Transformerless Z-source inverter is gaining interest as a competitive alternative for stand-alone as well as grid connected renewable energy system due to its inimitable benefits over the traditional voltage source and current source inverters. Advantages of Z-source inverter over the more widespread Voltage Source Inverter (VSI) and Current Source Inverter (CSI) topology have been explained by a number of researchers. However, there is a room for improvement and learning of quality work in designing of transformerless Z-source inverter interfacing with renewable energy resources. this thesis develops a multilevel inverter structure based on transformerless Z-source for single-phase standalone solar Photovoltaic (PV) application to enrich the research focussing on application of Z-source inverter for renewable energy interface. The first part of the thesis deals with the designing of single-phase two-level Z-source inverter interfacing with dc source. The designed system has been studied for harmonic distortions available in voltage and current at the output of inverter and also the DC component. The requirement for the output filter has been reduced and the output wave shape is sinusoidal. The impedance network used has been designed and its mathematical proof is also provided using state space analysis. a number of iterations are done in the MATLAB environment to verify the performance of the designed topology. A comparative performance analysis of three-phase ZSI and VSI has been provided to take forward this research. For both the topologies, software models have been provided and their results have been verified using a laboratory prototype. The results show the clear superiority of ZSI over the existing traditional inverter. The transformerless Z-source based system consists of MPPT and Silicon Carbide (SiC) MOSFET switching devices. Nowadays SiC MOSFET switching devices are gaining attention due to their higher thermal rating and capability of handling higher power density in comparison to traditional silicon MOSFET devices. Simulation studies are carried out for variety of conditions covering all the possible configurations to verify the performance of the developed transformerless Z-source inverter,.
Furthermore, this thesis deals with the designing of a novel transformerless Z-source solar photovoltaic multilevel inverter topology. The design of developed topology of transformerless Z-source inverters is based on the comprehensive study of existing Z-source multilevel inverters for solar photovoltaic applications. Many new topologies of Z source inverters are evolving. Based on the existing literature many configurations are designed, simulated and analysed to develop a novel transformerless topology. The developed topology has tested through simulation for both fixed DC source, when fed by a battery, as well as variable DC source using a solar photovoltaic array. The performance of the developed inverter has analysed using the current, voltage and power curves at the inverter output, operating at different temperature and insolation conditions through simulation. A comparative performance analysis has been performed by comparing the output of the designed inverter with the existing STP5000TL-20 5kW inverter placed at the rooftop of TERI School of Advanced Studies (TERI SAS), New Delhi. The developed Z-source multilevel inverter uses only seven semiconductor switches to achieve seven-levels of output waveform. It can work under different insolation levels. Due to this, harmonic content of low order are generated in case of traditional inverters, whereas it has been proved in this research that in case of Z-source inverter output THD is in permissible limits according to the IEEE standard 519-2014. For this comparison, the output of the developed inverter is obtained using the real time data of insolation, temperature, output power taken from the rooftop sensors and simulated for randomly selecting a day, a month and a year.
The designed inverter uses only 7 SiC semiconductor switches (that are predominantly higher in all the previous research studies) in its main circuitry and with a pair of dc sources as input and an impedance network to achieve seven-level waveform. This has thus reduced the volume of developed Z-source multilevel inverter (Causation; reduction in number of component count). Total Harmonic Distortion (THD) for voltage and current of the designed inverter is 3.41% and 1.30 % which is found to be within the permissible limits according to the IEEE standard 519-2014. DC component of the designed inverter is 0.844 which is also within the permissible limits according to CEA. The less number of components used can indirectly lead to a possible reduction in the procurement cost of the developed Z-source inverter.
The simulation results strongly indicate that the performance of the developed inverter is in close approximation with the theoretical and the experimental results.
Keywords: Z-source, Transformerless, Multilevel Inverter, Total Harmonic Distortion, Standalone, Single Stage