Page 242 - Handbook of Electrical Engineering

226 HANDBOOK OF ELECTRICAL ENGINEERING

The base current I base at 11,000 V is,

S f × 10 6 150 × 10 6
I base = √ = √ = 7872.9amps
3 × V base 3 × 11000
The base impedance Z base is,

V base 11000
Z base = √ = √ = 0.8067 ohms/phase
3 × I base 3 × 7872.9
This impedance has an X/R ratio of 10, its resistance R base and reactance X base are,

2

X base 2
2
2
Z base = R + X = + X
base base base
10

2 2
= X base 0.1 + 1.0 = 1.00499X base
0.8067
X base = = 0.8027 ohms/phase
1.00499
And
X base
R base = = 0.08027 ohms/phase
10
Transfer these components to the secondary circuit at 460 volts, and call them Z bs , R bs and X bs .
The transformation ratio u ps of the impedance is,

460 2
u ps = = 0.001749
11000 2
Z bs = Z base × u ps = 0.8067 × 0.001749 = 0.001411 ohms/phase

Similarly R bs = 0.000140 ohms/phase
And X bs = 0.001404 ohms/phase

The ohmic impedance of the load on the transformer Z flt seen at its secondary winding is
found as follows:-
Full-load current I flt of the transformer secondary winding is,

S t × 10 6
I flt = √
3 × V os

Where, S t is the MVA rating of the transformer, and V os is the open-circuit line voltage of the
secondary winding.
2.5 × 10 6
I flt = √ = 3137.8 amps/phase
3 × 460