Generation Getting Down to Zero

A

Thread Starter

ARK

Hi all,

We are facing a problem in one of our sites that the generation is getting down to zero from full load while in service without any external commands and the Generator CB is tripping on Reverse power protection. This is not happening on regular basis. Its just happening once in a while. Below are some of the points which require the expert's advice. Please enlighten us:

1. We assume that the above problem might be caused due to the failure of Governor or missing of generator CB input to the governor while in service (loose connection).

2. Governor service engineer has come and is arguing that there is no problem with the governor (Woodward 505E) and the relay is giving nuisance tripping output.

3. But in the relay records it is clearly showing that the power is reversed during all the trips that have happened so far. so this is ruled out.

4. what if Generator CB contact input is lost to the governor while the generation is at maximum and is connected with grid? How governor behaves in this case?

5. We had a turbine trip even during home load operation similar to the above condition i.e without any external trip commands. What if Generator CB contact input is lost under this condition? will it have any impact on governor control?

Please reply with your valuable suggestions.

thanks in advance.
ARK
 
ARK,

Most turbine governors will automatically reduce (and very quickly in most cases!) the energy flow-rate into the turbine when the generator breaker is determined to be open. This is done to prevent overspeeding the turbine and causing catastrophic damage, and even personnel death in some cases. If the control valves remained open at the same position when the generator breaker opens then the speed of the turbine will increase VERY fast.

You haven't said how quickly the load (generation) is reduced on these events, which could be some key to what's happening. If the load reduction is occurring as some controlled rate over time (minutes?) then it's not likely the problem is the loss of generator breaker status; it's most likely something else causing the governor to slowly, automatically reduce the steam flow-rate. If there is some signal from a DCS or boiler control system that is used to control load of the steam turbine-generator, then it could be that signal which is causing the problem. You need to know all of the inputs to the governor to determine what could possibly be the problem.

The good news is--the Woodward 505E is a fairly simple control system, and the manuals for them are also pretty straight-forward and relatively simple. You should also have a set of drawings which show all of the inputs to the 505E (and, again, there usually aren't too many of them as it's a fairly simple control system) and you should be able to use the manual and the drawings to eliminate possible causes until you arrive at the root cause.

Again, the rate at which generation is being reduced could be very helpful in determining what the problem could be. A loss of closed generator breaker signal would likely result in a very FAST load reduction, and possibly a trip of the generator breaker on reverse power. If the rate of load reduction is slower (many seconds or minutes) then it's not likely the loss of generator breaker status.

Please write back to let us know what you find!
 
B

Bob Johnston

Can you give us a little more information on what kind of control system you have? Is it possible to monitor the status of the generator breaker contact? Is it possible to see what command is driving the governor set-point down?
 
Thank you CSA sir for your reply,

As per the DCS trends the load is getting down to zero from full load within 5 Secs. As you said 505E is a simple governor and inputs are very limited and there are no other inputs except Load Raise / Load Low which could have any impact on the load. Also it can please be noted that we have not considered Analog input from DCS to control the load, the control is being done only through digital signals from DCS or through front panel keys.

We can understand that the loss of gen. CB input contact to the governer when it is in parallel with grid/other source can have an impact on load control. Does loss of the same contact will have any impact on solo running governor (i.e not in parallel with any other generator / grid) too?

Actually what i wanted to ask you is when the governor is running at full load in islanded mode without any other generators in parallel and during this condition if my gen. CB input to the governor is lost, what will be the consequences? will the governor shuts the valves? or it doesn't have any impact on the governor as it is running in speed mode?
 
Hi sir,

The governor what we are using is of Woodward make 505E. It doesn't have features to monitor or take record off the input and output statuses. That is why we are finding it difficult to conclude where the exact problem lies. Is it due to missing of gen. CB contact I/P or governer malfunction?
 
Not sure what unit mfg you have, but some types have a protective feature installed to unload the generator in case of overheating (some only potential). If the unit has a load compartment damper, or cooling fan, and it (damper closes {switch}), or it (fan stops {another switch}) the unit will unload accordingly-some faster than others.
 
ARK,

> Actually what i wanted to ask you is when the governor is running at full load in islanded mode without any other
> generators in parallel and during this condition if my gen. CB input to the governor is lost, what will be the
> consequences? will the governor shuts the valves? or it doesn't have any impact on the governor as it is running in speed mode?

When a generator and it's prime mover (the steam turbine in this case) are operating in islanded mode, the generator breaker should still be closed but the "tie" breaker (the one connecting the plant to a grid or utility) is open--and the governor should be in Isochronous Speed Control mode, controlling speed to control frequency.

However, when the generator breaker opens there is no more load--and in the case of a steam turbine the steam inlet valves should close VERY quickly to prevent an overspeed. Which is exactly the same as when the tie breaker is closed and the generator breaker opens--the load on the steam turbine goes to zero VERY quickly, and the governor should close the steam inlet valves very quickly to prevent an overspeed.

But you say the load drops off over a five second period. If that happens when the unit is islanded, then the frequency should be dropping very as the unit is unloaded.

I'd still recommend that you review the 505E manual and the configuration of the 505E. I don't believe there's any kind of time delay possible on steam inlet valve closure but there might be. But, you should be comparing the configuration parameters of the 505E to the possible configuration parameters to see if there's some other 5-second time delay to unload.

Please write back to let us know how you fare!
 
ARK,

I'm also going to ask about the size of the steam turbine (rating), and what kind of actuators are used on the control valves (high-pressure hydraulic; low-pressure hydraulic; pneumatic; other?).

I suppose it could be possible that if the generator breaker status is lost (indicating an open generator breaker) when the generator breaker is actually closed then depending on the actuator it could be possible that it would take some time (much longer than it should in my estimation) to actually close the inlet valves/stop valve.
 
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