Harmonic analysis of interphase currents in 12 pulse rectifiers

Many large PWM inverter drives use diode or thyristor front end rectifiers connected directly to the DC link capacitor with little or no dc link inductor relying on the source inductance to provide the minimum smoothing. High power inverters may even be 12 pulse to reduce the supply current harmonics but if the arrangement is bridges in parallel they could end up with unacceptably high ripple in the DC current supplied by each bridge. This paper uses matrix frequency domain modelling to solve for the harmonic currents in each of the two 6 pulse groups and is first verified with a single 6 pulse bridge before extending to 12 pulse. The procedure outlined here uses balanced supply voltages and equidistant firing but could be extended to unbalanced systems using the appropriate switching functions.