Voltage restrained Over Current relays:
The overcurrent relay is nothing but a relay operates when the current reaches the pickup value. The overcurrent relay is used to protect the alternator or generator against overloading and which trip the circuit breaker. Another important criteria in overcurrent element is protection against short circuit. The short circuit creates heavy fault current through the winding for few milliseconds. At that same time, the voltage starts reducing gradually as the current increases., but in this, condition the over current element cannot sense the fault current. In order to protect the generator winding the voltage restrained over current relays are implemented. The relay sense the generator current and voltage. Get More Electrical Concept And Interview Questions By Using This Link
Simple understanding (Voltage restrained Over Current relays):
Consider the generator running at 11kV and 1440 Amps…at the same time due to short circuit the current goes 2000 Amps and the voltage gradually decreasing from 11kV and for the fraction of time it stands at 8kV the current is 1500 Amps, at this condition the under voltage (the time setting is high for UV function) and overload protection will not trip the power system. So that, to protect the power system, this relay is employed.
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Normally the voltage reference for under voltage function and current reference for overcurrent function will be given to the relay. The overcurrent setting is set at a value less than the rated current of the generator. For the relay to operate, both the under voltage and the overcurrent need to occur at the same time. This can occur only at the instant of a short circuit.
Relay ANSI Code: 51VR
Used as: Backup Protection
Tripping equipment: 86M mater trip, 51V flagged, Under voltage operation, Generator Breaker trips (51)
Voltage restrained overcurrent protection provides improved sensitivity of overcurrent relaying by making the set overcurrent operating value proportional to the applied input voltage.