278 HANDBOOK OF ELECTRICAL ENGINEERING
Therefore,
Sgen=∑ni= 1Sgi=KgSloadWhereSgiis the rating of theithgenerator andKgis a marginal factor> 1 .0.
The generator unit transformers have a total capacityStgslightly higher thanSgen,
Stg=KtgSgenWhereKtgis a marginal factor> 1 .0.
Assume the leakage reactanceXtgof 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 capacityStmslightly higher than that of the
motorsShm,
Stm=KtmShm
WhereKtmis 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 transformersStdis at least twice that of
the load,
Std= 2 Ktd(Slm+Sls)
WhereKtdis a marginal factor to account for future increase in load, assumeKtdto be 1.3.
Assume the leakage reactanceXtdof 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 ratingSteof all the