SUDHIR KUMAR SRIVASTAV
Additional General Manager-RAPDRP
NTPC LIMITED, NEW DELHI
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.
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