A new switching loss reduced discontinuous PWM scheme for a unidirectional three-phase/switch/level boost-type PWM (VIENNA) rectifier

In this paper the applicability of discontinuous modulation for a three-phase/switch/level PWM rectifier system is investigated. As an analysis of the voltage formation of the system shows, the clamping in intervals (in each case, of one bridge leg to one bus of the DC output circuit) being characteristic for discontinuous modulation can occur in regions lying symmetrically around the maxims of the related phase currents. This results in a maximum reduction of the switching power loss. The increase of the effective pulse frequency (as compared to continuous modulation for equal switching losses) made possible thereby is calculated analytically. Furthermore, by digital simulation the RMS value of the mains current harmonics resulting for discontinuous modulation and increased pulse frequency is determined in dependency on the modulation depth. Also, it is compared to the harmonic characteristic for harmonic optimal continuous modulation. The load on the power semiconductors and on the output capacitors (characterized by the RMS value and the amplitude of the low-frequency harmonics of the capacitor current) is included into the comparison of the modulation methods. Finally, the time shape of the phase modulation functions (to be provided for the control of the voltage formation of the system based on a subharmonic oscillation method) for discontinuous modulation is calculated and the control of the potential of the output voltage center point is discussed.