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Transformer Impedance 2

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WDB

WDB

Electrical
Nov 23, 2000
2
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.

Thanks
WDB
 
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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.
 
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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
Thus:
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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