Gas Turbine tripping on Generator breaker and Bus-Tie opening


Thread Starter

Bilal Khurshid

We have two General Electric MS-5001P/Frame-5 Gas Turbines of 18MW each capacity installed at our fertilizer plant for power generation. One of these turbines runs on Isochronous mode with SpeedTronic Mark-II Control system while the other runs on Droop mode with SpeedTronic Mark-V Conrol system.

Recently we faced a problem on our Isochronous turbine. Accidental tripping of 51-V-3 (Over current with voltage restraint relay) resulted in opening (tripping) of both Bus-Tie breaker 124-2 (52 T) and Generator breaker 52G in the generator panel.

The turbine also tripped on the following causes:
1- Over speed Trip
2- Exhaust Temperature High

Why did the turbine tripped on the above causes?
We believe that the turbine should have continued running on FSNL under this scenario.

Please give your expert opinion on the subject problem.
> <b>We believe</b> that the turbine should have continued running on FSNL under this scenario.

Without a LOT more information about the load the Isochronous unit was carrying and the nature of the load (stable; large loads starting/stopping normally; etc.) and what load the Droop machine was carrying it's very difficult to say what should have happened or what did happen. We would also need to know what the load on the two units was after the tie-line breaker opened.

We also don't know what fuel the units were burning when the event occurred.

While it's possible to tune a GE-design heavy duty gas turbine such that it does not lose flame on a large load rejection there are many factors that affect the ability of the Speedtronic and the auxiliary systems to be able to prevent tripping on loss of flame or even exhaust overtemperature or overspeed.

It's unfortunate that GE does NOT block subsequent trips from being annunciated, meaning that if a unit trips on, say, overspeed, and then an exhaust overtemperature is detected the Speedtronic will annunciate both. But only the first condition (in this example, the overspeed) is the REAL reason the turbine tripped. You didn't provide enough information to be able to tell which condition actually tripped the turbine, because both conditions cannot trip the turbine--only one condition can even if other condition(s) is(are) detected after the first.

Have you ever experienced the same scenario in the past, and did the turbine remain at FSNL? Everyone would want their turbine to "survive" such a scenario and remain at FSNL so the breaker could quickly be closed again, but it just doesn't happen. The Speedtronic has to be tuned to allow it to happen, and even then, sometimes conditions during such events are not always identical and the unit may not remain running at FSNL even it was tuned to do so (albeit under a different set of conditions).

To tune a unit to remain running at FSNL after a load rejection requires rejecting load--something most plants are very nervous about doing for obvious reasons. In some parts of the world it's not possible to "dump" 18 MW of load suddenly without adversely affecting the system (there has to be 18 MW of capacity ready and waiting and possible of loading very quickly for there to be little to no affect on the system). So, for obvious reasons, very little testing and tuning--if any--is ever done.

And, yet people still "believe" their unit should survive any and every possible scenario.

Based on the information provided, it's time to change beliefs.
Although I am not an expert, it sounds as if the 50/51 relay and the 86G lockouts did EXACTLY what protective relays are designed to do. PROTECT! In my not expert opinion, the turbine tripping is the least damaging result that can happen even though is was an accidental tripping of the protective relay.
To check whether the unit "should" be at FSNL after a lockout operation, the best and only way is to look at the trip/ interlock logic diagram for your plant. This diagram would clearly show whether the unit should be in FSNL or trip after a lockout operation. After checking the diagram, then we can troubleshoot further to check for any wiring mistakes and etc.

Good luck!



If it is accurate that the 51 trip was accidental, then I would hazard that the cause of the isochronous machine shutting down was overspeed. I would also hazard that the overtemperature light was not a true condition but caused by the original trip.

If the trip was overspeed, (and there are two overspeeds - mechanical and electrical: as you did not mention resetting the mechanical I expect the trip was electrical) there could be a number of reasons.

(i) the electrical overspeed is set too low. the setting is critical in that one cannot set it too high or there is a risk of destruction of the rotating parts should an overspeed condition occur (which I believe happened here)


(ii) the response of the controls was not sharp enough and an overspeed condition actually occurred. The controls need to be calibrated to a great degree of accuracy to ensure that responses are fast and stable - and there are items both in the panel and on the skid which need calibration in conjunction with one another to ensure this.

The units were originally designed to take a load dunp and stay running. However if the controls are all set correctly, some wear and tear on the engine over the years could mean that the original control settings no longer work for a dump.

However, if the 51 trip was really an accident (i.e. there was no overcurrent) then you should not be too worried by the overspeed trip as it did what you wanted it to do. Of course, the machine would have to be restarted and I do not know what disruption was caused to the process.

I would suggest calibration of all the fuel controls and also checking of the electrical overspeed setting by an expert.

I have assumed in all of the above that the accidental tripping of the 51 was a trip of the relay without an overcurrent situation. If the accident was an overload with an actual high current, then there could have been an overtemperature of the turbine and depending on the sequence of the trip (protection relay/turbine controls) there could also have been an overspeed situation. If this was the case then in all of this the trips seem to have worked fine.

Hope this helps some.