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.

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