Loss due to Circulating Current in Parallel Transformers 1

[CKent]

I remember participating in a discussion sometime in Aug 2003 regarding paralleling transformer (see thread238-68183)...I'm doing a study now on technical loss...I have come to the issue of circulating current in parallel XFs...circulatin current (Ic) arises due to difference in the turns ration of the XFs in parallel...Ic adds up to the current in the XF with lower turns ration while it subtracts to the current in the XF with higher turns ration...this effectively changes the capacity of the XFs in parallel...however, in general I'm being confused on how this affects the losses of the combined parallel combination...of course the one with the lower turns ration will have a resulting additional loss due to this circulating current...however, isn't it that the other XF with higher turns ration effectively will have reduce loss because the current it its wdg will be lesser than the actual load...so assuming that the 2 XFs have equal resistance, isn't it that the effective loss of the parallel combination is actually equal as in the condition wherein there is no circulating current (in cases with equal turns ratio)...

In the thread I mentioned above, one of the respondents has this to say in paralleling XFs...
"But an advantage is the reduced losses, especially near full load of one transformer. This applies equally to I^2X and I^2R loss - on a 25MVA transformer, times about one hundred substations, it adds up. Halving the current quarters the loss. But the primary advantage is supply availability - it was this that was the primary driver for us doing it."

How could this be?

 

[CKent]

Thanks guys! It gets more interesting...

dpc, you actually have shown that what have been discussed here is true, as far as simulation is concerned...however, maintaining the different turns ratio and everything else from this simulation of yours, could you please vary the load...my initial knowledge is the same with stevenal, that is, the circulating current will be constant whatever the load is, since it is brought by the difference in turns ratio of the XFs in parallel...however, I also believe that waross has a strong point in that, secondary voltage between the XFs in parallell will be affected by the load they carry, and thus might have an effect on the circulating current.

 

[stevenal]

dpc,

I'm confused by your post. The lower impedance transformer when moved to a lower high side tap increased its share of the load from 2/3 to 41%? Sounds like a decrease to me.

I did some similar modeling in Aspen Power Flow, and found the lower impedance unit when tapped down on either the high or low side increased it's share of the real power. Aspen must use tap position to adjust the transformer impedance in its calculation. Changes in the resistive load did nothing to change the real power load sharing once impedance and ratios were fixed.

 

[waross]

dpc
I'm sorry, I got confused and may have misread your post.
Can you adjust your tap setting so that the open circuit voltage of the 6% transformer is the higher of the two and then put a load on it please. We want to try to increase the loading of the transformer that has the higher percent impedance.

Another simulation if you have time. can you parallel two identical transformers. Everything equal, no circulating currents. Then vary the ratio of resistance and inductance in one of the transformers, while leaving the impedance the same.
If this causes circulating currents, I may be on the verge of learning something.
I was taught that transformers divided the load in inverse proportion to their impedances. I am wondering if this is an approximation, rather than an absolute law.

Thank you for your time gentlemen.
yours

 

[amps21]

Intresting topic. I would like to opine also.

Circulating currents during parallel operation may also be caused by not only voltage difference/impedence values but also by different x/r ratios. Like waross says regulation of the transformer may cause change in the terminal voltage @ (when loaded) thereby causing voltages to be equal @ some point and hence ceasing the circulating current due to voltage difference BUT since the r/x ratios (between 2 units) may not be the same, leading to the currents being out of phase of the total load current. This circulating current cannot be ceased. The circulating current is because of the phase angle dfference between the 2 transformer currents. These currents can be calculated if the r & x values of the units is separately known.

As far as impedence variation by changing taps, this is possible if the taps are located physically on the regulated winding.(such as in smaller transformers). In large transformers @ higher voltages, the regulating winding is separately wound and hence will not effect the impedence envelope along the length of the winding.

 

[davidbeach]

Voltage difference between the two transformers will drive reactive currents, that which can not be absorbed by the load will be circulated between the two transformers. Phase angle differences will govern the power balance between the two transformers. (Yes, voltage also affects power, and phase angle also affects power, but the couplings stated are much stronger.)

So, two transformers of the same impedance but different X/R values (amps21, please, it is X/R not R/X) will share the power load unequally because of the different phase angles coming out of the two transformers.

It is all basic power system analysis, check "Transformer, Regulating" in the index of your power system analysis text or other power system analysis reference book.

 

[amps21]

David, Z needs to be split up either X/R or R/X. instead of using Z1+Z2 Sqrt((r1+r2)^2+(x1+x2)^2) for magnitude. You are right when calculating phase angles Atan of X/R

 

[bacon4life]

Getting back to the OP, Bung was talking about "runnning two transformers in parallel instead of one on / on standby". His statement is correct in that the load losses on each of 2 well matched 25MVA trasformers in parallel are 1/4 of a single 25MVA transformer. His post does not address losses of unmatched transformers, which this thread has been very interesting reading.

In addition to halving the total load losses, it would double the no load losses though. An economic analysis and the loss evaluation part of a transformer specification needs to take both effects into account.

 

[CKent]

dpc, what's up? I was just wondrin' if you were able to simulate my request during my last post as well as waross'request...

we are using PTI/PSSE, however, I can't seem to make simulation in which to reveal the circlating current (Ic) between the two XFs in parallel...the loadflow report in this PSSE always shows that the power in and out of 2 XFs in parallel is always equal...most of my readings tell that Ic is independent on the load, that it will remain constant no matter the load since it is due to the difference in votage ratio of the XFs in parallel...but, what Waross discuss makes me think...

My idea why the circulating current will not be affected by the load is that since the Xfs are in parallel, they will "always" have the same secondary voltage already (they are forced to)...the difference actually resulted to the circulating current...when loaded, they will share the load depending on their impedance, not voltage (since they are equal)...this will be in contrary to what Waross' claim that when loaded, the XFs with higher open circuit voltage (that is before they were paralleled) will take all the loads...

regarding the X/R ratio, I also have read that although it may have an effect on Ic, it is very minimal and might be insignificant. How true is this? Has anyone made a confirmation of this based on simulation of calculation? This is why I wanted to know if dpc was able to do the request of Waross'...because this, I think what Waross' wanted to know also...

 

[waross]

Gentlemen;
I think I owe some of you and this forum an apology. Thank you for your patience with my partial knowledge and the contributions you have made to my education.
I particullarly regret any negative affects my inaccurate posting may have caused to the stature of this excellent Forum.
I now must agree with stevenals suggestion that my theory was a simplification that did not take all the factors into account.
If I havn't worn out my welcome, may I ask a couple of questions.
1> Is the situation of parallel transformers with different turns ratios similar the situation with paralleled generators where attempting to raise the voltage on one changes the power factor but not the loading?
2> Is it correct that the reactive current is dependent on the turns ratios only?
3> Will the different tap setting help the transformers share the load current, (Understanding that the reactive currents may well outweigh any actual gain in capacity.)?
4> Do I understand from the simulation that setting the taps higher on the transformer with the higher percent impedance has two effects? First the load sharing is improved somewhat, (possibly related to the resistive component of the transformer impedance) but this advantage is outweighed by the resulting reactive currents.
5> In the instance of unequal voltages caused by unequal line impedances, If the impedances of the lines are added vectorily to the impedances of their respective transformers, could combined impedances be used as if they were the transformer impedances to evaluate the results of paralleling the combinations.
6> Will we get circulating currents in paralleled transformers with enequal X/R ratios?
respectfully.

 

[jghrist]

Beckwith Application Note 13, Section 4 has a good explanation of the problem of paralleling dissimilar transformers. See

http://www.beckwithelectric.com/infoctr/appnotes/App13.pdf

 

[waross]

Thank you very much for the information jghrist.
I think that I had the right idea with paralleling and rerating, but I really blew it when I suggested changing taps.
respectfully