Dear sir,

We have 2 Gas turbine-generator units (ModelG 6541), and a Steam turbine-generator that is combined-cycle power generation system. All are controlled by Mark VIe.

GT I and GT II are working in the island mode now, and they supply power to one factory.

After step-up to 60KV, the ST connects to the local grid of State.

In operation, we found that the frequncy and reactive power of ST are fluctuating violently without control.

The range of frequncy vary from 48Hz to 51Hz, sometimes the ST will trip because of Under frequncy or Over frequncy protection. Maybe the reason is the system is unstable.

However, we don't know why reactive power of ST is fluctuating violently, vary from (-12Mvar to 30 Mvar) when ST generate 13 MW.

Is there something wrong with our exciter control system? or something else?

 

In flight simulators controlled by analog computers (experience from forty years ago) most of the systems had one or more feedback loops, which are also common in power control systems.

These loops follow a chain of "cause-effect-cause-effect"€ that can be traced to the open circuit, faulty component, or false signal causing the problem:

In the case of false fluctuations, the fault can be located by the following steps:

1. Find a point in the system where the signal fluctuates the same as your unwanted variation or result;

2. trace the signal back (toward its source) through the circuits, subsystems, or modules until it disappears or changes; and

3. analyze the signals traced compared to their normal characteristics (assume if unknown).

 

Dear Roadz,

Your post is somewhat unclear, in that you say the frequency and load of the ST when connected to the grid fluctuates wildly, and then you say that when the ST is generating 13 MW the MVar output fluctuates wildly.

Your system is somewhat unusual in that in most combined cycle power plants all of the generators are connected to the same bus and that bus is also connected to the (national) grid, even if a portion of the power is consumed by a local load (the factory in your case). This makes the steam turbine more of a "follower" since it's so much smaller than either of the gas turbines and can't really have much of an effect on frequency.

In your case--and probably precisely because the (national) grid is so unstable--the two larger generator-sets (the gas turbine-generators) are supplying the factory in "island" mode, and the steam turbine is supplying "excess" power produced by the CCPP to the (national) grid. In this case, the steam turbine is very much smaller than the grid (the total of all of the other generators and their prime movers) and it's frequency--and speed--are controlled by the frequency of the grid. If the grid frequency is wildly unstable the steam turbine frequency will be wildly unstable.

And, since the steam turbine is likely operating in Droop Speed Control mode when it's synchronized to the grid when the frequency is fluctuating so, too, will the load of the steam turbine-generator. That's because the amount of steam entering the steam turbine is a function of the error between the turbine's speed reference and the actual speed, and the actual speed is a function of the frequency which is fluctuating because of the grid instability.

So, the amount of steam entering the steam turbine is fluctuating--which means the amount of electrical power being produced by the steam turbine-generator will also fluctuate. There's not much you can do about these frequency/load fluctuations when the grid frequency is so unstable. It's just a situation you have to put up with, unfortunately.

Now, as for the reactive power swings--reactive power is mostly a function of excitation, as you have rightly noted. It's hard to imagine that the grid is unstable at all times, so I'm presuming that for the most part the steam turbine operates fairly stably (that it's output and speed/frequency are relatively stable). And, that the reactive power is also relatively stable during these periods of load/frequency instability caused by the grid.

So, when is the reactive power unstable? All the time--regardless of whether the ST load/frequency is unstable or stable? Only sometimes when the ST load/frequency is stable? At all times when the ST load/frequency is unstable? At all times when the ST load/frequency is unstable?

What mode is the ST generator being operated in? Automatic? Manual? Does the ST control system have a VAr/PF control mode--and if so, which mode is active (if either of them are active)?

What is the grid voltage (the 60 KV system voltage) doing when the ST generator reactive power is unstable?

What kind of excitation system is in use on the ST generator? Are there any alarms being annunciated by the ST generator exciter? Are there any Diagnostic Alarms being annunciated by the ST generator exciter with the reactive power is unstable? (Or even when it's stable?)

 

Thank you for your time and answer!!!

As you said, our operation mode is unusual, because we are in Africa where lack of power. and the grid is unstable.

Maybe the ST in our plant operates in the "follwer mode", cause it's output is controlled by "the pressure of the main steam generated by the boil"

There are several other ways to control the output of ST, like "Var control" "PF Control", "MW control", but we select "pressure control".

The exciter is in the "Auto control mode". In this mode, Mark VIe can adjust the field automatically ,and operator also can do it in the computer.

The exciter model number is "DS3820SRVB", GE

It's doesn't happen all the time,but at least once per week.

Sometimes our generator protection, <b>loss of excitation</b>, will trip the circuit breaker when leading reactive power come in.