Design of harmonic filters to improve the operation of a PV system

Abstract

Renewable energy has been growing significantly, especially photovoltaic (PV) systems, PV system is a technology which can effectively help to overcome several issues. As the system is based on the conversion of the energy generated by the sun to electricity. The energy produced is clean, reliable and highly efficient. The main component of a PV system is the power electronics inverter, which is an electronic converter that converts DC current generated from the PV system to AC current fed to the local network at the point of connection. Because of the power electronic equipment present at the point of connection, there will be a generation of harmonic current which causes a power quality problem. Harmonics considerably affect the efficiency of the grid connected PV system. To achieve an acceptable distortion, increase the power quality and to reduce the harmonics, an LCL filter between the inverter and the grid has been designed. The mathematical concept is based on equations and the transfer function which describe the LCL filter. Transfer functions are the result of the ratios between various input to output Laplace-transformed complex currents and voltages. The most important transfer function for this LCL filter is from the inverter voltage to the grid current that is injected. It is important to note that the parameters mentioned above were designed under certain assumptions such as ideal power electronic switches, a constant DC link voltage and no perturbations in the output grid voltage. The passive damping is used to reach the stability in the voltage source inverter (VSI) based on LCL filter. By inserting passive elements into the filter structure, it changes the response and resonance frequency. So with parallel passive damping, the filter attenuation is decreased since this method adds a zero to the LCL filter transfer function. In the design and modelling approach, the filter is considered ideal, once magnetic and electrical losses are neglected. There are three major factors in the design which are: the grid side and inverter side inductance, as well as the capacitor. The capacitance value is a compromise between power factor decrease and injected current harmonic distortion. The expected value of the capacitance is by about 5% of the nominal power in order to avoid overrating the converter. In high power application like this study, the filter is not directly integrated into the converter. As the inductor current saturation is an important aspect. Then the calculation of the resonance frequency. It should be in a range larger than the grid frequency and smaller than the switching frequency, in order to avoid resonance in lower and higher harmonic orders. In this thesis, a performance evaluation of the proposed system under different conditions, in terms of the THD in the output stage of the inverters, was undertaken. MATLAB/Simulink software package is used for the modelling and simulation.

Development of the simulation for the whole grid connected PV system using MATLAB/Simulink as a simulation tool to investigate the use of the designed filter to mitigate harmonics power of the system according to IEEE 519 standard. The results obtained reveal several information. It has been showed how the irradiance impacted the PV power output because from when the irradiance dropped the power does the same vice versa. The filter decreased the harmonic distortion rate from 25 to 2.39. The filter is very resilient, even besides the changes of step reference; it was able to maintain the harmonic percentage under a certain level.