ΔΙΠΛΩΜΑΤΙΚΗ ΕΡΓΑΣΙΑ

Abstract
Purpose of this Diploma Thesis is the analysis of the leakage ground current shown in
photovoltaic systems connected to the grid due to the lack of an isolation transformer. The
two main factors affecting the amplitude and the frequency spectrum of leakage currents are
the technology of photovoltaic modules and the chosen inverter topology. Topologies such as
the Full Bridge, NPC etc. are investigated and compared in terms of the leakage currents in
the rest of this thesis.
More specifically, the first chapter sets out the reasons for omitting the isolation
transformer in modern photovoltaic systems and analyzes the factors affecting the value of the
parasitic capacitance and therefore the value of the ground currents.
The second chapter describes the simplified equivalent circuit of the photovoltaic
generator. It also presents all the inverter topologies that will be developed and explored in
terms of leakage currents and analyzes the harmonic components of output voltage and output
current of each topology. Furthermore, the function of the dc/dc boost converter and the
frequency response of the LC filter are being studied.
The third chapter analyzes all the individual control systems that are necessary for the
proper functioning of the entire PV system. Specifically, both current control techniques: PI
control and Hysteresis control are being studied. It also describes the operation of Phase
Locked Loop and Maximum Power Point Tracking devices.
In the fourth chapter, the simulation results are presented, the leakage currents are
analyzed in terms of their harmonic content and different inverter topologies are being
compared. A complete model of the solar system was implemented with the help of the
designing program PSpice.
Computer simulations are followed by experimental measurements on a grid
connected photovoltaic system installed at the National Technical University of Athens. The
experimental results are in the fifth chapter.
In the final chapter, conclusions of the study are drawn and ideas for future research
are being suggested.
Key Words
Photovoltaic (PV), parasitic capacitance, leakage current, common mode analysis, galvanic
isolation, transformer – Transformerless (TL), inverter.
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