The effect of grid operating conditions on the harmonic performance of grid-connected PV inverters

Abstract

Solar energy is a readily available alternative energy source that can be utilised to help reduce carbon emission; typically created by traditional, yet depleting, fossil fuel sources such as coal, gas and oil. For this reason, there has been a rapid increase in the installation and development of grid-connected Photovoltaic (PV) systems. Typically, grid-connected PV systems require a power electronic inverter to interface with the distribution grid. This thesis is concerned with the power quality interactions which arise between the distribution grid and the connected inverter. In particular, it considers the effect of grid operating conditions on the harmonic performance of the system. In the first part of the thesis, the mechanism of the interaction between the distribution grid and the grid-connected inverter system is investigated. Specific attention is given to researching the impact this interaction has on the current controller of the grid-connected inverter system. Initially, a simulation model is developed to investigate the effect that variation in the grid operating conditions has on the harmonic performance of the PV inverter current controller. Simulation results demonstrate that the magnitude and relative phase angle of the background grid voltage harmonics (with respect to the fundamental) has a direct impact on the inverter system output current harmonics. The second part of the thesis focuses on the experimental validation of these simulation results. An experimental grid-connected inverter system is described, and the harmonic performance is fully analysed and assessed. Results are shown to compare favourably with theoretical and simulation studies. During the experimental work, the inverter output low pass filter is found to interact with the grid impedance; creating a low impedance resonance whose frequency varies over time with respect to the grid impedance. For this reason, the final part of the thesis concentrates on the suppression of this resonance. A novel adaptive PI control scheme is devised, which is tuned via feedback from a resonance detection bandpass. The proposed control scheme is experimentally compared to a conventional PI control scheme and shown to offer superior harmonic and resonant characteristics.