in 12 - pulse and 18 - pulse ac / dc converter systems a single wye connected auto transformer using appropriate segments of voltage from each phase connected in fork fashion is used to develop 6 - phase and 9 - phase voltages in a symmetrical manner such that inherent compensation is made for the dc converter output voltage to be within 4 . 2 % of that of a 6 - pulse converter . additional design adjustments for step - up or step - down of voltage can be made without compromising the transformer symmetry . an unconnected but closed delta winding provides a path in which third harmonic currents , and multiples thereof can flow .