Single shaft load control 1

[npower0073]

Dear all. I want to find details as it concerns the load control on single shaft combined cycle power plants. I know that the steam turbine is engaged to the generaotr with a SSS clutch (synchro self sustaining clutch). Also i want to know the oeprating control principles for load controllers. Any help will be highly appreciated.

 

[rmw]

The gas turbine controller is the daddy of it all. The generator is maintained at synchronous speed by the grid and the turbine control signal is literally trying to make the generator speed up the whole grid if it could. That it can't is how electrical power is sent to the grid via torque to the generator.

Meanwhile, the generator is getting its torque contribution from two sources. If the steam turbine were to suddenly burst a line for example, the GT would have to provide all the torque necessary to maintain the generator at its present load (if it could).

Conversely, if you were to suddenly add steam to the header from some other imaginary source, the steam turbine (within it's capability to handle the steam flow-we are talking theory here) would try to add torque and the GT governor would sense that and back off the combustion controls.

That is all simlplistic, but that is what is going on. If you want to see it, go pull your safeties on your HRSG and watch the GT controls make up the difference (to their limits, of course.) Don't do this really, but think of what is going on in the system if you did.

I once was standing looking righ at the MW meter of a single shaft CCGT and out on the system (an islanded system) a squirrel fried itself between two phases and it caused an instantaneous spike of 6 MW. It only took a fraction of a second to cook the squirrel (the distribution crew later told us in the operating room what had hit the line).

I heard the unit 'groan' under the sudden instantaneous load (slight frequency change noted too) and I heard the unit 'surge' probably less than a full second later when the load just as suddenly disappeared. I was sucking in my breath hoping that it didn't overspeed and trip. The whole unit seemed to let off a 'sigh'. I can promise you that all that happened too fast for the steam turbine to have had any effect on the torque contribution to the generator. The CT did it all.

rmw

 

[davidbeach]

Sorry, but the squirrel would have resulted in a loss of load (MW). It may well have resulted in more current, but at a significantly lower power factor so the power went down and the speed of the generator went up. When the fault cleared the load came back and the generator slowed down.

 

[byrdj]

If the ST had EVA, it would slam the admission valves on the rapid loss of generator Voltage, reducing the current.

 

[rmw]

Sorry, but I saw the meter. For whatever reason I happened to be standing there (I think I had just arrived at the plant and was seeing what was going on with the unit-I was the start up engineer and it was still the OEM's unit) looking directly at the KW meter. The frequency meter was within my perifial vision-either directly to the right or left of the MW meter.

The load jumped 6 MW, which represented a 50% load change, from 12 to 18 faster than the eye can blink, as indicated by the instantaneous needle deflection in the positive direction, much to my surprise, amazement, and frankly, disbelief.

The turbine generator (two turbines on one generator) slowed perceptably (the groan), the gas valve noise changed as the fuel responded and immediately after the squirrel was cooked to a crisp and most probably vaporized, the turbine tried to overspeed. Again, very audiable-the unit was only about 10-20 ft behind me with no obstructions-the panel was on the open turbine deck.

Except for the 'groan', and the 'sigh' I would have suspected meter malfunction. But the unit clearly had a load hit it and it responded. First time I ever heard it, but I have heard it plenty of times since. I got to hear the 'sigh' again when I managed to trip the unit at full load some months later.

ByrdJ, this was one of GE's original Frame 5 STAG units and the valves to the ST were either open or closed, the ST was a complete slave to the steam produced by the HRSG (that was before the term HRSG was coined.) Actually, this ST had two admission points because the HRSG was a two drum affair with a HP and a LP section. Neither was regulated. Stop valves were open or closed. The municipality that owned it is connected to the grid today, but this was over 35 years ago and back then they were paranoid about connecting to the "big boys." They believed that somehow if they did that the utility would take them over. Go figure.

David Beach, I don't understand your statement. Since the squirrel would have been a purely resistive load, how did that change the power factor? Isn't a purely resistive load conducive to maintaining unity PF?

rmw

 

[electricpete]

If that resistive squirrel has a much lower impedance than the source impedance of the generator/transofmrer, then bulk of the voltage drop occurs accross the source impedance which is primarily inductive and does not dissipate real power. In simulations of bolted faults (or very low resistance faults), the generator initially speeds up for this reason (power out decreases and power in takes awhile to respond to the transient)

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[rmw]

What was the load swing I saw on the MW meter?

And, there is no doubt that the generator sped up because I thought sure it was going to overspeed. I am pretty sure that this was the only unit running at the time and an overspeed trip would not have been useful. But what was the groan (I have tripped units and heard adjacent units give that groan as they picked up the load) that I heard and still remember vividly to this day?

I had already tripped that unit once when it was the only unit supplying the city and I had enjoyed all of that I could stand. I ended up tripping it twice, once at full load. It tripped the other unit right off too. The town went black, hospital and all. That was why I was sucking in my breath.

rmw

 

[npower0073]

if i understand correctly in a single shaft CCGT it is the gas turbine again that makes all the critical load control and the steam turbine is aonly a slave, as it happens also in a multi shft unit. Are there any specifications for single shaft load control in pdf?