Step-type Voltage Regulator Impedance 1

[dpc]

I am trying to model some step-type voltage regulators for short-circuit calculations. I have a couple of questions:

Does anyone have a source for typical regulator impedances? This data is not on the regulator nameplate.

Due to the limitations of the modeling software, I need to model these as two-winding transformers with load tap changers, even though they are really autotransformers. Can someone explain how to translate an autotransformer kVA and impedance to an equivalent two-winding kVA and impedance for the purpose of short circuit calculations? (This has always confused me, for some reason)

Thanks.

 

[tgott]

From Westinghouse T&D Reference Book, Chapter 11, page 395
TABLE 13 - Impedance of Feeder Regulators - Percent On
Circuit KVA Base - Plus or Minus 10 Percent Regulation
Max. Min. Neutral Pos.
Induction
Single-phase..................... 2.5 0.7 1.5
Three-phase...................... 1.5 1 ...
Step-Type
Single-core....................... 0.4 0 ...
Two-core, three-winding......0.7 0.4 ...
Two-core, four-winding...... 1.1 0.5 ...

 

[tgott]

Also from Westinghouse T&D Reference Book Chapter 5, page 117. The ratio between winding kva (Up or Us) and circuit kva (Uc) is,
Up/Uc = Ea x Ip/Ea x Ia = Ip/ (N/N-1)Ip = N-1/N

For example, an autotransformer rated 1000 kva, with a circuit voltage ratio of 22 kv to 33 kv (N = 33/22 = 1.5) has an equivalent two-winding kva of
Up = Us = (N-1)Uc/N = (1.5-1.0)x1000/1.5 = 333 kva

To express the impedance in terms of Zps
Za = ((N-1/N)^2) x Zps

Sequence connections are shown in the appendix. I can e-mail the drawings to you if needed.

 

[dpc]

Thanks, tgott. I have the T&D Book, but I never thought to look in there. I think I'm in business now - thanks again.