Reverse Power Protection of An Alternator


Today's complex power scenario demands uninterrupted supply for consumer. Therefore generator is the ultimate source of today's life and its function is of heart of power system. Hence it needs the maximum protection from the occurring faults.The generator used is a synchronous machine which can either run as generator or motor depending upon the form of input energy. These generator are synchronized with the bus-bar that is the grid. These are number of faults which may cause heavy damage to generator and simultaneously to the economical situation. The faults are described in the table. Some Abnormal condition and protection system

Condition for Reverse Power

Following are the possible circumstances in which reverse power condition may arise.

i) When number of units are operating in parallel and failure of one unit may result in feeding power to the suffered unit from healthy units.

ii) When one of the unit from all connected parallel units is overloaded and its consequence would be reduced speed. Due to reduction of speed, frequency decreases and ultimately emf generated goes on decreasing and finally a critical stage is reached where is machine (alternator) is driven as motor.

iii) When load is suddenly thrown off, terminal voltage of bus-bar increases instantly. If this condition is not detected properly again alternator will run as synchronous motor.

When the input to the turbine is stopped the generator continuous to rotate as synchronous motor taking power from the bus-bar. During the motoring action of the generator the power flows from the bus-bars to the machine and this condition is called as REVERSE POWER.

Effects Of Reverse Power

During reverse power condition alternator runs as synchronous motor and the turbine acts as a load. Motoring protection is mainly for the benefit of the prime mover and load coming on the generator bus-bar while motoring reerse power protection measures the power flow from bus-bars to the generator running as a motor. Normally the power taken in most cases is low of the order of 2% to 10% of noted power, power factor and current depends on excitation level.

A single element directional power rely, sensing the direction of power flow in any one direction, phase is sufficient. The setting depend on the type of prime mover.






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