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Transformer Impedance

Transformer Impedance

Transformer Impedance

Given the %impedance of a power transformer, how can the actual impedance be determined?  The transformer in question is an autotransformer base rated at 900 MVA with an H impedance of 7.03%, the X of 54.8% and a Y of 48.03%. Voltage on H is 525 KV, on X is 240 KV and on Y is 25 KV (Y is the tertiary winding).  An x/r ratio is not known at this time.  


RE: Transformer Impedance

The high side H actual impedance may approximately be calculated as:
Zh=(7.03%/100%)x525000V/[900MVA/(sqrt3x0.525MV)]=37.29 Ohms
Similarly for X and Y, once the MVAx for X and MVAy for Y are known.

RE: Transformer Impedance

An easy way to calculate the impedance is from the formula
Z = (Z pu).(base Ohms)
Z = ((Z%/100).(kv)^2/MVA) Ohms phase to neutral
Zh = (7.03/100).(525)^2/900 = 21.53 Ohms (phase to neutral)

The other winding impedances will be calculated on the basis of their voltage and MVA ratings.

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