Switching group
The switching group is essential in three-phase transformers, providing information about the type of circuit and phase position, as well as the possible types of load of a three-phase transformer.
The code letters are:
- Y, y for the star connection,
- D, d for the delta connection,
- Z, z for the zigzag circuit,
- I, i for the open circuit of the windings.
- N, n indicates whether a star point is led out as an outer connection.
- a indicates an autotransformer, (which, as a three-phase transformer, is always designed in a star connection).
The upper case letter is generally used for the upper voltage winding, the lower case letter for the lower voltage winding.
The figure at the end indicates the multiple of 30°el. the voltage pointer of the output voltage lags the pointer of the input voltage in a counterclockwise direction.
If the orderer does not specify otherwise, BREMER three-phase transformers are manufactured in switching group YNyn0.
The selection criterion for the switching group is, among other things, whether the system feeding the input side has a star point and whether the load is a symmetrical load or whether single-phase loads are also to be fed. In the case of single-phase (asymmetrical) loads, this is also referred to as secondary-side star point loading.
Preferred switching groups for transformers:
- Yy0 – The secondary star point may only be loaded with the full load current if the mains feeding it on the input side has a star point conductor and this is firmly connected to the transformer star point. Otherwise, the star point of the output winding can only be loaded with approx. 10%.
- Dy5 – Secondary star point fully loadable
Other common switching groups for transformers:
- Dd0 No star points present
- Yd5 No secondary star point present
- Yz5 Secondary star point fully loadable
The core type power ratings of transformers with Z circuits are smaller than those of comparable transformers with Y or D circuits, so they are used less frequently.
Star points that have been led out (N or n indicator) cause additional costs compared to a winding without star points that have been led out. It therefore is advisable to specify in orders whether the star points must be accessible.
Examples of switching groups:
Example 1: YNyn0yn0
Star-star connection with 2 secondary windings and star points led out
- A transformer with such a vector group could, for example, be mounted in a machine tool and have a three-phase motor connected to one secondary winding and a B6 three-phase rectifier downstream of the other. The secondary windings can be loaded with approx. 10% of the rated output power in single phase for small control powers and lighting.
Example 2: Dyn5
Delta-star circuit with star point led out on the secondary side and phase angle 150°el. between input and output
- For example, three single-phase loads (e.g. three heating elements), which may not be switched on at the same time, could be operated on such a transformer. The transformer can be loaded with the rated current on each phase – regardless of whether the other phases are loaded.
Example 3: YNa0
Three-phase autotransformer