Home › Electrical Engineering Forum › General Discussion › what is % impedance of transformer?
 This topic has 8 replies, 7 voices, and was last updated 11 years, 6 months ago by Spir Georges GHALI.

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2011/04/11 at 10:10 am #10447jatin333Participant
what is % impedance of transformer? how we calculate %impedence of transformer,
what is purpose of % impedence?
2011/04/14 at 4:25 pm #11960adminKeymaster%impedence=impedence voltage/rated voltage *100
%Z=IZ/Vr *100
2011/04/15 at 1:01 pm #11962freak16ParticipantThe normal method
of expressing transformer impedance is as a percentage voltage drop in the
transformer at fullload current and this reflects the way in which it is seen by
system designers. For example, an impedance of 10% means that the voltage
drop at fullload current is 10% of the opencircuit voltage, or, alternatively,
neglecting any other impedance in the system, at 10 times fullload current, the
voltage drop in the transformer is equal to the total system voltage.Formula written by MH is perfect.
Thenormalmethod
ofexpressingtransformerimpedanceisasapercentagevoltagedropinthe
transformeratfullloadcurrentandthisreﬂectsthewayinwhichitisseenby
systemdesigners.Forexample,animpedanceof10%meansthatthevoltage
dropatfullloadcurrentis10%oftheopencircuitvoltage,or,alternatively,
neglectinganyotherimpedanceinthesystem,at10timesfullloadcurrent,the
voltagedropinthetransformerisequaltothetotalsystemvoltage.Expressed
insymbolsthisis:
Vz %Z
IFLZ
E
1002011/04/19 at 1:19 pm #11972erickenchParticipantTypical transformer impedances are around %6. Have you tried performing a short circuit test?
2011/04/25 at 5:29 am #12012jatin333Participantthanx guys
2011/04/27 at 1:10 am #12019Simon FosterParticipantAnother way of looking at this, and the way it's measured during factory acceptance testing is:
Short circuit is placed on the LV connections, then simply apply a voltage to the HV connections to create Full Load current in the LV winding.
The percentage of HV voltage required to do this is also the percentage impedance.
example:
600V required on HV to circulate Full Load current in the LV.
If HV rated Voltage is 12000V
The %Z is 5% or 600/12000
Hope this helps
Simon
2011/04/30 at 2:02 pm #12028adminKeymasterIt is voltage drop in % of nominal voltage
2011/05/03 at 1:38 pm #12048electricalexpert65Participantjatin333 said:
what is % impedance of transformer? how we calculate %impedence of transformer,
what is purpose of % impedence?
Have you ever wondered why the name plate LV Volatge in a transformer is 433V but the name plate voltage of an LV motor is 415V? Are they both not LV? Yes! the 433 V in a trf. name plate is the open circuit voltage or the no load voltage. But, when you load the transformer, a current would flow thro its windings. As the windings will have an impedance, this current and impedance would very definitely cause a voltage drop within the transformer windings. On emust know the magnitude of this voltage drop. The volatge drop within the transformer's LV windings, with rated load current flowing thro it, expressed as a percentage of the no load voltage of the teansformer is called its percentage impedance.
2011/06/06 at 4:14 pm #12123Spir Georges GHALIParticipantjatin333 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.

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