3-Winding Transformer Model
3-Winding Transformer Model
(OP)
I was provided the data to model a 230/34.35 kV 3 winding transformer for a wind farm with the +/- 12.5 % regulation and +/- 8 taps. The primary-secondary impedance value for the transformer is 11.80% at nominal and 12.4% (8R tap position, 258.75 kV) and 11.81% (8L tap position, 201.25 kV) all using 124 MVA base - Voltage control is on the primary (230 kV) and secondary (34.5 kV) is held constant. Recently, however, realizing that the voltage regulation should be on the secondary, the transformer mfr provided the impedance values of 15.72% (for the 8R, 30.667 kV), 11.87% (for Nominal, 34.5 kV), and 9.03% (for 8L, 39.481 kV). They suggested this as the same impedance value. I have 2 questions, 1. Is what they stated valid and if so, would the short circuit calculation at the lowest tap position still valid? 2. If you were provided the 2 sets of impedance data as above, which one would you use for the short circuit evaluation since the 9.03% impedance would represent a much larger than the 11.81% ? Thanks for any advice.






RE: 3-Winding Transformer Model
RE: 3-Winding Transformer Model
RE: 3-Winding Transformer Model
RE: 3-Winding Transformer Model
RE: 3-Winding Transformer Model
RE: 3-Winding Transformer Model
The first set of figure with the higher impedance (11.81) at the lowest tap (8L) was for primary voltage (230 kV) regulation between +/- 12.5% from nominal while holding the secondary constant. An engineer suggests the voltage should be held constant at the primary, and secondary voltage should be varied. Thus the mfr provided the later set of impedance to justify that if the 230 kV voltage is held constant, the 34.5 kV could be varied between 30.667 to 39.481 kV - at the lowest tap position (8L) this would constitute to the impedance value of 9.03%. That was what threw me off.
RE: 3-Winding Transformer Model
For example at 8R:
15.72%= 12.4*(30.667/34.5)^2
Which set of per unit impedances you choose to use will depend on which set of base voltages you use for the rest of the ETAP model.
As prc pointed out, it is very important to operate the transformer core within allowable flux levels and the windings within allowable current limits. If the transformer was designed for variable high side voltage, the core may saturate if operated at 230 to 39.481 instead of 201 kv to 39.5 kV. Likewise,the windings may be limited to 110 MVA if operated at 230 kV to 30.66 kV instead of at 258 kV to 34.5 kV.
RE: 3-Winding Transformer Model
%Z =Z/Zref where Z actual series impedance in ohms (R+jX) Zref = UxU/Sr where U is rated voltage or tapping voltage,Sr= rated power of the transformer.
RE: 3-Winding Transformer Model