We have four GE 6B units connected with redundant transmission grid system. Say for example grid A and grid B, both are dual circuit 132 KV system. On 17th June, 2018 outgoing feeder breaker (in our power plant) for grid A tripped for exceeding overcurrent limit. All GT's went to FSNL from Base Load and generated the following alarms:
1. Reverse Power Fault
2. Generator Breaker Tripped
3. Remote Breaker Open Detected.
Grid B was stable, there was no significant change in frequency, voltage etc. and Plant 132KV bus remained uninterrupted. GT breakers tripped due to reverse power! My question is why GTs felt reverse power if Grid B and 132 Busbar is stable??
In the Rung there is a relation between L20CBX and L83RBO (Remote Breaker Open Detected) which opens compressor bleed valve if L83RBO becomes "TRUE". Is this (opening of bleed valve for L83RBO=TRUE) a cause of GT load lowering upto reverse power and consequently trips 52G??
It would be necessary to see the one-line for your plant to try to understand what the exact configuration is and exactly how the GTs are connected to the bus(es). It's just not possible to say with any degree of certainty why the generator breakers opened on reverse power.
I have no personal experience with the L83RBO logic and would have to see the rung(s) to understand what causes the logic to operate to be able to make a guess as to why it is necessary for your site. I've heard of some sites with DLN combustor-equipped units that have something that sounds similar to what you tried to describe and I think it was to try to keep the units from losing flame on a load rejection (breaker open event).
GE logic has a bad habit of failing to block alarms which occur after some major event which are basically untrue (the alarm condition occurred but only because some event precipitated the alarm--the subsequent alarm conditions occurred but not individually). And, if the logic came from GE in Belfort, France, you can be pretty certain that it is needlessly and overly complicated.
Opening the compressor bleed valves during a load rejection should not cause a sufficient loss of load to result in a reverse power condition. It would, however, reduce the airflow through the combustors which would probably reduce the chances of flame-out on load rejection.
Lastly, it would also be necessary to have the complete, chronological order of alarms (Process and Diagnostic) that occurred during the event in order to be able to understand what might have happened. I have seen the "Remote Breaker Opened" alarm be driven by a protective relay somewhere else in the plant; without being able to see the application code running in the turbine control panels it would be impossible to say precisely what was driving that particular alarm.
I wish the news were better, and I hope as you uncover details about the event you will reply back to this thread to let others know exactly what happened.
Thanks for your response.
L83RBO is not driven by protective relay. GE has built this logic signal from turbine speed mechanism. I have seen GE has given the same logic for non -dln machine as well in some other power plant of our country. Opening and closing of bleed valve with generator breaker operation is logical but I don't understand the requirement of interlocks between L83RBO and L20CB1X? Even I don't understand why machine reduces load if it is connected in Electrical Ring Bus system?
Here is the Link below of our csp and font to read the rungs (unfortunately I can't draw rung here in this box!!!).
6FA, generation application connected to an isolated 600 MWp grid, MNQC liquid fuel, water injection for NOx control, TMR Mark V.
Many years ago we had an experience with L83RBO “Remote breaker open detected” after a fatal accident in one of our electrical substations which caused a big portion of our grid to switch off. This caused our units to speed up, and in one of them L83RBO was activated (L83RBO picks up if TNH > 101.5% and TNHA > 3%/sec). Per our CSP, L83RBO=1 puts L52ONLINE œ”Unit on line” to 0. L83RBO=1 has the same effect on our units as a sudden opening of 52G or 52L. L52ONLINE=0 (L83RBO=1) starts what I have always thought of as a load rejection logic on our units, interacting in FSRMIN, TNR, CSGRV’s blocks, in the water injection logic, in the load/speed logic’s trying to push down the load of the unit as quickly as possible. In this occasion the unit ended up tripping on loss of flame (in less than one second FSR ramped down from 62% (58 MW aprox.) to 15% (29 MW aprox.)).