Power variability of small scale PV systems caused by shading from passing clouds in tropical region

Due to the nonlinear characteristics of the photovoltaic (PV) cell and of the bypass diode used to mitigate against hot spots, not only the amplitude of the irradiance can influence the power output but also the irradiance distribution. For a small to medium PV installation, the power output could be significantly influenced when clouds pass over a solar power installation and temporarily block the incoming radiation from sun. Especially in tropical regions, shading events caused by passing clouds are significantly common. This paper examines the power variability of the small scale PV installations caused by the impact of passing clouds in tropical regions. An array of 16 irradiance sensors (4 by 4), which collect data at 1-second intervals, the irradiance difference between individual sensors in the sensor array is investigated. Three typical weather conditions, namely overcast, clear and cloudy, are tested in the experiment, with cloudy days producing many large irradiance change events resulting in significant variations among the sensors. The two diode model is used to model the PV array and its parameters are identified using an improved differential evolution algorithm. This method provides good accuracy of the PV array model. After applying the measured irradiance and temperature data to the PV array model, the average power variabilities of a series of polycrystalline and CIGS thin film PV panels are studied. The results show that for polycrystalline PV systems the average power variability caused by the shading effect of passing clouds in overcast, clear and cloudy days can reach 2.84%, 4.49% and 6.71%, respectively, for a 10 hour period, while for CIGS thin film PV system 1.69%, 4.19% and 5.61%, respectively. The results also show that for the tested systems polycrystalline PV arrays are more vulnerable to the variable irradiance conditions. We plan to use this data to develop strategies for the design of PV power plants, to mitigate against the shading effect caused by passing clouds in tropical regions.