What Are the Two Most Important Considerations in Dry-Type Transformer Wiring?

Time：2025-09-1    Auther：ZTelec-www.ztelectransformer.com

Proper dry-type transformer wiring is critical for safe operation and long-term reliability. Mistakes during installation can lead to equipment damage, power interruptions, or safety hazards. Among all factors, voltage matching and the winding connection method are the most crucial, while phase alignment, insulation, and grounding impact long-term stability. This guide explains these two core principles with practical tips for installation.

Voltage Matching

Voltage matching ensures the transformer can start and operate safely. Errors here can result in minor load malfunctions or catastrophic transformer failure. Both the turns ratio and the vector group must be verified before wiring.

1. Turns Ratio

The turns ratio is the rated voltage ratio between the primary (high-voltage) and secondary (low-voltage) windings. The primary voltage must match the supply grid exactly, while the secondary voltage must meet the load requirements.

Example: A 10kV/0.4kV dry-type transformer designed for a 10kV grid delivers 0.4kV (380V/220V) to the load. Connecting it to a 6kV grid reduces output to 0.24kV, preventing equipment startup. Connecting the low-voltage side to a high-voltage grid can destroy insulation, causing equipment failure.

2. Vector Group

The vector group indicates the phase relationship between primary and secondary voltages. It affects parallel operation feasibility and secondary voltage stability. Common groups include Dyn11 (primary delta, secondary star, 30° lag) and Yyn0 (primary and secondary star, in-phase).

Consequences of Errors:

Incorrect vector group prevents parallel operation and may generate circulating currents, burning the windings.

Abnormal secondary voltage phases can damage sensitive loads such as variable-frequency drives or precision instruments.

Always verify the rated voltages and vector group on the transformer nameplate before wiring. Approximate or substitute connections are strictly prohibited.

Winding Connection Method (Star Y vs. Delta Δ)

The winding connection method determines system voltage, neutral availability, and load balance tolerance. Transformers must be connected as per grid type and load characteristics.

1. Star Connection (Y)

In a Star (Y) connection, the ends of the three windings are joined to form a neutral point, while the start terminals provide the three-phase live lines. A neutral line can be drawn for a three-phase four-wire system.

Key Features:

Provides both line voltage (e.g., 0.4kV) and phase voltage (e.g., 0.23kV).
Neutral grounding improves system safety and reduces single-phase fault impact.

Typical Applications: Low-voltage side of transformers for mixed loads (lighting, household appliances, small motors), supporting single-phase and three-phase power.

2. Delta Connection (Δ)

In a Delta (Δ) connection, windings are connected sequentially to form a closed triangle. Only three-phase lines are provided, with no neutral.

Key Features:

Provides line voltage only, with strong tolerance to unbalanced loads.
Suppresses triplen harmonics, reducing grid interference.
Higher short-circuit impedance limits fault currents.

Typical Applications: High-voltage side (10kV, 35kV) or low-voltage systems with high harmonic sensitivity or unbalanced loads, such as large industrial motor systems.

Consequences of Incorrect Winding Connection

Delta wired as Star: Output voltage drops to ~58% of rated, preventing motors from starting and causing winding overheating.

Star wired as Delta: Output rises to ~173% of rated, risking instant burn-out of low-voltage loads and insulation failure, possibly causing short-circuit fires.

Before wiring a dry-type transformer, always verify the nameplate for voltage and vector group, then connect the windings according to the schematic in Star (Y) or Delta (Δ) configuration. These two principles—voltage matching and winding connection method—are essential for safe, reliable transformer operation, while other factors mainly affect long-term performance and stability.