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 3.2.7.8 Six-Phase Star. The six-phase star connection is shown in Figure 3-8. It provides
a means for converting three-phase to six-phase transformations and is used in many rectifier and
thyristor circuits where a path for DC current flow is required. The characteristic angle of a
six-phase system is 60 degrees.
3.2.8 Loading of Transformers. Ordinarily the kVA that a transformer should carry is limited
by the effect of reactance on regulation or by the effect of load losses on system economy. At
times it is desirable to ignore these factors and increase the kVA load until the effect of
temperature on insulation life is the limiting factor. High temperature decreases the mechanical
strength and increases the brittleness of fibrous insulation, making transformer failure
increasingly likely, even though the dielectric strength of the insulation material may not be
seriously degraded. Overloading of transformers should be limited by reasonable consideration
of the effect on insulation and, consequently, transformer life.
3.2.8.1 Kilovolt-Amperes (kVA) Ratings. Ratings in kVA or megavolt-amperes (MVA)
will include the self-cooled rating at a specified temperature rise, as well as the forced-cooled
ratings if the transformer is so equipped. As a minimum consideration, the self-cooled rating
should be at least equal to the expected peak demand.
3.2.8.2 Winding Temperature. The standard allowable average winding, of copper,
temperature rise (by resistance test) for the modern liquid-filled power transformer is either
55C/65C or 65C, based on an average ambient of 30C (40C maximum) for any 24 hour
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