# Re: what is % impedance of transformer?

Home Electrical Engineering Forum General Discussion what is % impedance of transformer? Re: what is % impedance of transformer?

#12123

jatin333 said:

what is % impedance of transformer? how we calculate %impedence of transformer,

what is purpose of % impedence?

Dear Mr. Jatin ;

In some case called the impedance of transformer that is given in ” % “, but the sientific name is ” Usc  or Ucc ” that means the percentage of primary voltage ( HV ) that should be applied to have the nimonal current ” In ” on short circuited secondary.

Normally, each transformer hase a Resistance ” Rtr ” and an Inductance ” Xtr “, where we can calculate by the following formula :

– Ztr = Usc x U²/Sn

– Rtr = Ptr / 3 x In²

– Xtr² = Ztr² – Rtr²

Noting that :

– Ztr : the impedance of transformer

– Usc : the voltage in percentage of transformer

– U : the voltage between 2 phases

– In : the nominal current for the transformer's secondary

– Ptr : the total loss at full load

As desiegner, we should calculate the ” Rtr  &  Xtr ” to the transformer ten add themm to the values ” R  &  X ” of cables to calculate the different values of short circuit currents.

Regards.