Longitudinal or percentage or biased differential protection of stator (MERZ PRICE PROTECTION)
The best form of stator protection against phase faults, occurring rarely, will be by differential relay, of longitudinal bias type. This type of relay can also protect the stator from earth faults.
The biased differential longitudinal relay is also called as percentage differential relay. This type of protection is used for generators of more than 1MW rating. This relay provided complete protection from phase to phase faults, whereas for earth or ground faults, it provides only 80 to 85% of the stator winding.
If the CTs on the line side have dissimilar characteristics from those mounted at the neutral end of the winding's, a relatively large spill current will flow through the relay operating coil. Under such circumstances, a percentage bias differential relay should be used.
percentage bias differential relay Merits
- This is the best form of stator protection i.e., for phase to phase faults, phase to ground faults and 3-phase faults.
- Uses instantaneous attracted armature type over current relays. The normal setting of the relay will be 10 to 40%.
- High speed about 15 ms static protection.
- Requires low setting
- Offers full stability on external faults.
Principle of Biased Differential Protection Relay for Stator Protection
The differential protection responds to the vector difference of two or more similar electrical quantities. The principle of operation is same as single phase differential relay protection. In this, each stator winding consists of two sets of CTs. The relay has two coils viz the restraining coil connected centrally in pilot wires and operating coil connected between the midpoint of restraining coil (biasing coil) and the neutral pilot wire. This figure above shows the detailed arrangement.
Under normal conditions, equal current flows through the secondary of the CTs and no current flows through the relay operating coil. If there is any fault in any one of the winding, the current through the CTs will not be equal and the balance is disturbed. The differential current flows through the operating coil of the relay and hence the relay will operate, thereby tripping the generator circuit breaker. The field is disconnected through suitable impedance. Under normal conditions, sometimes the current through the CTs will not be equal. For this biasing coil can be used to make the current through the two CTs equal (to avoid residual current).
Connection of CTs for differential protection of generators
The figure above shows that the CT connections for a star connected alternator. The CTs are connected in star and are provided on both the outgoing side and machine winding connections to earth side. The figure below illustrates the CT connections for a delta connected generator.
The CTs on delta connected winding side are connected in delta and CTs at outgoing are connected in star.
1. This protection cannot detect the inter turn fault.
2. Does not respond to through faults and overload.
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