In the case of a three-phase transformer, more coils are wound around the core, providing multiple options for interconnecting the windings. These connections determine the phase relationship between the respective voltages.
A three-phase transformer comprises three primary and secondary windings, each encircling the iron core, facilitating the flow of three-phase power. This arrangement can be achieved by externally linking the windings of three individual single-phase transformers. Both primary and secondary windings can be interconnected in either delta or star configurations.
In the delta configuration, one winding's polarity end is linked to another's non-polarity end, forming a closed ring connection among the three windings. Phases are derived from the common coupling points.
Conversely, in the star configuration, all non-polarity ends of the windings are connected to form a neutral point, with the polarity ends serving as phase terminals.
By varying the connections of primary and secondary windings, four configurations can be obtained, including delta-delta, star-star, delta-star, and star-delta. In configurations with similar winding connections (star-star and delta-delta), the secondary voltage waveform aligns entirely with the primary waveform, termed as the "no phase shift" condition.
In configurations with different winding connections (star-delta and delta-star), the secondary voltage waveform experiences a 30-degree phase shift relative to the primary waveform, posing challenges when paralleling two three-phase transformers. Ensuring no relative phase difference between the secondary voltage waveforms of the transformers is crucial to prevent short circuits upon energization.
To address the classification of winding connections in three-phase transformers, the International Electrotechnical Commission (IEC) introduced Vector Groups, which specify winding connection details and phase relationships between low-voltage and high-voltage windings.
Vector groups offer insights into winding configurations and phase relationships between the high-voltage (HV) and low-voltage (LV) windings. While understanding the configuration aspect is straightforward, utilizing the vector group symbol to determine phase displacement between the windings is crucial.
In vector groups, the high-voltage winding vector serves as the reference for determining the phase displacement on the low-voltage side. The angle displacement between these two vectors, measured anticlockwise, is depicted in clock-hour figures within the vector groups to denote the phase displacement between the windings.
As per the IS 2026 (Part 1V)-1977 standard, there exist 26 sets of connections such as star-star, star-delta, star-zigzag, delta-delta, etc. The displacement of the low-voltage winding vector ranges from zero to -330° in increments of -30°, depending on the connection method. However, it's uncommon for all 26 sets to be utilized. Most transformers introduce phase shifts of 0, -30, -180, and -330 degrees, corresponding to clock-hour notations of 0, 1, 6, and 11, respectively. These different clock-hour positions are determined with the high-voltage winding vector at the 0-hour position.
Interpreting the low voltage vector positions:
- Digit 0: Both winding vectors are in phase.
- Digit 1: LV vector lags HV vector by 30°.
- Digit 5: LV vector lags HV vector by 150°.
- Digit 6: LV vector lags HV vector by 180°.
- Digit 11: LV vector lags HV vector by 330°.
The formula "30 x Clock Handle Value" can be employed to ascertain the phase displacement between the two windings.
Now, let's consider an example, using one of the phasor symbols, to determine the type of winding configurations used and the phase displacement.
Example – Dyn1:
Analyzing the vector group according to specific rules:
- The first symbol is always a capital letter, indicating the HV side's winding configuration (D for Delta, S for Star, Z for Interconnected star, or N for Neutral configuration).
- The second symbol is always a small letter, indicating the LV side's winding configuration (d for Delta, s for Star, z for Interconnected star, or n for Neutral configuration).
- The third symbol represents the phase displacement between the two windings, expressed as a clock hour number (1, 6, 11).
- An additional symbol, "n," is added to signify that the neutral connection is taken out from the star-connected side.
Based on these rules, the transformer in this example (vector group = Dyn1) must have a delta-connected HV winding (D), a star-connected LV winding (y) with the star point brought out (n), and a 30° lagging phase shift as power flows from the HV side to the LV side (1). This denotes a delta-star transformer, as illustrated below:
Let's consider a winding configuration wherein the HV winding is connected in delta while the LV winding is connected in wye. This configuration belongs to the vector group of transformer Dyn1 where the LV lags the HV by 30°.
For transformers with additional windings, such as a 220/66/11 kV star-star-delta configuration, the winding details can be represented in the vector group by using symbols of the windings in descending voltage order. This implies dividing the vector group into two segments for a 220/66 kV star-star configuration and a 220/11 kV star-delta configuration, both referencing the same High voltage vector of 220 kV star winding. Consequently, the resulting vector group in this case would be Yy0 – Yd11; Yy0 (denoting star-star with no phase displacement), Yd11 (indicating that the delta LV winding has a 330-degree lag or a 30-degree lead). Various vector groups and their associated phase shifts are outlined in below table:
When parallel connecting two three-phase transformers, it's vital to ascertain the vector groups of both transformers. Transformers should only be connected if they possess identical vector groups; otherwise, substantial circulating currents might occur between the secondary windings of the transformers upon their energization.
Manufacturers typically indicate the vector group for a three-phase transformer on the transformer nameplate, facilitating quick and easy determination of the transformer's characteristics. By referencing the nameplate and identifying the appropriate phasor symbol, one can swiftly recognize the transformer configuration.
Transformers adhering to ANSI standards may not display vector groups but instead illustrate a vector diagram relationship of the windings and phases. The IEC method introduces vector groups to classify the winding configurations of three-phase transformers. Windings can be connected in delta, star, or zigzag configurations, and the polarity of the windings impacts their phase relationship, all of which can be easily conveyed through vector groups. Always ensure to review vector groups on transformer nameplates before connecting the transformer to the power system.