Reactive power Theory 5

[Sofistioelevib]

Hy Guys,
this is very simple post but it is not so clear with my knowledge.

When i want to change reactive power in a Generator i have to control the excitation current and this is a fact...


Looking at formulas and power triangle i can assume the S=V*I, P=S*cosfi, Q=S*senfi

What exactly happen during change in excitation? Because if excitation currant rise, the output voltage rise so S rise and even P and Q rise itself depending on change ov voltage output.

So how can be possible to change ONLY and ONLY power Q with no changing to the rest?

Thamk you..

 

[waross]

What is happening is that the current is getting out of step with the voltage.
The calculations are a little complicated.
VARs and Power Factor are names for partial solutions to the calculations based on the trigonometry of the phase angle difference between the voltage and the current.
VARs and Power Factor are imaginary concepts that nevertheless do an accurate job of describing the effect of phase angle differences between the voltage and the current.
Add or subtract VARs:- An imaginary description of a shift in phase angle.
Power Factor:- The cosine of the angle of phase displacement.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I have always found W/VA to be a more useful definition of power factor.

 

[HamburgerHelper]

Yeah, and you can tell off the top of your head how much you can reduce current and reduce your losses.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[waross]

That's my point. A lot simpler than calculating the reduction of current or the change in phase angle resulting from the connection of x micro-Farads of capacitance.
The current is what it is.
VARs is the result of current at a 90 degree phase angle to the voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

As an operator and not an engineer, I eventually tumbled to the fact that I had gotten to a place of understanding imaginary numbers bass-ackwards, as the saying goes, sort of by reverse engineering them [ha!] from what I was actually seeing happen.

For me and many of my cohort, when it came to learning how AC power systems operate, the explanations used for us rubes were as close to non-mathematical as you can get; I suppose the irony of it is that the handles that my generation of operators used to grasp this particular topic were essentially empirical / pragmatic / functional, with some of us sooner or later, to a greater or lesser degree, forming the mental constructs / conceptualizations that higher mathematics, coupled with physics, uses to describe reactive power system theory.

Things have of course changed considerably since that time, and for several years now the greater majority, but not all, of the up and coming entry level power system controller trainees we hire have been through college, emerging with Electrical Engineering Technician or Technologist diplomas. A few of the "not all" recent controller trainee hires either already are, or are varying degrees of partway through becoming, a journeyman [sorry, journeyperson] electrician, and who plan to attain full status as an electrician in parallel with being a controller-in-training.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Sofistioelevib]

Thank you guys thank to all