//Vector group is a key characteristic of transformers that defines the phase relationship and connection type between primary and secondary windings. Understanding vector groups is essential for selecting the appropriate transformer for specific applications and ensuring proper system operation.
What Is Vector Group
The vector group of a transformer describes the phase shift between the primary and secondary windings, as well as the winding configurations. The primary and secondary windings can be connected in star (wye), delta, or zigzag configurations, each with distinct properties.
Winding Configurations
Star
In a star connection, one end of each phase winding is connected to a common neutral point, forming a “Y” shape. This allows for a neutral connection and provides good voltage regulation under unbalanced load conditions. However, star-connected transformers are more susceptible to triplen harmonics and may require additional measures to mitigate their effects.
Delta
Delta-connected windings have each phase connected end-to-end, forming a closed triangle or “delta” shape. Delta connections provide a path for circulating currents, which helps cancel out triplen harmonics. This configuration is often used in power distribution systems to supply both single-phase and three-phase loads. However, delta-connected transformers lack a neutral point, which can limit their application in certain situations.
Zigzag
Zigzag connections, also known as interconnected star connections, are a combination of star and delta configurations. In a zigzag connection, each phase winding is split into two halves, with one half connected in star and the other half connected in delta. This arrangement provides a neutral point while also allowing for the cancellation of triplen harmonics. Zigzag transformers are commonly used in grounding applications and for supplying unbalanced loads.
The phase shift between the primary and secondary windings is another crucial aspect of the vector group. The most common phase shifts are 0°, 30°, and 150°.
Common Vector Groups for Distribution Transformers:
Dyn11
Dyn11 is a widely used vector group for distribution transformers. In this configuration, the primary winding is delta-connected, while the secondary winding is star-connected with a neutral brought out. The phase shift between the primary and secondary voltages is 30° (leading), denoted by the “11” in the vector group notation.
The delta-connected primary provides several advantages, such as the cancellation of triplen harmonics and the ability to supply both single-phase and three-phase loads.
Yzn11
The Yzn11 vector group features a star-connected primary winding with a neutral point, and a zigzag-connected secondary winding. Similar to the Dyn11, the phase shift between the primary and secondary voltages is 30° (leading).
The zigzag connection on the secondary side combines the benefits of star and delta connections. It provides a neutral point for grounding and allows for the supply of unbalanced loads while also canceling out triplen harmonics.
Dyn5
In the Dyn5 vector group, the primary winding is delta-connected, and the secondary winding is star-connected with a neutral. However, unlike Dyn11, the phase shift between the primary and secondary voltages is 150° (lagging), indicated by the “5” in the notation.
This phase shift is beneficial in certain applications where it is necessary to match the phase angles of connected equipment or to ensure proper parallel operation with other transformers. Dyn5 transformers are commonly used in power distribution systems where a specific phase shift is required for compatibility with existing infrastructure.
Factors Influencing the Choice of Vector Group
System Requirements
The vector group must be compatible with the existing power system’s phase shift and earthing requirements. For example, if the system demands a specific phase shift between the primary and secondary voltages, the vector group should be chosen accordingly. Similarly, the earthing arrangement of the transformer should align with the system’s grounding practices.
Load Characteristics
The nature of the load supplied by the transformer influences the choice of vector group. If the load is primarily balanced and three-phase, a delta-connected primary may be sufficient. However, if there is a significant presence of single-phase loads or unbalanced conditions, a star-connected secondary with a neutral becomes necessary. Zigzag connections are advantageous when supplying heavily unbalanced loads while mitigating harmonics.
Parallel Operation
When transformers are required to operate in parallel, their vector groups must be compatible to ensure proper load sharing and avoid circulating currents. Transformers with the same vector group can be paralleled directly, while those with different vector groups may require additional considerations, such as phase-shifting transformers or special connections.
Identifying the Vector Group
To determine the vector group of a transformer, several methods can be employed:
Nameplate Information
The transformer’s nameplate often includes the vector group information. It is typically represented by a combination of letters and numbers, such as Dyn11, Yzn5, or Dyn1. The letters indicate the winding configurations (D for delta, Y for star, and Z for zigzag), while the numbers denote the phase shift between the primary and secondary windings (in multiples of 30°).
Phase Relationship Tests
By conducting phase relationship tests, the phase shift between the primary and secondary windings can be determined. This involves applying a known voltage to the primary winding and measuring the phase angles of the corresponding secondary voltages. The phase shift can then be calculated based on the difference in phase angles between the primary and secondary voltages.
Winding Resistance Measurements
Measuring the winding resistances can provide insights into the winding configurations. By comparing the resistance values between different phases and between the primary and secondary windings, the presence of star, delta, or zigzag connections can be inferred.
Impact of Vector Group on Protection and Earthing
Earth Fault Protection
The choice of vector group affects the behavior of earth fault currents and the effectiveness of earth fault protection systems. In transformers with star-connected windings and a neutral connection, earth fault protection can be achieved through neutral overcurrent or residual earth fault protection schemes. Delta-connected transformers, on the other hand, may require alternative protection methods, such as restricted earth fault protection or zero-sequence current detection.
Neutral Earthing Arrangements
The presence and configuration of the neutral connection in a transformer are determined by the vector group. Star-connected windings allow for a neutral point that can be earthed directly or through an impedance, depending on the system requirements. Zigzag transformers provide a virtual neutral point, which can be used for earthing purposes. Delta-connected transformers lack a neutral connection, which may limit their suitability in certain earthing arrangements.
Zero-Sequence Currents
The vector group influences the flow of zero-sequence currents during earth faults. In star-connected transformers with a solidly earthed neutral, zero-sequence currents can flow freely, facilitating earth fault detection and protection. Delta-connected transformers, however, provide a high impedance path for zero-sequence currents, which can complicate earth fault protection. Zigzag transformers, with their inherent zero-sequence impedance, can help limit the flow of zero-sequence currents and provide a means for earth fault detection.
