WATTS the DEAL ? 2

[marada93]

I've got a question about watt production from an AC generator. I'm new here...so bear with me.

I understand allright about if I change excitation current I get a corresponding change in output voltage (and reactive power).

I understand that an increase or decrease of the prime mover will cause a corresponding change in watt output of the generator...I just don't understand why.

Can someone explain this to me in a a way a control room operator might be able to understand? Are there and good web sites that explain this?

Thanks

 

[busbar]

May want to review:
Miller & Malinowski, Power System Operation, Mc Graw-Hill 1994
Homer M. Rustebakke, Electric Utility Systems & Practices, Wiley 1983

http://www.fetchbook.info/search_00704

9779/tab_reviews.html

http://www.fetchbook.info/Electric_Utility_Systems_Practices.html

 

[jghrist]

Watts are a measure of real power. Power produced over a period of time is energy. Energy cannot be produced, only converted from one form to another (with the exception of nuclear energy where it is produced from mass), so to produce energy, you have to increase the power input from the prime mover, thus converting chemical energy (in the case of burning fuel) or potential energy (in the case of hydro) to electrical energy.

You can increase voltage or reactive power without increasing the prime mover power (except to make up for increased real losses).

 

[alehman]

By increasing the throttle, you increase the torque output from the prime mover (while maintaining constant speed). Power = torque x angular velocity. As you increase the torque on the generator shaft, you increase the electrical power output.

P = Vgen x Vload x sin(delta) / X
where
Vgen = Voltage of generator
Vload = Voltage of load
delta = phase angle displacement between the generator and the laod
X = reactance of the circuit between the generator and the load

As you increase torque, sin(gen-load) increases, increasing P.

 

[alehman]

My previous post assumes the generator is connected to a utility system. If it is serving only constant local loads then increasing prime mover power will increase frequency and voltage as well.

An analogy for the interconnected case - you can think of the power lines connecting your generator to the utility as a springy mechanical shaft. As you increase the torque, the input end of the shaft twists slighly ahead of the load end. This flexure is equivalent to the phase displacement which occurs on a power line.

 

[rodar]

Hi
To understand it helps to consider the load as unchangeable
by the generator. In other words the grid voltage and
frequency cannot be alterted. This is true because of the size of the power grid. As the mechanical power is increased the generator rotor phase angle relative to the
phase angle of the grid changes resulting in the generator
being harder to turn. It will maintain exactly the same speed with more torque input just the relative phase angle
will change for the rotor and more voltage is generated
causing more in phase current to leave the generator. In
other words more power out for more power in.
rodar

 

[marada93]

Ahhhhh...

Ok, thanks for all the help on this. I think I understand that if connected to a grid, the grid frequency pretty much determines the generator frequency. So an increase in the prime mover (inlet steam to the turbine) would cause the MW output of the machine to increase, but only if the voltage changed as well. This to increase the torque on the generator in order to keep it from slipping a phase.


In other words, if I made no change to machine voltage (say the voltage reg was in manual) and increase the prime mover, then load wouldn't increase. But voltage would begin to drop off. Am I understanding you guys correctly?

I sure hope so, because this is starting to make sense to me.

 

[marada93]

Just another follow up to see if I've got this.

Say I was in manual on the voltage regulator and began to increase the inlet steam to the turbine. Would it be safe to say that at some point the energy input to the turbine would eventually(and gradually) overpower the grids ability to maintain the turbine at synchronous speed? Or the phase displacement would finally be so severe you'd relay the machine off line. In other words, attempting to slip a phase...

 

[alehman]

Not quite. If you are connected to the grid, adjusting the excitation voltage changes the power factor at which you are operating. Higher excitation means more amps and more reactive power (VARS) is exported. You must stay within the ampere or power factor rating of the generator.

Maintaining constant voltage and increasing torque (prime mover throttle) increases real power (watts) output. The voltage doesn't necessarily change. If the prime mover is powerful enough or your interconnect circuit weak enough, you can overpower your generator or interconnect and the generator may slip. Slip can also result in sudden changes to the system operating condition.

 

[jghrist]

If you increase the prime mover power, the phase angle of your generator will increase relative to the other generators on the system. The phase angle governs the real power flow. The voltage governs reactive power flow.

If you increase prime mover power, and if the loads are fixed, other generators must decrease power output or frequency will increase.

 

[marada93]

I'm very much impressed with the quality of the responses to this post. I feel like I'm starting to get a grip on this as I read through your responses. I'm not quite there yet, but somethings flickering.

Voltage and reactive power seems easy to visualize and understand. Increase/decrease excitation current and you increase/decrease voltage and reactive power output.

I think what i hear each of you saying is that if we increase the prime mover, the generator rotor itself maintains the same speed...but advances slightly. Or tries to speed up somewhat with respect to the grid. In other words the volt/amp phase relationship is slightly ahead of the grid. Am i close on that?

It seems to me when i was youger i was told that the additional torque on the generator cause more magnetic lines of flux to cut through more surface area of the generator windings, thus increasing MW load. Maybe thats whats throwing me off.

Don't give up on me...I think I'm getting there. Keep in mind I'm only a control room operator trying understand what I'm doing.

Thanks

 

[alehman]

Your 3rd paragraph is almost correct. The generator voltage will lead the grid slighly. The current will lead or lag the voltage depending on the level of excitation, thereby controlling the power factor.

I'm not sure about the 4th. I guess you could visualize it that way, but I'm not sure I understand the point of confusion with this.