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One of the most important issues in the operation of a photovoltaic (PV) system is extracting maximum power from the PV array, especially in partial shading condition (PSC). Under PSC, P-V characteristic of PV arrays will have multiple peak points, only one of which is global maximum. Conventional maximum power point tracking (MPPT) methods are not able to extract maximum power in this condition. In this paper, a novel two-stage MPPT method is presented to overcome this drawback. In the first stage, a method is proposed to determine the occurrence of PSC, and in the second stage, using a new algorithm that is based on ramp change of the duty cycle and continuous sampling from the P-V characteristic of the array, global maximum power point of array is reached. P&O algorithm is then re-activated to trace small changes of the new MPP. Open loop operation of the proposed method makes its implementation cheap and simple. The method is robust in the face of changing environmental conditions and array characteristics, and has minimum negative impact on the connected power system. Simulations in Matlab/Simulink and experimental results validate the performance of the proposed methods.Index Terms-DC/DC converter, maximum power point tracking (MPPT), Partial shading condition, Photovoltaic power generation system. I.

This paper first discusses dynamic characteristics of wind turbines with doubly fed induction generator (DFIG). Rotor back electromotive force (EMF) voltages in DFIG reflect the effects of stator dynamics on rotor current dynamics, and have an important role on rotor inrush current during the generator voltage dip. Compensation of these voltages can improve DFIG ride-through capability and limit the rotor current transients. It is found that the electrical dynamics of the DFIG are in nonminimum phase for certain operating conditions. Also, it is shown that the dynamics of DFIG, under compensation of rotor back EMF and grid voltages, behave as a partially linearizable system containing internal and external dynamics. The internal and external dynamics of DFIG include stator and rotor dynamics, respectively. It is found that under certain operating conditions, the internal dynamics, and thus, the entire DFIG system becomes unstable. This phenomenon deteriorates the DFIG postfault behavior. Since the DFIG electrical dynamics are nonlinear; the linear control scheme cannot properly work under large voltage dips. We address this problem by means of a nonlinear controller. The proposed approach stabilizes the internal dynamics through rotor voltage control, and improves the dynamic behavior of the DFIG after clearing the fault.