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% Z of X-Mer and Its Calculation

Percentage impedance (%Z) of a transformer measures voltage drops at full load due to winding resistance and leakage reactance, impacting voltage regulation and short circuit levels. High %Z reduces short circuit current but leads to poor voltage regulation and efficiency, while low %Z results in better regulation but higher short circuit levels. The document explains how to calculate %Z and its significance in designing protection switchgear to withstand maximum fault currents.

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61 views3 pages

% Z of X-Mer and Its Calculation

Percentage impedance (%Z) of a transformer measures voltage drops at full load due to winding resistance and leakage reactance, impacting voltage regulation and short circuit levels. High %Z reduces short circuit current but leads to poor voltage regulation and efficiency, while low %Z results in better regulation but higher short circuit levels. The document explains how to calculate %Z and its significance in designing protection switchgear to withstand maximum fault currents.

Uploaded by

sachinyadavw98
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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Percentage Impedance of Transformer and Its Calculation

“The percentage impedance is nothing but a measure of the volt drops when the
transformer in on full load due to the winding resistance and leakage reactance expressed
as a percentage of the rated voltage.” Also in other words it is a percentage of the ratio
between the full load short circuit terminal voltage to nominal voltage of the transformer. It
is denoted by %Z. Z is called Impedance.

Secondly, it defines the voltage regulation of the transformer because of


maximum voltage regulation is directly proportional to the p.u impedance of the
transformer. So high p.u impedance will reduce the short circuit level and on the other it will
result in poor voltage regulation. And low p.u impedance will result in higher short circuit
level and good voltage regulation.

Percentage Impedance Calculation:

Take a single transformer. The primary of the transformer will be connected with the
variable AC voltage source (Variac) and the secondary terminal of the transformer will be
shorted as shown in the figure. The primary of the transformer will be energized from
low voltage level. Upon increasing the voltage across the primary, the
secondary current starts increasing. Raise the voltage in primary until to reach rated
secondary current in secondary winding. Now measure the voltage across the secondary
terminal. This is the voltage drop due to the internal resistance and reactance of the
transformer.

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Percentage Impedance Formula:

Percentage Impedance in Nameplate of an 11kV/415V Transformer

Significance of percentage impedance:


 Increasing % Z or percentage impedance reduces the short circuit current and,
 High Value of %Z percentage impedance causes more voltage drop and poor voltage
regulation. Finally, transformer delivers less efficiency.
 The %Z is used to design protection switchgear. Typically, it is used to calculate
maximum level of fault current. So that, we can design a switch gear to withstand that
maximum level of fault current.

Let see an example.

The short circuit current can be calculated from the ratio of full load current to the
percentage impedance value. Thus, the formula of short circuit current using percentage
impedance,

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Transformer rating:

 75 kVA
 240 V secondary
 5% impedance

Calculate the full load secondary current

Therefore, the short circuit current Isc is

Therefore, the maximum short circuit current is almost 20 times of its full load current.

Summary
So that we can design or choose our switchgear to withstand at 20 times of its full load
current for the transformer of 5% percentage Impedance. The rating is called kAIC. i.e. if we
have less than 5% i.e 2.5 % means the short circuit current still increase and again we have
to go for high rating of kAIC i.e. 40 times for 2.5 %Z.

----------------------------------------------
Md. Humayun Kabir Khan
Assistant Professor
Department of EEE, City University

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