Synchronous generator on an infinite bus

[ffp]

I belive you are drawing incorrect conclusions by mixing up many things at a time.

Yeah, I'll try to not mix things up. I too feel I'm going in circles, but mostly because I'm not focusing where matters.

I was re-reading the thread and I'll try to put what I think as simply as I can.

There are 4 scenarios:

A.1) generator alone with load gets turbine speed increased
A.2)generator alone with load gets field current increased

B.1) generator in grid gets turbine speed increased
B.2) generator in grid gets field current increased

I will make affirmations about the scenarios and would like to be corrected when wrong.

A.1) increasing turbine speed increases Ea (=KΦω), which increases Vt (=Ea-IajXs). Both terminal voltage (Vt) and active power (P) increases. Since the angle between Ia and Vt didn't change, reactive power (Q) remains unchanged.

A.2) Increasing If will increase Φ which increases Ea(=KΦω), which increases Vt(=Ea-IajXs). However, P (=τω) remains unchanged. For that to happen, the angle between Ea and Vt, ad well as Vt and Ia must change, so reactive power (Q) changes.

B.1) increasing ω (τω) governor means increasing P (=τω). Since the grid fixates ω and Vt, there is an increase in the opposing EM torque, which reduces ω, but makes τ increase, after a transitory. So the turbines speed is still synchronous speed, but it's torque is higher. Result is higher P and no change in Q.

B.2) increasing If increases Φ, which increases Ea(=KΦω). P(=τω) remains constant and Vt tries to increase. However Vt is fixated by the grid. So, there is a bigger stator current (Ia), and changes in angles between Ea-Vt and Ia-Vt: Ea-Vt reduces and Ia-Vt increases.This way, P remains constant, but Q increases (angle between Ia and Vt increases). What exactly causes this changes in angles, I'm not sure.There must be several wrong afirmatives there. If you could point them and correct then, or maybe do the same 4 scenarios from scratch, that'd be nice. I know that thinking in terms of all variables (Ea, If, τ,ω,Φ,P,etc) might not be the best way, but that's exactly what i need to understand.

 

[Tom.G]

Hi @ffp, I have been sometimes reading this thread, but not in detail. Also I am NOT at all an expert in this field!

Anyhow, here are a few observations:
a) there seemed to be a confusion about the definition of an Infinite Bus
b) there was maybe a confusion between an AC generator and a DC generator

I think in your assertion A2, that the reasoning and conclusion should be identical to assertion A1.

Assertions B1 and B2 are so far out of my knowledge that I can not make any statements about them.

If I have made an error in this post, I hope that the more knowledgable people here will set us both straight!

Cheers,
Tom

p.s. I am impressed by your persistence and amount of effort you have put in to understanding this subject. I sincerely hope that you can get a complete and correct understanding of the subject here.

 

[cnh1995]

A.1) increasing turbine speed increases Ea (=KΦω), which increases Vt (=Ea-IajXs). Both terminal voltage (Vt) and active power (P) increases. Since the angle between Ia and Vt didn't change, reactive power (Q) remains unchanged.

A.2) Increasing If will increase Φ which increases Ea(=KΦω), which increases Vt(=Ea-IajXs). However, P (=τω) remains unchanged. For that to happen, the angle between Ea and Vt, ad well as Vt and Ia must change, so reactive power (Q) changes.

In a stand alone generator, increasing field current or generator torque will change the terminal voltage and speed. Any changes in load impedance and load power factor will also affect terminal voltage and frequency. But I do not have much information on how the terminal voltage and frequency is regulated in a stand alone generator.

Result is higher P and no change in Q.

Higher P and lower Q. With an increase in the power angle, the reactive power is reduced. Look up the formula for Q. It depends on the cosine of the power angle.

B.2) increasing If increases Φ, which increases Ea(=KΦω). P(=τω) remains constant and Vt tries to increase. However Vt is fixated by the grid. So, there is a bigger stator current (Ia), and changes in angles between Ea-Vt and Ia-Vt: Ea-Vt reduces and Ia-Vt increases.This way, P remains constant, but Q increases (angle between Ia and Vt increases).

Yes.

What exactly causes this changes in angles, I'm not sure.

Read #16.