cogen and other systems paralleling with utility
cogen and other systems paralleling with utility
(OP)
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
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





RE: cogen and other systems paralleling with utility
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
RE: cogen and other systems paralleling with utility
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?
RE: cogen and other systems paralleling with utility
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.
www.woodward.com Also www.basler.com for info on AVRs.
I assume your plant is not yet installed. Be ready beforehand...
rasevskii
RE: cogen and other systems paralleling with utility
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
RE: cogen and other systems paralleling with utility
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 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
RE: cogen and other systems paralleling with utility
Rafiq Bulsara
http://www.srengineersct.com
RE: cogen and other systems paralleling with utility
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
RE: cogen and other systems paralleling with utility
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.
RE: cogen and other systems paralleling with utility
Bill
--------------------
"Why not the best?"
Jimmy Carter