Detailed Explanation of Transformer Connection Groups (English Version)

The connection group of a transformer is an important parameter in power systems, as it determines the phase relationship between the primary and secondary voltages, affects parallel operation, protection settings, and system stability. A clear understanding of transformer connection groups is essential for power system design, operation, and maintenance.

1. What is a Transformer Connection Group?

A transformer connection group refers to the wiring method of the primary (high-voltage side) and secondary (low-voltage side) windings and their relative phase difference. It typically consists of two parts:

1. Winding Connection Type: Such as star (Y), delta (Δ), etc.
2. Clock Notation: Used to represent the phase relationship between high and low voltage sides, expressed in "hours", with each hour representing a 30° phase shift, totaling 12 positions.

For example, "Yyn0" or "Dyn11" are common representations of transformer connection groups.

2. Common Types of Connection Groups

According to national standards such as GB/T 6451 and international standards like IEC, some commonly used connection groups include:

3. Representation Method and Clock Notation

1. Symbolic Representation

• First part: High-voltage winding connection (Y, D)
• Second part: Low-voltage winding connection (y, d)
• Third part: Number indicating the phase difference between high and low voltage windings (0–11)

Examples:

• Yyn0: High-voltage side is star-connected, low-voltage side is also star-connected with a neutral point; phase difference is 0°.
• Dyn11: High-voltage side is delta-connected, low-voltage side is star-connected with a neutral point; phase difference is 330° (i.e., 11×30°).

2. Principle of Clock Notation

The A-phase voltage of the high-voltage side is taken as the "12 o’clock" position. The A-phase voltage of the low-voltage side is positioned clockwise around the clock face based on its phase shift relative to the high-voltage side. Each 30° represents one hour. For instance:

• If the low-voltage A-phase lags the high-voltage A-phase by 330°, it corresponds to the "11 o’clock" position.
• If they are in phase, it is the "0 o’clock" position.

4. Characteristics and Applications of Different Connection Groups

5. Principles for Selecting Connection Groups

When selecting a transformer connection group in practical engineering, the following factors should be considered comprehensively:

1. System Requirements: Is a neutral point required? Is third-harmonic current allowed?
2. Parallel Operation Conditions: Transformers operating in parallel must have the same connection group; otherwise, circulating currents may occur.
3. Load Characteristics: Are there unbalanced loads or harmonic sources?
4. Protection Configuration: Some connection groups impose specific requirements on relay protection device design.
5. Economy and Reliability: Different wiring methods affect equipment cost, operational losses, and system stability.

6. Conclusion

The connection group of a transformer is a key indicator of its electrical performance, directly influencing system operation safety and efficiency. Understanding the structural characteristics, phase relationships, and application scenarios of different connection groups helps make informed decisions in power system design, operation, and maintenance. With the development of smart grids, the understanding and application of connection groups will become increasingly refined and standardized.