Current handling capability of the neutral point of a three-phase/switch/level boost-type PWM (VIENNA) rectifier

In this paper the stationary operational behavior of a three-phase/switch/level PWM rectifier is analyzed for asymmetrical loading of the output partial voltages. For the considerations a sinusoidal mains current shape, resistive behavior of the mains current fundamental and constant pulse frequency are assumed. Based on analytical calculations it is shown that the average value of the neutral point current (which has to be formed for asymmetrical load) has an approximately linear dependency on the distribution of the overall relative on-time of the switching states of the system being redundant concerning the voltage formation between begin and end of a pulse half interval. The maximum admissible load of the neutral point (capacitive output voltage center point) is calculated. This load is given as a function of the amplitude of the mains current and the voltage transfer ratio of the system. The calculations are checked by results of a digital simulation of the system behavior. Furthermore, the increase of the rms value of the ripple of the mains current of the rectifier is analyzed as it results in forming of an average value of the neutral point current. As a basis for comparison, the ripple rms value is used which results in harmonic-(sub)optimal control and negligible average value of the neutral point current. Finally, the current stress on the power semiconductors and on the output capacitors of the system are compiled in the form of diagrams which can be applied directly for dimensioning the system