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Ring bus rearrangement and zones

Short answer

For fault on BC: if breaker 7 fails, single bus trips 4,5,6 losing load 1; ring bus trips 6,8 and 4, also losing load 1. If breaker6 fails, single bus is unaffected; ring bus trips5 and remote3, load 1 stays. Ring bus zones overlap at breakers, no separate bus zone.

The system of fig. 1(a) is rearranged as shown in fig. 1(b) where the bus arrangement at B has been changed to a ring bus arrangement. (i) Compare the operation of the two systems when a fault occurs on line BC with a. a failure of breaker 7, b. a failure of breaker 6. (ii) Sketch the zone of protection. [Fig. 1(a): Bus A is fed through breaker 1. Two parallel lines run from A to bus B: upper line with breaker 2 at A and breaker 4 at B, lower line with 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 ends at breaker 8 at bus C; Load 2 is supplied from C through breaker 9.] [Fig. 1(b): Same system, but B is a ring bus with four breakers 4, 5, 6 and 7 in the ring. The line from breaker 2 joins the ring between breakers 4 and 5, the line from breaker 3 joins between breakers 5 and 6, Load 1 is taken from the ring between breakers 4 and 7, and line BC leaves the ring between breakers 7 and 6, ending at breaker 8 at bus C; Load 2 is fed from C through breaker 9.]

The ring at B (Fig. 1(b)), taken in the order 4 → 5 → 6 → 7 → 4: the line from breaker 2 joins between 4 and 5, the line from breaker 3 between 5 and 6, line BC between 6 and 7, and load 1 between 7 and 4.

(i) Comparison of operation for a fault on line BC

Normal clearing

  • Fig. (a): breakers 7 and 8 trip. Load 1 continues.
  • Fig. (b): line BC is fed from the ring through 6 and 7, so breakers 6, 7 and 8 trip. The ring is opened, but load 1 is still fed through 4 from the line of breaker 2; no load is lost.

(a) Failure of breaker 7

ArrangementBack-up actionLoad 1
Fig. (a) single busBreaker-failure relay trips all bus-B breakers 4, 5, 6 (or remote 2, 3 trip)Interrupted
Fig. (b) ring busThe breaker on the other side of 7 in the ring, i.e. 4, trips (with 6 and 8)Interrupted, because load 1 lies between 7 and 4

In the ring bus fewer breakers trip and the lines from A remain connected to the ring, but load 1 is still lost because it is placed next to breaker 7.

(b) Failure of breaker 6

ArrangementBack-up actionLoad 1
Fig. (a)Breaker 6 is the load-1 breaker and is not involved in clearing a BC fault; 7 and 8 clear itNot interrupted
Fig. (b)The breaker-failure relay of 6 trips the other breaker at that ring node, 5, and also trips the remote breaker 3 at bus A (by direct transfer trip, or by remote back-up at A), because the line from 3 still feeds the fault through the stuck breaker 6. Breakers 7 and 8 have already tripped. The line from 3 is isolatedNot interrupted; load 1 is fed through 4 from the line of breaker 2

Conclusion: in a ring bus, a stuck breaker is isolated by opening the breaker on the other side of its ring node, plus the remote end of any line connected at that node. So a breaker failure removes at most one extra circuit, instead of the whole bus as in Fig. (a). To keep load 1 in service for any single breaker failure during a BC fault, load 1 should not be placed next to line BC; it should lie in the ring between the two lines from A (ring order: line 2, load 1, line 3, line BC).

(ii) Zones of protection

              [1]
   Bus A ──┬───┴───┬──        Bus A zone: 1, 2, 3
          [2]     [3]
           │ AB1   │ AB2
           │       │
   Ring B: │       │
     N45 ●─┴─[5]───┴─● N56
         │           │
        [4]         [6]
         │           │
     N74 ●────[7]────● N67
         │           │
      Load 1      line BC ──[8]── bus C ──[9]── Load 2
  • Bus A zone: 1, 2, 3.
  • Line AB1 zone: 2 and ring breakers 4, 5 (includes node N45).
  • Line AB2 zone: 3 and ring breakers 5, 6 (includes node N56).
  • Line BC zone: ring breakers 6, 7 and breaker 8 (includes node N67).
  • Load 1 zone: ring breakers 7 and 4 (includes node N74).
  • Bus C zone: 8, 9; load 2 zone beyond 9.
  • In a ring bus there is no separate bus zone: each ring node between two breakers belongs to the circuit connected to it.
  • Each zone overlaps its neighbour at the breaker between them.

✓ Answer checked by Claude Opus 5.5 (high effort)

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