Primary and backup protection

Primary protection is the first line of defence: the protection installed for a particular zone (line, transformer, bus, generator) that is expected to clear any fault in that zone quickly and with the smallest possible outage.

Back-up protection is the second line of defence: a protection that operates, after a deliberate time delay, only when the primary protection or its breaker fails to clear the fault.

Why primary protection can fail

• Failure of the relay itself or its setting.
• Open or shorted CT/PT secondary leads.
• Failure of the DC tripping supply or trip coil.
• Circuit breaker mechanism stuck (breaker failure).
• Failure of the communication channel in pilot schemes.

Necessity of back-up protection

• No protective equipment is 100 % reliable, and an uncleared fault can burn equipment, cause fire, collapse system voltage and make generators lose synchronism.
• It protects the system while primary protection is out for maintenance.
• It covers parts of the system not fully covered by primary protection (e.g. the remote end of a line).
• It must be time-graded so that primary protection always has the first chance.

Example

 Gen   A          B          C
 ──●───┼──[CB1]───┼──[CB2]───┼──[CB3]──► load
       R1         R2         R3

• A fault F occurs on section B–C (beyond CB2).
• The primary protection is relay R2 with breaker CB2; it should trip instantly.
• If R2 or CB2 fails, relay R1 at the upstream station (set with a higher time, e.g. 0.4 s more) trips CB1 as remote back-up.
• The cost is that section A–B and its loads are also disconnected, but the fault is removed and the rest of the system survives.

In modern substations, breaker-failure (local back-up) protection is also used: if CB2 has not opened within about 150–200 ms after its trip signal, all other breakers on the same bus are tripped.