Page 292 - Handbook of Electrical Engineering

278 HANDBOOK OF ELECTRICAL ENGINEERING

Therefore,
n

S gen = S gi = K g S load
i=1
Where S gi is the rating of the i th generator and K g is a marginal factor > 1.0.
The generator unit transformers have a total capacity S tg slightly higher than S gen ,

S tg = K tg S gen

Where K tg is a marginal factor > 1.0.
Assume the leakage reactance X tg of each generator unit transformer to be 0.08 per unit, and
ignore the resistance.

11.5.2.3 High voltage motors and their transformers

The high voltage motor unit transformers have a total capacity S tm slightly higher than that of the
motors S hm ,
S tm = K tm S hm
Where K tm is a marginal factor > 1.0.
Assume the leakage reactance of each motor unit transformer to be 0.06 per unit, and ignore
the resistance.

11.5.2.4 Lower voltage distribution transformers

Assume that the lower voltage switchboards are each fed by two transformers and that the bus-section
circuit breaker is normally open. In this configuration each transformer carries half the load on its
switchboard. Therefore the total capacity of the distribution transformers S td is at least twice that of
the load,
S td = 2K td (S lm + S ls )

Where K td is a marginal factor to account for future increase in load, assume K td to be 1.3.

Assume the leakage reactance X td of each transformer to be 0.055 per unit, and ignore the resistance.

11.5.2.5 Equivalent transformer

Suppose a main switchboard feeds load through transformers of different ratings and impedances.
For the purpose of estimating fault current at an early stage in a project it is reasonable to combine
all the distribution transformers into one equivalent transformer. The equivalent rating S te of all the