- Differential Protection Definition: Differential protection is a method used to clear internal faults in the stator winding of a generator or alternator.
- Current Transformers: Two sets of current transformers (CTs) are used, one on the line side and one on the neutral side, and their characteristics must match to avoid relay malfunctions.
- Stabilizing Resistor: A stabilizing resistor in series with the relay prevents operation due to external faults or CT saturation.
- Percentage Biasing: Percentage biasing in differential relays helps manage spill current from mismatched CTs, preventing unwanted relay operation.
- Relay Operation: The differential relay operates during internal faults when the operating coil’s torque overcomes the restraint coil’s torque, ensuring reliable protection.
Differential protection is primarily used to clear any internal faults in the stator winding of a generator or alternator.
The differential protection is provided in the generator by using longitudinal differential relay.
instantaneous attracted armature type relays are commonly used because they operate quickly and are not affected by AC transients in the power circuit.
There are two sets of current transformers, one CT is connected to the line side of the generator and other is connected to the neutral side of the generator in each phase.
The characteristics of all current transformers (CTs) must match for each phase. If there’s a mismatch, the differential relay might malfunction during external faults or normal operation. A stabilizing resistor in series with the relay prevents it from operating due to external faults or CT saturation, ensuring reliable protection.
It is always preferable to use dedicated current transformers for differential protection purpose because common current transformers may cause unequal secondary loading for other functionalities imposed on them. It is also always preferable to use all current transformers for differential protection of generators or alternators should be of same characteristics. But practically there may be some difference in characteristics of the current transformers installed at line side to those installed in neutral side of the generator.
These mismatches cause spill current to flow through the relay operating coil. To avoid the effect of spill current, percentage biasing is introduced in differential relay.
A percentage biased differential relay has two restraint coils and one operating coil per phase. The operating coil’s torque closes the relay contacts to trip circuit breakers, while the restraint coils’ torque prevents this by acting in the opposite direction. During an external fault, the restraint coils increase the relay setting to prevent operation. During an internal stator fault, the restraint coils are ineffective, and the relay closes its contact when the operating coil current exceeds the setting.
Differential current pickup setting/bias setting of the relay is adopted based on the maximum percentage of allowable mismatch adding some safety margin.
The spill current level for the relay is to just operate it; is experienced as a percentage of the through fault current causing it. This percentage is defined as bias setting of the relay.
