Generator general protection

Due to current decrement in the Generator during fault conditions the Back-up protection with IDMT relay for a generator in the Electrical protection Scheme becomes complicated. Accordingly, the setting of the associated IDMT relay is set in such way that the said relay will not operate for a system fault but will respond when fault current is fed into the Generator which acts as a back-up to the main Generator Protection. However, if the supply is only from Local Generation, the current will decay to a level where it is insufficient to activate the O/C relay to trip. Therefore, a relay which responds to current as well as the level of the voltage is required. The operation principle of such setting that the IDMT relay with less setting than the full-load current is facilitated with an arrangement which increase the setting to above full-load current when full system voltage is presents. Consequently, full load current can follow without relay operation under normal conditions where the voltage is present, since the fault will remove/drop the voltage where the relay will be enabled to assume lower setting.

Based on the above, the conclusions are that Relays assigned for such type of protection may be either:-
a)-Voltage restrained, or
b) - Voltage controlled (setting changeable by voltage operated attracted armature).

Moreover, in accordance to my limited knowledge may I have this opportunity to highlight the following tips as guidelines for Genset general protection?

A) - Since the basic query was about the Overcurrent (genset), Overcurrent protection (O/C) has two aspects which are of completely different functions therefore it may be necessary to differentiate between them, these aspects are
i). Short Circuit. As above.
ii). Overload, to be detected thermally. Pls refer to item # B.3 below.

B) - Genset protection may include and not limited to:-
1. Main Generator protection that include insulation failure and short circuit which require immediate disconnection while the Unsatisfactory operating conditions will be dealt by the operator for attended plant otherwise to be included in the main protection scheme if not attended. However, now a day most of standby Genset are provided by Local control panel to address these unsatisfactory conditions as appropriate.

1. .Earth fault protection. Max. Earth fault current to be restricted to a fraction of FLC. IDMT can be used.

1. Overload protection. On large M/Cs resistance thermometers/Thermistors/thermocouples/ /PT100 are embedded in the stator windings.

1. Stator protection and interturns fault, due to insulation failure. Biased differential protection.

1. Rotor earth fault protection. Negative biasing Method can be used.

1. Unbalance load. Normally Genset will take 15 p.u (15%) of the Negative Phase Sequence (NPS) continuously without damage, the required time setting from NPS current rating I²t = 20. Relays of IDMT characteristic can be used for this purpose.

1. Loss of field-Under current relay can be used.

1. Over speed – In addition to the governor, a centrifugal switch might be provided and/or sensitive under power relay and/or magnetic pickups/Proximity sensors.

1. Overvoltage- High Speed Voltage regulator is applicable.

1. Failure of prime mover, Reverse Power relay is applicable.

Cross Reference: Protection of Industrial Power Systems.