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Differential protection of power transformer

Short answer

Differential protection compares CT secondary currents from both sides of the transformer. Under normal or external fault conditions, currents balance and no current flows in relay. For an internal fault, imbalance current operates the relay. CT connections correct phase shift, and percentage bias with harmonic restraint ensures stability against inrush and CT mismatch.

With a neat sketch explain the operation of differential protection scheme for a power transformer.

Differential (Merz-Price) protection of a transformer compares the currents entering and leaving the transformer. For a normal load or an external fault these currents balance, so no current flows in the relay. For an internal fault they do not balance, and the difference current operates the relay.

      HV side                     LV side
 ──CT1──┬──[ POWER TRANSFORMER ]──┬──CT2──
   (Y)  │      (Δ / Y)            │  (Δ)
        i1                        i2
        │      ┌──────────┐      │
        └──────┤ OC  │ RC ├──────┘
     pilot     │  relay   │   pilot
               └──────────┘
   OC = operating coil (carries i1 − i2)
   RC = restraining coil (carries (i1+i2)/2)

Operation

  1. CTs are placed on both sides, and their secondaries are joined by pilot wires so that, under healthy conditions, the secondary currents circulate and the relay operating coil carries almost no current.
  2. External fault / load: I1 and I2 rise together and the circulating currents stay equal, so the relay does not trip (it is stable).
  3. Internal fault (winding to earth, phase-to-phase, turn-to-turn): extra current flows from one or both sides into the fault, the secondary currents differ, and (i1 − i2) flows in the operating coil. When it exceeds the setting, the relay trips the HV and LV breakers.

Special points for transformers

  • CT ratios: chosen so that the secondary currents are equal, i.e. in inverse proportion to the voltage ratio.
  • CT connections: CTs on the star side are connected in delta and CTs on the delta side in star, to correct the 30° phase shift and to block zero-sequence current for external earth faults.
  • Percentage (biased) relay: the restraining coil makes the trip setting rise with through current, so CT mismatch, tap changing and CT saturation do not cause false tripping.
  • Magnetising inrush: at switch-on, a large inrush current flows on one side only. It is rich in second harmonic, so harmonic restraint is used to block tripping.

Use: standard main protection for transformers above about 5 MVA.

✓ Answer checked by Claude Opus 5.5 (high effort)

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