Paralleling Transformers

[JJHorak]

We are adding a 2nd 150/280MVA auto-xfmr to a 230/115kV substation. The xfmrs are to be run in parallel. Each has a delta tertiary. Through fault impedances are nearly identical on a 150MVA base. The intent is for each xfmr to be on taps so that ideal voltage ratios are identical.
There is one difference: Due to bus arrangement difficulties, there is a difference in which phase is connected to which bushing:
Auto 1: A to H1/X1, B to H2/X2, C to H3/X3 (1993 vintage)
Auto 2: C to H1/X1, A to H2/X2, B to H3/X3 (2008 vintage)
The phasing issue has been worked out.
The design has been questioned by some field people: Someone wondered if some unknown problem might arise, such an interplay of the excitation currents between the two xfmrs. My reaction is that I do not think any problems will arise, but maybe I just have insufficient experience. Does anyone have any experience with a problem that might arise?

 

[BJC]

Are you paralleling the tertiarys?? The installations I have seen did not. Both were taken into the control building and a transfer switch used on to power critical loads.

 

[cranky108]

There will be some, but it should be managable. The problem comes in mainly when you have LTC's on the transformers.

If you are concerned about the LTC: I would think problems would happen if you have significent system unbalance. And with modern theory of not needing to transpose lines that is a real possibility.

This mainly applies to the cross connected CT method, and may not apply to the other methods.

 

[PaulSaul]

I guess since the phase sequence is d same, there should not be any problem once the CT secondary conections are also connected same in the control panel

 

[BJC]

PG&E was paralleling a lot of 400 MVA transformers in Substations. I cant remember how many taps they had but it was like 12 above and 12 below. They Load taps and were PLC controlled. The PLc would switch the taps up and down.
They replaced a set up that used a transforme with minimal taps and an in series voltage regulator ( autotransformer).
I don't remeber seeing the tertiarys paralleled.

 

[jghrist]

There are some good papers on paralleling transformers on the Beckwith website

http://www.beckwithelectric.com/lc/controls/apptips.htm

 

[prc]

You must check from rating plate that voltage ratio at all taps are same.Check from test report that impedance between H-L at rated and extreme tappings are same or are within 10 %.In case you want parallel operation on tertiary side,then you should check tertiary limb impedance too. Verify OLTC control scheme for their compatability.