Vector Group

[BritEng]

We are installing a 24 pulse 6.6kV VFD to drive a compressor motor and the input transformer to the VFD has the vector group Yd0.75d11.75d0.25d11.25. I've never seen anything like this before and wondered if anyone could shed some light on it. HV winding is star connected (6.6kV) with 4 delta secondaries (1907.2V) with shift angles of +22.42°, -7.46°, +7.46° and -22.42°.
I'm struggling to understand how we get these values for the phase shifts.

Regards

Paul

 

[davidbeach]

Each leg of a zig-zag winding has two half windings that are electrically 120 degrees apart; thus a delta zig-zag can avoid a phase shift. Now put that "bend" somewhere other than the middle of the winding; any angular displacement is possible (within reason). The transformer nameplate should provide a fair amount of information.

 

[prc]

The vector group number is selected based on the clock position of secondary voltage vector with the primary voltage vector at 12 o'clock position. So considering the secondary vectors at -22.42 degrees (0.75 o'clock -one hour gap=30 degrees) +7.5 ( 11.75 o'clock) -7.5( 0.25 hours) and +22.42 degrees ( 11.25 O'clock)