Abstract: In order to ensure that the photovoltaic system under a drastically-changing light can still output its maxi- mum power,an adaptive variable step-size incremental-resistance maximum power point (MPP) tracking (MPPT) algorithm is proposed.In the algorithm,a threshold function is adopted to automatically divide the operation area of the photovoltaic system into the fixed step-size region and the variable step-size region according to the changes of external environments.In the fixed step-size region,a method of tuning initial fixed step sizes is proposed based on the approximate linear relationship between the short-circuit current and the maximum power point current.In the variable step-size region,by drawing on the relationship between the step conversion function and the output power,a scaling factor of the variable step-size strategy is determined to ensure the convergence of the proposed al- gorithm.Finally,a Matlab/Simulink model of the photovoltaic system and a 5kW photovoltaic system controlled by DSP (TMS320F28035) are established to conduct a series of experiments.Simulation and experimental results in- dicate that the proposed algorithm can improve the dynamic response speed of the photovoltaic system under a dras- tically-changing light by 73%，and can achieve a steady-state tracking accuracy of 98. 9%.

Key words: photovoltaic generators, maximum power point tracking, adaptive variable step size, incremental re- sistance, incremental conductance