Q10Power System Protection

The essential qualities required of a protective relay are: selectivity, the ability to discriminate between a fault within its own protected zone (requiring operation) and a fault or disturbance elsewhere in the system (requiring the relay to remain stable/non-operating, leaving that fault to be cleared by the relay whose zone it actually falls within); speed, operating as quickly as possible once a fault is detected, to minimize equipment damage, limit the duration of abnormal voltage/current stress on the system, and preserve transient stability; sensitivity, the ability to reliably detect and operate for the smallest fault condition (lowest fault current or smallest deviation from normal) that the relay is intended to protect against, without requiring an excessively large fault magnitude to trigger operation; reliability, the assurance that the relay will operate correctly and dependably whenever a genuine fault within its protected zone occurs (dependability), while also not operating unnecessarily for normal system conditions or faults outside its zone (security); and stability, the relay's ability to remain non-operative during normal load conditions, permissible transient disturbances (such as inrush current or a stable power swing), and through-faults occurring outside its protected zone, avoiding unwanted (nuisance) tripping that would unnecessarily interrupt healthy parts of the system.