Transformerless inverters for grid-connected photovoltaic systems

Abstract

Nowadays, solar energy generation has gained significant popularity in light of the increasing energy demands. Transformerless inverters have become an important alternative to interface photovoltaics with the grid for converting DC electricity to AC, since they are more reliable and have higher efficiencies of up to 98%. However, they are prone to leakage currents and their use is regulated by the local grid codes of individual countries. This project is concerned with the analysis and comparison of the operation of the H4, H5 and HERIC transformerless inverter topologies. The main objective of the project is the design, modelling and simulation of a 2.5kVA single phase inverter implemented with these three topologies. Simulations results will include a comparison of efficiencies at different power injection levels; common mode voltages and leakage currents. For the implementation of the transformerless inverters connected to the grid, first it was needed to design and simulate the Photovoltaic Array which was going to serve as the DC source for the inverters. The second phase was to design and simulate the inverter topologies under different DC input power levels. In order to perform these tests, control loops for the grid connected operation of the inverter were also designed and modelled. Third stage was to investigate their Common Mode Voltage and leakage currents. In order to achieve these tests, stray capacitances were used at the input side of the inverter. Finally, the simulation of the inverters including switching and conduction losses of the IGBTs and diodes were considered in order to determine the efficiency curves of the three topologies. In addition, the European Efficiency was also considered which takes into consideration the operation of the PV array.