Transformers and Unbalanced Currents

The phase currents in the primary side will be different, and there isn't any particular problem with this.

All that has happened is that the star point of the primary windings, unlike that on the secondary windings which is connected to the neutral wire on the secondary side of the transformer, is contained entirely within the transformer.

so currents will be different ? so according to which principle the currents will shared?

i guess u meant the primary currents will differ from secondary ones??? so the 1 turn ratio is in valid here?

In principle, the primary currents on a 1:1 voltage ratio hypothetical three-phase transformer will be slightly larger than those on the secondary, as there will be losses in the transformer. The supply voltage and the secondary voltage would be the same, therefore the primary current will be higher than the secondary current to provide the power that is dissipated as losses.

The reason for using a 1:1 isolating transformer with a secondary neutral conductor would be a mystery unless it were intended to supply at least one single phase load!

Next stop: Wikipedia - "three phase power".

It is uncommon for the current in any three phase system to have the same current in each phase. There will always be a current flowing at the star point. It just so happens that in the original question, the star point of the secondary windings is connected to a neutral conductor. The only purpose of a neutral conductor is to enable the supply of a range of single phase loads from the secondary side of the transformer. Therefore it follows that the current in each phase conductor on the secondary side of the transformer will differ as single phase loads go on and off, therefore the current in the phase conductors on the primary side will differ too.

This isn't unusual, or something to worry about. It's commonplace.

Try reading the Wikipedia pages on three-phase power, perhaps?

hello chap, i think you talk about wikipedia 3 phase power system and nothing more:)

i ask you "in ideal conditions(forget magnetizing current!) secondary and primary currents need to be same but one side is not floating and the other one is floating so the currents cant be the same, it sounds a little bit weird, because one side is subjected to kirchoff law and the other one behaves differently than turn ratio law u got it? okey forget it...

Indeed in the 1 to 1 wye to wye transformer as described above the currents in the prim windings would not be a 1 to 1 reflection (plus the transformation losses) of the currents in the sec windings. Let's imagine the secondary windings are loaded 1A, 10A, and 100A - very plausable situation. But, it is impossible indeed for the currents in the primary windings of that transf to be , let's say, 1,1A, 11A, and 110A ! There would not be that gross difference of the currents in the prim wind (as some "levelling" takes place) - and there are some sophisticated formulas for calculating them the real currents.

tell me pls, is it possible that if secondary has a floating neutral, line currents sum to zero? if there is an imbalance (sum of the currents not zero)so how can total current at wye point is zero? is there a circulation of a zero sequence current in the secondary circuits?

There need to think of +ve, -ve and zero sequence currents on both primary and secondary sides. Once there is neutral there is flow of zero sequence currents on a star-connected transformer. As there is no star point on the primary the phase currents will include zero sequence currents also which may be called circulating currents.

And the transformation ratio will hold 1:1 once we consider primary / secondary each phase (not line)

so you say if neutral is floating at wye connection, the line currents sum up to zero but there is a zero sequence current circulating through the phases because of unbalanced load ???The contradiction in my mind lies here, now the neutral is grounded , the line currents are unbalanced and there exists an imbalance current, at the same moment i broke the neutral line and what happens to the line currents?? line currents change ??? since kirschoff law will enforce this line currents will change or the currents will remain the same but there exist an extra circulating current so as to realize the kirschoff law?? i cant solve this part...

If the neutral breaks, then the loads, which were connected to phase voltage (let's say 240V and let's imagine there were just one single load in every phase), turn out to be connected in parallel to the line voltage (let's say 400V). If by chance the loads are equal in their power drawn, nothing special, they will just draw less current as now to each of them 200V is applied instead of the prior 240V. But this is not predominantly the case in life, and the loads have different powers, and the less powerful load simply burn as more voltage turn up to be applied to it.

With the 1 to 1 transf (wye to wye) with the netral in the sec windings, with loads accross phases let's say 1, 10, and 100A, I imagine the currents in the prim neutralless wind might be something like 20, 40, and 60A. ( One needs to look up for the formula(s)).

Sorry, a correction, loads are of course connected in line - not in parallel.

Hi Yuri,

you have the idea as to what happens when a neutral is lost in ideal conditions wrong. if the N is lost with three equal loads, say three heater elements, they will continue to work exactly the same as they will each still have 230V across them.

You are right about unbalanced loads in that the lightly loaded phase will be subject to an increased voltage and typically burn out.

regards

Chas

(Ia + Ib + Ic = 0) means that there are no zero currents at primary indeed. In this case, primary side currents are composed of negative and positive sequence currents. Therefore, system can be decomposed to positive and negative sequence circuits. However, a zero sequence circuit can not be constructed since there is no neutral connection at primary side. I think this shows that there will no zero sequence currents at secondary.

dear guest, i think what you said is interesting, so with contrary to what masdal said the prim. and sec. currents will be same (practically) because if there is no zero sequence current on secondary side it means there is no a neutral current and the other phase currents sum up to zero as primary currents. Am I right?

here you two contradict each other guys, here arises a new question, what is "zero sequence current exactly? if we can get a real answer we can decide who is right..

Dear Jinxnao,

positive, negative and zero sequence components are the 3-3 transformed form of three phase a, b, c parameters. A 3x3 transformation matrix is defined for this transformation. Zero sequence component is simply the addition of three phase values (Ia+Ib+Ic, for zero sequence component of three phase currents). You can look at "symmetrical components" concept for further info.

sorry, correction for the last sentence:

neutral currents flows through secondary windings as In = Ia+Ib+Ic as phasor and this induces zero sequence emf at primary windings that can not generate zero sequence currents at primary because of the lack of primary neutral connection. To sum up, system can be analyzed as positive and negative sequence circuits, however, to analyze the secondary currents and voltages, the effect of the secondary zero sequence circuit must be superposed.

yeahhh ! you are now completely rgiht my friend! congratulations....

To find out the individual phase current, you would need to consider angle - and depending upon the load and transfortmer reactance and unbalance, it can be significantly different....Haseeb

I have a 3 phase corner ground delta. My current is very unbalanced. I have a 75 HP air compressor that when set to 120 PSI will trip out the overload that is set at 100 amps. I get readings of 70 amps 80 amps and 110 amps on the motor. This is the typlical unbalance in my system. My utility provider had an expert here last year and he said he didn't see anything wrong. DUH. Everwhere I have worked you could check a motor and the current was always balanced. I have worked at two steel mills and a brewery. Presently at a small steel mill.