IEC 5 or VDE C Group of coupling of 3-phase transformers

Picture 1: IEC 5 or VDE C Group of couplings of 3-phase transformers


On Picture 1 are shown the 3 types of coupling of three-phase transformers. According to IEC standard, this group of couplings is defined as Group 5, while according to VDE standard this same group of couplings is defined as Group C. The vector diagrams and the connection schematics are also shown on the figure for both, primary and secondary voltages or high and low voltages. The phase shifting per phase between the secondary and primary coils for this group is 150 degrees or 5 hours (according to the IEC standard this is represented as distance expressed in hours between the clock arrow for hours and the clock arrow for minutes which is always at fixed position at 12).

So, according to these standards this Group of couplings consists three couplings marked as:

1. Dy5 (IEC) or C1 (VDE);
2. Yd5 (IEC) or C2 (VDE);
3. Yz5 (IEC) or C3 (VDE);


As we can see, the IEC marking for the couplings is more descriptive than the VDE marking, since the mark actually represent the primary and secondary coupling connection and the number represents the phase shifting between them. So, the Dy5 coupling means delta-star connection with 150 degrees (5 hours) phase shift between secondary and primary voltage per phase, Yd5 is star-delta connection and Yz5 is star-zickzak connection.

In practice, there is a common need for two transformers working in parallel connection on the same network. One of the conditions for this parallel working is that secondary voltages per each phase must be in phase (no time delays) between the transformers. However, for this time synchronization between same phases from the two transformers it is not required the both transformers to have the same coupling. According to this, the transformers which belongs to the same group of couplings may work in parallel, of course, if the other conditions are also satisfied. So, two transformers which has any of the IEC Group 5 or VDE Group C coupling can work in parallel.