Additional General Manager-RAPDRP
NTPC LIMITED, NEW DELHI
Generator Differential Protection
An electrical fault between phases of Generator winding causes heavy flow of fault current inside the generator, resulting highly extensive damage to machine. The damage may be in coil, in core or in both. Any damage to the core of generator is most severe condition.
To avoid above damage, we uses Generator Differential Protection, which act on differential current between Neutral & Phase current of Generator, as shown in diagram. If a fault occurs inside the stator winding between CT1 & CT2, a distinct difference will be there between the current at the neutral end & phase terminal ends of the particular winding. This difference is detected by Differential relay (87G).
The current entering and leaving the protected object are determined by CT and compared by relays by means of a circuit as shown in diagram.
During normal condition any current flowing in CT1 will flow out from CT2 & there will not be any difference between CT1 & CT2 currents. Also there polarity will be same, so differential current through relay 87 G will be zero & relay will not operate.
A fault inside the protected zone is fed from either one side or both sides depending upon the current source present, thus producing a difference current in the differential circuit. If this differential current exceeds a set percentage (normally 10%) of the normal current flowing in the protected object, the relay pick up and initiate tripping of the generator, thus protecting generator from severe damage. Balancing resistances are used to avoid mal-operation of relay during transient conditions.
Test Procedures:
1. Relay Test: Remove the relay from cabinet. Test its accuracy of operation through secondary current injection test at 25%, 50%, 75%, 100% & 125% of rated current.
2. CT test: Following tests to be conducted on CT:
Ø CT polarity test: To check about proper connection.
Ø CT ratio test: Through primary injection method & secondary current to measure at relay terminal.
Ø CT saturation test
Ø CT secondary circuit insulation resistance test.
Ø CT secondary one end earth connection check.
3. Online protection test: Short any two phase of Generator bus duct between CT1 & CT2 as shown in drawing. Roll the turbine, slightly increase the excitation till fault set current & let the relay operate. This test to be done in strict supervision of system expert, with proper care & monitoring, to avoid any damage to machine.
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