- 2074 Bhadra · 8 marks
Converting IDMT scheme to differential protection
Short answer
An IDMT overcurrent scheme can be converted to differential by connecting CTs on both sides through pilots and placing relays in the difference path. Biased relays with harmonic restraint correct for phase shift and inrush. Advantages over IDMT: instantaneous tripping, absolute selectivity, high sensitivity, no need for grading.
Describe how an overcurrent and earth-fault protection scheme by IDMT relays for a transformer could be converted into the form of differential protection. What are the advantages over the other protection schemes as applied to transformer?
An IDMT overcurrent and earth-fault scheme on a transformer uses CTs and relays on each side separately. By connecting the same CTs (on both sides) together through pilots and placing the relay in the difference path, the scheme becomes a differential (Merz-Price) scheme.
Conventional IDMT O/C and E/F scheme
HV CTs ─► 2 × O/C relays + 1 × E/F relay (residual)
LV CTs ─► 2 × O/C relays + 1 × E/F relay (residual)
Each relay sees only its own side current
These relays are graded with downstream relays, so they are slow (0.5–2 s) and cannot tell an internal fault from an external one.
Steps to convert into differential protection
- Use CTs on both sides of the transformer and choose their ratios so that the secondary currents are equal at full load (CT ratios in inverse proportion to the voltage ratio).
- Correct the phase shift: connect the CTs on the star winding in delta and the CTs on the delta winding in star (for a Δ/Y transformer). This also removes zero-sequence current, so external earth faults do not cause tripping.
- Join the CT secondaries by pilot wires so that currents circulate between the two CT groups.
- Move the relays from the CT secondaries into the spill (difference) path between the pilots, one relay per phase.
- Replace plain O/C elements with biased (percentage) relays that have operating and restraining coils, and add harmonic restraint against magnetising inrush.
- Earth-fault element: a restricted earth-fault (REF) relay can be formed from the three line CTs and a neutral CT of the star winding.
CT(Y-conn)─┐ pilot ┌─CT(Δ-conn)
├──[87]────┤
│ spill │
HV ═══[ TRANSFORMER Δ/Y ]═══ LV
(HV delta winding: CTs in star; LV star winding: CTs in delta)
Advantages over other schemes
| Point | Differential | IDMT O/C & E/F |
|---|---|---|
| Speed | Instantaneous (1–2 cycles) | Time-delayed, graded |
| Selectivity | Absolute: only internal faults | Relative: needs grading |
| Sensitivity | High; detects low-level winding faults | Must be set above load |
| Load current | No effect | Limits setting |
| Coordination | Not needed with other relays | Needed |
| Damage | Minimum, fast clearance | More damage, longer arc |
Other advantages: the zone is clearly defined by the CT positions; it protects the windings and the bushings/connections between the CTs; and it works at any loading level. The O/C and E/F relays are then kept only as back-up.
✓ Answer checked by Claude Opus 5.5 (high effort)
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