2+1 CC both GTs tripped and Steam Turbine was still producing 30MW


Thread Starter


I was wondering why when both GTs were tripped and both diverter dampers have reached the close position, the Steam Turbine was still synchronised on grid producing 30MW! It shouldn't trip as well the Steam Turbine due to GTs trip and diverter damper close or....?

Hussein Al Nawasrah


Even though the steam turbine kept running and synchronized, I am sure it didn't go for long, and all that depend on the logic that control your process. Some steam turbines trip due to the decrease of the inlet temperature and pressure under a certain limit. so since the drums were pressurized the steam turbine could keep running for a short period of time.


This could be one of "those" conditions that was never envisioned by the plant designers that happens only when the planets are aligned in just the right way. (And, usually, if it happens once it seems to happen again. And again.)

It's really difficult to say what should have happened or what could have happened based on the information provided. As was said, even if the steam turbine generator breaker didn't open it's not likely that the steam turbine remained running for very long without any waste heat from the GTs to produce steam. A lot of steam turbines in combined cycle are just "followers"--meaning the control valves just get opened wide at some point in the start-up and left wide open, and the steam turbine just produces whatever power it can from the steam temperature and -pressure that's available from the HRSGs.

Did the operators open the steam turbine generator breaker or did reverse power open the breaker?

If this truly is one of "those" conditions that wasn't considered by the plant designers then what usually happens--after careful consideration of precisely what happened and what plant personnel believe should have happened and what the utility may wish to happen then what usually happens is that the plant controls are re-programmed to take this condition into account and take the desired action (which will usually be forgotten about if the same condition ever arises again--because most people aren't informed of the changes and only remember what happened the last time!).

At any rate, based on the information provided it's really not possible to say what should have happened. There may have been some other circumstances which we aren't aware of which changed how the plant would have reacted.

Again, after gathering lots of data and performing careful review and analysis if its determined that the plant should have reacted differently then what usually happens is that it's re-programmed. The thing that DOESN'T happen when the plant is re-programmed is that the operators aren't made fully aware of the change(s) and expect something different (more like what happened the first time) and so take action on their own, when perhaps they shouldn't have--and that's when the real "fun" begins!
Famous last words prior to a destructive overspeed event, "the turbine tripped, but I saw MWs and opened the breaker".

Maybe there was a stuck valve?

> Did the operators open the steam turbine generator breaker or did reverse
> power open the breaker?

You said both GT's were tripped and their diverter dampers were closed and the steam turbine was still on line and producing 30 MW.
Obviously there is still steam going through the steam turbine - 30+ MW worth. The steam has to be coming from some place! Is there an auxiliary boiler on line? What is the steam turbine rated at? (Or, what percent of rating is 30 MW?) Do you have any data recorded that might show where the steam was coming from? You really haven't provided very much information to go on here.
> Did the operators open the steam turbine generator breaker or did reverse power open the breaker?

Operator tripped the ST before at 5MW

the steam was going to the ST from both HRSGs there is no aux. blr on line! The thing is that both GTs tripped and diverter dampers both closed. I shouldn't receive a trip signal to both HRSGs and as a result a trip signal to ST?
> Even though the steam turbine kept running and synchronized, I am sure it didn't go for long, and all that depend
> on the logic that control your process. Some steam turbines trip due to the decrease of the inlet temperature and
> pressure under a certain limit. so since the drums were pressurized the steam turbine could keep running for a short period of time.

I agree with all above!It happen exactly like this, operator just tripped the ST at 5MW.

So you think is correct the logic: 'If in 2+1 CC configuration both GTs tripped and both diverter dampers reach the close position, not any trip signal to send on HRSGs, but keep ST working online until a trip signal is received to ST due to inlet pressure or temperature'?

Your question can only be answered by the people who are responsible for the operation of the plant at your site who should be experienced and capable of analyzing and understanding the events and how they believe the plant should be operated. If the plant doesn't have this kind of experience and expertise, then an outside consultant should be brought in to assist with the analysis and make recommendations. The answer depends on lots of factors we do not--and because we are not at the plant, cannot--know.

Does it seem unusual for the steam turbine to remain running when the two GTs are tripped and the diverter dampers have moved to the position to try to maintain the heat in the HRSGs? Yes. It is wrong? We can't know the answer to that question at your site; we simply don't have the all the information and all the facts.

Many times when a plant is designed and built the designers produce a document that is included with the manuals supplied with the plant that describes intended operating philosophy and sequences. There have been MANY times when the Commissioning or Start-up Managers have over-ridden that documentation because THEY believe the plant should be operated differently. Also, MANY Plant Managers have also over-ridden plant design documents and forced changes to be made to implement how they believe the plant should be operated.

We don't know if either, or both, of the above may have occurred during the commissioning of the plant where you are now working. Do you? Has anyone consulted the plant manuals to see what the designer intended (if anything at all)? Because these kinds of schemes and scenarios are usually anticipated and planned for--usually, but not always. And, even if they are over-ridden but rarely documented during commissioning. I've even been to plants where, after scratching my head for a few days trying to understand how some sequencing was modified from what it's been for decades, one, sometimes two or more, plant technicians will quietly approach me to tell me that so-and-so (Plant- or Operations Manager) demanded the sequencing changes some time back. And, quite often it turns out that so-and-so Plant/Operations Manager is the one screaming the loudest about the problems the sequencing change has created--and conveniently forgotten they ordered the change some time ago.

Look, if you're asking for someone on a World-Wide Web site to say definitively the scenario you described--briefly--is right or wrong, well, we can't. I mean, someone could based on their experience at this-or-that plant, but it may not be totally applicable to your plant and circumstances (of which we know very little about the particulars of the circumstance--like, why did BOTH the GTs trip, for example).

It does seem unusual that the steam turbine remained on line, producing power for the grid as the HRSGs cooled, but it wouldn't have remained on line for long, unless--as has been suggested, there was another heat/steam source we are unaware of. Eventually, it would have tripped (hopefully) on reverse power.

So, was it wrong? Maybe. Might it have damaged something? Not unless the steam turbine auxiliary motors (condensate pump; vacuum pump; gland seal steam exhaust/cooler; L.O. pump; hydraulic pump; etc.) lost power when the GTs tripped--which it doesn't sound like they did. Again, there's a LOT of information we don't--and can't--know.

Should it be changed? Possibly. Only the people at your site, with knowledge of the entire plant operating philosophy and configuration with time to review and analyze all the data which can be gathered about the event in question can make the final determination.

In my opinion, venting steam through relief valves or dumping it to a condenser through a bypass on a steam turbine trip are both undesirable. So, by allowing the steam turbine to "draw down" or reduce the steam pressure by using the steam to make electricity as the HRSGs cooled was probably not a bad thing, at all--from this perspective--and maybe even a blessing. At many plants I've been to when safeties blow operators and managers tend to get really agitated and forget that the objective is protect plant equipment and personnel and they just start yelling and can't concentrate and usually make mistakes, forgetting to follow standard operating procedures because of that noise coming from the safeties!

So, if a trip was issued to the steam turbine at the time the both GTs were tripped and the diverter dampers were being re-positioned, what would happen to the steam that's "captured" in the boilers? Would boiler safety valves open to relieve the pressure? Are there mufflers on the safeties? Does the steam get routed to the condenser via bypass valve? Is there sufficient cooling water flow to the condenser during an emergency condition such as this to condense the steam which is being dumped to the condenser without damaging the condenser tubes and/or breaking the condenser rupture disc?

So, there are lots of ways to look at what happened. That's why a careful and considered analysis of the events is necessary to determine if any changes need to be made. You may just find out that even if this scenario wasn't anticipated by the plant designers that it was acceptable under the circumstances--and may even have been the right thing for your plant!
In my opinion, if both GT's are tripped, the steam turbine should be tripped without even waiting for the diverter dampers to close. Once the GT's have tripped, the HP steam temperature will drop rapidly and this is bad for the steam turbine. As long as the HP steam bypass is active, you should not lift any safety relief valves.

You probably should get agreement from the steam turbine supplier and the HRSG supplier before making changes to the trip logic, but I know any multishaft combined cycle units that I designed the controls for tripped the steam turbine if all gas turbines were tripped (unless there was another steam source available and on line).