Under-voltage Protection

Under-voltage protection on low-voltage systems is usually accomplished by an integral under-voltage release on the circuit breaker. Above 1000V, a separate relay would be required. It is commonly employed on marine systems:

1. To prevent closing of the circuit breaker when the generator is to be paralleled with other generators or the shore supply and the terminal voltage is less than 70%.

2. To ensure that loads, particularly motors are disconnected during a temporary loss of supplies. (Otherwise the generators could be tripped due to over-current conditions when supplies are restored owing to the total starting current of all connected motors).

3. To provide a back-up to short-circuit protection.

An under-voltage release is fitted to all generator breakers and some main feeder circuit-breakers. Its main function is to trip the breaker when a severe voltage dip (around 70%) occurs. The under-voltage release on a generator circuit-breaker prevents it from being closed when the generator voltage is very low or absent.

As an example of providing a back-up to short-circuit protection, let us assume that during generator paralleling procedures, an attempt was made to close the wrong circuit-breaker - the circuit-breaker of a stopped (and dead) generator; if this circuit-breaker was closed, the dead generator would be the equivalent of a short-circuit fault on the bus bars an cause a blackout. The under-voltage relay then prevents the closure of the circuit-breaker of dead generator.

The under-voltage trip on the generator breaker must not be operated if a feeder fault occurs. Consequently, it must be 'time-delayed' to permit the prior operation of feeder protection circuits. A time delay is also necessary to prevent operation when voltage transientsoccur during load-switching or synchronising. A voltage setting in excess of 80% would not normally be adopted because of the possibility of the generator being 'lost' when starting large motors.

Under-voltage protection is also required for motor starters. The starter contactor normally provides this protection as it drops out when the supply voltage is lost or is drastically reduced. The starter circuit will not normally allow the motor to re-start when the voltage supply is restored except when special automatic re-starting facilities are provided. Under-voltage protection can be electromagnetic or electronic.

Checking and calibration of generator under-voltage relays can be done accurately by voltage injection. A known variable voltage is directly applied to the under-voltage relay to check (a) the voltage at which the relay 'pulls in' and (b) the voltage at which the relay,out'.

Under-voltage relays of generators are usually 'slugged' to prevent spurious or ‘misance' tripping during transient voltage dips (typically 15%) caused by large motor-ting currents. Whether under-voltage protection can provide effective back-up protection under short-circuit conditions is debatable. Often only the voltage across two phases is monitored and therefore the majority of phase-to-phase faults would be detected only when developed into three-phase faults. It is notable that this form of protection is relatively uncommon in industrial systems except on motor feeders.

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