How do IDMT relays work?
An inverse definite minimum time (IDMT) relay trips faster as the current rises. The pickup setting Is sets where it starts timing, the time multiplier (TMS) moves the whole curve up or down, and the curve type sets its shape. Standard inverse suits networks where fault current changes little with location. Very and extremely inverse curves grade better with fuses and with downstream devices where fault current falls steeply along the circuit.
| IEC curve | k | α |
|---|---|---|
| Standard inverse (SI) | 0.14 | 0.02 |
| Very inverse (VI) | 13.5 | 1 |
| Extremely inverse (EI) | 80 | 2 |
| Long time inverse (LTI) | 120 | 1 |
What is a grading margin?
For a fault downstream, the device closest to the fault must trip first, and the next device upstream must wait long enough not to trip as well. The margin covers the downstream breaker's opening time, relay overshoot, CT errors and a safety allowance. 0.3 to 0.4 s is typical between electromechanical-style settings, and numerical relays with fast breakers may use about 0.25 s.
Why use a CB instead of fuses on larger transformers?
For a fault on the LV main board, the HV fuse often melts before the LV ACB's short-time delay ends, so the whole transformer is lost instead of only the LV breaker tripping. A CB with a relay can be set with a time delay that grades with the ACB, and it also allows earth fault, temperature and gas relay trips. This is one reason utilities move from F units to V units above roughly 1000 to 1250 kVA.
ExpertDelta-star transformers change how phase-to-phase and earth faults appear on the HV side. An LV phase-to-phase fault shows up as 1.15 times the equivalent three-phase current in one HV phase, and LV earth faults are not seen at all by HV earth fault relays. Use restricted earth fault (REF) on the LV winding for sensitive transformer earth fault protection.
Regional notes
GCC
Utilities set the primary feeder relays and publish the settings envelope that consumer substations must grade with. Transformer feeder protection in utility RMUs is often fixed by the utility standard.
Australia and New Zealand
Network operators specify protection for HV customers and require a protection study for HV installations. Selectivity on the LV side follows AS/NZS 3000.
Asia
Utilities such as TNB and SP Group publish protection settings requirements for consumer substations and expect a coordination study with the submission.