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Three-Phase Photovoltaic Systems: Structures, Topologies, and Control 83

Increasing irradiance

Power

(a) Voltage

Increasing temperature
Power

(b) Voltage

FIGURE 4.13 Variation of the power–voltage curve influenced by irradiance (a) and by temperature (b).

optimization, etc., as well as PV model based, which are theoretically well suited for tackling the
nonlinear problem of MPPT [55, 56]. Indeed, good performances of these methods are reported;
however, they share a common drawback in that they require detailed knowledge of the system they
are applied to, which therefore decreases their general applicability.
The MPPT methods that are mostly preferred by the industry are the hybrid or enhanced hill-
climbing ones, which have a P&O algorithm as their core, with an improvement/enhancement in
conjunction, to overcome its limitations in variably and/or partially shaded conditions.
With the rise of large-scale PV plants as well as microinverters and DC modules, distributed
MPPT systems have gained more attention (see Figure 4.15c) [57–60]. The main MPPT structures
are outlined in Figure 4.15.
In Figure 4.15, the location/granularity of the MPP trackers in the PV systems is also determined
by the use of converter system layout (see Figure 4.2). In fact, an important factor in choosing the
PV plant converter layout is the desired level of resolution of the MPPT.
The central (or multistring) (a) and string (b) MPPT structures are the most common and can be
considered as the traditional configuration. As mentioned before, module-level (or even submodule-
level) MPPT is a promising structure, considering the potential for higher yield due to localized
MPPT and condition monitoring.
Another possible MPPT structure is the hybrid solution, where only a part of the string is
equipped with module-level MPPT [61]. This can be a cost-effective solution to minimize shading
losses; however, they are more beneficial for, for example, rooftop installations. In this case (and
for layout (c) in case a central MPPT is present), the interaction between the local and central MPP
tracker has to be dealt with as discussed in [20, 62, 63].