Introduction to Electrical Power and Power Electronics

128 Introduction to Electrical Power and Power Electronics

5.3 SELECTION OF KILOVOLT-AMPERE RATING

The transformer size (kilovolt-ampere rating) for a given load prole is determined

based on the following factors:

• The peak load it would supply, taking into account the National Electrical Code

(NEC) load factors if numerous small loads with intermittent use are connected

to it. Large loads are accounted for their individual on and off timing.

• For transformers supplying harmonic-rich power electronics loads, the

additional harmonics heating is accounted for by selecting correct K-rating

(discussed in Section 16.3.3).

• About 30% margin is then added for the future growth, which will make the

transformer operate at about 1/1.3 = 0.77 or 77 ...