Grid-connected Quasi-Z-Source Photovoltaic Inverter with Energy Storage

Abstract

In a small to medium scale PV inverter system, energy storage functionality such as battery is significant to ensure the energy is efficiently stored, used and delivered to the load. As for the grid-connected system, having a bidirectional power flow in the system is an advantage in case the power from the PV array is insufficient to satisfy the demand from the load. The grid power can also be used to charge the battery storage in case the energy stored is at the critical level and cannot be recovered immediately due to low/zero level of sun irradiation. In relation to this, the quasi-Z-source inverter (qZSI) topology, apart from offering a single stage DC-DC-AC conversion, offers a flexible way on how the battery storage can be introduced to the system. In this thesis a new method of connection and control method for battery storage is proposed which can be connected to the system easily with minimal modification with main advantage of controlled current for the charging and discharging operation. Apart from this, bidirectional power control between the PV source, battery storage, load and the grid is also investigated. As for the implementation, a complete grid-connected quasi-Z-Source PV inverter system with battery storage is modelled and designed. Verification is carried out firstly through the simulation of a 5 kW system with transformerless connection to the grid and a scaled down version of 0.5 kW system with transformer as a comparison with the experimental hardware prototype. A hardware prototype at 0.5 kW is built for experiment purpose. From both simulation and experimental results, it is shown that the proposed battery storage method works satisfactorily with the inverter and the bidirectional power flow control can be realized successfully. The work in this thesis is significant specifically for the PV inverter system using the Quasi-Z-Source topology which so far has not been explored in the literature. It has demonstrated that this topology is suitable for commercial and industrial implementation and has equivalent performance as other conventional topologies used for grid-connected PV inverter system.