Zones of protection and their rules

A zone of protection is the part of the power system (generator, transformer, bus, line or motor) protected by a particular protection scheme. A fault anywhere inside the zone makes all the circuit breakers on its boundary trip, so that the zone is fully isolated. The boundary of a zone is defined by the location of the CTs and the circuit breakers.

Protection zones of a sample power system

 ┌──────────────┐
 │  G    Gen    │ 1 Generator zone
 └──┬───────────┘
 ┌──┴─[CB]──────┐
 │  Gen-transf. │ 2 Transformer zone
 └──┬───────────┘
 ┌──┴─[CB]──────┐
 │  HV bus      │ 3 Bus zone
 └──┬───────────┘
 ┌──┴─[CB]──────┐
 │  Line        │ 4 Transmission line zone
 └──┬───────────┘
 ┌──┴─[CB]──────┐
 │  Rx bus      │ 3 Bus zone
 └──┬───────────┘
 ┌──┴─[CB]──────┐
 │  Step-down Tr│ 2 Transformer zone
 └──┬───────────┘
 ┌──┴─[CB]──────┐
 │  Feeders/Mtr │ 5 Feeder / motor zone
 └──────────────┘
 Each box is a zone; adjacent boxes
 overlap around the circuit breaker.

Rules for drawing zones

1. Each zone is drawn around one main element: generator, transformer, bus, line or motor.
2. Every element of the system must lie in at least one zone; there should be no blind (unprotected) spot.
3. Adjacent zones overlap, and the overlap region contains a circuit breaker. This is done by placing the CTs of the two zones on opposite sides of the breaker.
4. A fault inside the overlap region trips the breakers of both zones; the overlap is kept as small as possible so that only a small extra part is disconnected.
5. Zone boundaries are set by CT location; a zone includes the breakers that must trip to isolate it.

Using zones makes the scheme selective: a fault on the line trips only the two line breakers, while generator, transformers and buses stay in service.