Hello Guys,
Recently we experienced an interesting event at our site. We have a CCPP at our site operating in Island mode. There are two Hitachi H-25 GTs and one ST which make up the combined cycle. There is also a small Tornado GT of 5MW capability. We operate with one H-25 GT on Iso mode and rest of the turbines in Droop mode.
One good morning at 05:20 am, the lube oil pump of our Isoch turbine tripped due to motor trouble. As per defined SP and delay, standby lube oil pump started and took over. Between the 1.5 sec episode of primary pump tripping and secondary pump taking over, the lube oil pressure fell as below as 0.10MPa. The tripping SP is 0.06MPa so there shouldn't have been any problem whatsoever during the auto cut-in of pumps. Well, interestingly, there is a whole lot that happened during these 1.5 sec that has left us with our heads scratching. I'll summarize the SOE below:
Before going further, I should mention that we do have reverse power protection through G-60 relay but there is a 5 sec delay before tripping so we never got that far.
It is quite apparent that all this mess happened due to strange response by SRV. Its sudden closing during the first second wreaked havoc. Since hydraulic oil pressure never went below 9MPa, the only cause that could have caused SRV to close is Trip Oil Pressure. But The trip oil pressure went from 0.45MPa to 0.23MPa (The dump valve setting is at 0.14MPa ) so, SRV should never have closed in the first place. The detailed study of trend has shown us that when the lube oil pump tripped, in the first second the trip oil pressure went from 0.45MPa to 0.36MPa . During this, SRV had abruptly closed to zero. Trip Oil pressure further went from 0.36MPa to 0.23MPa in the next half second but during this time, SRV was recovering back to its original position. This is the part which has us in splits. We can imagine that dump valve actuated pre-maturely and caused SRV to close but it should have stayed close if the pressure was decreasing to 0.23MPa. Why did the SRV recover when trip oil pressure was going from 0.36 to 0.23?
I understand there maybe some mechanical issue with the SRV but we had only just started the said turbine 5 days before the event after undertaking its shutdown where SRV was thoroughly checked. Its calibration verified, stroke checked and servo replaced. SRV was working perfectly fine and has been working fine after the event also with no abnormality whatsoever.
Need your guys opinion and expertise on this. Any data you require, I'll be happy to provide.
Recently we experienced an interesting event at our site. We have a CCPP at our site operating in Island mode. There are two Hitachi H-25 GTs and one ST which make up the combined cycle. There is also a small Tornado GT of 5MW capability. We operate with one H-25 GT on Iso mode and rest of the turbines in Droop mode.
One good morning at 05:20 am, the lube oil pump of our Isoch turbine tripped due to motor trouble. As per defined SP and delay, standby lube oil pump started and took over. Between the 1.5 sec episode of primary pump tripping and secondary pump taking over, the lube oil pressure fell as below as 0.10MPa. The tripping SP is 0.06MPa so there shouldn't have been any problem whatsoever during the auto cut-in of pumps. Well, interestingly, there is a whole lot that happened during these 1.5 sec that has left us with our heads scratching. I'll summarize the SOE below:
- Droop GT was running at 22.5 MW, Isoch GT was at 20 MW and Tornado was at 4 MW
- Isoch GT's lube oil pressure was also 0.18MPa, Hydraulic oil pressure was 9.9 MPa and trip oil pressure was 0.45MPa
- System frequency was at 50 Hz
- At around 5:21:06, Isoch GT main lube oil pump tripped on overload and standby pump started as per cut-in logic
- This caused the Lube oil pressure to drop to 0.1Mpascal and Trip Oil pressure to drop to 0.23MPa. Hydraulic Oil Pressure remained above 9MPa
- At the same time, the SRV lost position from 45% to -1.0375% in one second which resulted in dip in P2 pressure from 2MPa to 1.2MPa. After 1 second, SRV began to recover and returned to its original position in next 0.5 seconds
- During the SRV closing, Isoch GT started slowing down and shedding all its load to the Electrical grid
- Isoch GT gave all its 20 MW load and became motorized by further taking 3.7 MW during reverse power phase.
- Isoch GT remained motorized for approx.. 1.5 sec during which the additional load was taken by droop GT and Tornado
- Frequency of the network dropped as well to 43.35 Hz which resulted in underfrequency protection actuation and 32 MW of load was removed from the network
- With 32 MW load removed, Isoch GT normalized and the frequency restored back again to 50 HZ
- The overall time between lube oil pump tripping and underfrequency actuation was less 2 sec
Before going further, I should mention that we do have reverse power protection through G-60 relay but there is a 5 sec delay before tripping so we never got that far.
It is quite apparent that all this mess happened due to strange response by SRV. Its sudden closing during the first second wreaked havoc. Since hydraulic oil pressure never went below 9MPa, the only cause that could have caused SRV to close is Trip Oil Pressure. But The trip oil pressure went from 0.45MPa to 0.23MPa (The dump valve setting is at 0.14MPa ) so, SRV should never have closed in the first place. The detailed study of trend has shown us that when the lube oil pump tripped, in the first second the trip oil pressure went from 0.45MPa to 0.36MPa . During this, SRV had abruptly closed to zero. Trip Oil pressure further went from 0.36MPa to 0.23MPa in the next half second but during this time, SRV was recovering back to its original position. This is the part which has us in splits. We can imagine that dump valve actuated pre-maturely and caused SRV to close but it should have stayed close if the pressure was decreasing to 0.23MPa. Why did the SRV recover when trip oil pressure was going from 0.36 to 0.23?
I understand there maybe some mechanical issue with the SRV but we had only just started the said turbine 5 days before the event after undertaking its shutdown where SRV was thoroughly checked. Its calibration verified, stroke checked and servo replaced. SRV was working perfectly fine and has been working fine after the event also with no abnormality whatsoever.
Need your guys opinion and expertise on this. Any data you require, I'll be happy to provide.