Output Power Variation and Mismatch Losses of Photovoltaic Power Generators Caused by Moving Clouds

Photovoltaic (PV) systems are affected by irradiance fluctuations, mainly caused by overpassing cloud shadows, suffering from fluctuating output power. With high PV penetration levels, these fluctuations can lead to power system instability and problems in power quality. Further, overpassing cloud shadows cause partial shading (PS) which is the main cause of mismatch losses in PV systems. Mismatch losses occur in every PV system when interconnected PV cells have different electrical characteristics at a specific instant. Mismatch losses are mainly caused by PS but also by other differences in the operating conditions of PV modules, module damages and manufacturing tolerances. Moreover, PS can lead to failures in maximum power point tracking thereby causing extra losses.

In this thesis, the output power variation and mismatch losses of PV arrays caused by the edges of moving cloud shadows are studied by simulations based on a comprehensive analysis of the measured irradiance data of the solar PV power station research plant of Tampere University of Technology. Shadings caused by moving clouds, especially the characteristics of irradiance transitions caused by the edges of cloud shadows, are analysed. For that purpose, methods to identify irradiance transitions and shading periods caused by moving clouds in measured irradiance data and a method to determine apparent shadow edge velocity were developed. A mathematical model of irradiance transitions caused by moving clouds to be used in the simulations of PV system operation was developed and verified. A parametrisation method of irradiance transitions was also developed to make the simulations of PV system operation computationally less demanding.

The study of the output power variation and mismatch losses of PV arrays is conducted using the developed mathematical model of irradiance transitions and an experimentally verified MATLAB Simulink model of a PV module. The output power variation and mismatch losses of various electrical PV array configurations are studied during the irradiance transitions identified in the measured irradiance data. The effects of irradiance transition characteristics and the layout and geographic orientation of PV arrays on the output power variation and mismatch losses are studied and the overall effect of the mismatch losses caused by moving clouds on the energy production of PV plants is determined.

It is shown that the electrical configuration of PV arrays has only minor effects on the output power variation and mismatch losses of the arrays. Furthermore, it is shown that the mismatch losses caused by moving cloud shadows have only a minor effect on the overall efficiency of PV arrays. Even that can be largely eliminated by minimising PV string diameters.