While the system protection design is segmented into distinct protective zones, backup protection is also provided for the entire system. This ensures fault clearing capability in the event that the primary protective devices fail. The common backup protection is time-delay relays that trip breakers in series near to the source. Other methods should be available to provide system protection whenever any breaker fails to open and clear a fault from the system.
The diagram above shows a portion of a small power system. Two generators feed power to a 13.8 kV generator bus. Two transformers transmit power from this bus to two transmission lines and ultimately to the remote loads. The generating station has high-voltage switchgear which feeds power to these transmission lines and routes that power over alternate paths during emergencies or during routine dispatching operations. Circuit breakers are provided to permit any part of the system to be removed from service and still deliver power to the major loads. These circuit breakers are the devices which are controlled by the protective relays. The relay scheme may be quite elaborate, with various levels of back-up and overriding protection, covering all possible contingencies. Manual control of each breaker is provided as well.
Each protection zone is indicated by the dotted lines on the diagram.
