The YNd vector group – for example YNd11 – describes a very common combination of windings in power transformers: the high-voltage side is connected in star with the neutral point brought out, while the low-voltage side is connected in delta. This configuration combines the advantages of both connection types. Not least because of this, the YNd vector group is so widespread, as it elegantly combines the technical advantages of both basic connections in a single transformer.
What exactly does YNd11 mean?
The “Y” stands for a star connection on the high-voltage side, “N” for the neutral point being brought out, “d” for a delta connection on the low-voltage side, and the number “11” indicates the phase shift in multiples of 30°. YNd11 therefore means: 11 × 30° = 330° phase shift (equivalent to -30°). This vector group is standard in many transmission grids.
Key Characteristics at a Glance
- High voltage: star connection with the neutral point brought out
- Low voltage: delta connection
- Neutral-point earthing possible – important for protection concepts
- The delta-side winding absorbs third-harmonic currents
- A classic vector group for power transformers in the transmission grid
- YNd11 is the standard group in the European transmission grid
- Neutral earthing transformers are often added to the vector group as needed
Frequently Asked Questions
Why is the delta side often the low-voltage side?
The delta connection filters third-harmonic currents and ensures a stable neutral-point potential. In large transformers, it is often used on the low-voltage side, because smaller conductor cross-sections there must carry high currents at the same time – the delta connection distributes these evenly.
How important is the correct vector group for parallel operation?
For multiple transformers to operate in parallel, the vector group, voltage ratio, and impedance voltage must match. Even different vector group numbers lead to circulating currents that overload the units – careful planning is therefore essential.
How is the vector group verified in practice?
A vector group test measures the phase relationship between the primary and secondary sides. If it does not match the specified vector group, parallel operation with other transformers is not possible. This test is part of every factory test according to IEC 60076.
What alternatives to the YNd vector group are there?
For special applications, vector groups such as Dyn5, Yzn5, or Dy11 are used, each with different phase shifts and properties. The choice depends on grid requirements, the protection concept, and parallel operation requirements. In standard transmission grid applications, YNd11 remains the dominant choice.
Are you planning to use power transformers in a YNd configuration, or do you need advice on parallel operation? Talk to our specialists.