Discussion in 'The Projects Forum' started by Novice Powers, Feb 26, 2012.

1. ### Novice Powers Thread Starter New Member

Feb 26, 2012
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If i have three single phase transformers of different capacities, say two 50 kVa transformers and one 75 kVA, connceted in a star-star configuration, and i also have the line and phase voltages and currents on the secondary side of the bank taken by a clip-amp meter or some other device, how do i determine the total load the bank is supplying?

For example i may have measurements on the secondary side of the bank as follows :

Line Voltage (Red-Blue phases) = 228 V
Line Voltage (Red-White phases) = 230 V
Line Voltage (White-Blue phases) = 227 V

Line/Phase Current I (Red Phase) = 15 A
Line/Phase Current I (Blue Phase) = 14 A
Line/Phase Current I (White Phase) = 15 A

How do i determine if the bank is overloaded from these readings?

Thnx

2. ### #12 Expert

Nov 30, 2010
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Just doing 15A x 230V gives 3450 watts. Way below 50kVa and I don't even know the correct math!

I suspect you want the math formulas. Somebody else will be along in a while and do that for you.

3. ### Novice Powers Thread Starter New Member

Feb 26, 2012
8
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You are correct, that was just a for-instance set of figures. What I am really after is the method of calculation, like which values/readings (from the example figures) do you use to determine the total load the bank is doing and why use those, along with any related theory ofcourse.

Thanks for your response, helped me to clarify what exactly i was trying to get at

4. ### #12 Expert

Nov 30, 2010
16,655
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A quick web search tells me that power is radical3 times volts times amps, but that's about power to the load.

Each leg of a 50kVa transformer can provide 50,000 volt-amps, right?
50,000 VA/230 volts = 217.39 amps allowable.
Can it be that simple?

no. all 3 legs combined make 50kVa.
if power is radical3 V A then you'd have to do this for each leg and add them together...I think.
so...15amps times 230 volts times radical3 times 3 phases makes 17.927 kVa.

Can somebody else confirm this?

Last edited: Feb 26, 2012
5. ### Novice Powers Thread Starter New Member

Feb 26, 2012
8
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Well two 50 kVA transformers and one 75 kVA in a three phase star-star configuration gives me a total capacity of 175 kVA I think, I would just like to know how to use those values in calculation to tell if the load exceeds the total capacity of the bank.

Also how could i tell if just a single transformer (example, the lighting transformer: say the 75 kVA) in the bank is overloaded using the kind of readings i gave as example in the initial post?

6. ### #12 Expert

Nov 30, 2010
16,655
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The smallest transformer is always the limiting factor.
This is absolute if all the loads are 3 phase loads with each leg being very close to equal current.
If you're going to put some single phase loads from the 75 kVa transformer to ground (common) I think they would be calculated without the radical3 factor because there is no phase relationship in a single phase load. Do the radical3 for phase to phase loads and just volt-amps for a single phase load.

One thing I feel sure about is that an amp-clamp and a volt meter is all you need. Your phase angles are locked. The amp-clamp will show true amps, not phase adjusted power factor amps, so it will be valid. The radical3 factor is the power factor adjustment for phase to phase loads.

Last edited: Feb 26, 2012
7. ### Novice Powers Thread Starter New Member

Feb 26, 2012
8
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So i guess that brings me down to the root of my question, now in a star-star configuration (assuming each transformer has two secondary terminals X1 and X2) the X1's from the all the transformers would be tied together and connected to earth forming the star point while the X2's would give your line voltage and current outputs.

So lets say we want the total load supplied by the 50 kVA transformer who's X2 terminal is the Red phase output of the bank, do i multiply Ir ( Line current from the X2 terminal of the transformer) by 228V, or by 230V? (with the radical3 ofcourse) ?

And then do i do similar calculations for all three transformers (with no radical3 for the transformer with only single phase load) and them add them up to get the total three phase load the bank is supplying?

8. ### #12 Expert

Nov 30, 2010
16,655
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To figure the load on the transformers, simple volt amps measured at each leg would be the VA of the leg. After all, the label says 50kVA, not killowatts. With you using a simple two winding transformer for each leg, just measure the voltage with the volt meter and measure the amps with the amp-clamp and multiply them together for volt-amps. Do not use the radical3 factor on this. The radical3 converts from VA to watts, and you want VA.

I think I have this correct now.
anybody else confirm?

ps, I'm not really fast at typing. I've been guessing your next question and typing the answer while you were typing the question.

Last edited: Feb 26, 2012
9. ### GetDeviceInfo Senior Member

Jun 7, 2009
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In a star configuration with neutral, you can draw in imbalanced current up to your transformer rating. Line current is phase current. Phase voltage is sqr root 3 of line voltage. KVA rating of the phase transformer divided by phase voltage equals max rated current. Pulling full rated current will imbalance the primary supply. Your code must be followed so you may have a higher cost on wiring/conduit/over current/disconnects/ etc. It's all kind of pointless unless you have a specific need to service. Watts have nothing to do with it, that's a load thing, which a transformer is not (less efficiencies). If your talking 'load', then look at your load.

#12 likes this.
10. ### shortbus AAC Fanatic!

Sep 30, 2009
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Just a dumb thought here - How can you have three "single phase" transformers hooked to "three phase"? Using single phase transformers on three phase doesn't make them three phase. There is no 'magnetic' linkage between them so their out put is no longer three phase.

Or am I misreading the question?

11. ### GetDeviceInfo Senior Member

Jun 7, 2009
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3 phase transformers are simply 3 single phase transformers grouped together. They produce the classic 3 phase displacement because that's what there being supplied with.

12. ### Novice Powers Thread Starter New Member

Feb 26, 2012
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Thanks a million guys for the response.

Just trying to figure out one thing, as i am not sure i am following;

If I am given the readings in a manner like i have outlined in my original question, what would be the correct way to use those readings in a calculation, to determine the total load the transfomer is doing, what do i multiply by what and what do i add to what, to find the load the bank is doing.

I could allways compare the result of my calculation to the capacity of the bank (that is 150 kVA,) to determine if it is overloaded)

Just want to be clear as day.

13. ### shortbus AAC Fanatic!

Sep 30, 2009
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Thanks for the answer, but I assumed he was talking about 3 separate single phase transformers in a "bank" not three secondaries wound on one core. My bad.

14. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Hi shortbus,

The OP was actually talking about three single phase transformers connected in a three phase configuration [star-star in this case].There's nothing unusual about that option. The three windings don't have to share a common core to perform as a three phase isolation / distribution transformer. There are economies to be achieved in winding entirely on the one core but it's not essential for normal operation.

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15. ### Novice Powers Thread Starter New Member

Feb 26, 2012
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thats correct tnk, three single phase transformers in a star-star connected bank, so if anyone is willing to lay out the correct calculation for me (using the figures given in the initial post and the power formulas), that would be great,

it would go a long way in aiding my full understanding and helping me to apply the theory i have in practical situations. Thanks

Jun 7, 2009
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17. ### t_n_k AAC Fanatic!

Mar 6, 2009
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You need to keep in mind the point that kVA ratiing of itself is insufficient to deduce the current rating for the 1-phase transformer windings. Normally one would quote the kVA rating for a rated primary voltage. One may then deduce the rated currents for both primary and secondary windings provided the transformer primary-to-secondary turns ratio is known.

Also it may be possible to have other 3-phase configurations depending on the primary voltage rating. A delta connection may be possible if the primary winding is capable of withstanding the supply side line-to-line voltage.

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18. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Hi Novice Powers,

You are very keen to have someone set down the calculations / working for you. Is this homework? If you are a professional engineer / technician you should be able to sort this out yourself.

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19. ### Novice Powers Thread Starter New Member

Feb 26, 2012
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I have my take on the whole thing, just want to make sure my understanding is concrete. So i have done my own sorting out, just wanted the input of someone with a tighter grasp of the theory cause i am not confident i had it down to the point where i would be able to apply it to different situations.

So it isnt home work, just going for a more complete understanding by having someone lay it out correctly and then analyzing that to get a firm grip on things

So this is what i would do: I would simply take the line voltage reading for the red phase transformer multiply it by the measured line current for that same red phase transformer and multiply that by root 3 to get the load the transformer is doing.

I would do the same thing for the blue and white phase transformers and add all three result and that will be the load the bank is supplying as far as i am concerned. I would then compare that to the capacity of the bank to see if it is overloaded.

Am i correct?

Last edited: Feb 28, 2012
20. ### t_n_k AAC Fanatic!

Mar 6, 2009
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No that's incorrect.

Just to get a couple of points straight first.

For this discussion let's assume the 3-phase voltages are perfectly balanced.

When one speaks of a line voltage that is normally interpreted as the line-to-line voltage. In a 3-phase system that would be the voltage between any two line connections: Red-to-Blue, Blue-to-White [per your terminology] etc. Presumably the phase rotation [0°,120°,240°] would be Red-Blue-White in your terminology, although that's not particularly important for this discussion.

The phase voltage would the voltage between any line connection and the neutral connection....hence Red-to-Neutral, Blue-to-Neutral, etc.

The Volt-Amperes drawn by any single phase transformer in your voltage balanced 3-phase Star-Star configuration would be

VA=(Line-to-Neutral Voltage) x (Line Current)

or

VA=[(Line-to-Line Voltage)/√3] x Line Current

So if the Red Line-to-Neutral voltage was 231V and the Red line current was 10A then the Red phase transformer would be supplying 2.31kVA.

If reference was made to the Red-to-Blue line-to-line voltage in the same situation, then this would be 400V. The VA draw from the Red phase transformer would still be the same having regard to to second 'formula' given above.

Supplementary Note:

Again at the risk of being pedantic I would re-iterate that the single phase transformer VA rating has relevance only with regard to rated winding voltage. I'd emphasize that as follows.

Suppose I take the 50kVA unit. Let's say it has a rated primary voltage of 231V. Lets also assume the primary to secondary ratio is 230V:120V. This means the rated [permissible] current draw on the secondary side would be 50,000/120=416.7A. For completeness this also means the primary winding current rating would be 50,000/230=217.4V.
Suppose the same 50kVA transformer was (for some reason) used in a situation in which the primary side was actually fed from a supply voltage of 120V [less than the rated value]. The secondary voltage would then be 62.6V. One might be tempted to say that, given this is a 50kVA unit then the permissible current draw on the secondary side would now be 50,000/62.6=798.6A. This would be a grave mistake if put into practice. The secondary winding conductors are only rated for 416.7A. Hence one must always consider the kVA rating with reference to the rated winding voltages for either primary or secondary.

Last edited: Feb 28, 2012