Analysis and Modeling of Transformerless Photovoltaic Inverter Systems

The need for a cleaner environment and the continuous increase in power demands makes decentralized renewable energy production, like solar and wind, more and more interesting. Decentralized energy production using solar energy could be a solu-tion for balancing the continuously-increasing power demands. This continuously increasing consumption overloads the distribution grids as well as the power stations, therefore having a negative impact on power availability, security and quality. One of the solutions for overcoming this is the grid-connected photovoltaic (PV) system.
PV inverter systems can be improved in terms of efficiency using transformerless topologies, but new problems related to leakage current need to be dealt with.
The work presented in this thesis deals with analyzing and modeling of transfor-merless PV inverter systems regarding the leakage current phenomenon that can damage solar panels and pose safety problems.
The major task of this research was the investigation and verification of trans-formerless topologies and control strategies to minimize the leakage current of PV inverter topologies in order to comply with the standard requirements and make them safe for human interaction.
The thesis is divided into two parts: Part I - Report and Part II - Publications. Part I is a summary report of the work done throughout the research and contains 6 chapters.
Chapter 1: Introduction, focuses on the background and motivation regarding the research done in this thesis. Furthermore, the objectives and limitations of the project are enumerated. The chapter finishes with the outline of the thesis.
Chapter 2: Overview of grid connected PV systems, gives an overview about grid connected PV inverters, focusing on transformerless inverters and related safety issues. The parasitic capacitance of several commercial mono- and multi-crystalline PV panels has been measured, and an appropriate value has been defined for use in the simula-tions. Also, two commercial current sensors that can be used for leakage current measurement, have been tested and the results are presented in Appendix A.
A detailed investigation of different inverter topologies regarding the ground lea-kage current is described in Chapter 3: Investigation of transformerless topologies, showing the ground voltage and leakage current for the analyzed topologies, concluding with whether the topology is suited for transformerless PV systems.
Chapter 4: Common mode voltage in PV inverter topologies, explains the com-mon-mode behavior of single and three-phase PV inverter topologies by presenting a comprehensive analysis of the single and three-phase transformerless converter with respect to the problem of the leakage current that flows through the parasitic capacit-ance of the PV array.
In Chapter 5: H-Bridge Zero Voltage Rectifier topology, a new inverter called H-Bridge Zero Voltage Rectifier (HB-ZVR) is proposed, where the mid-point of the DC link is clamped to the grid only during the Zero Voltage period by means of a diode rectifier bridge and one switch. A comparison of known transformerless topologies and the HB-ZVR is performed using simulations, focusing on the voltage to earth harmon-ics and ground leakage current. Furthermore, experimental results are shown, confirming the simulations, and finally, the efficiency curve of the compared topologies is detailed.
In Chapter 6: Conclusion, the final conclusion is presented, based on the theoreti-cal and experimental results performed. Also a list is given, detailing the contributions presented in this thesis. Additionally, guidelines for future work are given.

The second part of the thesis: Part II - Publications contains the papers that have been published during the period of the research. The articles describe in detail the methods, simulations and the experimental results that make up the backbone of the work described in this thesis.