Performance Investigation of Quasi Z-source Inverter with Energy Storage Facility for Standalone PV Systems

Abstract

Traditional two stage converters in grid connected or standalone applications are being replaced with single stage converters like quasi impedance source inverters (Quasi-Z source inverter). The impedance network coupling the source to the inverter bridge allows boosting the input voltage through utilizing shoot-through states. The conventional PWM inverters' switching methods are also applied to the QZSI; the modulation index and the periods at which shoot-through occurs determine the RMS voltage of the inverter's output. A battery assisted QZSI has been introduced the literatures to reduce the fluctuation in the output voltage of solar panels during day time. The QZSI has been also used as battery charger and the control strategy applies maximum power point tracking MPPT technique. Most of the literatures in this respect discussed the power flow control and battery management. Also, they mainly focus on achieving a constant dc-link voltage, constant capacitor voltage, or maximum power point tracking (MPPT). These studies did not disclose a dynamic model-based controller design for the batteryassisted QZSI neither took into consideration the variation from traditional QZSI. For example, the inductors currents are no longer equal during battery charging and discharging, and the capacitor voltage is clamped by the parallel battery. It limits the conventional control method ability to achieve a constant dc-link peak voltage. In this thesis, a QZSI with energy storage system will be developed for standalone applications. A mathematical model of battery assisted QZSI and its switching technique have been developed. A controller based on the battery assisted Quasi Z-Source Inverter model has been designed to achieve both maximum power point tracking (MPPT) from the solar panels and to control battery State of Charge (SOC), throughout shoot through duty ratio (D) and Modulation index of the inverter (M). The simple boost modulation technique will be adopted for the inverter switching strategy. The performance of the designed system has been verified using simulation