per unit impedance values of a transformer

[tariro]

how do you calculate the per unit impedance value of a transformer given the percentage impedance value? How do you also estimate the per unit vales of resistance and reactance from the percentage impedance value?

 

[Skogsgurra]

Per Unit is same thing as percentage. Only a factor 100 different.

So, if you have 5.6 % that will be equal to 0.056 p.u.

The impedance of a power transformer (10+ kVA) is almost purely reactive. You can safely assume close to 90 degrees impedance. If you need better values, you have to ask the manufacturer for data,

Gunnar Englund

http://www.gke.org

 

[jghrist]

I think skogsgurra must have meant (10+ MVA). Distribution transformers have X/R ratios ranging from 1 to 6.

You can calculate %R from the load (copper) losses divided by the rating times 100. %X is sqrt(%Z²-%R²).

 

[Skogsgurra]

Thanks, jghrist. But I actually meant kVA. It all depends on what you mean with "almost purely reactive" - a matter of discussion. Someone?

Gunnar Englund

http://www.gke.org

 

[rbulsara]

Also if you know actual R in ohms from copper losses, to get the p.u. value, you need to divide it by the base Z which is base ( rated) L-N voltage divided by the base current. The base current is the rated current.

 

[varri79]

Just to summarize,

%impedance means, the ratio of impedance voltage drop at full load to the normal voltage, when multiplied by 100.

Let us take LV side of the trafo as base values,

%Z= I Z ÷ V, where Z is actual impedance or leakage impedance referred to LV side, and V & I are rated values of LV side.

then,

Z = %Z V ÷ I --- referred to LV side of trafo

%R = Full Load Cu losses ÷ trafo rating in VA ,
(R = Full Load Cu losses /I*I - ref to LV)

%X = sqrt {%Z*%Z - %R*%R}


P.U. impedance = % impednace / 100
or
actual impedance / base impedance.

Where, base impedance = V / I (rated values of base side i.e LV side )

or

KV* KV*1000 / KVA, ( where KV is LV side voltage).

hope this help you.

Varri

 

[cuky2000]

See the enclose figure to determine approx. the X/R Ratio

X~Z ==> R~Z/(X/R)