what does this phrase mean "a transformer with 5% impedance'
what does this phrase mean "a transformer with 5% impedance'
(OP)
Hi all,
what does this phrase mean "a transformer with 5% impedance'? Is it just the p.u. impedance? Also, what are the formulae for calculating that percentage impedance?
Thanks,
Andrew
what does this phrase mean "a transformer with 5% impedance'? Is it just the p.u. impedance? Also, what are the formulae for calculating that percentage impedance?
Thanks,
Andrew






RE: what does this phrase mean "a transformer with 5% impedance'
Zbase = kV^2 / MVA
Z p.u. = Z in ohms / Zbase
Please see any power engineering text for more detail on per-unit method.
RE: what does this phrase mean "a transformer with 5% impedance'
RE: what does this phrase mean "a transformer with 5% impedance'
It is the percentage voltage applied on the primary to produce a full rated load current in the 'short circuited secondary'.
RE: what does this phrase mean "a transformer with 5% impedance'
RE: what does this phrase mean "a transformer with 5% impedance'
Zpu=Rpu+jXpu
pu=per unit
It may also be presented as impedance in series with a voltage source.
RE: what does this phrase mean "a transformer with 5% impedance'
Z % = Z p.u. x 100
Zbase = kV^2 / MVA
Z p.u. = Z in ohms / Zbase
///kV is the secondary rated voltage since the primary voltage must be higher for the transformer secondary to deliver the rated kVA, Voltage and Current.\\\
RE: what does this phrase mean "a transformer with 5% impedance'
RE: what does this phrase mean "a transformer with 5% impedance'
%Z is not calculated, but rather measured by test, as indicated above, by shorting the seconday terminals and applying just enough voltage that will create the rated full load current in the secondary. The ratio of this voltage to per rated primary voltage times 100= %Z.
The meaning or signinficance of the Z% impedance is that it helps determine the maximum short circuit current the transformer can deliver. Assuming that the primary is connected to a infinite source, the maximum Isc= (rated full load amps*100/ percent impedance)
or Isc = FLA/Z in per unit , 5%=0.05 per unit.
For example, a 500kVA,480V secondary tranformer (600A FLA)with 5% Z will can deliver maximum of 600/.05 =12000A of SCC.
Hope this helps.
RE: what does this phrase mean "a transformer with 5% impedance'
RE: what does this phrase mean "a transformer with 5% impedance'
Clarification to jbartos's comment - per-unit or % impedance is the same from either the primary or secondary. Calculate Zbase for the voltage rating of the winding being considered. The measured impedance in ohms divided by Zbase for the side you are measuring will give you the same impedance regardless of which winding you measure from.
RE: what does this phrase mean "a transformer with 5% impedance'