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Concept
Vector group
Summary
In electrical engineering, a vector group, officially called a connection symbol, is the International Electrotechnical Commission (IEC) method of categorizing the high voltage (HV) windings and low voltage (LV) winding configurations of three-phase transformers. The vector group designation indicates the windings configurations and the difference in phase angle between them. For example, a star HV winding and delta LV winding with a 30-degree lead is denoted as Yd11.
The phase windings of a polyphase transformer can be connected internally in different configurations, depending on what characteristics are needed from the transformer. In a three-phase power system, it may be necessary to connect a three-wire system to a four-wire system, or vice versa. Because of this, transformers are manufactured with a variety of winding configurations to meet these requirements.
Different combinations of winding connections will result in different phase angles between the voltages on the windings. Transformers connected in parallel must have the same vector group; mismatching phase angles will result in circulating current and other system disturbances.
The vector group provides a simple way of indicating how the connections of a transformer are arranged. In the system adopted by the IEC, the vector group is indicated by a code consisting of two or three letters, followed by one or two numeric digits. The letters indicate the winding configuration as follows:
D or d: Delta winding, also called a mesh winding. Each phase terminal connects to two windings, so the windings form a triangular configuration with the terminals on the points of the triangle.
Y or y: Wye winding, (also called a star). Each phase terminal connects to one end of a winding, and the other end of each winding connects to a common central point, so that the configuration resembles the letter Y. The central point may be connected outside of the transformer to a system neutral.
Z or z: Zigzag winding, or interconnected star winding.
Official source
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