Methods of providing backup protection

Back-up protection operates, after a time delay, only when the primary protection or its breaker fails to clear a fault. The common methods are:

1. Remote back-up: relays at the adjacent (upstream) station act as back-up. They see the fault but are time-graded so that the primary relay has the first chance.
   • Example: zone-2 and zone-3 of a distance relay, or a time-graded IDMT relay at the source end.
   • Independent of the primary station's CTs, DC supply and breaker, so very reliable; but it is slow and disconnects a larger part of the system.
2. Relay back-up (local duplication): a second, independent relay at the same station (preferably a different principle, fed from separate CT cores and DC supply) trips the same breaker. Example: main-1 and main-2 distance relays on EHV lines.
3. Breaker back-up (breaker failure protection): if the breaker does not open within a set time (about 150–200 ms) after the trip signal while current still flows, the breaker-failure relay trips all other breakers on the same bus that feed the fault.
4. Centrally coordinated back-up: in modern substations with a central computer, the system state is monitored and, on failure of primary protection, the most suitable breakers are tripped.

 A          B          C
 ─[CB1]────┼──[CB2]────┼── F
  R1 (remote back-up)  R2 primary + local
                       BF relay trips bus B

Usually remote back-up is used on distribution systems and local relay + breaker back-up on EHV systems.