Physics of ... Tap-Changer

A number of forum threads discuss how the step-up transformer connecting an Alternator to the Grid is able to control voltage-alignment using a Tap-Changer. Also discussed is the question, "What happens when tap-position is altered?" Thus far, in my opinion, there has not been a viable conclusion.

Therefore it is the intent of this multi-part thread to explain how tap-change impacts reactive-current, hence kVAr flow. I will use the concept of a tap-changer as a Turns-Ratio-Controller.

Caveat:
This presentation/participation does requires a rudimentary knowledge of Vector Algebra. A Vector is a quantity having both magnitude and direction, such as Voltage, Current, and Impedance. Let’s start with the very simple network consisting of just three components:

o Gen’r connected to a bus having a Voltage magnitude, Vs, and direction θsº.
o Load connected to a bus having a Voltage magnitude, Vr, and direction θrº.
o Cable whose reactance, Xc, connects the two busses.

Concept of Load-Flow (or Power-Transfer)
Personally, I prefer the former, because 'Load', to me, denotes having two vector-components:

o Real, having as its magnitude, W or Ps, and direction, 0º.
o Reactive, having as its magnitude, VAr or Qs, and direction, 90º.

An Aside
One vector, being at 0º, and the other 90º means, mathematically, they are in ”quadrature”!

Basic Equations
There are just two, but quite crucial, equations representing Ps and Qs. They are,<pre>
o Ps = The Real-component, W, is transferred from the Gen-Bus,
Vs&#8736;&#952;sº, to the Load-Bus, Vr&#8736;&#952;rº,
= (Vs*Vr/Xc)*Sin(&#952;rº-&#952;sº)
o Qs = The Reactive-component, VAr, is transferred from Gen-Bus,
Vs&#8736;&#952;sº, to the Load-Bus, Vr&#8736;&#952;rº,
= (Vs/Xc)*[Vs–Vr*Cos(&#952;rº-&#952;sº)]</pre>
Before continuing with Part 2, please calculate Ps and Qs for the following three Exercises:<pre>
.... | Vs | |&#8736;&#952;sº| | Vr | |&#8736;&#952;rº| | X |
Ex1 | |1.00| |0.00| |1.00| |0.00| |0.10|
Ex2 | |1.05| |0.00| |1.00| |1.00| |0.10|
Ex3 | |1.05| |0.00| |1.00| |22.0| |0.10|</pre>

I think you ll will be pleasantly surprised at the results. And, it be a major step for understanding Part 2.

Please post your answers to Control.com or send them to me at: [email protected].

Ps: If you see the symbol '&#9633;’ substitute the angle 'theta'
 
>Hmmm... no 'takers'!

Using my trusty HP15C LE, I came up with: <pre>
.... | Vs | |&#8736;&#952;sº| | Vr | |&#8736;&#952;rº| | X |
Ex1 | |1.00| |0.00| |1.00| |0.00| |0.10| Ps = 0.00, Qs = 10.00
Ex2 | |1.05| |0.00| |1.00| |0.00| |0.10| Ps = 0.00, Qs = 11.03
Ex3 | |1.05| |0.00| |1.00| |22.0| |0.10| Ps = 3.93, Qs = 10.22
</pre>Close?
 
There should be a minimum of seven (7) replies before we're treated to Part 2.

And, there's been a new term introduced to the discussion that hasn't been defined: voltage-alignment. Perhaps a simple explanation of this term may help move some people off the dime, so to speak.
 
CSA,

I could have said adjust, maintain or regulate. More importantly, you are correct... the math must be completed before Part 2 is made known.

I know you will be surprised about what it reveals.

Regards,
Phil
 
CSA... No! Seven replies are not required! Thus far, there have been two; one On, the other Off-Forum

Frankly, I am very suprised about the gloomy response to a basic tenet of power generation.

Regards,
Phil
 
Phil Corso,

I'm certain there is interest. Perhaps it's just the way the information is being offered?

You seem like a very knowledgeable person, but the way you dole out your knowledge can be a little off-putting.

What's the matter with trying to remember how you first learned the topic and what made things click in your mind so that the maths and vectors and all would verify your understanding and could be used to predict? I know; some people did it bass-ackwards--they learned the maths and vectors first. But most of them have long forgotten all of that, and can barely explain watts and VArs. And I've never met anyone other than an electrical engineer who could "explain" how to use a tap-changer so that anyone (myself included) could understand it or even use the explanation to go do more research to get a better understanding.

The best explanation of VArs I ever received was from a retired power plant operator who had been through an apprenticeship that was specifically designed to impart basic physical principles and operational procedures. Most of us didn't get that class or training, and he told me he spent decades sitting in the Control Rooms of various plants trying to explain VArs to others so they could understand it--and he was <b>VERY GOOD</b> at it--without a single vector or cosine or formula. Though he could do the basic power triangle and power factor explanation, also--using maths and basic geometry--but none of his explanations started with them.

We're not all about designing and predicting--though some of that is good knowledge to have, too. We're mostly about understanding basic principles in layman's terms before we jump into the maths and vectors. None of this stuff is as complicated as the maths and vectors can make it seem. And, again--they're basically useful for designing and predicting and in some cases (granted, important cases) for understanding faults and problems.

But, for most of us here--and I think I can safely speak for a lot of the readers and posters and respondents--we just want to get a good, basic working grasp of how the equipment works and so that we can operate it properly and make it perform as it should. We don't need to know how to design and build a bicycle or a car to drive/ride one. But we do need to know the basics, and in time, the more we learn the better driver we can become, and the better we can tell when it's not working right or needs repair--and possibly avoid catastrophic damage, or even injury or death.

How about the basics--in concise terms, not dribs and drabs punctuated with exclamation points and questions and questions and questions. It seems that people respond really well to providing good solid information, even some background and history, and when the original posters don't provide sufficient information for us to provide a short concise answer how about a list of the possibilities and let them--as they are wont to do--say, "Yeah! That's what I'm talking about! I want to know more about [this or that]! That sounds like my problem! How do I solve it?" Or, "Yeah; I really need to know more about how [this or that] works--or is supposed to work; can you help me?"

But these terse replies with questions, always questions; they just don't seem to work very well.

We aren't in high school or college or university here. You may have been a professor previously (it certainly seems so--you remind me of a particularly curmudgeonly one I had), and I'm sure some of your students were well-educated and have gone on to do very good, even great, things. We're just simple power plant operators who didn't get very good training--if any--in charge of keeping the lights on and not blowing anything (or anyone) up. We don't need tests or quizzes--we need information, and if someone wants the maths and vectors then they'll ask for it and you can take the discussion off-line.

Help us help others, without the questions and quizzes and tests.

Thanks,
CSA
 
Tap Changers...?

Are we talking about the method of selecting different winding impedance within a transformer to satisfy this statement: "maximum power is transferred within a coupled AC circuit, when the reactive and inductive impedance are closely matched"

> Help us help others, without the questions and quizzes and tests.
 
TBONE,

Aye, mate! That be the same thing.

Can you add to the discussion of operation and basic principles?

We need some assistance.

Thanks!
 
CSA... the equations were in the original post. Solution did not require complex math.

Following are the conclusions one would have uncovered if they had been solved... using just High-SchoolmMath.


1) Power is transmitted if the two "Bus-Angles" differ.

2) VAr is transmitted if the two "Bus-Voltage" magnitudes differ.

3) Power and VAr are transmtted simultaneously, if both Magnitude and Angle, differ.

Wishing all Control.Com Staff, Members, and Posters, a very Happy, Prosperous, and more importantly, Healthy, New Year!

Phil
 
RootBoy... Just caught a major error that I made in the "Qs equation":

ORIGINAL
Qs = (Vs/Xc)*[VsVr*Cos(&#952;rº-&#952;sº)]


CORRECTED
Qs = (Vs/Xc)*[Vs - Vr*Cos(&#952;rº-&#952;sº)]

Please accept my apologies for the error.

Sincerely,
Phil Corso
 
>RootBoy... Just caught a major error that I made in the "Qs equation":

Ah, no problem. I'll try it again later (I'm at work at the moment, and I shouldn't be playing with math, let alone hanging out at Control.com. :) )
 
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