Control of Three-phase Cascaded Multilevel Converter Based Power Electronic Transformer under Unbalanced Input Voltages

As one of the most important parts of the future smart grid system, the three-phase cascaded multilevel converter based power electronic transformer (PET) is mainly composed of three parts: the three-phase input rectifier, middle dual active bridges (DAB) and output inverter. The three-phase input rectifier stage adopts pulse width modulation (PWM) rectifier. But there exists harmonic power in the power grid under unbalanced voltages which can affect the grid power quality seriously. Positive and negative sequence separating method is presented to solve the issue. The current references under positive and negative sequence d-q axis are obtained by using the relation of voltage, current and power, through which the control method is improved and simulation model is set up. Phase shifting control is adopted in DAB stage of three-phase PET. The result shows that the new method can control the input current, decrease the harmonic power, keep the DC voltage of rectifier stage steady and ensure that the whole system runs at unity power factor under unbalanced grid voltages.