Capacitor voltage control for MPPT range expansion and efficiency improvement of grid-connected Quasi Z-Source Inverter

In a grid-connected PV system using a QZSI(Quasi Z-Source Inverter), the capacitor voltage of an impedance network is controlled to be constant to track the MPP(Maximum Power Point) of a PV array and generate the reference output current. For higher efficiency of the system, if possible, the reference voltage of the capacitor should be set at a lower value. On the other hand, the reference voltage of capacitor has to be set at a higher value to have margin for a shoot-through reference and to inject a current into the grid. Although the capacitor voltage of an impedance network is controlled to be constant, the output voltage of an impedance network varies according to the shoot-through time ratio. This may result in an unstable operation. To overcome these matters, in this paper, a method to keep the output voltage of an impedance network constant is proposed. This results in an expansion of MPPT range and a higher efficiency of QZSI.