ECN Forum Forums General Topics of the Trade General Discussion Area transfomer taps

"That is correct, taps are for voltage adjustment, NOT phase angle or power factor correction."

This wouldn't necessarily be true in cases of a co-gen facility.

[This message has been edited by rat4spd (edited 09-18-2005).]

"This wouldn't necessarily be true in cases of a co-gen facility."

You've got my curiousity up. How do the taps apply to phase shifting in co-gen applications?

Well, first let me say, that I 100% concur that the primary use of the tap changer is to correct voltage.

That being said, in the case of our facility, we have a small power plant with a 7.5 MW generator, tied in with the city utility.

So no matter what our generator is making, relative to what we are buying, the VARS demand is the same of course as long as load is constant.

Now, the utility, as miserly as they are don't want to supply any more VARS than they have to. So our load tap changer (LTC) is controlled by a VARS controller. The VARS controller does NOT tap relative to voltage. What it does though, is tap to maintain the utility supplied VARS at less than 1 MVAR. And because of this small voltage shift, if the LTC drops city supplied voltage, in Isochronous mode on our generator (if I remember correctly) the generator will then assume VARS from the city. On the triangle, if you increase VARS, you increase VA, and therefore lower PF, for our side.

So one might say it is a form of PF control for the city, in this instance because it's sole purpose is to lower VARS for the utility, which increases PF, albeit a very small amount because of the total load they have. But it does noticably affect our PF.

An example of the voltage drift occurs at night when demand decreases. The LTC being in a happy steady state mode with <1 MVAR on it will see VARS decrease as evening progresses due to load shutdown, which of course causes voltage to drift from say 7400V up to 7500V (on a 12.5KV system, our meters must read phase to ground?). System voltage will stay there until one of several things happen.

1. Daytime load requires enough VARS that the LTC must correct, thus shifting voltage back down, but not necessarily to the normal 7400.

2. An operator intervenes to adjust voltage manually using the LTC, but we never do this.

3. An operator "drives" votage down using the voltage regulator on the turbine. Note that this does not directly affect voltage because the city determines that. However, the lower of field current in an isochronous generator will lower VARS on the generator, which will migrate to the city, which may or may not cause the LTC to move. Sometimes you need to send a good amount of VARS to the city to do that.

In the end, the LTC will tap sending VARS back to our generator at a new steady state voltage.

Please keep in mind that this is based on what I believe is the correct application of power factor. Of course, as one of my co-workers likes to point out, I am wrong from time to time, so if I am wrong somewhere in here, I'd like to correct my understanding.

[This message has been edited by rat4spd (edited 09-18-2005).]

Rat

What is your method of generation? Are you baseline? Peaker?

HP

"Are you baseline? Peaker?"

Actually, we're neither. We are a small university power plant that "co-generates" alonside the local utility. We can't make all of our demand most of the time, so we buy some amount continuously. How much we generate is a complicated affair based on how much steam is demanded from the university campus. Our turbine is an extraction/condensing turbine, exhausting 125# steam after the 1'st two stages to supply the heating and "cooling" needs of the campus. Plus right now, the utility has several of their base load units off-line, which causes prices to rise quite a bit.

This forces us to generate enough electricity to get under our "firm." Firm is the amount of electricity we cannot make based on the peak demand of the year minus generation credited to us via what's called an urge test. This means we must generate enough electricity to force our incoming "buy" to less than about 4 MW at this moment.

we are in effect a very small ant in terms of the grid, because we generate such a small amount, and we never export.

Actually though, you could say we are baseloaded. The generator is always in isochronous droop, constant power control. This means that no matter what total system load is doing, combined with the extraction steam flow, the throttle will do whatever they need to do to maintain a constant power. Although operator intervention is required from time to time, because of the changes in steam demand. There is a small range that this works in, and if it's exceeded, pressure will drop on the extraction side.

This load control is actually controlled by measuring the backpressure of the extraction (AKA process steam in some industries).

[This message has been edited by rat4spd (edited 09-18-2005).]

I've gots lots of experience with LTC's, but almost none with generation, so I'm looking at this from the utilities side rather than the generators side.

Which is another way of saying I'm confused!

Are you essentially over exciting the generator to go leading? If so, isn't this done within the generator, and not with the LTC?

Or am I missing this altogether?

"I've gots lots of experience with LTC's, but almost none with generation, so I'm looking at this from the utilities side rather than the generators side.
Which is another way of saying I'm confused!

"Are you essentially over exciting the generator to go leading? If so, isn't this done within the generator, and not with the LTC?

Or am I missing this altogether?"

Don't feel bad, because it's hard for me to understand sometimes too. In effect what we have is something almost abstract that almost know one here (power plant) knows anything about. I think if I knew any less, I'd be completely confused about it.

I think though that the situation here is an exception to what is the norm. The reason is, there are two generators in isochronous generation, but we are importing electricity, and not exporting it. Therefore VARS are a concern, which is another way of saying PF is a concern.

To control the VARS, we use the tap changer on VAR control which changes voltage up or down to send VARS back to us.

I wish I could explain the cooperation between the automatic operation of the voltage regulator of the generator and the automatic control of the LTC, but that's the one area I need to figure out.

I can say that aside from manually moving the LTC to change voltage, we would use the generators voltage regulator to over-excite the gen to bring voltage up which usually happens immediately. To lower voltage, we drive down the voltage regulator, or under-excite it to export VARS to the city, thus causing the VARS regulator to exceed setpoint, causing the LTC to tap to lower voltage, which moves VARS back to our generator.

I'm pretty rusty on the lead vs. lagging PF in how it works.

I wish I could explain this better, but in the end the LTC is in VARS regulate, not voltage regulate.

Maybe someone more freshly familiar with this can explain it better. My understanding is probably rudimentary based on the actual effects of the operation rather than the proper fundamentals electrically.

I have heard it said that the purpose of VAR regulating with the LTC is so that the city utility maintains a leading PF.




[This message has been edited by rat4spd (edited 09-19-2005).]