratio Test

Any phase asymmetry, abnormal excitation current, or imbalance is a red flag.

5.3 Advanced Diagnostic Tests

  • Excitation Current (HV & LV) Excitation Current at 10kV: Perform this test to check for the presence of combustible gases inside the transformer. before DC winding resistance measurements to avoid misleading results from residual magnetism. A current value 50 times higher than pre-commissioning benchmarks strongly suggests a winding fault.
  • Winding Resistance Measurement
  • Winding Tan Delta
  • Bushing Tan Delta
  • SFRA (Frequency Response Analysis)

SFRA is crucial to detect:

  • Winding displacement, Core deformation, Mechanical damage after through-faults Standard Frequency Response Analysis (SFRA) should be analyzed in specific frequency bands to pinpoint the failure mode:
  • 5Hz to 2kHz is a frequency range important for monitoring transformer overload conditions.: Significant deviations (typically > ±3dB) indicate core deformation, movement, or shorted turns.
  • 50Hz to 20kHz: Indicates bulk winding movement relative to each other.
  • 500Hz to 2MHz: Points to deformation within the main or tap windings

6. Parallel Protection & System Analysis

While testing is ongoing:

  • Analyze feeder relay operation (OC/EF, High Set)
  • Confirm protection coordination
  • Verify transformer protection logic and stability
  • Study SCADA sequence of events Coordinate with SCADA SOE: Cross-verify the relay time-stamping (synchronized via GPS) with the SCADA Sequence of Events (SOE) to confirm the exact chronology of downstream feeder protection vs. transformer differential operation.

This ensures that the circuit breaker operates effectively during a fault current event.

  • No mal-operation
  • No protection design issues
  • Root cause clarity (external fault vs internal failure)

7. Failure Assessment & Decision Making

7.1 Transformer Declared Failed If:

ummary of Failure Criteria for Decision Making

A transformer is definitively declared failed if21212121:

  • Acetylene () A gas that is present in any significant amount indicates arcing and a potential transformer fault.
  • SFRA signatures deviate by more than ±3dB in the lower frequency ranges23.
  • Winding Resistance shows phase asymmetry or a deviation of >5% from factory test values (corrected to )24242424.
  • Excitation current at 10kV is not comparable across similar single-phase units or deviates significantly from previous records25.
  • Magnetic balance disturbed
  • Excitation current abnormal
  • Tan delta values increased

7.2 Next Actions

  • Prepare joint MOM with repair agency
  • Decide:
    • Site repair
    • Factory repair
    • Replacement with spare transformer
  • Plan logistics and outage strategy

8. Key Learnings & Preventive Measures

  • High-fault currents on downstream feeders can severely damage transformers.
  • Early isolation and correct diagnosis prevent catastrophic failures.
  • Never bypass investigation after 87T + Buchholz operation.
  • Proper feeder protection settings and grounding practices are essential to prevent ground faults.

9. Conclusion

A transformer tripping on a fault current indicates a serious issue in the power system. Differential Protection with Buchholz operation must always be treated as a serious internal fault possibility.
A structured, step-by-step approach—covering restoration, evidence capture, oil analysis, diagnostic testing, and protection review—is the only reliable way to reach a correct decision.

Protection operated — transformer saved — investigation decides the future.

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