Page 123 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

110 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
®

All MPPT methods presented in this section are suitable to accommodate a constant-power-mode
control scheme [5], except the constant-voltage, constant-current, ANN-based and single-sensor
MPPT techniques, since, due to the types of sampled parameters, which are employed in these
methods, they do not comprise the sensors required to facilitate the measurement of the PV output
power.

5.4 MPPT METHODS FOR PV ARRAYS OPERATING UNDER
NONUNIFORM SOLAR IRRADIATION CONDITIONS
When the individual modules of the PV array receive unequal amount of solar irradiation, the power–
voltage characteristic of the PV source exhibits multiple MPPs as their positions change frequently
under the influence of stochastically varying meteorological conditions. In such circumstances, the
target of an MPPT process is to derive, among the individual local MPPs of the PV source, the
global MPP where the overall power production of the PV array is optimized. Multiple alternative
techniques have been developed in the past, which are suited for application under nonuniform solar
irradiation conditions (see Figure 5.4), and their operating principles are described and compared
in the following.

5.4.1 PV Array Reconfiguration

In order to increase the power, which, for example, is supplied to a constant resistive load by a PV
array operating under partial shading conditions, the use of a matrix of power switches has been
proposed in [59]. Using this matrix, the connections between the PV cells/modules are dynamically
modified, such that the PV strings comprise PV cells/modules operating under similar solar irradia-
tion conditions.
The PV array reconfiguration method has the disadvantages of higher implementation complex-
ity and cost due to the high number of power switches required but increases the energy production
of the PV array. According to [6], since the power–voltage curve of the PV array after reconfigura-
tion may still exhibit local MPPs, a power converter executing one of the global MPPT algorithms
presented in the following should be connected at the output of the PV source, in order to optimize
the generated power.

5.4.2 Evolutionary MPPT Algorithms

In this class of MPPT techniques, the MPPT process is treated as an optimization problem, where
the optimal value of the decision variable is calculated in real time, such that the objective function,
which corresponds to the power–voltage curve of the PV source, is optimized. Thus, various
alternative evolutionary optimization algorithms, which in some cases have been inspired from
biological and natural processes, have been applied for that purpose. A generalized flowchart of an
evolutionary algorithm for implementing an MPPT process is shown in Figure 5.14a. Initially, the
designer specifies the values of the optimization algorithm operational parameters, which define
the speed and accuracy of convergence to the global optimum solution. During the execution of the
optimization/MPPT process, multiple sets of values of the decision variable are produced in a way
defined by the operating principle of the specific optimization algorithm, which has been employed.
By appropriately controlling the power converter, the PV source is set to operate at alternative oper-
ating points corresponding to each of these sets (e.g., PV array output voltage levels V 1 − in the
V k
power–voltage curve shown in Figure 5.14b). At each operating point, the power generated by the
PV source is measured (i.e., the objective function of the optimization problem is evaluated) and
compared to the power produced at other operating points (e.g., PV array output power levels P 1 −
P k
in Figure 5.14b). This process is iteratively repeated, until a convergence criterion has been satisfied,
which indicates that the position of the global MPP on the power–voltage curve has been derived