Common-mode voltage reduction modulation techniques for three-phase grid connected converters

This paper experimentally investigates the performance of three-phase voltage source PWM converter, with the grid interfaced photovoltaic energy conversion system being the main application. In such applications the ground leakage current (common mode current, CMC) should be much less than an ampere and this is difficult to obtain in transformerless (direct) connected systems. With the target being reduction of the common mode voltage (CMV) and CMC, the converter performance is investigated thoroughly. Conventional PWM methods [space vector PWM (SVPWM) and discontinuous PWM (DPWM)] and recently developed reduced common mode voltage PWM (RCMV-PWM) methods [active zero state PWM (AZSPWM) and near state PWM (NSPWM)] are considered. The performance of a 1kW rated PV system has been experimentally investigated. It is shown that the CMV and CMC of the tested RCMV-PWM methods is significantly less than conventional methods. In particular, NSPWM yields the best overall performance including low ground leakage current, low inverter output (phase current) and input (DC link current) ripple, and low switching losses. Accounting for the parasitic capacitance effect, the resonant frequency of the common mode circuit is identified and it is used in the converter design for the purpose of avoiding resonances involving large CMV-CMC. The paper aims help the design engineer select the appropriate PWM method for grid connected applications and provides some design rules of thumb.