Input voltage control of a three-phase Z-source inverter in photovoltaic applications

Z-source inverter (ZSI) is a recently proposed buck-boost type inverter topology that has wider input voltage range compared to a commonly used voltage-source inverter (VSI). In photovoltaic (PV) applications, the wide input voltage range increases maximum power point tracking efficiency of the interfacing converter and makes it more suitable for different photovoltaic generator configurations. This paper presents a method to derive detailed small-signal model for a grid-connected three-phase ZSI and furthermore describes how a non-linear behavior of a photovoltaic generator (PVG) can be included in the model. It is shown that a right-half-plane (RHP) zero appears in grid current control related transfer function when the PVG is operating at voltages below the maximum power point voltage. The effect of the zero can be mitigated using properly designed input voltage control and without the input voltage control the inverter is prone to instability. The appearance of the RHP zero and its effect to control dynamics is verified by simulations and experimental measurements.