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We work in a combined cycle power plant where there are 4 gas turbines and 1 steam turbine running successfully for last 20 years. The model of the gas turbine is GE PG6551B associated with Alstom T190-240 generator operated by MarkV Control System. During December 2015, we have upgraded turbine parts to 32k parts (including turbine buckets, fuel nozzles, combustion liner, flame scanners etc) and MarkV to MarkVIe control system. After that, the gas turbine base generation increased to 38+ MW from 34MW. It was doing well then. But after 9 months (September,2016), we have faced generator + turbine trip running at base load due to Generator differential protection (We are using LGPG111 for combined generator protection) while other units remained unaffected (no Grid related issues).
That time we checked the stator current (IST1,2,3) curves, visually inspected the generator, did PI test, measured stator winding resistance and found nothing abnormal and started/synchronized the unit thinking of false alarm. But after that tripping due to generator differential protection started happening on irregular intervals sometimes twice in a month even in part load. As a matter of concern, we hired professionals to test the generator in depth (TAN delta, Partial Discharge, PI etc), and they certified that the generator is in very good condition. We have checked the generator CT secondary loop to relay panel and found OK, even we replaced the CT cables right from generator junction box to A520 panel. We have tested the generator CTs (ratio, saturation) as well as generator protection relay (LGPG111) by hired professionals and found all in very good condition. We have earthing resistance near the generator and the earthing resistance is found 2 ohm. Tripping due to generator differential protection become a nightmare to us now and don't know what to do next.
Before installing turbine 32k parts and MarkVIe, we never faced generator differential trip issue and we know that generator differential protection has nothing to do with 32k parts or MarkVIe.
That time we checked the stator current (IST1,2,3) curves, visually inspected the generator, did PI test, measured stator winding resistance and found nothing abnormal and started/synchronized the unit thinking of false alarm. But after that tripping due to generator differential protection started happening on irregular intervals sometimes twice in a month even in part load. As a matter of concern, we hired professionals to test the generator in depth (TAN delta, Partial Discharge, PI etc), and they certified that the generator is in very good condition. We have checked the generator CT secondary loop to relay panel and found OK, even we replaced the CT cables right from generator junction box to A520 panel. We have tested the generator CTs (ratio, saturation) as well as generator protection relay (LGPG111) by hired professionals and found all in very good condition. We have earthing resistance near the generator and the earthing resistance is found 2 ohm. Tripping due to generator differential protection become a nightmare to us now and don't know what to do next.
Before installing turbine 32k parts and MarkVIe, we never faced generator differential trip issue and we know that generator differential protection has nothing to do with 32k parts or MarkVIe.