Electrical Engineer with overall 23 years of diversified experience in Projects, Design, Engineering, Installation, Testing, Commissioning & Maintenance of electrical system in petrochemical & fertilizer plants.

1. Ashish Kumar Ganguli
Date: 21.01.2017
PERCETANGE IMPEDENCE OF TRANSFORMER
&
IT'S IMPORATANCE IN POWER SYSTEM STUDIES.
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BY :
ASHISH KUMAR GANGULI
Bachelor of Engineering - Electrical
MOBILE : 0091-9109782929
EMAIL : ashish_ganguli06@yahoo.co.in
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2. Ashish Kumar Ganguli
Date: 21.01.2017
DEFINITAION:
Percentage impedance of a transformer is the percentage of rated voltage applied at one
side (primary winding) to circulate rated current on transformer keeping its other side
(secondary winding) under short circuit conditions. It is marked on transformer
nameplate.
EXPLANATION:
If we apply rated voltage at primary winding of a transformer keeping its secondary
winding short circuited, then amount of current at both windings will be extremely
high as compared to the rated current. This current is called short circuit current and its
magnitude is very high due to zero impedance offered by the load (secondary winding
is short circuited).
Consider,
V : Rated Primary Voltage of Transformer
Z% : Percentage Impedance of Transformer
Irated : Rated Primary Current of Transformer
Zactual : Actual Impedance of Transformer (referred to primary winding)
Based on above explanation, we are applying a percentage (Z %) of rated primary
voltage. Therefore applied voltage is equal to V x Z% / 100. This voltage drives rated
current in transformer winding.
By Ohms Law:
Voltage = Current x Actual Impedance
V x Z% /100 = Irated x Zactual
Z% = Irated Zactual x 100 / V
Now, if we reduce the applied voltage on the
transformer primary i.e. we apply percentage of
rated voltage in transformer primary, current on
both windings will also reduce. At a particular
percentage of rated voltage, rated current will flow
on transformer windings. This percentage of rated
voltage at one side of transformer which circulates
rated current on transformer windings keeping its
other side winding short circuited, is called
percentage impedance of transformer.

3. Ashish Kumar Ganguli
Date: 21.01.2017
ROLE OF Z% IN SHORT CIRCUIT CALCULATION
Percentage impedance of transformer plays extremely vital role is network calculation
i.e. short circuit calculation and voltage drop calculation.
We can find the short circuit contribution of a transformer as per below explanation:
As considered above, secondary winding of transformer is short circuited.
Now,
If Voltage (V Z% /100) is applied on Transformer Primary, Rated Current (Irated) flows.
If Voltage (V) is applied on Transformer Primary, Current (Irated x 100 / Z%) will flow.
Therefore, when we apply rated voltage at primary winding of a transformer whose
secondary winding is short circuited, short circuit current (Irated x 100 / Z%) will flow on
transformer windings. Value of Z% is same for both windings as it is percentage of
rated voltage. However value of Irated will be different for primary and secondary
winding. Accordingly value of short circuit current will also be different for primary
and secondary windings.
Short Circuit Current (Isc) = Irated x 100 / Z%
If percentage impedance of a transformer is less, its short circuit current will be more
which will produce more stress in insulation. This is a negative factor. On the other
hand, if percentage impedance of a transformer is less, voltage drop on transformer
winding will be less which will facilitate better voltage regulation. This is a positive
factor. Therefore, percentage impedance of transformer has to be precisely selected to
maintain a proper balance between fault level and voltage regulation.
PERMITTED TOLERANCE IN Z%
Percentage impedance of transformer is specified at the time of ordering. But it must
be noted that IEC 76 permits +/- 10% tolerance in percentage impedance at
manufacturer's end. Therefore, if we order a transformer with 8% percentage
impedance, its actual percentage impedance after manufacturing may be any value
between 7.2% (-10% of 8) to 8.8% (+10% of 8), unless it is specifically agreed with
manufacturer at the time of ordering. Tolerance in percentage impedance must be
considered for power system calculations and accordingly system fault level & voltage
regulation must be finalized.

4. Ashish Kumar Ganguli
Date: 21.01.2017
ROLE OF Z% IN PARALLEL OPERATION OF TRANSFORMERS
If the ratios of kVA rating to percent impedance of two transformers operating in
parallel are equal, they will share equal load. However if ratio is different, they will
share unequal load. This may result in overloading of one transformer.
In order to analyses this criterion; let us consider two transformers operating in parallel
as shown in below diagram.
I1 : Load current from transformer 1
I2 : Load current from transformer 2
IL : Total load current (IL = I1 + I2)
Z1 : Actual impedance of transformer 1
Z2 : Actual impedance of transformer 2
Z1% : Percentage Impedance of transformer 2
Z2% : Percentage Impedance of transformer 2
kVA1 : kVA rating of transformer 1
kVA2 : kVA rating of transformer 2
kVAL : Total load kVA

5. Ashish Kumar Ganguli
Date: 21.01.2017
We know that
V x Z% / 100 = Irated Zactual
Therefore,
Z1 = (V x Z%1)/ (100 x I1)
Z2 = (V x Z%2)/ (100 x I2)
If we substitute value of actual impedance (Z1 & Z2) as per above equation, we get
If we multiply both sides with kV, we get
If ratio kVA1 / Z1% and kVA2 / Z2% are equal, then kVA1 and kVA2 will be same. That
means, both transformers will share equal load i.e. 50% of total load. This factor must
be analyzed before paralleling two transformers to avoid unequal load sharing and
overloading of one transformer.