3-Winding Transformer Equivalent Model
3-Winding Transformer Equivalent Model
(OP)
Hi everyone,
I'm confused about how to model a 3-winding transformer as three 2-winding transformers in star-formation. I was under the impression that it is a simple Delta-to-Star conversion of the impedances, but it is not according to some.
I've got an Autotransformer with a delta tertiary winding 330/132/22kV, 490MVA with the following impedances based on its rate:
Zps = 0.2081 pu
Zpt = 1.0045 pu
Zst = 0.7530 pu
If I follow a conventional Delta-to-Star transformation, then:
Zp=(Zps*Zpt)/(Zps+Zpt+Zst) = 0.10635 pu
Zs=(Zps*Zst)/(Zps+Zpt+Zst) = 0.01967 pu
Zt=(Zpt*Zst)/(Zps+Zpt+Zst) = 0.38481 pu
However, according to a different formula it should be:
Zp = (Zps + Zpt -Zst) / 2 = 0.2298 pu
Zs = (Zps + Zst -Zpt) / 2 = -0.0217 pu
Zt = (Zpt + Zst -Zps) / 2 = 0.7747 pu
Which of these calculations is right and on what bases?!
Thanks in advance for your help.
-Radi
I'm confused about how to model a 3-winding transformer as three 2-winding transformers in star-formation. I was under the impression that it is a simple Delta-to-Star conversion of the impedances, but it is not according to some.
I've got an Autotransformer with a delta tertiary winding 330/132/22kV, 490MVA with the following impedances based on its rate:
Zps = 0.2081 pu
Zpt = 1.0045 pu
Zst = 0.7530 pu
If I follow a conventional Delta-to-Star transformation, then:
Zp=(Zps*Zpt)/(Zps+Zpt+Zst) = 0.10635 pu
Zs=(Zps*Zst)/(Zps+Zpt+Zst) = 0.01967 pu
Zt=(Zpt*Zst)/(Zps+Zpt+Zst) = 0.38481 pu
However, according to a different formula it should be:
Zp = (Zps + Zpt -Zst) / 2 = 0.2298 pu
Zs = (Zps + Zst -Zpt) / 2 = -0.0217 pu
Zt = (Zpt + Zst -Zps) / 2 = 0.7747 pu
Which of these calculations is right and on what bases?!
Thanks in advance for your help.
-Radi






RE: 3-Winding Transformer Equivalent Model
regards,
RE: 3-Winding Transformer Equivalent Model
I can still accept a negative reactance (as shown above), but will also yeild negative resistance, which is hard to accept!
Thanks
RE: 3-Winding Transformer Equivalent Model
RE: 3-Winding Transformer Equivalent Model
In factory we find out the resistance value of each circuit by measuring losses by pair and applying formula similar to the one you mentioned for impedances to arrive at losses of each circuit.From that R is calculated out. Then from measured impedance of each circuit, reactances are calculated.