Transformer Impedances

[tlr7616]

Hi Everyone,

Perhaps I a bit rusty but in any case here is my situation:

I am trying to complete a generation interconnection data sheet for submission. I have a project that will be using a 27/230 kV step-up transformer. It will be rated at 150 MVA. Now, given that the client has actually NEVER built anything like this I am stuck with providing impedances on something that doesn't yet exist. The simplest way to do this (I think) is to hit a WECC load flow case and find something that is close in voltage class and MVA capacity. OK, done, no problem. I have a per unit R and X.

Now, it has been a looonnng time--how do I convert these per unit quantities into things like winding resistance? I know how to take a per unit X and turn it into percent impedance, but I am somewhat uncertain on how winding resistance shakes out. Any advice?

Thanks!

 

[jghrist]

What, precisely, do you need to provide on the generation interconnection data sheet?

What, precisely, have you gotten from the WECC load flow case? Per unit R and X of what?

 

[tlr7616]

Ok, here is waht I need to provide:

Primary Winding (i.e. H Winding):

Winding Resistance in ohms
Impedance in Percent
MVA Base
Tested Taps

Secondary Winding (i.e. X Winding):

Winding Resistance in ohms
Impedance in Percent
MVA Base
Tested Taps

The actual data sheet to be filled out is available at the following URL:

http://www.caiso.com/docs/2002/06/11/2002061110504928318.doc

I am thinking about using that R, X, and B in per unit quantities from a WECC load flow case and then calculate the ohmic values from those. R=0.0380, X=0.12837, and B=0 for a 150 MVA 230/25 kV transformer on a 100 MVA system base. The CAISO staff is of little help on what they really mean in regards to tested taps--I probably haven't found my way to the right person yet.

I am not really a transformer person but rather a plannig engineer--load flow and transient stability studies. I suppose I have been spoiled by having these numbers provided for so long, I mean does anyone actually have winding resistances of transformers available off of the top of their heads? I figure the answer was always as low as possible.

Anyway, hopefully this will enable someone to re-educate me...

Thanks again!

 

[bacon4life]

Tested taps probably refers to which taps the impedance was tested on. For example your 230/27 kV transformer might also have 235.75/27 kV and 241.5/27 kV taps. The actual impedance in ohms changes depending on the tap. And of course you also need to convert the per unit value if the transformer base is not the same as the system base.