Nadstrujna zaštita u elektroenergetskom sustavu

Overcurrent protection owes its application from the fact that in the event of fault the current would increase to a value several times greater than maximum load current. Overcurrent protection is a basic protection against overloads, short circuits and earth faults, and is used as additional protection of electrical machinery and distance protection of high voltage power lines. When both overcurrent and earth – fault protection are required using relays, it is usual to provide two phase relays instead of three phase relays with one earth – fault relay from economic considerations. Overcurrent relays are intelligent electronic devices (IED) used to protect the power system. The types of overcurrent relay are: instantaneous overcurrent relay, definite time overcurrent relay and inverse time overcurrent relay. Use of time, current and a combination of both time and current are three ways to discriminate adjacent overcurrent relays. Normally the time delay step lies between 0,3 s and 0,5 s. Overcurrent relays need to provide reliable, selective, fast, stable and sensitive protection and are used only when the cost of overcurrent relays are justified. In order to provide an effective overcurrent protection it is first necessary to collect system data and then to determine expected faults for each protected element of a power system.