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Biomass boiler
Boiler Load control on AVR

I work at a straw fired power plant, we run on turbine following meaning the boiler takes priority over the MW's we produce.

I was thinking if we drop boiler load, steam flow etc but maintain pressure and temperature. why does our generated MW's also drop? Why does the AVR not continue to provide the same voltage? Obviously the turbine does not slow down at all.

Does the boiler control communicate with the AVR or is it something more complicated than that?

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The basic formula for electric power (3-phase) is:

P = Vt * Ia *3 * PF

where P = Watts (or kW, or MW)
Vt = Generator Terminal Voltage (Volts)
Is = Armature Current (Amperes)
3 = 1.732
PF = Power Factor (a value between 0 and 1.0)

You should have noticed that the generator terminal voltage stays relatively constant; most synchronous generators can only tolerate approximately plus-or-minus 5% change in terminal voltage. The value of the square root of 3 is a constant--it never changes. And the value of the Power Factor is related to the mix of Watts and VArs being produced by the generator. And the power plant makes the most money when the VAr production is closer to zero, which means the Power Factor is closer to 1.0. (Typically, the power factor of most synchronous generators when synchronized to a grid producing power is between 0.9 and 1.0--again, because the power plant makes the most money when its making more Watts than VArs.)

You should have also noticed that as the amount of Watts being produced by the turbine-generator increases the amount of armature (stator) current increases, and when the Watts being produced decreases the armature amps decrease. In the basic power formula voktage usually doesn't change by more than 5%, the value of the square root of 3 never changes, and the value of the Power Factor is usually pretty close to 1.0. So, the only real variable is armature current. Therefore to make more power one has to increase the armature current while holding generator terminal voltage constant and keeping the power factor close to 1.0 which means producing as few VArs as possible.

Electric motors are devices for converting amperes to torque. That's a given most people have no problem with. And, generators produce the Amperes used by motors to convert to torque to do work (pump water, or move air, or compress refrigerant to cool beverages and food and hot rooms or buildings). And wires are how the amperes get from the generators to motors. So far, so good. Right?

Well, here's where many people have an issue: Electric generators are devices for converting torque into amperes. And the devices coupled to the generators provide the torque that generators convert into amperes. As you already noted: The speed of the turbine and generator doesn't change when the generator is synchronized to a grid with other turbines and generators. And when the torque being produced by a turbine coupled to a generator increases, instead of the turbine and generator speeding up the generator converts the extra torque into Amperes. And if the torque is then reduced, instead of slowing down the amount of Amperes decreases.

And to make a turbine produce more or less torque one changes the amount of steam (in your case) flowing into the steam turbine. That can be done by changing the steam turbine control valve opening, or by changing the firing rate of the boiler, or by some combination of the two. Hotter steam will also help make more power as it flows through the steam turbine, as will higher pressure sream. At a biomass plant the steam turbine control valve is usually open just about as much as possible so that as much steam as possible can flow into the steam turbine. And the firing rate of the boiler controls how hot the steam is as well as the pressure.

Now, if one holds the boiler firing rate fairly constant and changes the position of the steam turbine control valve one can affect the pressure of the steam flowing into the steam turbine (and being produced by the steam turbine). Isn't this fun?

The last thing we need to do to tie all this together is to talk about the AVR and what affect it has on the generator. Changing the AVR setting changes the generator terminal voltage (ultimately) and we already said that most of the time a synchronous generator runs at a fairly constant generator terminal voltage. So, to keep the generator terminal voltage constant the AVR output needs to remain fairly constant. And another effect of changing AVR output is to change the VArs being produced by the generator--and we also said the power plant makes more money when it makes less VArs and more Watts. So, changing the AVR output is not the proper way to change the power (Watts) being produced.

To make more Watts one needs to get hotter and/or higher pressure steam into the steam turbine, or to increase the flow-rate of steam into the steam turbine when it is at the proper temperature and pressure. So increase the firing rate of the boiler and/or open the steam turbine control valve further (but not too much!)--those are the proper methods of increasing Watts. Not changing the output of the AVR.

Hope this helps. This is basic AC (Alternating Current) power generation fundamentals. You can find no shortage of information on the World Wide Web using your preferred browser and search engine. There are even videos on YouTube.

But, and you won't hear this said or find it written very often, but generators are devices for converting torque to Amperes which are transmitted by wires to motors (primarily) where they are converted back into torque to do useful work. (The highest consumer of electric power for decades was to pump water--clean or not-so-clean. Today its probably to power air conditioners.) And no matter what electricity is doing at the end of a wire it's doing some work someone thinks is useful (is a video game useful, or a cable news show about politics useful--they both are, to someone or to many people). And it's all made possible by converting torque into amperes--which is what generators do. The prime movers (steam turbines; gas turbines; wind turbines; reciprocating engines--they're all forms of prime movers which can produce torque to drive a generator) provide the torque and the generators convert that torque to Amperes which then gets converted back into torque (real or virtual).

And that's a fact.