SUDHIR KUMAR SRIVASTAV
Additional General Manager-RAPDRP
NTPC LIMITED, NEW DELHI
UAT Differential Protection
Unit Auxiliary Transformer (UAT) is directly connected with Stator winding of Generator, without any breaker in between. So it is necessary to connect the UAT protection with generator protection & any fault in UAT should trip the turbine along with Generator Circuit Breaker & Excitation system.
To protect UAT, a biased differential relay (87UAT) is connected between primary & secondary winding of UAT though respective CTs. During normal condition any current flowing in primary winding CT will flow out from secondary winding CT & there will not be any difference between CT currents. Also there polarity will be same, so differential current through relay 87UAT will be zero & relay will not operate.
A fault inside the protected zone i.e. Transformer 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 turbine along with Generator Circuit Breaker & Excitation system.
Overhead Line Differential Protection
For power evacuation, there is overhead line between Generator Transformer & Switchyard without any breaker in between. So any fault in this over head line should trip the turbine along with Generator Circuit Breaker & Excitation system through generator protection system.
To protect this, an un-biased differential relay (87L) is connected between GT secondary & switchyard though respective CTs. During normal condition any current flowing in GT secondary CT will flow out from switchyard CT & there will not be any difference between CT currents. Also there polarity will be same, so differential current through relay 87L will be zero & relay will not operate.
A fault inside the protected zone i.e. overhead line 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 turbine along with Generator Circuit Breaker & Excitation system.
GT Differential Protection
Generator Transformer (GT) is directly connected with Stator winding of Generator, without any breaker in between. So it is necessary to connect the GT protection with generator protection & any fault in GT should trip the turbine along with Generator Circuit Breaker & Excitation system.
To protect GT, a biased differential relay (87GT) is connected between primary & secondary winding of GT though respective CTs. During normal condition any current flowing in primary winding CT will flow out from secondary winding CT & there will not be any difference between CT currents. Also there polarity will be same, so differential current through relay 87GT will be zero & relay will not operate.
A fault inside the protected zone i.e. Transformer 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 turbine along with Generator Circuit Breaker & Excitation system.
GT Overall Differential Protection
Since stator winding of Generator is directly connected with Generator Transformer (GT) & Unit Auxiliary Transformer (UAT) without any breaker in between, it is proper to have a protective circuit which protect combined unit of Generator stator winding, GT, UAT & overhead lines.
To protect the combined unit, a biased differential relay (87) is connected between CTs of neutral side of Generator, Secondary side (LV side) of UAT & CT at entry of switchyard. The connection of circuit is such that any current IN, through generator neutral CT should be balanced out through sum of outgoing current through UAT secondary CT & switchyard CT. During normal condition any current flowing in generator neutral CT will balance out with CT currents of UAT secondary & switchyard, so there will not be any difference between CT currents. Also there polarity will be same, so differential current through relay 87 will be zero & relay will not operate.
A fault inside the protected zone i.e. combined unit 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 of the normal current flowing in the protected object, the relay pick up and initiate tripping of the turbine along with Generator Circuit Breaker & Excitation system.
Biased setting of 30% is used to prevent the relay operation in case of through fault when the CT may saturate and produce an erroneous secondary current.
Remarks: In some cases, GT restricted earth fault protection is also connected to protect the system from earth fault in HV winding of GT.\
Fault analysis with operation of above protection system
Case-1:
Only UAT differential & GT overall differential protection operated:
i. Maximum chance is that fault is in UAT.
ii. Check the healthiness of UAT differential & GT overall differential protection system.
iii. Test the connected CTs in UAT.
iv. Test the bushing & other insulating device connected with UAT.
v. If all above is found OK, we presume that fault is in UAT & then we should do regress test of UAT for fault finding. If found faulty, replace it with new one. If spare UAT is not available, disconnect it from Generator and charge other circuit for normal operation with second UAT & station transformer supply (available at every station for running of station auxiliaries).
Case-2:
UAT differential operated but GT overall differential protection not operated:
i. Either GT overall differential protection system is faulty or UAT differential protection system is faulty.
ii. Check the healthiness of UAT differential & GT overall differential protection system.
iii. If GT overall differential protection system is faulty, we should conduct test as described in case-1 point iii to v.
iv. If UAT differential protection system is proved to be faulty, we may presume that UAT is healthy. We should rectify the fault of protection system & charge the circuit for normal operation.
Case-3:
Only Overhead Line & GT overall differential protection operated:
i. Maximum chance is that fault is in overhead line.
ii. Check the healthiness of overhead line differential & GT overall differential protection system.
iii. Test the connected CTs in overhead line.
iv. Test the bushing & other insulating device connected with overhead line.
v. If all above is found OK, we presume that fault is in over head line. If found faulty, rectify the fault and charge other circuit for normal operation.
Case-4:
Overhead Line differential operated but GT overall differential protection not operated:
i. Either GT overall differential protection system is faulty or overhead line differential protection system is faulty.
ii. Check the healthiness of overhead line differential & GT overall differential protection system.
iii. If GT overall differential protection system is faulty, we should conduct test as described in case-1 point iii to v.
iv. If overhead line differential protection system is proved to be faulty, we may presume that overhead line is healthy. We should rectify the fault of protection system & charge the circuit for normal operation.
Case-5:
Only GT differential & GT overall differential protection operated:
i. Maximum chance is that fault is in Generator Transformer.
ii. Check the healthiness of GT differential & GT overall differential protection system.
iii. Test the connected CTs in GT.
iv. Test the bushing & other insulating device connected with GT.
v. If all above is found OK, we presume that fault is in GT & then we should do regress test of GT for fault finding. If found faulty, replace it with new one.
Case-6:
GT differential operated but GT overall differential protection not operated:
i. Either GT overall differential protection system is faulty or GT differential protection system is faulty.
ii. Check the healthiness of GT differential & GT overall differential protection system.
iii. If GT overall differential protection system is faulty, we should conduct test as described in case-1 point iii to v.
iv. If GT differential protection system is proved to be faulty, we may presume that GT is healthy. We should rectify the fault of protection system & charge the circuit for normal operation.
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