However, there is one issue to be considered when designing a protection for a generator. In the event of a short circuit, the fault current is very high for a few milliseconds after a fault. This heavy current causes the generator voltage to drop. This drop in voltage causes the current to decay. Therefore, a high overcurrent setting may not operate in the event of a short-circuit.
To solve this problem, voltage dependent overcurrent relays bias the overcurrent setting with the measured voltage. That is, at normal voltage, the overcurrent relay operates if the current exceeds the setpoint. However, if there is a voltage drop, the overcurrent setting also progressive decreases according to the biasing. Thus, at lower voltages, the current required to operate the relay is very low.
A variation of the voltage-dependent relay is the voltage controlled. This relay has an undervoltage setting and a overcurrent setting. The overcurrent setting is set at a value less than the rated current of the generator. For the relay to operate, both the undervoltage and the overcurrent need to occur at the same time. This can occur only at the instant of a short circuit.