Page 135 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

122 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
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duty cycle progressively converge to the appropriate values such that the PV source operates at
the corresponding MPP. The tracking speed can be adjusted by modifying the value of param-
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eter “perturbation step” of the P&O MPPT algorithm embedded MATLAB functional block in
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the MATLAB /Simulink model. At steady state, an oscillation around the corresponding MPP is
observed in the plots in Figure 5.22. The amplitude of this oscillation also depends on the value of
the “perturbation step” parameter.
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The ANSYS Simplorer model shown in Figure 5.23a simulates the operation of a system com-
prising a PV array connected to a DC/DC power electronics converter and including an MPPT
controller. This model, as well as the one illustrated in Figure 5.23b, is part of collection of tutorial
examples developed for ANSYS , Inc. by Steve Chwirka. The models incorporate, at subsystem
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level, various techniques, such as lookup tables, nonlinear dependent sources, equation blocks, and
the VHDL-AMS language (IEEE standard multi-domain and mixed signal language). Examples
include the definition of solar irradiance characteristics, the variation of the PV cell resistance, and
output characteristics with temperature. The equation based VHDL-AMS language approach allows
the model to be scalable from cell to module to array level.
The results illustrated in Figure 5.23a, for the system operating connected to a stand-alone resis-
tive load, show that the use of an MPPT controller is advantageous in terms of increased output
power and system efficiency, even when considering inherent power losses of the DC/DC converter.
At the highest system level, this can translate in reduced PV array size, and hence cost, for specified
rated power.
The single-phase AC grid connected PV system depicted in Figure 5.23b, which incorporates a
DC/AC power electronics converter, provides additional functionality, such that during the day the
load can be supplied in combination by the PV array and the grid and any excess PV power is fed to
the grid. Through the integration of a battery, further capability can be added, such that part or all of
the PV energy is stored for local use during sun shading or at night, resulting in a reduction of the
energy demand from the grid.

5.6 SUMMARY
The power–voltage curves of PV modules/arrays exhibit a point where the PV-generated power is
optimized. Under uniform solar irradiation conditions, this point is unique, while in the case that dif-
ferent amounts of solar irradiation are incident on the individual PV modules of the PV array, then
multiple local MPPs may also exist. Thus, the control unit of the PV energy conversion system must
execute an MPPT process in order to operate the PV source at the point where the generated power is
maximized. This process enables to optimally exploit the installed PV capacity, thus increasing the
energy conversion efficiency of the overall PV system and, simultaneously, enhancing the economic
benefit obtained during the PV system lifetime.
The P&O and DMPPT methods have already been incorporated into commercial PV power con-
verters [94, 95]. However, a wide variety of methods have additionally been proposed in the sci-
entific literature during the last years for performing the MPPT process in a PV system. Aiming to
assist the designers of PV power processing systems to select the most suitable MPPT method, the
operational characteristics and implementation requirements of these techniques have been ana-
lyzed in this chapter.
In case the PV system has to operate under nonuniform solar irradiation conditions, the MPPT
methods that have been developed for PV arrays operating under uniform solar irradiation condi-
tions should not be applied, since they do not guarantee that the global MPP will be derived.
The operational complexity of each MPPT method affects the implementation cost of the corre-
sponding control unit. Digital control units are most frequently employed in the modern PV power
processing systems. The MPPT techniques relying on the execution of an optimization algorithm
(e.g., P&O, InC, evolutionary algorithms) are more easily integrated into such devices, compared
to the techniques that require the addition of specialized analog and/or digital control circuits for