Electrical Substations - Energy Conversion and Management - page 50

Fig. 5.2 Ratio r versus load factor x if the maximum transformer efficiency condition is imposed

be selected. Finally, provided that the other selection parameters are satisfied, the
nameplate power needed for maximum efficiency can be determined, with due
consideration to the load average power value. Table 5.1 lists standard transformer
parameters. Notice that standard type transformers with reduced losses are to be
preferred; only for very large machines or for special applications should

non-standard machines be considered.
Table 5.2 and Fig. 5.3 show the influence of no-load and load losses when the
load is changing and the value of r ratio is defined. The point of balance between
losses corresponds to the maximum efficiency.
Economic evaluations for transformer selection based on energy saving can be
made as explained in Chap. 20 . As a general rule, notice that in normal conditions
the annual losses cost is equal to roughly 10-30 % of the capital cost of the
transformer itself. It is worth remembering that in sizing transformers attention
must be paid to correlated costs, such as control and protection switchgear, voltage
level, cables, foundations, space occupied, etc., which are approximately equal to
the cost of the transformer itself, so that the incidence of annual losses is cut in half.
Oversizing transformers may thus be more expensive with respect to switchgear