Side-by-side comparison
| Parameter | Overcurrent Relay | Differential Relay |
|---|---|---|
| Operating Principle | Operates when current exceeds set pick-up threshold | Operates on difference (spill) current between two ends |
| Relay Standard | IEC 60255-151 (IDMT curves) | IEC 60255-187 (transformer differential) |
| Zone of Protection | Feeder, bus, motor — non-unit | Strictly between CTs at two ends — unit protection |
| Selectivity | Time-graded; upstream relay has longer delay | Instantaneous; inherently selective to zone |
| Typical Pickup Current | 50–200% of CT primary; e.g. 200 A on 400/1 CT | Differential: 15–30% of rated current (bias characteristic) |
| Speed of Operation | 0.1 s to 1.5 s (IDMT); 0.02 s instantaneous (50) | 20–60 ms for internal fault |
| Magnetising Inrush Immunity | Not applicable | 2nd harmonic restraint (15–20%) blocks trip on inrush |
| CT Matching Requirement | Single CT per phase | CTs at both ends must be matched; phase correction for Dyn transformer |
| Application Examples | 33 kV feeder, induction motor (49/50/51) | Power transformer (87T), generator (87G), busbar (87B) |
| Cost | Low — SEL-300G, L&T MCOMP, ~₹15,000–40,000 | Higher — SEL-387, ABB RET670, ~₹80,000–3,00,000 |
Key differences
An overcurrent relay (51/50) is non-unit protection — it cannot distinguish whether current flows through or into the protected equipment, so it must time-grade with upstream relays, adding delay. A differential relay (87) is unit protection — it only responds to faults inside the winding between its CTs and trips in under 60 ms regardless of through-fault current. The differential relay requires 2nd harmonic restraint (15–20% of fundamental) to avoid a false trip on transformer magnetising inrush, a feature overcurrent relays never need. CT ratio mismatch and vector group correction are critical commissioning checks for 87T that have no equivalent in overcurrent schemes.
When to use Overcurrent Relay
Use an overcurrent relay for feeder and motor protection where time-graded co-ordination with upstream devices is acceptable. Example: a 51 relay (IDMT, normal inverse, TMS 0.3) on a 33 kV outgoing feeder co-ordinates with the 132 kV incomer relay to provide backup protection.
When to use Differential Relay
Use a differential relay for protection of large power transformers, generators, and buses where a fast, selective unit trip is mandatory. Example: an ABB RET670 87T relay protects a 100 MVA 220/33 kV transformer, tripping both HV and LV circuit breakers in 25 ms on any internal winding fault.
Recommendation
For transformer, generator, and busbar protection, choose differential (87) — speed and selectivity are non-negotiable at those asset values. Choose overcurrent for feeder and motor protection where time-grading is acceptable and cost matters. On any transformer above 5 MVA, both relays are applied: 87T as primary, 51 as remote backup.
Exam tip: Examiners ask students to draw the percentage-bias differential characteristic (operate vs restrain regions) and explain why the slope is set at 20–40% — know that the slope accounts for CT ratio error and off-nominal tap position.
Interview tip: An interviewer at a protection engineering company will ask why magnetising inrush does not trip a differential relay — cite 2nd harmonic blocking (15–20% threshold) and explain that inrush waveform is rich in 2nd harmonic while a real internal fault current is predominantly fundamental.