Analysis and control of a three-phase, unity power factor Y-rectifier

This paper presents the analysis and operation of a three-phase pulsewidth modulation rectifier system formed by the star-connection of three single-phase boost rectifier modules (Y-Rectifier) without a mains neutral point connection. The current forming operation of the Y-Rectifier is analyzed and it is shown that the phase current has the same high quality and low ripple as the Vienna Rectifier. The isolated star point of Y-Rectifier results in a mutual coupling of the individual phase module outputs and has to be considered for control of the module de link voltages. An analytical expression for the coupling coefficients of the Y-Rectifier phase modules is derived. Based on this expression, a control concept with reduced calculation effort is designed and it provides symmetric loading of the phase modules and solves the balancing problem of the dc link voltages. The analysis also provides insight that enables the derivation of a control concept for two phase operation, such as in the case of a mains phase failure. The theoretical and simulated results are proved by experimental analysis on a fully digitally controlled, 5.4-kW prototype.