Line harmonic currents in 12 pulse rectifiers with unbalanced supply voltages

Abstract

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 necessary smoothing. However if high power inverters may go 12 pulse to reduce the supply current harmonics with the bridges arranged in parallel, they could end up with unacceptably high ripple in the DC current supplied by each bridge. This work uses matrix frequency domain modelling to solve for the harmonic currents in a 6 pulse bridge with unbalanced AC supply voltages and unequal overlap angles. It is then extended to each of the two 6 pulse bridges in a 12 pulse system. The resulting waveforms compared with those from microcap time stepping simulation. The procedure outlined here uses equidistant firing but permits the investigation of unbalanced supply voltages and computes the appropriate overlap angle for each phases and then derives the corresponding switching function for voltage and current.