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Zones and backup for breaker 7 failure

Short answer

Zones: bus A (1,2,3), line AB1 (2,4), line AB2 (3,5), bus B (4,5,6,7), line BC (7,8), load 1 (6), load 2 (8). If breaker 7 fails, breaker-failure relay trips 4,5,6; load 1 is interrupted. To avoid interruption, use double-bus with bus coupler or two series breakers on BC, so bus B remains healthy and load 1 stays fed.

Consider the system portion shown in figure below. (i) Sketch the zone of protection. (ii) Describe a possible backup scheme for failure of breaker 7 for a fault on line BC; is load 1 interrupted? (iii) Describe a means of clearing a fault on line BC without a momentary interruption of load 1. [Figure: Bus A is fed through breaker 1. Two parallel lines run from bus A to bus B: the upper line has breaker 2 at A and breaker 4 at B, the lower line has breaker 3 at A and breaker 5 at B. Load 1 is supplied from bus B through breaker 6. Line BC leaves bus B through breaker 7 and reaches bus C, from which Load 2 is supplied through breaker 8.]

Assumption: breakers 1–8 are as described; each line has a breaker at both ends and bus B connects breakers 4, 5, 6 and 7.

(i) Zones of protection

       [1]
  ┌─────┼──────── Bus A zone ──────┐
  │  ───┴──┬──────────────┬───     │
  └────────┼──────────────┼────────┘
          [2]            [3]
  ┌ Line AB1 ┐      ┌ Line AB2 ┐
  │    │     │      │    │     │
  └───[4]────┘      └───[5]────┘
  ┌────────┼──────────────┼────────┐
  │  ──────┴───┬──────┬───┴──      │ Bus B zone
  └───────────[6]────[7]───────────┘
           Load 1 ┌──┼────────┐ Line BC
                  │  │        │ zone
                  └─[8]───────┘
                     │ Bus C / Load 2
 Each zone overlaps its neighbours at
 every breaker (1,2,3,4,5,6,7,8).

Zones: bus A (1, 2, 3); line AB-1 (2, 4); line AB-2 (3, 5); bus B (4, 5, 6, 7); line BC (7, 8); load 1 feeder (6); load 2 / bus C (8).

(ii) Back-up for failure of breaker 7 (fault on line BC)

  • Normally relays of line BC trip breakers 7 and 8.
  • If breaker 7 fails to open, the fault is still fed from bus B.
  • Local back-up (breaker failure protection): after about 150–200 ms, the breaker-failure relay of 7 trips all other breakers connected to bus B: 4, 5 and 6.
  • Remote back-up (alternative): zone-2/zone-3 distance relays or time-graded overcurrent relays at bus A trip 2 and 3.
  • In both cases bus B is de-energised, so load 1 is interrupted (in the local scheme breaker 6 is tripped; in the remote scheme bus B loses supply).

(iii) Clearing BC fault without momentary interruption of load 1

The fault current from bus B must be interrupted by a breaker other than the one that isolates load 1. Possible means:

  • Provide two breakers in series on the BC line at B (or a breaker-and-a-half / ring bus arrangement), so that if breaker 7 fails the second breaker clears the fault and bus B remains healthy.
  • Use a double-bus with bus coupler arrangement: line BC and load 1 on different buses; the breaker-failure relay of 7 trips only the bus coupler and the breakers of BC's bus.

With these arrangements a fault on BC, even with a stuck breaker, is removed without de-energising the bus that supplies load 1.

✓ Answer checked by Claude Opus 5.5 (high effort)

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