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Overview of PV Maximum Power Point Tracking Techniques 113

i
i
The next search interval is decided by comparing the PV output power at c 1 and c 2 as follows:
+
+
+
+
i 1
i
i 1
i
i 1
i
i 1
i
i
i
if P pv() < () , then c 3 = and c 4 = c 4 , or else c 3 = and c 4 = c 2 . This search process is
c 1
c 3
P pv
c 1
c 2
i
i
i
continued until the variable n of F n is reduced to zero or the distances between c 3 , c 4 and P pv() ,
c 3
i
P pv() , respectively, drop below predefined thresholds. The MPPT algorithm based on the Fibonacci
c 4
sequence does not guarantee convergence to the global MPP.
5.4.4 Stochastic and Chaos-Based MPPT Algorithms
Random search method has been applied in [72] for deriving the global MPP of a PV array with par-
tial shading. Using this approach, the duty cycle of a DC/DC power converter is iteratively modified
using random numbers, such that it progressively moves toward values that operate the PV source at
points providing a higher output power.
The chaotic-search global MPPT process presented in [73] is based on two recursive functions
(i.e., dual carrier) in order to perform iterative fragmentations of the PV array power– voltage char-
acteristic. For that purpose, sequences of numbers are generated through the use of appropriate func-
tions, which correspond to alternative operating points on the power–voltage characteristic of the
PV array. By measuring the power generated by the PV array at these positions, the global MPP is
detected. In [74], the global MPP tracking process for flexible PV modules, which also exhibit local
MPPs on their power–voltage curves, is performed using a combination of the dual-carrier (i.e.,
using two recursive functions) chaotic-search and PSO optimization algorithms.
Due to their operational complexity, a microcontroller- or DSP-based control unit is required for
executing these global MPPT algorithms.
5.4.5 Distributed MPPT
In the case that the PV source, which is connected to the power converter of the PV energy production
system shown in Figure 5.1, comprises strings of series-connected PV modules, then a bypass diode
is connected in antiparallel with each PV module in order to conduct the string current in cases of
partial shading conditions. In contrast to this design approach, in the distributed MPPT (DMPPT)
architectures, a separate DC/DC power converter is connected at the output of each PV module of
the PV array.
In the current equalization DMPPT topology, the DC/DC converter connected at the output
of each PV module is power supplied by the DC bus of the PV string. A diagram of this topol-
ogy based on the architecture proposed in [75] is depicted in Figure 5.15. Under partial shading
conditions, the nth PV module produces a current equal to I pv n, and the corresponding DC/DC
converter is controlled to supply an additional current, which is equal to I s − I pvn, , such that the
total string current is equal to I s . At the same time, the output voltage of each PV module is
regulated such that operation at its MPP is ensured [76]. The position of MPP is different for each
PV module, depending on the geometry of the shading pattern on the PV array. Each DC/DC con-
verter is required to supply only the equalization current, thus operating at a low power level with
relatively low power losses.
In order to enable multiple strings, each employing the current equalization topology described
previously, to be connected in parallel without forcing their PV modules to operate away from their
MPPs, the current equalization topology presented earlier has been extended to the shunt-series
compensation topology. A diagram of this topology based on the architecture proposed in [77] is
shown in Figure 5.16. In this architecture, a current-compensating DC/DC converter with MPPT
controller is connected in parallel with each PV module and a voltage-compensating DC/DC con-
verter is connected in series with each PV string, which balances the deviation of the total voltage
produced by parallel-connected strings, thus enabling the individual PV modules to operate at their
own MPPs.