Paralleling Substation Transformers
Paralleling Substation Transformers
(OP)
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
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






RE: Paralleling Substation Transformers
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.
RE: Paralleling Substation Transformers
RE: Paralleling Substation Transformers
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).
RE: Paralleling Substation Transformers
The best solution would be to purchase a replacement transformer with the same impedance and taps.
RE: Paralleling Substation Transformers
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