Page 53 - Solutions Manual to accompany Electric Machinery Fundamentals

282  1317 
R   0.0122 pu X   0.0572 pu
EQ EQ
23,040  23,040 
The per-unit, per-phase equivalent circuit of the transformer bank is shown below:
I P I S
jX EQ
R EQ

+ 0.0122 j0.0572 +

V P R C jX M V S

63.6 j18.3

- -

(b) If this transformer is operating at rated load and 0.90 PF lagging, then current flow will be at an
angle of cos  1  9 . 0  , or –25.8. The per-unit voltage at the primary side of the transformer will be

V  V I Z 1.0 0       25.8  1.0 0.0125  j 0.0588  1.038 2.62  
P S S EQ
The voltage regulation of this transformer bank is


1.038 1.0
VR   100%  3.8%
1.0
(c) The output power of this transformer bank is
P OUT  V I cos    1.0  1.0  0.9  0.9 pu
SS
The copper losses are

P  I R   1.0 2  0.0122   0.0122 pu
2
CU S EQ
The core losses are

2  V 1.038 2
P  P   0.0169 pu
core
R 63.6
C
Therefore, the total input power to the transformer bank is
P  IN P OUT  P  CU P core  0.9 0.0122 0.0169  0.929


and the efficiency of the transformer bank is

P 0.9
  OUT  100%   100% 96.9%

P IN 0.929
2-13. A 14,400/480-V three-phase Y--connected transformer bank consists of three identical 100-kVA
8314/480-V transformers. It is supplied with power directly from a large constant-voltage bus. In the
short-circuit test, the recorded values on the high-voltage side for one of these transformers are

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