Parallel XFMR and Motor Operation

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mityeltu

Senior Member
Location
Tennessee
Ok, we are making a modification at the plant that involves 2 480v boards that are interlocked. Normally the tie breaker is open. Each board is fed by a separated 6.9kV/480V XFMR. The cable runs for each are not the same length.

The qauesiton was raised, what would happen if we parallel these transformers onto the bus. I know there will be a higher than expected SC current possible, which also raises arc flash concerns, and there will likely be circulating currents due to slight mismathces in phase and impedance, but the qauestion was about the impact on the attached motors.

Will the motors suffer from this configuration? As I see it, each phase will have a slight "ripple" from the slight phase mismatch of the 2 seaparate power feeds. This will be something like harmonics, but I don't know really how to evaluate this condition or how to quantify it.

Do you folks have any ideas?
 

GoldDigger

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Location
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Ok, we are making a modification at the plant that involves 2 480v boards that are interlocked. Normally the tie breaker is open. Each board is fed by a separated 6.9kV/480V XFMR. The cable runs for each are not the same length.

The qauesiton was raised, what would happen if we parallel these transformers onto the bus. I know there will be a higher than expected SC current possible, which also raises arc flash concerns, and there will likely be circulating currents due to slight mismathces in phase and impedance, but the qauestion was about the impact on the attached motors.

Will the motors suffer from this configuration? As I see it, each phase will have a slight "ripple" from the slight phase mismatch of the 2 seaparate power feeds. This will be something like harmonics, but I don't know really how to evaluate this condition or how to quantify it.

Do you folks have any ideas?
Phase differences between the two transformers (small) will cause the actual phase to be the average of the two. I do not this causing harmonics at all.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
I would have to agree with GoldDigger, I think the most critical points you have already mentioned and those need to be addressed. Usually in this situation, the board has not been designed to handle the combined SC current when paralleling the feeders. If your looking for redundancy in case of loss of source, then you can add reactors to limit the SC current. The circulating current will be seen as heat in the transformer. If the impedance's of transformers are the same or very close this will be minimized.
 

beanland

Senior Member
Location
Vancouver, WA
Protection

Protection

If you parallel the two transformers you will need to visit the protection because a transformer fault can now be fed from both the 6900V and 480V systems. Also, the increased fault current of the parallel transformers will change the tripping behaviour of the breakers so you may have miscoordination issues.

Otherwise, I agree that there may be some circulating currents and some unbalance in loading but paralleling transformers is done all the time.
 

mityeltu

Senior Member
Location
Tennessee
Thank you all. I understood the issue of the sc and arc flash. It is not the intent ro parallel the transformers, so this would be entirely undesirable. There are currently kirk-key interlocks on the breakers to prevent this, but part of the design change is to remove the interlock becasue it will not work with the new replacement breakers.

I sincerely appreciate all the input.

Thank you.
 

earlson

Member
Can we use transformer in parallel without the use of synchronizer? Say for example. 2nos of 1MVA 13.8Kv/400-230v 60hz Transformer supplying a common bus of a LVSG Panel. What issues will occur or parameters to be considered when we do this ?


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Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
Can we use transformer in parallel without the use of synchronizer? Say for example. 2nos of 1MVA 13.8Kv/400-230v 60hz Transformer supplying a common bus of a LVSG Panel. What issues will occur or parameters to be considered when we do this ?


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You need a synchronizer to put in parallel two synchronous generators or a generator with the Grid. You have to fit the driver speed in order to achieve the grid frequency and the excitation in order to match the grid voltage and at the end to reduce the angle between these two voltages.
If you want to put two transformers in parallel supplied from a common source you have already the same frequency. If the both connection type are the same or compatible check the voltage difference in order to match the taps.
See[for instance]:
http://www.mgu.bg/sessions/08/3/kpopesku_vkozma3.pdf
 

ronaldrc

Senior Member
Location
Tennessee
You need a synchronizer to put in parallel two synchronous generators or a generator with the Grid. You have to fit the driver speed in order to achieve the grid frequency and the excitation in order to match the grid voltage and at the end to reduce the angle between these two voltages.
If you want to put two transformers in parallel supplied from a common source you have already the same frequency. If the both connection type are the same or compatible check the voltage difference in order to match the taps.
See[for instance]:
http://www.mgu.bg/sessions/08/3/kpopesku_vkozma3.pdf

Ok, we are making a modification at the plant that involves 2 480v boards that are interlocked. Normally the tie breaker is open. Each board is fed by a separated 6.9kV/480V XFMR. The cable runs for each are not the same length.

The qauesiton was raised, what would happen if we parallel these transformers onto the bus. I know there will be a higher than expected SC current possible, which also raises arc flash concerns, and there will likely be circulating currents due to slight mismathces in phase and impedance, but the qauestion was about the impact on the attached motors.

Will the motors suffer from this configuration? As I see it, each phase will have a slight "ripple" from the slight phase mismatch of the 2 seaparate power feeds. This will be something like harmonics, but I don't know really how to evaluate this condition or how to quantify it.

Do you folks have any ideas?

If both Transformers are identical in every way or a matched set I only see one problem that bothers me.

You said both wouldn't be on at the same time anyway, there just feeding the same buss.

Nothing has been mentioned about the switching of the secondarys feeding this common buss. The secondarys need to be inter locked
so both secondarys can not connected at the same time doing so would back feed 480 volts to the line side
of the non energized Breaker terminals.

Ronald :)
 

GoldDigger

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Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Nothing has been mentioned about the switching of the secondarys feeding this common buss. The secondarys need to be inter locked
so both secondarys can not connected at the same
time doing so would back feed 480 volts to the line side
of the non energized Breaker terminals.

Ronald :)

Feeding to the line side of a non-energized breaker could be difficult unless you are referring to non-energized meaning opening a disconnect upstream of that transformer but not upstream of the other.
If the primary breaker on each transformer is interlocked to open the corresponding secondary, that will also prevent voltages on the load side of an open primary breaker.
Paralleling transformers in use will clearly need additional interlocking if they are fed from different service wires or supplies. (If that is even permitted outside POCO). Among the requirements there may be a network protection (reverse power flow detection) relay.

And for the OP's concern, it now sounds like either he does not really intend to parallel the two routinely, or he wants to use a make-before-break intertie means, possibly manual.
 

ronaldrc

Senior Member
Location
Tennessee
Feeding to the line side of a non-energized breaker could be difficult unless you are referring to non-energized meaning opening a disconnect upstream of that transformer but not upstream of the other.
If the primary breaker on each transformer is interlocked to open the corresponding secondary, that will also prevent voltages on the load side of an open primary breaker.
Paralleling transformers in use will clearly need additional interlocking if they are fed from different service wires or supplies. (If that is even permitted outside POCO). Among the requirements there may be a network protection (reverse power flow detection) relay.

And for the OP's concern, it now sounds like either he does not really intend to parallel the two routinely, or he wants to use a make-before-break intertie means, possibly manual.


My BAd

I didn't notice it was a 6900 volt primary I thought we where talking about a 480 volt primary
and secondary of maybe 24or 36 volts when he said boards.

But my answer is still the same except the line side of the non energized breaker would be 6900 volts.

Does that make more sense?

Ronald :)
 

earlson

Member
Thanks for the comments but to make my question simple, it goes like this. 2 identical transformer is fed from the same primary 6.9kv supply. And feeding common secondary 400v bus of an LVSG board and both are ONLINE. Without tie breaker or whatsoever to isolate them from 1 another. Any technical issues here?


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GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
Thanks for the comments but to make my question simple, it goes like this. 2 identical transformer is fed from the same primary 6.9kv supply. And feeding common secondary 400v bus of an LVSG board and both are ONLINE. Without tie breaker or whatsoever to isolate them from 1 another. Any technical issues here?


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Other than the safety-related issues such as total fault current available, backfeeding, etc, there will be no technical issues caused solely by paralleling the transformers. The output will very close to twice the normal current handling capacity of a single transformer and there should be no harmonics or other circulating currents that you would not also see with a single transformer.
Add similar or identical length and gauge wiring from each transformer to the common bus connection, and you will have covered most of the bases.
If one is much farther from the bus connection than the other, coil up enough wire on the close one to make it match.

Would I personally be comfortable doing it? Probably not.
 

Sahib

Senior Member
Location
India
My thought is during simultaneous start of motors on the secondary side of the two transformers, at least one transformer may be driven into its saturation region due to circulating current, thereby liable to produce harmonics. This may be checked in the field with an oscilloscope, for example.
 

GoldDigger

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Location
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Retired PV System Designer
My thought is during simultaneous start of motors on the secondary side of the two transformers, at least one transformer may be driven into its saturation region due to circulating current, thereby liable to produce harmonics. This may be checked in the field with an oscilloscope, for example.
As long as the transformers are identical and wired with low enough and equal wire resistances, I do not see any reason for circulating currents between the two transformers.
The only possible problem I can see would be if one motor were connected with short leads to one transformer and the other with short leads to the other transformer instead of being connected after the point where the two secondaries were paralleled.
 

Sahib

Senior Member
Location
India
The only possible problem I can see would be if one motor were connected with short leads to one transformer and the other with short leads to the other transformer instead of being connected after the point where the two secondaries were paralleled.

Of course, the motors are connected after the point where the two secondaries were paralleled.

The OP stated that there is a slight mismatch between transformer impedances.

So the best course of action is to check for harmonics in the field after the two transformers are paralleled, even when there is no appreciable harmonics during single transformer operation.
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
Of course, the motors are connected after the point where the two secondaries were paralleled.
The OP stated that there is a slight mismatch between transformer impedances.

Actually he stated "there will likely be circulating currents due to slight mismathces in phase and impedance, " but he never said that he knew the transformers were mismatched or to what degree.
Given that they are connected to the same source, I do not see where small mismatches in phase would come from. Large ones, yes. :)
It definitely will not hurt to look at the voltage and current waveforms under differing loads while the two are paralleled.
 
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