ABSTRACT:
The
modularity and decreased filter size properties have made cascaded multilevel
inverters (MLIs) more applicable than conventional two-level inverters,
especially in high and medium power photovoltaic (PV) applications. However,
partial shading of PV modules will affect negatively the output power of the
unshaded modules. There are several existing solutions in the literature to
address this challenge, however almost all of them suffer from complex
implementations, low efficiency, and high cost. This paper presents a new
simplified feedforward distributed maximum power point tracking (MPPT) method
for three-phase grid-connected cascaded MLIs. The cascaded MLI provides a
modular, and highly efficient single stage power conversion for PV systems. The
proposed distributed MPPT method is depending on the phase-shift pulse width
modulation (PSPWM) method with a simplified implementation. The proposed method
is developing a feedforward signal that is proportional to the maximum power of
the individual module. Then, the current controller, and the modulating signal
are multiplied with the proportionality factor of the module maximum power.
Furthermore, a modified modulation compensation method without using
proportional-integral (PI) controllers is introduced to solve the problem of
the unbalanced three-phase PV output currents that results from PV power
mismatches and shading. A case study is implemented for 15 kW PV system to
investigate the performance of the proposed method. In addition, comprehensive
comparisons with the previous attempts in the literature are provided to verify
the superior performance of the new proposed control method.
KEYWORDS:
1. Distributed
maximum power point tracking (MPPT)
2. Cascaded
multilevel inverter
3. Phase-shift
PWM (PS-PWM)
4. Photovoltaic
(PV)
SOFTWARE:
MATLAB/SIMULINK
CONCLUSION:
This
paper presents a distributed MPPT method for grid-tied PV systems with cascaded
H-bridge inverters. The proposed method is based on employing a feedforward
control method for extracting the MPPT of individual modules through adapting
the percentage sharing of each module. The proposed method is superior solution
for PV system grid integration due to its simple implementation, signal stage power
conversion, no added complexity with increasing the number of connected
modules, and it eliminates the need for individual control loop for each
module. The proposed distributed MPPT method is validated through various
operating scenarios of the selected case study. The results show the ability of
the proposed distributed MPPT to extract MPPT of each PV module at normal and
partial shading conditions. Moreover, the proposed modulation compensation
method shows the injection of balanced output currents, even if at partial
shading condition. The relative performance of the proposed distributed MPPT method
is also compared with the most notable methods in the literature according to
various performance criteria, including ability to extract individual MPPT,
implementation complexity, system cost, THD content and balance output
currents, and system efficiency. The comparisons show the validity and
superiority of the presented distributed MPPT method over the notable
candidates in the literature.
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