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ideal requirements for paralleling. In actual practice good paralleling is obtained even though the
actual transformer conditions deviate by small percentages from the theoretical ones.
(a) In order to insure maximum use of transformer capacity, and to prevent overloading
of one transformer inadvertently, the criterion generally accepted is that the circulating current
for any combination of ratios and impedances should not exceed ten percent of the full-load rated
current of the smallest unit. This can generally be accomplished if the impedances are within 7.5
percent of each other, as this is the standard ANSI tolerance from a nominal specified value for a
group of transformers that are manufactured to the same specifications. The X/R ratio of the
transformers should also be within 7.5 percent of each other. A more detailed discussion of
parallel operation of transformers with equations to evaluate whether parallel operation can be
safely achieved, is contained in Chapter 5 of Transformers, Second Edition, K. L. Gebert and K.
R. Edwards, American Technical Society, Chicago, IL.
(b) When it is desired to parallel transformers having widely different impedances,
reactors or autotransformers having the proper ratio should be used. If a reactor is used, it is
placed in series with a transformer having a lower impedance. It should have an impedance
value sufficient to bring the total effective percent impedance of the transformer and reactor up to
the value of the percent impedance of the second transformer. When an autotransformer is used,
the relative currents supplied by each transformer are determined by the ratio of the two sections
of the autotransformer. The autotransformer adds a voltage to the voltage drop in the transformer
with the lower impedance and subtracts a voltage from the voltage drop in the transformer with
the higher impedance. Autotransformers for use in paralleling power transformers are designed
specifically for each installation.
3.2.10 Classifications. Transformers have many classifications which are useful in the
industry to distinguish or define certain characteristics of design and application. Distribution and Power. Transformers may be classified according to the rating
in kVA. The distribution type covers the range of 3 kVA through 500 kVA, and the power type
all ratings above 500 kVA or over 67 kV system voltage. Insulation. Transformers may be classified by insulation type, as liquid and dry.
Liquid insulated can be further defined according to the types of liquid: mineral oil, askarel, or
other synthetic liquids. The dry-type grouping includes the ventilated and sealed gas-filled
types. Substation or Unit Substation. The title substation transformer usually denotes a
power transformer with direct cable or overhead line termination facilities. This distinguishes it
from a unit substation transformer designed for integral connection to primary or secondary
switchgear, or both, through enclosed bus connections. The substation classification is further
defined by the terms primary and secondary. The primary substation transformer has a secondary


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