A study of the harmonic content of distribution power grids with distributed PV systems

A photovoltaic system transforms solar radiation into electrical energy using so-called PV panels. A key component of this system is the power electronics subsystem which enables maximum power extraction from the available solar irradiation, as well as con-nection to the AC power grid. However, the current and voltage waveforms at the point of common coupling (PCC) with the power grid contain a degree of harmonic distortion which, in some instances, may surpass that recommended by existing standards. The presence of high harmonic distortion in an electrical installation significantly decreases power quality and the renewable energy sources’ power electronics carries the potential to yield high harmonic distortion. This thesis reports on an investigation of some of the factors that impact adversely the quality of the current and voltage waveforms in an electrical power distribution network with distributed photovoltaic systems. These factors include irradiance levels, imperfect conditions of the filtering system, resonant conditions, load imbalances and selection of the inverter’s switching frequency. To quantify current and voltage harmonic injections, a two-stage model of a photovoltaic array was designed in Simulink in order to show the impact of a single photovoltaic system. The basic PV system model is then applied to a model of an electrical power distribution grid, with several distributed PV units. The study indicates that irradiance is the primary factor influencing THD and that at low PV power outputs, harmonic emissions may exceed the recommended harmonic distortion limits, particularly when resonant conditions exist at the output of connection of the PV plant. Different MPP control methods employed in the DC-DC conversion stage were also investigated and it is observed that they do not seem to have much impact on THD. This applies in the absence of partial shading, an issue which was not considered as part of this research project. As expected, the use of well-designed filters is the key to keeping harmonics emissions low. Nevertheless, perfect filtering does not exist in actual installations and the study also investigates the impact of imperfect filtering parameters and filter branch failure, on the voltage and current waveforms at PCC