Requirements, coordination and protection zone

Basic requirements of a good protective scheme

1. Selectivity – only the faulty section is isolated.
2. Speed – minimum fault-clearing time to limit damage and keep stability.
3. Sensitivity – operates on the smallest expected fault current.
4. Reliability – operates correctly whenever needed and never falsely.
5. Stability – no operation for external faults, load swings or inrush.
6. Simplicity – fewer components, easy testing and maintenance.
7. Economy – cost matched to the value of the protected equipment.

Coordination of protective devices

Coordination is the process of choosing the settings (pick-up current and time) of relays, fuses and breakers in series so that the device nearest to the fault operates first, and each upstream device acts only as time-delayed back-up.

• Achieved by time grading, current grading or a combination (IDMT relays).
• A grading margin (coordination time interval) of about 0.3–0.5 s is kept between successive relays, allowing for breaker opening time, relay overshoot and errors.

Example: On a radial feeder A→B→C with relays R1, R2, R3, relay R3 at the far end is set at 0.1 s, R2 at 0.5 s and R1 at 0.9 s for a fault at C. R3 clears the fault; R2 and R1 only reset.

Protection zone

A zone of protection is the region of the system (generator, transformer, bus, line, motor) guarded by a particular protection; its boundary is fixed by the CTs, and all breakers on that boundary trip for a fault inside it.

• Every part of the system is covered by at least one zone.
• Neighbouring zones overlap around a circuit breaker so there is no blind spot.

 ┌─Gen─┐┌─Transf─┐┌─Bus─┐┌──Line──┐┌─Bus─┐
 G ──[CB]─────[CB]────[CB]──────[CB]──── load
      overlaps at each CB