The Most Complete Transformer Vector Group Collection with Winding Connection Diagrams（2026）

Transformer vector groups play a crucial role in determining the phase relationship between primary and secondary windings, directly impacting system compatibility and performance. Understanding these groups is essential for ensuring proper transformer selection, parallel operation, and minimizing harmonic interference. This article presents a comprehensive collection of transformer vector groups along with detailed winding connection diagrams. Whether you are an engineer, technician, or industry professional, this guide will help you navigate the complexities of transformer configurations and make informed decisions for various electrical applications.

What is a Vector Group?

Transformer Vector Groups specify the connection types of the primary (high-voltage, HV) and secondary (low-voltage, LV) windings—such as Delta (D), Star (Y), or Zigzag (Z)—as well as the phase displacement between their respective voltages. This phase difference is conventionally represented using the "clock method," where the HV winding serves as the reference pointer fixed at 12 o'clock, and the position of the LV winding indicates the magnitude of the phase shift, with each hour mark representing 30° (e.g., 1 o'clock represents a 30° lag, 6 o'clock represents 180°, and 11 o'clock represents a 330° lag or 30° lead). For instance, in the widely used distribution transformer vector group Dyn11, "D" denotes a delta-connected HV side, "y" denotes a star-connected LV side, "n" indicates an accessible neutral terminal, and "11" signifies that the LV voltage leads the HV voltage by 30°. Other commonly applied vector groups include Yy0 (Star-Star) and Dd0 (Delta-Delta), both exhibiting 0° phase displacement, as well as Yd1 (Star-Delta), which features a 30° phase lag.

Why specify the vector group?

The Vector Group design of transformers is essential to ensure that transformers can operate correctly within different power systems and work effectively with other transformers or loads. Different vector groups have different phase relationships, and selecting the appropriate vector group is crucial for maintaining the stability, reliability, and efficiency of the power system. The main reasons include:

1. Phase Difference Matching: Different vector groups create various phase differences by altering the winding connection methods (such as star or delta connections). For example, a common vector group like Dyn11 indicates that D refers to a delta connection on the high-voltage side, y refers to a star connection on the low-voltage side, and the number 11 means the phase on the low-voltage side lags the high-voltage side by 30 degrees. Proper phase difference design ensures that when transformers are connected in parallel, no phase issues arise, avoiding mutual interference.
2. Compatibility for Parallel Operation: When multiple transformers need to operate in parallel, the vector group design ensures that the phase difference between them is consistent, preventing short circuits, damage, or equipment failure. Careful selection of vector groups is required when using transformers in parallel to ensure correct phase matching.
3. Voltage Balance: In a three-phase power system, a correctly designed vector group helps prevent voltage imbalances between transformers, which could lead to power transmission issues. Voltage imbalance may affect motor efficiency and even cause equipment damage
4. Protection and Control System Requirements: Different vector group designs help ensure the proper operation of protection devices. For example, differential protection devices typically require the vector groups of transformers to be the same; otherwise, they may trigger false trips.
5. Different Application Needs: Different types of power systems (such as transmission, distribution, or substations) or different loads (such as industrial or residential loads) may have varying vector group requirements. By designing different vector groups, transformers can meet specific application needs, optimizing the overall performance of the system.

Characteristics of each Connection Group

1.Yy Connection Groups:

Yy0: Both the primary and secondary windings are connected in star. The corresponding line - to - line voltages are in - phase, with a phase difference of 0°. In the clock - notation method, when the primary - side line - voltage phasor is used as the minute hand pointing at 12 o'clock, the secondary - side line - voltage phasor also points at 12 o'clock.
Yy4: When the primary - side line - voltage phasor (as the minute hand) points at 12 o'clock, the secondary - side line - voltage phasor points at 4 o'clock. The secondary - side line - voltage lags behind the primary - side line - voltage by 120°.
Yy8: The secondary - side line - voltage lags behind the primary - side line - voltage by 240°. It is equivalent to the position where the minute hand is at 12 o'clock and the hour hand is at 8 o'clock in the clock - notation.
Yy6: Here, the secondary - side line - voltage lags behind the primary - side line - voltage by 180°. That is, the minute hand is at 12 o'clock and the hour hand is at 6 o'clock.
Yy10: The secondary - side line - voltage lags behind the primary - side line - voltage by 300°, corresponding to the 10 - o'clock position on the clock.
Yy2: The secondary - side line - voltage lags behind the primary - side line - voltage by 60°, similar to the situation where the minute hand is at 12 o'clock and the hour hand is at 2 o'clock.

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2. Yd Connection Groups
Yd1: The primary side is connected in star and the secondary side is connected in delta. The secondary - side line - voltage lags behind the primary - side line - voltage by 30°. In the clock - notation, when the primary - side line - voltage phasor points at 12 o'clock, the secondary - side line - voltage phasor points at 1 o'clock.
Yd5: The secondary - side line - voltage lags behind the primary - side line - voltage by 150°, corresponding to the 5 - o'clock position on the clock.
Yd9: The secondary - side line - voltage lags behind the primary - side line - voltage by 270°, equivalent to the 9 - o'clock position on the clock.
Yd7: The secondary - side line - voltage lags behind the primary - side line - voltage by 210°, that is, the minute hand is at 12 o'clock and the hour hand is at 7 o'clock.
Yd11: The secondary - side line - voltage lags behind the primary - side line - voltage by 330° or, in other words, leads by 30°. It is a commonly used connection group, often used in lines where the low - voltage is higher than 0.4 kV.
Yd3: The secondary - side line - voltage lags behind the primary - side line - voltage by 90°, corresponding to the 3 - o'clock position on the clock.

3. Other Connection Groups:

Yyn0: Yyn0 (Star-Star Connection, 0° Phase Shift)
Both the high-voltage and low-voltage windings are connected in star (Y) configuration.The neutral point of the low-voltage side (yn) is usually grounded, allowing for a three-phase four-wire system, the phase shift is 0°, meaning the voltage on both sides remains in phase.Suitable for applications where the neutral grounding is essential, such as distribution transformers. However, it has poor unbalanced load handling and is susceptible to third harmonic currents, requiring proper grounding or compensation.
Yyn0 is suitable for balanced loads and is commonly used in small distribution transformers.

Dyn11: (Delta-Star Connection, -30° Phase Shift)

The high-voltage side is connected in delta (D) configuration, while the low-voltage side is connected in star (Y) configuration with a neutral point (yn)，the low-voltage side voltage lags the high-voltage side by 30° (i.e., a -30° phase shift), offers better unbalanced load handling as the delta-connected high-voltage side provides a circulating path, reducing the impact of third harmonics.Commonly used in distribution systems requiring high power quality, such as industrial and commercial power supply.

Dyn11 is ideal for unbalanced loads and has better harmonic suppression, making it a popular choice in distribution systems

Field Testing & Diagnostics Note: When transitioning from vector group verification to insulation health assessment, the winding configuration directly impacts your test setups. For instance, knowing whether a transformer is Delta (D) or Winding-Neutral (Yn) helps engineers properly configure a Tan Delta Tester to isolate leakage currents using UST or GST modes. Similarly, when deploying a partial discharge tester, understanding the vector group ensures correct sensor placement on the corresponding high-voltage bushings or neutral lines to accurately capture localized ionization signals.

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