Page 108 - Solutions Manual to accompany Electric Machinery Fundamentals

10,566 V
X   8.52 
Su
1240 A
The base impedance of this generator is

3 V 2   3 7044 V 2
Z   ,base   5.95 
base
S base 25,000,000 VA
Therefore, the per-unit unsaturated synchronous reactance is
8.52 
X Su ,pu  5.95   1.43

(b) The saturated synchronous reactance at a field current of 380 A can be found from the OCC and the
SCC. The OCC voltage at I = 380 A is 14.1 kV, and the short-circuit current is 1240 A. Since this
F
generator is Y-connected, the corresponding phase voltage is V   14.1 kV/ 3  8141 V and the

armature current is I  1240 A . Therefore, the saturated synchronous reactance is
A
X  8141 V  6.57 
S
1240 A
and the per-unit unsaturated synchronous reactance is
8.52 
X   1.10
,pu
S
5.95 
(c) The saturated synchronous reactance at a field current of 475 A can be found from the OCC and the
SCC. The OCC voltage at I = 475 A is 15.2 kV, and the short-circuit current is 1550 A. Since this
F
generator is Y-connected, the corresponding phase voltage is V   15.2 kV/ 3  8776 V and the

armature current is I  1550 A . Therefore, the saturated synchronous reactance is
A
8776 V
X   5.66 
S
1550 A
and the per-unit unsaturated synchronous reactance is
5.66 
X S ,pu   0.951
5.95 
(d) The rated voltage of this generator is 12.2 kV, which requires a field current of 275 A. The rated
line and armature current of this generator is
25 MVA
I  L   3 12.2 kV  1183 A

The field current required to produce a short-circuit current of 1183 A is about 365 A. Therefore, the
short-circuit ratio of this generator is

SCR  275 A  0.75
365 A

(e) The internal generated voltage of this generator at rated conditions would be calculated using the
saturated synchronous reactance, which is about X  6.57  if the field current is 380 A. Since the
S

power factor is 0.9 lagging, the armature current is I A  1183 25.8 A . The phase voltage is

V   12,200 / 3 0 V    7044 0 V . Therefore,

102