More Transformer Loading

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Ed MacLaren

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Here is a completely different transformer loading question for those of you that like this sort of thing.
(Perhaps it will put an end to the "other one". :) )

What is the load in KVA on the transformer after the installation of the power factor correction capacitor bank?

There are no other loads connected. Use 480 volts as the motor voltage.

Ed

PF3.gif


[ March 11, 2005, 09:32 PM: Message edited by: Ed MacLaren ]
 
Re: More Transformer Loading

I don't quite have Ed's gift of graphics, but while you're working on his, consider this version of the circuit we've been talking about.

The transformer is ideal; i.e., it is lossless. The turns ratio (N) is 1:1. The primary source is an infinite sink.

What are the RMS values of the primary voltages; the line currents 1, 2 and 3; and the primary phase currents A, B, and C?

threephase%20holt.gif
 
Re: More Transformer Loading

Sneaky rbalex, I have the answers, but I will wait for someone else to jump in so I can criticize if they make a mistake.

On Ed's problem, for once I agree with Bob the consultant. Real power is unaffected, so it is simply a ratio of PFs.
 
Re: More Transformer Loading

The problem isn't sneaky; in fact, it's actually rather a "classic." Most of the answers are intuitive and can be determined by inspection; however, one of the intuitive answers is often wrong.

The results illustrate the "practical" value of using "balanced" three-phase circuits and that RMS values aren't vectors.
 
Re: More Transformer Loading

Well, it appears that my question wasn't very challenging for you guys, with the very first response being the correct one. :)

I'm interested in the answer to your question, Bob. Are you going to post the solution?

And by the way, there's nothing wrong with your graphics.

Ed
 
Re: More Transformer Loading

Originally posted by Ed MacLaren:
...I'm interested in the answer to your question, Bob. Are you going to post the solution?
Click to expand...
Of course, but not till someone actually posts an answer. It is NOT a trick question, but the answer is usually surprising. It can be analyzed with "symmetrical components" but there is actually a much simpler solution.

...
And by the way, there's nothing wrong with your graphics.

Ed
Click to expand...
Thanks :D
 
Re: More Transformer Loading

OK, I will stick my neck out.

First off, xfrmr A is open, no load current, take it out.

Second, the primary phase voltages are 120V.

Third, the primary line currents are 10, 10, and 20A. Anyone doubt that?
 
Re: More Transformer Loading

I'll say -
Primary phase voltages are 120V.

Primary line currents are 10a, 10a, and 17.3a.

Ed
 
Re: More Transformer Loading

I'm going with Ed:

120v across primary windings

current in line 1 = 10 amps

current in line 2 = 10 amps

current in line 3 = 17.3 amps
 
Re: More Transformer Loading

Is the wye voltage 120V phase to phase as described, or phase to neutral as shown?

It is getting late and my head cold is bothering me, but my gut feel answer is I1 = I2 = 10A, I3=0. Maybe tomorrow I can really think about it.
 
Re: More Transformer Loading

Jim,

The wye voltage is phase to neutral as shown. Maybe I should have clarified the primary/secondary winding turns ratio is 1:1. So far, most folks have taken it that way, but it would be reasonable to have taken it as the primary/secondary phase voltage ratio.

[ March 13, 2005, 11:28 PM: Message edited by: rbalex ]
 
Re: More Transformer Loading

I'm going with Rattus.

On the wye side, there is only one current. It is the 30? shifted current (assuming the load to be 0?).

On the delta side, with N=1, wye winding voltage and current = delta winding voltage and current.

So, delta V phase to phase = 120 volts.

And the current in delta winding B and winding C are each 10 Amps, and they are in phase with each other. No current is in delta winding A.

The tricky bit is the dot convention, or the direction of the current in delta windings B & C. The current in B is 10 A and the current in C is 10 A and they relate to the angle of the 10 Amp current in the wye windings B & C. But the delta winding currents are in opposite directions, so Line 1 = 10 A, Line 2 = 10 A & Line 3 = 20 A.
 
Re: More Transformer Loading

This is a bit easier if you omit xfrmr A and redraw using standard xfrmr symbols and keeping the dots straight.
 
Re: More Transformer Loading

Wow I can't believe I answered like that. I need to remember not to operate a computer while taking medication.
 
Re: More Transformer Loading

Now my question is, "What is the loading on each of the three transformers". And, why do you say that?
 
Re: More Transformer Loading

For the loading debate to continue please keep this in mind:

From IEEE C37.12.80-2000
"Standard Terminology for Power and Distribution Transformers":
3.4.5 Load (Output).: The apparent power in megavolt-amperes, kilovolt-amperes, or volt-amperes that may be transferred by the transformer.
Click to expand...
From IEEE C37.12.00-2000
Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers
4.1.6.1 Capability
Transformers shall be capable of
a) Operating continuously above rated voltage or below rated frequency, at maximum rated kVA for any tap, without exceeding the limits of observable temperature rise in accordance with 5.1 1.1 when all of the following conditions prevail:
</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">1) Secondary voltage and volts per hertz do not exceed 105% of rated values.</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">2) Load power factor is 80% or higher.</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">3) Frequency is at least 95% of rated value.</font>
<font size="2" face="Verdana, Helvetica, sans-serif">b) Operating continuously above rated voltage or below rated frequency, on any tap at no load, without exceeding limits of observable temperature rise in accordance with 5.1 1.1, when neither the voltage nor volts per hertz exceed 110% of rated values.
Click to expand...
For what its worth, the IEEE C57 series of standards is the UL basis for listing them.
 
Re: More Transformer Loading

Okay, so Rattus and Al are making me think.... and I am going down to the lab right now to hook this thing up and take some amp readings

oh oh! And oscilloscope graphs too!

I'll get back with the results
 
Re: More Transformer Loading

OK mens as my old First Sergeant used to say, it is time to own up. Al and I are right, but I must admit that in private discussions with rbalex, I got it wrong the first time. I will blame that on him though for not drawing conventional transformer symbols where it is obvious that there is a 180 degree flip in one of the transformers.

Rattus
 
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