Understanding Vector Groups
A vector group describes the winding connection (Delta/Y/Star/Zigzag) and phase displacement between primary and secondary voltages. The correct vector group is critical for system compatibility, parallel operation, and harmonic management.
Common Vector Groups
| Vector Group | HV Winding | LV Winding | Phase Shift | Typical Application |
|---|---|---|---|---|
| Dyn11 | Delta (Δ) | Star (Y) + Neutral | 30° lag | Distribution transformers (most common) |
| Dyn5 | Delta (Δ) | Star (Y) + Neutral | 150° lag | Distribution (alternative) |
| YNd11 | Star (Y) + Neutral | Delta (Δ) | 30° lead | Transmission, generator step-up |
| Yy0 | Star (Y) | Star (Y) | 0° | Small capacity, balanced loads |
| Dd0 | Delta (Δ) | Delta (Δ) | 0° | Industrial, motor loads |
| Dz0 | Delta (Δ) | Zigzag (Z) + Neutral | 0° | Unbalanced loads, shore power |
Why Dyn11 is the Standard
- Harmonics: Delta primary blocks 3rd harmonic currents from entering the grid
- Single-Phase Loads: Star secondary with accessible neutral enables L-N connections
- Protection: Phase shift simplifies directional relay coordination
- Earth Fault: Path for zero-sequence currents through neutral grounding
Parallel Operation Rules
Transformers operating in parallel MUST have:
- Same vector group (phase displacement must match)
- Same voltage ratio (within ±0.5% tap range)
- Similar impedance (within ±10%) for proportional load sharing
Special Cases
- Data Centers: Dy11y11 dual-secondary for 2N redundancy
- Solar Farms: Dyn11 or YNd11 depending on inverter topology
- VFD Applications: Dy5y5 multi-pulse arrangements for harmonic cancellation