PHASE ANGLE CONTROL TRANSFORMERS BASIC INFORMATION AND TUTORIALS

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.

INSTRUMENT TRANSFORMER METERING IN METALCLAD SWITCHGEAR

In 1958 the Meter and Service Committees of EEI and AEIC issued Guide for Specifications for Revenue Metering Facilities Installed in Metalclad Switchgear. This guide states the principal objectives to be attained:

1. That a separate sealable compartment be provided exclusively for revenue metering equipment when mounted within the switchgear;

2. That space be provided within the compartment sufficiently large to accommodate separately the installation of any standard current transformers and any standard voltage transformers required for metering;

3. That space be provided within the compartment for the installation of separate, isolated voltage transformer fuses, where required;

4. That, where required, adequate space and panel facilities be provided within the compartment to permit the installation of all necessary meters, instruments, auxiliary devices, or test facilities, of any type, whether they be front connected, back connected, surface mounted, or flush type;

5. That the arrangements be such that the secondary wiring may be installed in a manner to facilitate checking of connections.

By following these specifications, control of all metering transformers and conductors rests with the utility company.

When extremely high-capacity current transformers are used, it is essential that spacing of bus bars be adequate to avoid interference between individual transformers.

There are many advantages to be gained by mounting instrument transformers in the customer’s switchgear. Protection, appearance, and, in many cases, economy, may be the result.