Paralleling Substation Transformers

[duh]

Problem:
Have 1 existing 18 MVA (ONAN), 132:13.2kV (nominal), z = 8%, OLTC = 21 steps ±10% dy1 transformer. To supply the load, this transformer will have to be paralleled with a new transformer. The existing parallel scheme used is a "master-follower" arrangement. Typically, we order transformers rated for 138kV (nominal) because that is the common supply voltage on our transmission system. Also common is an OLTC = 33 steps ±10%. I need help understanding what the specs of the new transformer should be.

The best solution would be to have a transformer that will work for this installation, while still being able to be used elsewhere in the system (following the common specs above). I am concerned that these are opposing criteria I have laid out, making a solution difficult.

Any insight would be helpful. Or, any idea on transformer application reference guides that may address this issue would also be appreciated.

TIA,
Pete

 

[jghrist]

I suggest reading Beckwith Application Notes AN #11: Introduction to Paralleling of LTC Transformers by the Circulating Current Method

http://www.beckwithelectric.com/infoctr/appnotes/App11.pdf

and AN #13: Advanced Paralleling of LTC Transformers by the Circulating Current Method

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

for a comprehensive explanation of the subject.

The first assumes the ideal case of two or more transformers of equivalent turns ratios and impedances. The second deals with complications that arise when this is not the case. If it is not the case, there will be circulating currents and the problem is to minimize them.

 

[apowerengr]

I'd ask Beckwith (or whoever supplies your parallel balancing control) how they handle two parallel LTC's with different step percentages. Many controls simply keep LTC's with the same step percentages within one step of each other thus minimizing circulating currents (assuming you've done your homework regarding no-load taps and impedances). At 10 steps = 10% on the existing unit a 5/8% step on tap 9 (one step off) would have 5.625% or a gap of approx. 4.4% and there it would sit - high circulating currents. I've had opportunity to set units with 2:1 load capacity to run in parallel but the LTC's had the same 5/8% step increment. I also assume you will specify an impedance so your downstream equipment can handle the fault current resulting from the parallel combination or do your common specs preclude this? You might need to program a PLC for the parallel-balance vs. an off-the-shelf control.

 

[duh]

Thanks, as additional background, the existing unit actually had a sister unit which it operated in parallel with. This unit has failed, and so we are in the process of replacing it with a new one. I do not believe that the fault current will vary much, since this project is a replacement project.

I have looked at the Beckwith application notes, but found that they are in reference to the circulating method. The paralleling scheme we are implementing is a simple master / follower (ie. Master A raises, therefore Follower B raises; Master A lowers, therefore Follower B lowers). It seems that if a "mis-stepped" replacement transformer is ordered, we would need to revise this scheme (perhaps to the circulating current method).

 

[jghrist]

I don't see how a master/follower scheme would work with dissimilar tap voltages. I guess that if there are taps close enough together that circulating currents would be acceptable, a PLC could be used to to restrict the taps to certain particular combinations. I'm not sure that this would work for all possible loads and voltages.

The best solution would be to purchase a replacement transformer with the same impedance and taps.

 

[isquaredr]

Hi duh
For your existing tap control scheme to work you need identical transformers ie rating, number of taps & impeadance. If this criteria is not met you will be faced with large ciculating currents.
If you install a different spec transformer and the %v/tap & impeadance are not too disimilar to your existing transformer you may get away with it as long as you change the control scheme.
Supertapp or microtapp relays from siemens ex vatech/reyolle will do the job