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 or load-side voltage rating greater than 1000 V, whereas the secondary substation transformer has
a load-side voltage rating of 1000 V or lower.
3.2.11 Special Transformers. There are various transformers used in special applications,
such as autotransformers, grounding, constant-current, furnace, and self-protected transformers.
3.2.11.1 Autotransformers. Autotransformers are constructed with a single winding per
phase, such that part of the winding is common to both primary and secondary sides. The
common portion is called the common winding, and the remainder is called the series winding.
The high-voltage terminal is called the series terminal, and the low-voltage terminal is called the
common terminal. Part of the power passes from one winding to the other by transformation,
and the rest passes directly through without transformation. A solid connection exists between
the primary and secondary circuits. This is generally of little consequence with low-voltage
circuits, but with high-voltage systems the neutral point must be grounded for safe operation.
Autotransformer voltage transformation ratios rarely exceed 2:1. They are commonly used to
connect two transmission systems at slightly different voltages, frequently with a delta tertiary
winding. Similarly, autotransformers are used for generator step-up transformers to feed two
different transmission systems.
3.2.11.2 Grounding Transformers. A grounding transformer is intended primarily for the
purpose of providing a neutral point for grounding purposes. One such application could be
grounding a Delta-connected system. Grounding transformers may be either of the Zigzag- or
Wye-Delta type.
(a) The type of grounding transformer most commonly used is a zigzag three-phase
transformer with no secondary winding. The internal connection of this transformer is illustrated
there is no fault on the system, only a small magnetizing current flows in the transformer
windings. The transformer impedance to ground current, however, is low so that it allows high
ground currents to flow. The transformer divides the ground current into three equal in-phase
current components, flowing in the three windings of the grounding transformer.
(b) A Wye-Delta transformer can also be used as a grounding transformer. in this case
the delta must be closed to provide a path for the zero-sequence current, but can be made up at
any convenient voltage level. It may or may not be used to serve other loads. The wye winding
must be of the same voltage rating as the circuit which is to be grounded.
3.2.11.3 Constant-Current Transformers. A constant-current transformer automatically
maintains an approximately constant current in its secondary circuit under varying conditions of
load impedance when its primary is supplied from an approximately constant-potential source.
The moving coil design is the most commonly used type, having separate primary and secondary
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