Switching Groups and Circuits

Three-phase transformers, are marked as follows according to the approach that is commonly used worldwide and is easier for the user to understand.

The 1st capital letter describes the primary input winding and the 2nd lower case letter the secondary output winding.

If no other order specifications with a more precise assignment of the upper and lower voltage positions have been formulated, the above-mentioned type of marking is selected! Three-phase transformers are preferably manufactured in Dyn5 if no specifications are available.

Load capacity of the neutral conductor (star point)
In the star-star connection (Yy), the star point may only be loaded with the full rated current if the neutral conductor of the feeding network is connected to the transformer star point on the primary side. If this is not the case, the neutral point can only be loaded with approx. 10% of the phase current.

The same rule applies to three-phase autotransformers designed in a star-saved circuit (YNa0). If a fully loadable star point is required, but this is not available on the line side, the double zigzag connection (ZZan0) must be selected.