Basic requirements of a protection scheme

A protection scheme must detect every fault quickly and remove only the faulty part, while staying stable during normal operation. The basic requirements (qualities) are:

1. Selectivity (discrimination): only the circuit breakers nearest to the fault should trip, so that the healthy part keeps supplying load. Example: for a fault on one outgoing 11 kV feeder of a substation, only that feeder breaker should open, not the incoming transformer breaker.
2. Speed: the fault must be cleared as fast as possible to limit damage to equipment, reduce burning at the fault point and keep generators in synchronism (transient stability). Example: a 132 kV line fault is cleared in about 3–5 cycles (relay ≈ 1 cycle + breaker 2–3 cycles).
3. Sensitivity: the relay must operate for the smallest fault current expected (e.g. a high-resistance earth fault at minimum generation). Example: a restricted earth-fault relay on a transformer is set at 10–20 % of rated current so it can see winding faults near the neutral.
4. Reliability: the scheme must operate correctly every time it is needed (dependability) and must not operate when not needed (security). Good design, quality CTs/relays, DC supply and regular testing give reliability.
5. Stability: the scheme must remain inoperative for faults outside its zone (through faults), for load swings and for magnetising inrush. Example: a transformer differential relay must not trip for an external fault on the LV feeder or for inrush current at switching-on.
6. Simplicity and economy: the scheme should be simple to maintain and its cost should be justified by the value of the protected equipment. A small distribution transformer is protected by fuses, while a large generator gets differential, earth fault, loss-of-excitation and other relays.
7. Adequateness: it should cover all the likely faults and abnormal conditions (overload, unbalance, over-voltage) of the equipment.

These requirements partly conflict (more speed and sensitivity can reduce stability), so the designer reaches a balance suitable for each element.