Over Current Protection
Every alternator has an over current protection. With the help of this trip,the alternator and distribution system can be protected from various faults but the main thing to be considered in this method is to maintain power to the distribution system till the time the alternator trips on any other protection devices.
For this reason,the protection device has been designed in such a way that in case the over current is not high enough, a time delay provided by an inverse definite minimum time (IDMT) relay occurs, which prevents the alternator from tripping in case the over current values reduces back to normal within the IDMT characteristics
But in case of a major fault such as short circuit, the alternator will trip instantaneously without any delay, protecting all devices on the distribution system.Overload of alternator is caused either due to increased switchboard load or serious fault causing very high current flow.
If sudden over load occurs then, the load is reduced with the help of preferential trips which removes non essential load such as of air conditioning, ventilation fans etc., from the switchboard.These preferential trips are operated by relays which are set to about 110% of the normal full load of alternator.