Transformerless Grid-Tied Impedance Source Inverters for Microgrids

Renewable energy source (RESs) diffusion into the power system is continuously increasing, where the world cumulative installed capacity of solar and wind energy sources increased from around 63.2 GW in 2005 to around 903.1 GW in 2017 according to International Renewable Energy Agency (IRENA). The energy utilization from these RESs implies the use of what is called power conditioning stage (PCS). Such PCS acts as an interfacing layer between the RES side and the customer side, i. e. the load or the grid. These PCSs can utilize many different configurations depending on the employed RES, where the two-stage architecture is commonly used with solar photovoltaic (PV) systems due to the low or variable output voltage. Such two-stage architecture is usually implemented using a boost converter in order to regulate the PV source output voltage and maximize the output power, and a voltage source inverter (VSI) in order to achieve the inversion operation. On the other hand, impedance source inverters represent a different family of the existing PCSs, which are called single-stage power converters as they embraces the boosting capability within the inversion operation. This family of PCSs is seen as an interesting and competitive alternative to the twostage configuration, which are mandatory for low or variable voltage energy sources, such as PV and fuel cell energy sources. Therefore, these impedance source inverters have been utilized in many different applications, such as distributed generation and electric vehicles. This family of PCSs, i. e. impedance source inverters, has experienced a fast evolution during the last few years in order to replace the conventional two-stage architecture since the first release of the three-phase Zsource inverter (ZSI) in 2003. Consequently, many research activities have been established in order to improve the ZSIs performance from different perspectives, such as overall voltage gain, voltage stresses across the different devices, continuity of the input current, and conversion efficiency. Among these different topological improvements, the conventional ZSI and the quasi-ZSI (qZSI), are the most commonly used structures. Accordingly, the objective of this thesis is to study and reinforce the performance of this family of PCSs. Hence, the work in this thesis starts first by addressing the challenges behind eliminating the low frequency transformer in grid-tied PV systems in order to improve the conversion system efficiency, where a new measurement technique for the dc current component is proposed in order to effectively mitigate this dc current component. Then, the performance of the classical impedance source inverters has been assessed by studying all the possible modulation schemes and proposing a new one, under which the efficiency of these classical impedance source inverters have been improved. Furthermore, the partial-load operation of these impedance source inverters, considering the three-phase qZSI, has been studied and the possible ways of achieving a wide range of operation have been investigated. Due to the seen demerits behind the classical impedance source inverters, an alternative new topology, which is called split-source inverter (SSI), is proposed, under which these demerits have effectively been mitigated or eliminated. Then, the challenges behind grid-tied operation of this single-stage dc-ac power converters has been investigated considering the SSI topology. It is worth to note that all the prior mentioned contributions have been validated experimentally. Finally, this thesis is divided into two chapters, where the first chapter introduces an extended summary of the work done concerning the thesis topic, while the second part includes some selected papers from the publications that have been developed during the doctoral study. These selected papers give all the details of the work done in each section in the extended summary.