Outage of Generator Transformer due to High DGA

BRIEF HISTORY 200 MVA 400 kV GT erected at site. After satisfactory pre-commissioning checks GT was successfully commissioned This GT was commissioned along with other GTs and was under full load operation for one year. During DGA monitoring some of the critical fault gases were showing high concentration. Outage of GT was taken for investigation. Following actions completed to diagnose the fault.

DGA monitoring during operation Date/ gases (ppm) H2CH4COCO2C2H4C2H6C2H2O2 (%)N2 (%)TGC (%) DGA interpretation :- High concentration of Hydrogen (H2) gas. Acetylene (C2H2) increased after August 2011 to 14 ppm (rate of rise 1.3 ppm per day). All other fault gases are within limits. Nitrogen (N2) & Oxygen (O2) and total gas content (4.6%) are also within reasonable limit. Sudden rise in acetylene gas during full load operation necessitated internal inspection.

Low Voltage Testing of GT after Shut Down S.N.DescriptionObservationRemarks 1.Ratio, resistance and magnetizing current on HV & LV windings, IR values. Results found satisfactory. All the results matching with pre-commissioning and factory results. 2.Tan delta and capacitance test on 420kV bushing / pF Results are OK No appreciable change in the parameters. Bushing is healthy. 3.Tan delta and capacitance test of windings HV-Earth 2.06%, LV- Earth 0.64% & HV-LV 0.29% HV to earth values are very much on higher side. 4.BDV of tank oil Diverter tank 65kV 28kV Main tank oil OK but OLTC oil showing deterioration. 5.Isolation of Core Clamping & Core laminations w.r.t. tank/earth Isolation meggered better than 500 meg. Ohm Isolation is satisfactory.

Internal inspection S.NDescriptionObservationRemarks 1.Wall shunts, yoke shunts connections. Connections & yoke shunts isolation OK Shunts found healthy. 2.HV lead and copper tube with braid connection. Lead & copper braid connection found intact. No abnormality noticed. 3.Locking beam of end frame on HV side. One of the bolt of cross beam HV side found deposited with carbon particles. No heating/ melting marks noticed. Deposited particles removed by oil washing and cleaning. 4.General inspection of tank bottom Metallic soot's found on the bottom tank. Some of them found to be magnetic. 5.OLTC leads and bevel gear assembly. All the leads of selector and diverter contacts found in tact. Top gear assembly found cracked. Ingress of moisture in diverter through gear.

Carbon particles deposit on bolts & beam

Damage of upper gear body

ACTION TAKEN Thorough cleaning of active parts and rinsing with hot processed oil. Top OLTC gear replaced. OLTC diverter taken out & overhauled. Oil replaced. Boxing of GT for vacuuming with new gaskets. Desired dew point of GT achieved after three cycles of vacuuming and N2 purging. Hot and processed oil filled and circulated to achieve BDV > 75kV and moisture <8ppm.

Transformer testing All the low voltage test including tan delta completed and values found within limit. Tan delta / capacitance of HV winding to earth improved from 2% to 0.342% and 5793pF. GT commissioned and all the parameters including DGA found within limit. Acetylene gas content was found within 0 to 3.19ppm but stabilized. GT operating satisfactorily.

Conclusion Sudden increase of C2H2 gas was due to minor arcing outside the active part i n presence of metallic soot which later burnt out. OLTC oil got contaminated due to moisture ingress leakage through crack in upper gear unit. High tan delta values of HV winding was due to OLTC. Transformer commissioned after replacement of OLTC gear and is operating satisfactorily.

Feed Back DGA problem was attributed to improper cleaning of pipe work during erection of transformer. OLTC upper gear body cracked due to improper handling and fitting during erection. Thorough cleaning of bottom tank & pipe work during erection. Rinsing of core & winding before boxing. Damage of OLTC fittings like upper gear and OLTC head cover to be examined and replaced them if required. OLTC oil BDV to be checked six monthly and to be replaced if found deteriorated.

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