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Balancing Output to Demand in an Electrical Power Plant

11/14/2010 8:48 AM

By what method do the operators of a power plant keep the output matching the demand on the grid? I understand the concept of having spinning reserve and bringing on-line, power generating capacity based on most efficient resources first, but how is the output fine tuned to maintain a product within an acceptable margin of error in voltage? If a turbine/generator unit is producing power, does it do so within a narrow RPM range, or can it spin faster and the output be regulated on the electrical output side? I did some research but didn't find an answer.

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Guru
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#1

Re: Balancing output to demand in an electrical power plant

11/14/2010 9:00 AM

The turbine RPM's stay constant.

The steam turbine plants that I frequent (as a contractor) run at 1800 RPM's +/- 4 RPM's.

As demand increases/decreases as detected by the PLC (program logic control) system, steam is throttled up/down to maintain 1800 RPM's.

I was told by an operator that increasing or decreasing RPM will alter the (required) 60 Hz wave.

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#2
In reply to #1

Re: Balancing output to demand in an electrical power plant

11/14/2010 9:19 AM

Thanks for the quick answer, it sounds like the load acts as a "brake" to keep the RPM's relatively constant?

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#3
In reply to #2

Re: Balancing output to demand in an electrical power plant

11/14/2010 5:32 PM

Yes, you've got it, sort of.

The increased electrical load puts a mechanical load on the generator/turbine, in turn, the PLC calls for more steam to the turbine which drives the generator.

The opposite applies to reduced electrical load.

Regards - KJK

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#5
In reply to #3

Re: Balancing output to demand in an electrical power plant

11/15/2010 9:02 AM

Power generation is done using synchronous generators. The name Sync is because of this machine to operate with a constant speed synchronized with the frequency of the alternating voltage applied to the terminals of the same.

So the load does not control the speed of the motor.

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#7
In reply to #5

Re: Balancing output to demand in an electrical power plant

11/15/2010 9:32 AM

Greetings Poison,

Not only do I agree with your first two sentences completely but that is what I said in a very general way in post #1.

In post #3, I provided further general explanation of the same.

Your third sentence "So the load does not control the speed of the motor" indicates some confusion on your part as there was no discussion regarding "the speed of the motor".

The topic of this thread is "Balancing Output to Demand in an Electrical Power Plant"

Perhaps going back and re-reading this thread from the beginning will enable you to attempt at a grasp of better understanding of the topic at hand.

Regards - KJK

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#8
In reply to #7

Re: Balancing output to demand in an electrical power plant

11/15/2010 10:53 AM

Sorry KJK/USA, it appeared that I was responding to your comment. I was referring to this statement...

"it sounds like the load acts as a "brake" to keep the RPM's relatively constant"

My apologies for the confusion.

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#4

Re: Balancing Output to Demand in an Electrical Power Plant

11/14/2010 5:41 PM

The grid is monitored and controlled by a separate body referred to as system control, and is in contact with all power stations, it controls the system speed, voltage and the addition and removal of generating plant.

Regards JD.

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#6

Re: Balancing Output to Demand in an Electrical Power Plant

11/15/2010 9:23 AM

The power stations on a grid are divided into several categories. Plants designated as "base loaded" produce all the power they can because they are the cheapest producers. They try to run at 100% all of the time, assuming that there is enough connected load on the grid to take all of the output from the base loaded. Obviously, it there is not enough load, at least one of the base loaded units must cut back. Nuclear power plants, hydoro-electric power plants and coal fired power plants are typical base loaded units. When natural gas is cheap, then combined cycle plants are used as base loaded units as well.

There is an entity for every grid calle the "Load Dispatcher". This organization make decisions concerning who is going to make the power. In a competitive environment, power producers bid, and the dispatcher takes the lowest bid for the next increment of power needed. Otherwise, the Dispatcher simply orders the lowest production cost unit to come on line or increase power.

One or more units on a grid are designated for load following (or load control or frequency control) duty. They run up and down in power as required to maintain the power generated equal to the power consumed. An increase in load on the grid will decrease the frequency of the grid as all machines will tend to slow down. The load control units will respond by opening their throttles to produce more power and bring the grid back up to 60 Hz (or 50 Hz).

Another important quantity is the grid voltage. The grid voltage depends on the level of field excitiation of the generators. There is reactive power in addition to real power in an AC distribution system. Both the reative power and the real power need to be controlled.

Within normal limits, the real power produced is controlled by the throttle and the reactive power "produced" by a generator is controlled by the field excitation.

The reactive load is shared between generators by adjusting the field excitiations, just as the real power is shared by adjusting the throttles. If the field excitation of a generator is increased, it increases the voltage of the grid and also causes that generator to assume more of the reactive load (and the other generators less).

Naturally, if there is only one generator on a grid (isolated grid), then the real load and reactive load that the generator sees is completely determined by the connected load. This one generator, obviously, must make all throttle and voltage regulation changes necessary to maintain the frequency and voltage of the isolated grid.

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#9

Re: Balancing Output to Demand in an Electrical Power Plant

11/15/2010 11:43 AM

Did all that get you there Packy?

There are 2 basic controls

the power input to the generator to regulate the rpm [hz]

the excitation of the field regulate the voltage

It sounds so easy...:D

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#10
In reply to #9

Re: Balancing Output to Demand in an Electrical Power Plant

11/15/2010 8:50 PM

Hey Edge Bro, Yeah, found all the above answers helpful. Need to do some further study on electrical generation basics. My skillsets are more along the lines of the guy who builds the box the generator comes in. Packy

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