Tap-changing equipment is sometimes
used in a loop system, for phase-angle control, for the purpose of
obtaining minimum losses in the loop due to unequal impedances in the
various portions of the circuit.
Transformers used to derive phase-angle
control do not differ materially, either mechanically or
electrically, from those used for in phase control. In general,
phase-angle control is obtained by interconnecting the phases, that
is, by deriving a voltage from one phase and inserting it in another.
The simple arrangement given in figure
below illustrates a single core delta-connected autotransformer in
which the series windings are so interconnected as to introduce into
the line a quadrature voltage.
One phase only is printed in solid
lines so as to show more clearly how the quadrature voltage is
obtained. The terminals of the common winding are connected to the
midpoints of the series winding in order that the in phase voltage
ratio between the primary lines ABC and secondary lines XYZ is unity
for all values of phase angle introduced between them.
As large high-voltage systems have
become extensively interconnected, a need has developed to control
the transfer of real power between systems by means of
phase-angle-regulating transformers.
The most commonly used circuit for this
purpose is the two-core, four-winding arrangement. The high-voltage
common winding is Y-connected, with reduced insulation at the neutral
for economy of design, and a series transformer is employed so that
low-voltage-switching equipment may be used.
Phase-shifting regulating transformers;
single core delta-connected common winding for low-voltage systems.
