Can Unbalanced load on transformer cause voltage drop?

I have a 75KVA 600V delta to 120/208V wye dry type aluminum transformer with a fairly high impedence rating of 7.5%.
There is a load of approx 90A across phase A and B and considerably smaller loads on across BC and CA. The 90 amp load is one piece of equipment.
I have 600V on H1 H2 H3 but only 200V across the AB phases. The starpoint is firmly bonded to the ground, and all connections look good.
Is there a tendency for imbalanced loads to further increase voltage drop in transformers? I would suspect that I would have closer to 208V on the secondary if the load was 90A on all three phases.
doug G
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in article snipped-for-privacy@4ax.com, . at snipped-for-privacy@telus.net wrote on 5/11/04 11:40 PM:

This sounds like a job for Captain Symmetrical Components! :=)
When I was preparing for my PE exam, every previous exam that I looked at had a question on symmetrical components. I got to where I could do em all. But, as my luck would have it, I got the first exam without symmetrical components.
Bill
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| I have a 75KVA 600V delta to 120/208V wye dry type aluminum | transformer with a fairly high impedence rating of 7.5%. | | There is a load of approx 90A across phase A and B and considerably | smaller loads on across BC and CA. The 90 amp load is one piece of | equipment. | | I have 600V on H1 H2 H3 but only 200V across the AB phases. | The starpoint is firmly bonded to the ground, and all connections look | good. | | Is there a tendency for imbalanced loads to further increase voltage | drop in transformers? I would suspect that I would have closer to 208V | on the secondary if the load was 90A on all three phases.
I calculated the voltage drop at 7.5% impedance on 75 kVA to be about 3.9 volts on line to ground, and about 6.7 volts on line to line, when the current is at 90 amps. That's assuming zero resistance/reactance everywhere else, which would certainly not be the case. Other things could add a little more voltage drop and bring it down to 200 volts.
You might check to see if your transformer has adjustment taps on the primary. Most transformers at that high kVA rating do have some taps. Connect the 600 volts at a -2.5% tap. That should give you a small boost on the 208Y/120 side, to about 213Y/123 in theory. Then your voltage drop due to load will be less.
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Thanks for the reply.
How is that calculated? I'm guessing that you assume that at full load (208A) you get 7.5% drop and at 90 amps (43% load) you get 43% of the drop expected with full load?
Crappy transformer. Must be the aluminum windings. Its already on the 585V tap, which would give me 213V no load. I have one more tap at 570V that I can use to give me 219V or 123V to N. Thats the way I is going to have to go I guess. Looks like this machine will be getting its own transformer.
doug
On 12 May 2004 09:04:03 GMT, snipped-for-privacy@ipal.net wrote:

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| Thanks for the reply. | | How is that calculated? | I'm guessing that you assume that at full load (208A) you get 7.5% | drop and at 90 amps (43% load) you get 43% of the drop expected | with full load?
The full load current is 7.5% of the fault current. You can figure the impedance in ohms given the current and voltage.
| Crappy transformer. Must be the aluminum windings. Its already on the | 585V tap, which would give me 213V no load. I have one more tap at | 570V that I can use to give me 219V or 123V to N. Thats the way I is | going to have to go I guess.
It's probably designed to be a hot transformer. The high impedance suggests this possibility.
You're starting to get above nominal voltages. Are you trying to get up to 240 volts? What is the load on this.
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I am trying to get 208V from a 208V transformer. I was under the assumption that a 75KVA 600 - 120/208V transformer would deliver 75Kva at 208V. With 600.00V input they are delivering more like 40 - 50 Kva at 200V.
I have 2 of the same transformer that are doing this with similar loads. I took the cover off them and took some current readings. One has approx 150A on the highest leg and the other transformer has 180A. Both transformers are running unbalanced because there is a large single phase load on each 3 phase panel. There is a lot of heat being generated too.
The loads are basically ballasts for high output UV lamps for curing inks on screen printing press's, with a few small motors for fans and conveyors etc. I do not know if these ballasts are electronic or not. I intend to find out tomorrow.
Here is a link to the technical specifications for these transformers. I think the answer can be found in the % regulation section.
http://www.bemag.ca/design/english/3PhaseAlStd/BA3075V.pdf
doug
On 13 May 2004 06:01:11 GMT, snipped-for-privacy@ipal.net wrote:

To send me e-mail remove the sevens from my address.
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| I am trying to get 208V from a 208V transformer. I was under the | assumption that a 75KVA 600 - 120/208V transformer would deliver 75Kva | at 208V. With 600.00V input they are delivering more like 40 - 50 Kva | at 200V. | | I have 2 of the same transformer that are doing this with similar | loads. I took the cover off them and took some current readings. One | has approx 150A on the highest leg and the other transformer has 180A. | Both transformers are running unbalanced because there is a large | single phase load on each 3 phase panel. There is a lot of heat being | generated too. | | The loads are basically ballasts for high output UV lamps for curing | inks on screen printing press's, with a few small motors for fans | and conveyors etc. I do not know if these ballasts are electronic or | not. I intend to find out tomorrow.
This could be the problem. If the load is very non-linear, it can be causing a substantial pulse load on the transformer if it is also a major part of that load.
A normally resistive load is ideal. But when a load draws all of its current only in a short pulse during each cycle, then it is causing more of a demand on the transformer and wiring than what it would be if averaged out.
A simple explanation works like this. Suppose you have 3 banks of heaters that pull 90 amps of power each. You switch one bank on for 1 second at a time, and cycle through each so that only one bank is on at one time. You get the same amount of heat as if you had just one bank on steady. But now, suppose you turn all 3 banks on at the same time for 1 second, then everything off for 2 seconds and repeat. You get the same heat again, but this time you are drawing 270 amps 1/3 of the time. Consider that impedance loss affects the power in proportion to the _square_ of the current. At 3 times the current you have 9 times the loss. Average that 9 times loss over the 3 seconds in each cycle, and the average loss is 3 times as much.
When you have electronic loads that pull current in short pulses on each AC cycle, it's still the same kind of problem. The math is more complicated, but it puts more stress on the wiring and the transformer windings, and results in more loss, more heat, and potentially even damage or fire.
This problem is even worse with 3 phase power when using loads that connect between any hot phase and neutral. That puts current pulses on the neutral that do not balance out. You can end up with three times the current on the neutral ... and that's just average ... the pulsed current problem can make it worse if the pulses are narrow enough.
Since your loads are connected phase to phase, the neutral overload shouldn't be much of an issue. But you can still cause a 15% extra overload just due to the pulses not matching between phases (they don't average together like sine waves would). But you still could have the I^2*R problem with deep pulses.
If you can get an oscilloscope reading of the current waveform on each of the phases and the neutral, that might be more informative.
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wrote:

One thought - discharge lamps can give problems due to 3rd harmonics adding on the Neutral to give very large neutral currents (this is a disadvantage of ballasts on a 3 phase supply). It may be that you are experiencing excessive volt drop due to this. Put a good ammeter on your Neutral and see what is going on there. I would expect that the Neutral connections are rated at full phase current rating, but in some cases it is better to use an oversize neutral. Does the transformer manufacturer have anything to say about harmonic loads?
I hope this helps, Dave
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On Fri, 14 May 2004 08:48:15 +0100 Dave I <sdrfasflo> wrote:
| One thought - discharge lamps can give problems due to 3rd harmonics adding | on the Neutral | to give very large neutral currents (this is a disadvantage of ballasts on | a 3 phase supply). | It may be that you are experiencing excessive volt drop due to this. | Put a good ammeter on your Neutral and see what is going on there. | I would expect that the Neutral connections are rated at full phase current | rating, | but in some cases it is better to use an oversize neutral. | Does the transformer manufacturer have anything to say about harmonic loads?
Don't forget about the I^2*R impact of high current pulses, too. This affects single phase, as well as three phase whether delta or wye, in addition to the triplen buildup on the neutral of wye.
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