Effects of unbalanced mains voltage conditions on three-phase hybrid rectifiers employing third harmonic injection

Multitudinous industry applications require single-phase or three-phase rectification circuits. Hybrid rectifiers offer fairly flexible fields of applications (compared to purely implemented passive or active rectifiers) due to their optional active converter topology. The low harmonic input stage, however, is in general exposed to unbalanced AC-side mains voltage conditions. Depending on location and/or facility, input voltage variations can be 3% or higher (short-term). Imbalance in mains input voltages on hybrid rectifiers which are employing the third harmonic injection principle are therefore analysed in detail. It is shown that unbalanced AC-side input voltages not only directly influence THD of input currents and power factor of the system, but also increase current stress of active switches and passive components. Basic considerations are discussed from a mathematical point of view and are finally confirmed by simulation and experimental results of a 10 kW/10 kHz laboratory prototype (passive three-phase diode bridge rectifier equipped with `Flying' Converter Cell (FCC) active current injection cell).