Detailed explanation of transformer coupling groups
In the power system, the Transformer Connection Group is the key parameter that determines whether the equipment can operate safely, operate in parallel, and the quality of power. It defines the wiring method of high and low voltage windings and the phase relationship of line voltage, which directly affects the setting of relay protection, the design of grounding system and the harmonic suppression effect. This article will systematically analyze the core rules and selection strategies of the transformer coupling group based on the national standard GB/T 1094.1.
The transformer connection group is colloquially referred to as the "wiring group" of transformers. It consists of letters + numbers, which not only explains how the high-voltage side and the low-voltage side are connected (star, triangle, or zigzag), but also clarifies the phase difference between the voltages on both sides.
The correct selection of the coupling group is a prerequisite to ensure that the parallel operation of the transformer does not produce circulation and the protection does not malfunction.
Labeling rules for connection groups (GB/T 1094.1)
According to national standards, the transformer coupling group adopts a unified "letter + number" labeling method, the specific meanings are as follows:
Letter Part: Winding Connection Method
- Capital letters: Indicates the high-voltage winding connection method
- Lowercase letters: indicates the low-voltage winding connection method
Correspondence between common code names：
- Y / y: Star connection (Star / Wye)
- D / d: Triangle connection (Delta)
- Z / z: Zigzag connection (Zigzag)
- Suffix n: Indicates that the neutral point of the winding has been drawn out, which can be used for grounding or three-phase four-wire system
Digital Part: Clock Notation
In order to visually express the phase relationship between the high and low voltage side line voltages, the standard uses clock notation：
Fix the voltage phasor of the high-voltage side line at 12 points on the clock surface as the phase reference;
The number of hours pointed to by the voltage phasor of the low-voltage side line is the number of the connection group;
Each hour scale on the clock surface corresponds to an electrical angle of 30°, so the digital range is 0~11.
Typical Example: Dyn11
Take the common Dyn11 as an example：
- Figure D: The high-voltage winding is connected in a triangle
- Figure y: The low-voltage winding is connected in a star shape
- n: Low voltage neutral point leads to ground
- Figure 11: The voltage of the low-voltage line hysteresis is 330° on the high-voltage side (11×30°)
This group is widely used in 10kV distribution transformers due to its good harmonic suppression and high ground protection sensitivity.
Comparison of the advantages and disadvantages of commonly used winding connection methods
In actual projects, different connection methods will directly affect the electrical performance and scope of application of transformers. The main characteristics are compared as follows：
| Connection Type | Core Advantages | Core Disadvantages | Typical Application Scenarios |
|---|---|---|---|
| Y/y | Neutral wire extractable for three-phase four-wire systems; low phase voltage reduces insulation cost. | Zero-sequence flux path causes noticeable 3rd harmonics; weak unbalanced load tolerance. | Rural grids, small industrial plants with balanced loads. |
| D/d | Circulating path for 3rd harmonics ensures good sine waveform; large zero-sequence impedance for stable operation. | No neutral wire (cannot directly supply single-phase loads); higher insulation requirements. | Distribution transformers, large motor drives. |
| Z/z | Best unbalanced load tolerance; stable neutral point potential. | Complex structure, high copper usage and manufacturing cost. | Rectifier systems, railway traction, highly unbalanced load scenarios. |
Mainstream connection groups and engineering application scenarios
According to different voltage levels and load characteristics, there are mainly the following types of coupling groups commonly used in engineering:
The first choice for 10kV power distribution
Main application: 10kV/0.4kV power distribution system for residential areas, commercial complexes, industrial parks, etc.
Advantages: Suitable for triangular wiring on the low-voltage side, effectively suppressing the 3rd harmonic.
Strong anti-unbalanced load capacity.
In the event of single-phase ground failure, the protection sensitivity is high, and it is suitable for mixed loads of power and lighting.
Economical and applicable solution
Main applications: agricultural networks, retrofits of old systems, and small projects with limited budgets Please pay attention.
Attention: Simple structure and low cost
The neutral current should generally be controlled within 25% of the rated current, otherwise it is prone to neutral drift, local overheating and other problems, which affect the power quality.
high voltage transmission and large unit boost
Main application: main transformer of 35kV and 110kV substations, as well as step-up transformer of thermal power plants and large-scale units.
Advantages: Use high-voltage side star wiring to reduce insulation costs.
The triangular wiring on the low-voltage side of the switch is conducive to harmonic filtering.
Especially suitable for large-capacity transmission and harmonic sensitive load access.
Grounding and contact transformer for high voltage system
Main application: high-voltage power grid contact transformer, systems that require direct grounding of high-voltage neutral points.
Advantages: Use the high-voltage neutral point to ground directly to ensure reliable relay protection in the event of single-phase ground failure.
It is most commonly used in system communication and high-current grounding systems.
