Electrical Machine - IES Academy

India’s No. 1
IES Academy
Transformer
Chapter-1
transformers A and B are equal. The total output kVA is reduced; because I is the
phasor sum and not the arithmetic sum, of Ia and Ib.
Ia < I/2 and Ib < I/2 and the total kVA of output is less than the sum of kVA ratings
of the individual transformers. In general when reactance to resistance ratios is not
equal, their full load kVA output is less than the sum of their individual transformer
kVA ratings. Also, a transformer with greater leakage impedance angle operates
at a poor pf as compared to the other with a lower leakage impedance angle.
In case of equal voltage ratio and equal leakage impedances, no circulating current will
flow.
x x
(b) Let zea
> zeb
=
r r
,but
ea eb
ea
eb
When E = E , I z = I z and since z > z , I < I
a b a ea b eb ea eb a b
Both the transformers operate at the same power factor and total output kVA is increased.
Since I is the arithmetic sum Ia and Ib , the total output kVA is equal to sum of kVA ratings of
the individual transformers.
The kVA supplied by transformers A and B are EaIa and EbIb × 10 –3 respectively.
Here zea
> z
eb
, Ia< Ib
, therefore EaIa < EbIb. It means that kVA shared by transformer A is less
than that shared by transformer B.
Note: — That transformer having greater equivalent leakage impedance shares less kVA (here
EaIa ×10 –3 ) and transformer having lower equivalent leakage impedance shares greater kVA.
Transformers of different kVA ratings can be operated in parallel provided their
equivalent leakage impedances in ohms are inversely proportional to their
respective kVA ratings.
1 1
zeaα
and zebα
Sar
Sbr
zea
Sbr
=
z S
eb
ar
I z
I
I
a ea b eb
a
b
S
=
S
= I z
ar
br
Parallel operation of transformers
xea
xeb
with zea
> zeb
but =
r r
The transformer with smaller leakage impedance will get saturate first.
ea
eb
www.iesacademy.com E-mail: iesacademy@yahoo.com Page-21
25, 1 st Floor, Jia Sarai, Near IIT. New Delhi-16 Ph: 011-26537570, 9810958290