cogen and other systems paralleling with utility

[buddy91082]

We are thinking of putting in a cogen plant to provide heating as well as electricity. The cogen can provide around 30% of our electrical needs and will paralleling with the utility.

My question is how does the utility and cogen know how much each needs to generate to supply our facility. For example, if the cogen run at full capacity supplying 30% of our load, how does the utility know they need to provide the other 70%?

This could apply to pv and wind systems paralleling to the utility.

Thanks.
c

 

[rasevskii]

The Cogen would (preferably) use a synchronous generator that runs in parallel to the utility supply. Frequency stability is provided by the utility system. The governor system of the Cogen would regulate MW output according to the available energy supply of the Cogen itself. The utility would make up the difference between this and the requirement of the facility (your consumers). This happens by itself, automatically, as the system runs in synchronism.

All powerplants run basically this way. It is more or less irrelevant what kind of generation is involved. Assumption: The Cogen is very small compared to the utility system, in other words your Cogen output would have no effect on the utility system frequency.

See the recent Thread on this forum by user "Ters" regarding connecting Wind Turbines onto a system, and the comments by other users.

What kind of Cogen is involved?

rasevskii

 

[buddy91082]

Thanks. It's 4 2.5MW synchronous units.

I guess i am not understanding how it "happens by itself". If the utility is much larger than the cogen, then why doesn't it overpower the cogen and supply all of the load?

 

[rasevskii]

Well, how can I explain this...Your generators each run synchronized (in Lock-Step) with the utility frequency. That is you had to synchronise each generator (by means of a synchroscope or other such equipment) to the system before closing the generator breaker. After this the speed (Hz ) cannot change. The output MW of each generator will be determined by its fuel input controlled by its governor system. In other words, YOU can take over the load otherwise supplied by the utility by increasing the power output of each generator. That is how AC power systems work. Many hundreds of generators working in parallel, all in Lockstep with each other. If one generator increases output, the others will back off, in principle.

Read up a bit on governors and things like droop... Check the Woodward web site, there is a lot of info there on this subject.

http://www.woodward.com

Also

http://www.basler.com

for info on AVRs.

I assume your plant is not yet installed. Be ready beforehand...

rasevskii

 

[waross]

What rasevskii said.
When a generator is working in "Island" mode or by itself, with a steady load, varying the power input will vary the speed and the frequency.
In island mode varying the excitation will vary the voltage.
When in parallel with the utility, things change as soon as the switch is closed. Now, varying the power input changes the power output of that generator. Increasing the power input will tend to increase the system frequency but the effect is in inverse proportion to the relative size of the plant compared to the system capacity and is very slight. In addition, the system controls back of slightly and there should be no net effect.
The excitation or voltage control is similar, in that the effect on the system is negligible and is corrected by the system settings, however, the excitation setting does control the reactive power of the generator. A normal excitation setting will be enough to supply the magnetizing current of the generator. Less excitation and the power factor will drop and you may incur power factor penalties. Over excitation will export reactive power in a similar manner to a capacitor bank.
Load control: The simplest control mode for a generator is droop mode. At 3% droop, the no load speed/frequency will be 61.8 Hz. (or 51.5 Hz.) As the load increases the speed/frequency drop to 60 Hz at full load. When a droop controlled generator is paralleled with the utility the load on the generator is controlled by the speed/frequency setting. At a setting of 60 Hz. the generator is not producing real power, but may be used as a synchronous condenser to export VARs. As the speed setting is increased the load increases until at a setting of 61.8 Hz (103%) the output is 100%. If (or more accurately WHEN) a breaker trips and the load is lost, the governor will limit the speed/frequency to 61.8 Hz to avoid overspeed damage. However if the governor is slow to respond, a load loss may result in an overspeed trip.
There are more sophisticated computer control systems available now but droop mode control worked fine for about 100 years or more and is possibly the easiest way to explain how a basic system may work.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Essentially you "try" to run the gen slightly above the grid frequency, but since the frequency is locked up by grid, the result will be the power being pushed back in to the grid, which consumes more fuel. This ramping signal to overspeed can be controlled by a kW feeback singnal, thus telling a generator to keep its output at a set limit. In fact that is how synchronous generators' ramping for soft loading, unloading and base loading is controlled.

For induction generators, they always run at their rated load once they go slightly above the sych speed, there is no real load control on it. They turn into motor at subsync speed.

As to

how does the utility know they need to provide the other 70%?

Once a cogen (or any other generating plant for that matter) is connected to a grid, the electrons do not necessarily know where to go, the cogen at that point simply becomes another generation plant on the grid and the "grid" is providing "all" the load to your facility. You get the "credit" for power fed by the cogen, by metering it. To a cogen the grid is just a big power sink, it is not choosing any load.





Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

To explain the latter half of my post, the "Net" metering is seeing the net result of (all facility power coming from the utility minus the power going back into it by the cogen). So the cogen is not deciding which portion of your facility is to be served.

Rafiq Bulsara

http://www.srengineersct.com

 

[itsmoked]

Before you think too much about doing this make sure you look into the air quality board,(bunch of despots), as they get involved in anything that has "exhaust" as your generators most certainly will.

Also 10MW could trigger utility upgrades that might be charged to your facility, so of course, you should talk with your utility early on.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[buddy91082]

Thanks for the replies.

So for an analogy:

The utility is a large wave coming to shore and the generator is a water hose showering water in the ocean. The water from the hose is absorbed by the wave. Or something like that.

 

[waross]

Almost. The utility is a large wave filling a bucket (your plant load) and your generator is a water hose showering water into the bucket. If, at times, the hose overfills the bucket, the overflow runs out into the wave. At other times the wave tops up the bucket.

Bill
--------------------
"Why not the best?"
Jimmy Carter