ANGULAR DISPLACEMENT OF THREE PHASE TRANSFORMERS BASIC AND TUTORIALS

Angular displacement is defined as the phase angle in degrees between the line-to-neutral voltage of the reference-identified high-voltage terminal and the line-to-neutral voltage of the corresponding identified low-voltage terminal.

The angle is positive when the low-voltage terminal lags the high-voltage terminal. The convention for the direction of rotation of the voltage phasors is taken as counterclockwise.

Since the bulk of the electric power generated and transmitted is three-phase, the grouping of transformers for three-phase transformations is of the greatest interest. Connection of three-phase transformers or three single phase transformers in a three-phase bank can create angular displacement between the primary and secondary terminals.

The standard angular displacement for two-winding transformers is shown in Figure above. The references for the angular displacement are shown as dashed lines.

The angular displacement is the angle between the lines drawn from the neutral to H1 and from the neutral to X1 in a clockwise direction from H1 to X1.

The angular displacement between the primary and secondary terminals can be changed from 0 to 330 degrees in 30deg steps simply by altering the three-phase connections of the transformer.

Therefore, selecting the appropriate three-phase transformer connections will permit connection of systems with different angular displacements.

Figure shows angular displacement for common double-wound three-phase transformers. Multicircuit and autotransformers are similarly connected.

THREE PHASE TRANSFORMER POLARITY EFFECTS AND STANDARD ANGULAR DISPLACEMENT BASICS AND TUTORIALS

THREE PHASE TRANSFORMER POLARITY EFFECTS AND STANDARD ANGULAR DISPLACEMENT BASIC INFORMATION
Polarity Effects And Standard Angular Displacement Of Three Phase Transformers


Polarity Effects
Any combination of additive and subtractive units can be connected in three-phase banks so long as the correct polarity relationship of terminals is observed.

Whether a transformer is additive or subtractive does not alter the designation of the terminals (X1, X2, etc.) thus correct polarity will be assured if connections are made as indicated in the diagrams.

The terminal designations, if not marked, can be obtained from the transformer nameplate which shows the schematic internal-connection diagrams diagramming the actual physical relationship between the high and low voltage terminals.

If subtractive-polarity transformers are used in threephase banks secondary connections are simplified from those shown for the additive-polarity units.

The additivepolarity connections, for standard angular  isplacement, are somewhat complicated, particularly in cases with delta-connected secondary, by the crossed secondary interconnections between units.

For this reason simplified bank connections, which give non-standard angular displacement between primary and secondary systems, are sometimes used with additive-polarity units.

Standard Angular Displacement
Standard angular displacement or vector relationships between the primary and secondary voltage systems, as defined by ANSI publications, are 0° for delta-delta or wye-wye connected banks and 30° for delta-wye or wye-delta banks.

Angular displacement becomes important when two or more three-phase banks are interconnected into the same secondary system or when three-phase banks are paralleled. In such cases it is necessary that all of the three-phase banks have the same displacement.

The following diagrams cover three-phase circuits using:

1. Standard connections—where all units have additive polarity and give standard angular displacement or vector relation between the primary and secondary voltage systems (as defined by ANSI publications).

2. Simplified connections for the more common three phase connections with the delta-connected secondary— where all units have additive polarity but give nonstandard angular displacement between the primary and secondary voltage system.