Isoch vs. Droop for generator operation 4

[GBell]

Would like a down and dirty explanation or references to basic explanation of modes
of generator operation. Particularly "blackstart" of gas turbine power plant in a
municipal power setting, the operation of 3 generators 20MW+20MW+40MW.
What do I expect to happen with no existing grid to sync to?

 

[peterb]

Isochronous = constant speed operation. This mode is used when a generator is operating on its own and is used to maintain the frequency at rated 60 or 50 Hz.
Speed Droop = generator governor operates to decrease speed with increasing load. This is the mode that is commonly used to operate generators in parallel, as it allows them to share load in proportion to rated load.
Blackstart = generator can be started with no external power (from lights out (black) condition). This will usually imply either a battery or air receiver - for your GTs, this will imply a diesel start, which is in turn started by battery.

With no existing grid, your units will probably need to be set up so that when a single unit is operating, it is in the isochronous mode; when other units are synchronized, the control can be switched to droop, or one unit can remain isochronous to maintain system frequency and the others be set for droop. Other options could include constant load (kW) control.
Note that your voltage regulators must also be set up to allow reactive load sharing between the units.
I suggest that you check the Woodward Governer website for more details:

http://www.woodward.com/ic/pubpage.cfm

 

[rhatcher]

peterb's post got a star from me. GBell's hypothetical case does bring some thoughts to mind though. If I understand this correctly the scenario is that the entire grid is black, meaning that the 4 municipal generators are down and the main grid to which the municipal grid is connected is down. I do not know how this type of scenario would play out am very curious. My guess as to how it would go is as follows:

Presumably, you would isolate from the main grid and blackstart the 4 municipal generators as described by perterb. The blackstart would be in stages, light off one generator, load it up and stabilize it, synch in the second, increase the load to stabilize the pair, etc...This would require segregating the municipal load to bring it on in stages with the generators. Eventually, everything would be on line and the municipal grid would operate as a self contained unit.

The part where I get stuck at is the is the relationship between the municipal grid and the rest of the world (main grid). At some point the main grid would come on line and it would be desirable to resume the tie to it. To do so would require synchronizing the municipal grid with the main grid.

How would this be accomplished? Or...have I missed something here?

 

[peterb]

rhatcher, I think that you have correctly identified the scenario.
The answer to re-synchronizing the muni supply to the grid is that you would adjust speed/voltage on the smaller system and synch at the point of separation, preferably using a synch check relay to supervise the operation. The only trick here is that you are adjusting the parameters of a group of generators - takes some coordination, but is not really that difficult to achieve. Of course, you need the appropriate synch instrumentation and control of the synchronizing breaker at the separation point, which may well be remote from the control location.

 

[GBell]

In this "blackstart" scenario the city would be isolated from any normal
161KV feed and would lockout the possilbilty of having the grid return
and be out of sync with the city's 69KV. The 161 to 69 stepdown X-formers
(supplies from the outside world) would be locked out.
I am trying to get a feel for what the operator will experience as load is
picked up around the city. What size increments can a 40 MW generator
pickup without dragging down the Hz? Will the operator have to manually
increase the power (fuel flow) to the gas turbine? What part will my governors
play in maintaining the plant?
As of now in our scenario, I have my plant running, self sustaining the plant load,
(we have actually tested that with one LM2500 gas turbine) and now begin to pick
up circuts around the city. What do you think?
Thanks
GBell

 

[rhatcher]

Thanks for the answer peterb. Another star for you...

For GBell, the question is one of the capabilities of turbine and turbine controls. I do not have a feel for the ability of the LM2500 to absorb step load increase. The only generators I have experienced with these as prime movers were shaft generators on navy ships. A shaft generator is one attached to the propellor shaft, so the ratio of prime mover power output to generator power output is a number >>>>1. As such, these puppies would pick up full load without a blink.

For you the situation is obviously much different. However, I would be surprised if you do not have info from the manufacturer on the performance of the unit as I have never seen a GE unit with less than an encycolpedia sized reference set of operating and maintenance instructions.

 

[peterb]

Your'e welcome, rhatcher.

To GBell -
Trying to address your latest questions (responses in bold)-

I am trying to get a feel for what the operator will experience as load is
picked up around the city. What size increments can a 40 MW generator
pickup without dragging down the Hz? As rhatcher suggests, consult your operator manual - there will be a load ramp curve in there somewhere. Typical numbers for step load change would be of the order of around 5-10%.

Will the operator have to manually
increase the power (fuel flow) to the gas turbine? What part will my governors
play in maintaining the plant? No need for manual increase. The governors will maintain speed or load proportion by increasing fuel flow automatically. The first unit on line will probably be operating isochronously to regulate freqquency, subsequent units will operate in droop mode.
As of now in our scenario, I have my plant running, self sustaining the plant load,
(we have actually tested that with one LM2500 gas turbine) and now begin to pick
up circuts around the city. What do you think? Sounds like a good start. What you then need to think about is starting a other units in sequence when the load gets high enough to sustain them on line sharing load. Resynchronizing to the grid is done when stable grid operation is restored -- you probably would then want to retain your units on line for a period to offload them in stages and cool them down somewhat before shutdown. Again, when all else fails, read the instruction manual.

 

[jbartos]

Suggestions:
1. Visit

http://www.energyonline.com/blueprints/bluprintsint4j.asp

http://www.wapa.gov/media/pdf/sep24p6.pdf

etc. for more info
2. With a little repetition, several possibilities exist depending on loading, power availability need, reliability, safety, etc.
One may start all generators simultaneously, unloaded, and gradually add/synchronize one loaded after one loaded together with approximately the same load to maintain the system as stable as possible. Drooping, isochronous mode, steady state incremental speed operation, etc. would depend on the preference; however, considering the generator MW disparity, the isochronous mode appears to be better. Generators do not have to be in sync with the grid if they form the autonomous power system; however, when the utility connection is to be performed, the synchronization will have to be performed.

 

[GBell]

Gentlemen-
Have contacted Woodward (gas turbine controls division bought by GE) and have
torn into my own operator's manuals. Everyone's input has been helpful.
Many thanks
GBell

 

[SooryaShrestha]

peterb, sorry being late. I was away for sometime.
I have a little doubt. Please clarify.
In the case of synchronising with grid when the utility power becomes available at 161 kV, the present case of GBell with three generators of two 20 Mw and one 40 Mw units will not be easy to synchronise. The small system of 80 Mw is also running independently. The frequency of 80 Mw system may be changed by the 40 Mw unit in order to bring it to grid frequency, though with difficulty or may be not possible.
In general what we do with different islands formed in the case of system trips with weak links should be applicable. This means each unit should be again synchronised when the grid power returns by segregating the loads. Have I been able to put my view?