  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.
