Minimizing the current harmonics RMS value of three-phase PWM converter systems by optimal and suboptimal transition between continuous and discontinuous modulation

A given converter voltage space vector can be realized by a three-phase PWM converter system by switching only two bridge legs. There, the third phase is clamped to the positive or negative DC link voltage. This method is called discontinuous modulation, due to the discontinuous shape of the generating phase modulation functions. As a closer analysis shows, the discontinuous method allows an increase in the effective system pulse frequency, dependent on the phase angle between converter output voltage and output current. The basis for a comparison with the continuous method is chosen as equal average switching losses of any converter bridge leg. For harmonic-optimal operation one must change between continuous and discontinuous modulation (or between various variants of the discontinuous modulation), depending on the load status. The harmonic losses of the control methods are calculated directly in the time domain. For this purpose, the space vector calculus is applied and approximations are used which are sufficiently exact for PWM converter systems with high pulse frequency