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 period. Since temperature distribution is uneven in most transformers, hottest-spot
insulation temperature usually determines insulation life and, therefore, transformer loading. A
65C rated transformer in a 30C ambient environment is usually rated for a hottest spot
temperature of 110C.
3.2.8.3 Overloads. Transformers have certain overload capabilities, varying with ambient
temperature, pre-loading, and overload duration. These capabilities are defined in ANSI Std.
C57.92, Guide for Loading Mineral-Oil-Immersed Distribution and Power Transformers, and
ANSI Std. C57.96, Guide for Loading Dry-Type Distribution and Power Transformers.
3.2.9 Parallel Operation. The theoretically ideal conditions for paralleling transformers are:
(a)
Identical turns ratios and voltage ratings.
(b)
Equal percent impedances.
(c)
Equal ratios of resistance to reactance.
(d)
Same polarity.
(e)
Same phase angle shift.
(f)
Same phase rotation.
3.2.9.1 Single-Phase Transformers. Only the first four conditions for paralleling
transformers (listed above) apply to single-phase transformers, as there is no voltage
transformation related to phase angle shift or phase rotation. All six conditions apply, however,
to paralleling three-phase-banks of single-phase transformers. If the turns ratios are not the same,
a circulating current will flow even at no load. If the percent impedance or the ratios of
resistance to reactance are different, there will be no circulating current at no load, and the
division of load between the loaded transformers will no longer be proportional to their kVA
ratings.
3.2.9.2 Three-Phase Transformers. While the same transformer paralleling conditions
apply to three-phase transformers, consideration must be given to the phase angle shift and to
phase rotation. The transformers must have the same winding arrangement.
(a) Certain transformer connections as the Wye-Delta or Wye-Zigzag produce a 30
shift between the line voltages on the primary side and those on the secondary side.
Transformers with these connections cannot be paralleled with transformers not having this shift
such as Wye-Wye, Delta-Delta, Zigzag-Delta, or Zigzag-Zigzag.
(b) Phase rotation refers to the sequence in which the terminal voltages reach their
maximum values. In paralleling, those terminals whose voltage maximums occur simultaneously
are paired.
3.2.9.3 Power Transformer Practice. The preceding discussion covered the theoretically
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