Droop/ Isoc Control 1

[blacklabs]

I understand all of the components of these control modes, but I can't seem to pull it all together in my mind.
Suppose I am running a gas turbine facility with 5 GT's connected to a large grid. Also suppose an ncrease in load which causes a slight decrease in frequency. How will my 5 GT's, all in 4% droop, respond? How will the machine(s) in isoc respond? Will my machines each assume a share of this load, droop in frequency and slowly work to bring fequency back to the desired 60 hertz?
Thanks for any help. Jim

 

[davidbeach]

What production mode are the generators in? Are you trying to load follow with the turbines, or are you running at a fixed output? The isochronous/droop combination seems more like you are running without any outside connection.

 

[blacklabs]

We are running at a fixed output. We are dispatched hourly and average about 500 MW output.

 

[davidbeach]

In base load (fixed output) with PF or Var control, change in frequency of the system should not have an appreciable impact on the generators. They should not be in either isochronous or droop control, but rather they will run at the same speed/frequency as the surrounding utility. Any speed deviation will result in pole slipping and a slipped pole can mess up your entire day. The governor of each machine will be targeting a specific power input to produce the desired power output, and the excitation system will control the field current (by controlling the field voltage) to control the Var production to maintain the desired PF or Var output.

 

[dpc]

You can run machines in droop while on the grid. You don't want to be in isochronous mode if on the grid unless you're big. Your governor will being going balls to the wall trying to force the frequency to where it thinks it should be, but it won't be able to change it.

Normally, most smaller machines on the grid will be on droop with the frequency control assigned to specific larger machines. But while tied to the grid on droop, your machine frequency doesn't really change as you change load. The angle between the system and the generator changes.

 

[blacklabs]

Thanks dpc. I understand why we wouldn't be in isoc, it doesn't seem like droop mode even matters in our case. We set the output on each machine, put in var control and pump out MW's.

 

[dpc]

Yes, as long as you are tied to a stiff grid, you won't see much "droop", but technically, you're still in droop mode.

Check out the Woodward Governor or Basler websites for technical papers on generator operation.

 

[powerjunx]

blacklabs,

dpc suggested woodward...i guess this may help you, to expand your query. try this; MSLC/DSLC(Master synchronizer and load control/Digital synchronizer and load control) from woodward.

 

[waross]

When on the grid the output will be dependant on the fuel input. The grid will control the frequency and the throttle will control the output.
However, when the machine trips off the grid the governor will control the frequency. The amount of droop will determine the frequency rise of the set when the breaker trips.
Also, some plants run in droop mode and adjust the load by setting the frequency slightly above grid frequency. The amout of droop determines the sensitivity of the frequency setting in this mode.
Eg; For 60 Hz. and 1% droop set the frequency to 60.3015 Hz. for 50% load.
For 60 Hz. and 3% droop set the frequency to 60.914 Hz. for 50% load.
respectfully

 

[GTstartup]

4% droop means that for a 4% increase/decrease in frequency the GT's will increase/decrease 100% in load. Or any proportional equivalent. e.g 1%Hz=25%MW. All within the GT allowable parameters. 4% droop is the standard setting in the US. Droop setting has no impact on what happens when the generator breaker opens as it is disabled as soon as that happens and normally below a certain load. Also there is normally a deadband to prevent fluctuating load with small changes in frequency.

Hope this helps to explain the droop part of your question.

 

[waross]

That was quite interesting. I am wondering where in the world the load is controlled by a variable grid frequency???
yours

 

[zeusfaber]

Used to be everywhere I think (dragging up twenty year old memories of what was old history then).

4% droop is what you get from the standard pattern Watt governer (the one that gave us that wonderful expression "running balls out").

Provided the load doesn't change faster than you can schedule extra plant to keep the frequency within the statutory bands, this works reasonably well.

A.

 

[ScottyUK]

Absolutely true: the large generating sites of yesteryear each had two clocks: a chronometer maintaining Greenwich time (or whatever local time was), and the station clock driven by a synchronous motor from the mains. The station clock fell behind during the day when loads were heavy and the frequency dropped a little, and caught up overnight when load was lighter and the frequency was slightly high.

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I don't suffer from insanity. I enjoy it...

 

[waross]

Thanks zeusfaber and ScottyUK
Yes, we still used this system until recently with the small plant on the island. When a breaker tripped, the no load speed increase was dependant on the droop. We used to run a lot less than 4% but everytime someone decided that a governor needed a rebuild and sent it out for repairs it would come back so far out of calibration that parallel operation became more and more difficult. We now have one large set with an isochronous governor. We never did have the second clock. The locals knew not to use an old synchronous type clock in the house.
However we are talking about the action of present day machines connected to what I have assumed was a stable grid.
I asumed that we were discussing a situation where the system was controlling the frequency.
respectfully

 

[GTstartup]

Warcross.

Don't know if you were referring to my post but the answer to your question is The United States of America. As I say, there is a deadband so that this feature only kicks in when the frequency is outside of specified limits (normally 1%). And yes, this happens once in a while.

 

[GTstartup]

As a little bit of back up info here is a exerpt from the performance standard for governers on gen sets from the Electrical Generator Systems Association. (US)

3.7 Speed Regulation (Droop).7 Speed Regulation (Droop). Speed regulation is the speed change with increase in power output of the prime mover. It is expressed as the percentage change in speed corresponding to the change from zero to full power output.

 

[waross]

Hello GTstartup
Thanks for the reply. The diesel sets I install have 3% droop. They are manufactured in IEC land and the droop would be close to 3.7% at 50 Hz.
On the diesel sets efforts are made to eliminate deadband. I had one set with less than 1000 hrs. that was running perfectly until the pump was rebuilt by the vendor. The set would pick up the load, but when the load reduced, the fuel setting would stick and the set would eventually shut down on overspeed.
I have yet to see a governor on a diesel with intentional deadband or with provision to bypass the droop control if the load is lost. I understand that the electronic governors on the diesel sets have a slight lag so that the governor doesn't try to respond to the speed surges caused by each cylinder firing. This can also be achieved with a very small amount of deadband. In theory there is a lot of difference between lag and deadband, but in practice, on a diesel generator, the effect would be about the same.
Can you tell me what is the control mode when the droop is disabled? Also, how do you bring a unit up to speed and synchronise it if the droop is disabled with the breaker open? When I least expect it, I may be called upon to look at my first Gas turbine, and any information I can get will be valuable to me.
Thanks again.

 

[davidbeach]

When running alone, the governor should be in isochronous mode so that it maintains set frequency for all loads. When coming up, it would be in isochronous mode prior to the breaker closing. If the generator is connected to a dead load, not parallel with any other source, it should remain in isochronous mode. The mode selection gets a bit more complicated when the generator is to be paralleled with another source.

If that other source is a utility grid, the generator will be run to supply a fixed or variable watt output. Fixed is the simplest, but variable may be necessary to limit power back out onto the grid or to maintain a minimum import level.

If that other source is one or more local generators with no grid connection, the group need to share the load reasonably. One way of doing that is to place all units, or all but one unit, in droop. There will be better frequency control if one unit is in isochronous mode with the others in droop, but the load sharing will not be as equable as it would be if all were in droop. All in droop will provide worse frequency control than having one in isochronous but better load sharing. The other way of load sharing is a single controller overseeing all of the generators, acting as a single isochronous controller for the whole group, actively controlling the governors of all machines in the group.

 

[waross]

Hello davidbeach
Thank you.
That was my understanding, but I appreciate having my information confirmed by you.
Respectfully