Case Studies on Generator Earth Fault Protection

SUDHIR KUMAR SRIVASTAV

Additional General Manager-RAPDRP

NTPC LIMITED, NEW DELHI

sudhirksri1@gmail.com

Case studies on Generator Earth Fault Protection

Case Study-I:

·         While in operation, suddenly 200 MW unit trips on Generator Earth Fault protection.

·         On inspection it is found that both E/F protection i.e. Stator Main Earth fault protection & Stator Stand by earth fault protection got operated.

·         Disconnected the associated equipments & exhaustive testing done on Generator, GT, UAT & all associated equipment. No abnormality found.

·         Re-started the unit & syncronised with grid on full load.

·         After 08-10 hours of normal running, the unit again tripped on same protection.

·         As the unit was tripped in day time, we rushed to site & started preliminary checks. We found that one surge arrestor, connected with generator bus duct, was much hotter compared to other surge arrestors. While testing, it is found that the test report of surge arrestor was normal.

·         We presume that, due to aging effect the leakage current of surge arrestor should have been increased causing heating of arrestor & the leakage current further increased with increase of temperature through cascading effect on live condition. This may be the reason for E/F operation.

·         We replaced the surge arrestor & re-started the machine with full load. No tripping observed after words.

·         Finally we concluded that, at normal temperature & test voltage the surge arrestor would have working properly. But on rated voltage with passage of time there must be increase in leakage current resulting heat generation in surge arrestor which keeps on increasing of leakage current & heat causing malfunction of surge arrestor.

Case Study II:

·         While in operation, suddenly 200 MW unit trips on Generator Earth Fault protection.

·         On inspection it is found that both E/F protection i.e. Stator Main Earth fault protection & Stator Stand by earth fault protection got operated.

·         Disconnected the associated equipments & exhaustive testing done of Generator, GT, UAT & all associated equipment. No abnormality found.

·         Re-started the unit & syncronised with grid on full load.

·         After 10-12 days of normal running, the unit again tripped on same protection.

·         As it was rainy season, we suspect of some insulation puncture in generator bust duct.

·         We opened the generator bus duct & found that ample amount of water was present in the duct, so the gap between live bus & earth point (i.e. water level) was reduced. Also due to water inside bus duct the insulating property of the air inside the bus duct may have very poor.

·         So we presume that, any vibration or any disturbances may have caused insulation breakdown between live bust & earth point in the bus duct, resulting fault current tracking and operation of E/F protection relay.

·         We removed the water & re-started the machine with full load. No tripping observed after words.

Case Study-III:

·          While in operation, suddenly 200 MW unit trips on Generator Earth Fault protection.

·         On inspection it is found that only Stator Stand by earth fault protection got operated & Stator Main Earth fault protection was not operated.

·         To remove the possibility of malfunction of protection system, we first target to test the healthiness of stator E/F protection system.

·         We found Stator Main Earth fault protection system totally healthy.

·         While checking Stator Stand by earth fault protection system, we found that VT secondary (used for Stator Stand by earth fault protection system) of one phase got burnt. So open delta voltage of three VT were in the range of phase voltage, causing operation of Stator Stand by earth fault protection.

·         We replaced the burnt VT.

·         To take precaution we performed testing of Generator, GT, UAT & all associated equipment & no abnormality found.

·         Finally we re-started the machine with full load. No tripping observed after words.   

Stator Earth Fault Protection

SUDHIR KUMAR SRIVASTAV

Additional General Manager-RAPDRP

NTPC LIMITED, NEW DELHI

sudhirksri1@gmail.com

Stator Earth Fault Protection

Stator winding faults: These types of faults occur due to the insulation breakdown of the

stator coils. Different types of stator windings faults are:

a) Phase to earth fault

b) Phase to phase fault

c) Inter turn fault

Phase to earth fault are limited by Grounding Transformer connected to the neutral of stator winding. There are fewer chances for the occurrence of the phase to phase and inter turn faults. The insulation between the two phases is at least twice as thick as the insulation between one coil and the iron core, so phase to phase fault is less likely to occur. Inter turn fault occurs due the incoming current surges with steep wave front. High impedance reduces the fault current and thus it is very difficult for differential protection system, to detect the high impedance faults. So the differential protection does not work for the high impedance grounding.

To overcome this limitation, an Inverse Definite Minimum Time over voltage relay (64G1) is connected to the secondary side of neutral grounding transformer & another Inverse Definite Minimum Time over voltage relay (64G2) is connected across an open delta of generator PT secondary winding.

Stator Earth Fault Main Protection (64G1): The secondary winding of neutral grounding transformer is shorted through loading resistance(R).  Inverse Definite Minimum Time over voltage relay (64G1) is connected across loading resistance(R). The separate relay to the ground neutral provides the sensitive protection. But ground relay can also detect the fault beyond the generator, so the time co-ordination is necessary to overcome this difficulty.

For an earth fault in stator winding, the E/F current flows in primary winding of neutral grounding transformer. As a result, a voltage is developed across the loading resistance (R) which activate earth fault sensing relay (64G1).

When the earth fault occurs at terminal end of stator winding, full voltage is developed at neutral of stator winding & maximum voltage is developed across neutral grounding transformer, resulting faster operation of inverse definite minimum time over voltage relay. But when the fault is near neutral, very less voltage is developed at neutral of stator winding & very less voltage is developed across neutral grounding transformer, resulting slower operation of inverse definite minimum time over voltage relay.  If the fault occur very near to the neutral i.e. less than 5% of stator winding, there is chance that relay may not operate.

So with this protection system only 95% of the stator winding is protected and 5% of the stator winding starting from neutral point remains unprotected because a fault in this portion will generate too low voltage for relay operation.

Stator Earth Fault Standby Protection (64G2): The Inverse Definite Minimum Time over voltage relay (64G2) is connected across an open delta of the generator VT secondary winding. When there is no E/F, the phasor sum of voltages of all three phase generator VT secondary will be zero, resulting non-operation of relay.

When there is E/F in one phase, the voltage of that phase will be less resulting unbalance phasor sum of voltages of all three phase generator VT secondary and a voltage will appear at relay, causing operation of relay & tripping the system.

Neutral third harmonic under voltage: There is the third harmonic present between the neutral and the ground , and protection schemes takes advantages of this and respond to the under voltage between the neutral and the ground.

100% protection scheme: This scheme provides complete protection of the stator winding by injecting the signal between the stator winding and monitors it for change. 95% scheme and third harmonics protection scheme provide protection only at rated speed and rated voltage but the 100% scheme also provide protection at standstill.

Overview of 100% Stator Ground Fault Protection

Third Harmonic Neutral Undervoltage Scheme. In the late 1970’s, a major European manufacturer introduced a third-harmonic neutral under voltage relay that in conjunction with the traditional 60Hz overvoltage protection, could provide stator ground fault protection over the entire stator winding. The third harmonic was measured across the generator neutral grounding resistor. The scheme’s basic concept is that when a generator stator ground fault occurs near the generator neutral, the third-harmonic voltage goes to zero. If the generator has enough third harmonic neutral voltage present during normal operation, such generators are candidates for 100% schemes using third-harmonic neutral detection.

Third-Harmonic Ratio Scheme. In the early 1980’s, a second third-harmonic scheme was developed by an American manufacturer. This scheme compared the third harmonic at the neutral and terminals of the generator. The major advantage of the scheme was that it was more secure than simply using third harmonic under voltage measured at the generator neutral. It required a broken-delta potential connection on the generator terminals to measure the terminal value of third-harmonic voltage. This required the installation of a VT-—the primary winding of which needed to be wye-grounded. Many generators, especially smaller units, required the addition of this VT since these generators used open delta-phase VT connections. This additional cost and wiring complexity reduced the number of people that used the scheme.  

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UAT, OH Line, GT & GT overall differential protection

SUDHIR KUMAR SRIVASTAV

Additional General Manager-RAPDRP

NTPC LIMITED, NEW DELHI

sudhirksri1@gmail.com

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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