Reverse Power Protection of Generator

A synchronous Generator / Alternator is expected to supply active power to the system in normal operating condition. If the turbine i.e. prime mover fails the Generator / Alternator connected to the system will continue to operate as synchronous Motor drawing active power from the system. This reversal of power flow due to loss of prime mover can be detected by reverse power relay.

The  consequences  of  generator  motoring  and  the  level  of  power  drawn  from  the  power system will be dependent on the type of prime mover as under this condition prime mover acts as a load for synchronous Motor. For steam turbines, the motoring power is around 0.5-3 % of rated power of Generator. Under the failure of prime mover, due to motoring of turbine windage loss will be more in turbine blades as there is no steam to cool it down. Thus it will lead to damage of turbine.

Reverse Power element of Numerical Relay calculates the three phase active power using its current and voltage input based on the following formula,

P = V_aI_aCos�_a + V_bI_bCos�_b + V_cIcCos�_c

The Numerical Relay is connected with the convention that the forward current is the current flowing from the generator to the busbar. This corresponds to positive values of the active power flowing in the forward direction. When a generator is operating in the motoring mode, the machine is consuming active power from the power system and if this active power crosses the set value, then after the set time delay the relay will operate to trip the Breaker.

Normally reverse power setting is kept less than 50% of the motoring power. For example if the motoring power of steam turbine is 4% then reverse power setting shall be kept less than 2%. It shall also be noted that reverse power protection is provided with a time delay of around 5 s to prevent spurious operation due to disturbances or following synchronization.

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Causes of Reverse Power in Generator:

As discussed earlier, one cause of reverse power flow in a generator is failure of prime mover. Now failure of prime mover may be because of failure of Governor or failure of Governor Valve or maloperation of Boiler Pressure Control System.

Another cause of reverse power flow occurs during synchronization of Generator. Let us assume that Generator is to be synchronized with the Grid. The general practice for synchronizing a Generator is to close the breaker when the needle on the synchroscope is moving clockwise and crossing 11 O�clock position as shown in figure below.

Picture taken from Yokogawa Meters and Instruments Corporation

As shown in figure, the position of needle is at 12 O�clock. Suppose the frequency of Generator is less than the frequency of Grid, in this case the needle of synchroscope won�t move in clockwise i.e. fast direction rather it will move anticlockwise direction i.e. toward slow direction. If we close Generator circuit breaker in such condition, then the Grid will try to take the Generator to synchronous speed by feeding power to the Generator and motoring of Generator will take place. Thus in such case also reverse power will flow.