Evaluation of measured signals and control scheme design for a series Z-source inverter

The Z-source inverter (ZSI) provides a wide range of AC output load currents due to its ability to both buck (DC to AC), by lowering the modulation index (MI), and boost, by increasing the shoot-through duty ratio (D_S) in the modulation waveform, thereby raising the bridge voltage (V_S). There are therefore two degrees of freedom in controlling the AC load current fora fixed DC link, namely MI and D_S. The series ZSI proposed by Y. Tang et al.(2009) and by J. Ding et al. (2010) provides the same boosting capabilities as the parallel Z-source but has the advantage of reduced voltage stress on the Z-source network capacitors, and no inrush current on connection with the DC link. In this presentation, a three-phase series ZSI has been implemented in hardware in order to evaluate certain measurements and estimation methods as control signals. On the basis of the open-loop experimental results which have been obtained, a control method is proposed for the series ZSI which maximises the range of AC load current capable of being produced. The control scheme adopted also allows for limitations in calculating the shoot-through control signal (D_S) based on the bridge voltage (V_S), particularly when MI is low, by using voltage control to generate D_S for the modulator only in a boost mode which maximises the amplitude of the reference waveform for the switching signals.