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180 CHAPTER 5 DMPPT PV System: Modeling and Control Techniques

basis of a reasonable compromise because the lower T r , the higher the rate of change
of irradiance variation that can be tracked [1] and, at the same time, the higher the
amount of available energy that is wasted during the scan when the operating point
of the PV system is not the optimal one.

3.2 HMPPTF TECHNIQUE
In this subsection, the HMPPTF technique is presented and discussed [60,63]. Such
a technique is based on the “one-shot” estimate of the optimal operating range of the
inverter input voltage and of the optimal operating voltages of the PV modules based
on the measurement of the short circuit currents of the PV modules. The main advan-
tage of the proposed technique is just represented by the speed of identification of a
set of optimal operating points for the inverter and for the PV modules, which in turn
allows to obtain a marked increase of the speed of tracking both of the inverter
(which performs the CMPPT function) and of the DC/DC converters (which perform
the DMPPT function). Moreover, a further advantage is represented by the possibil-
ity to avoid mistakes of the inverter CMPPT technique; that is, the operating value of
the inverter input voltage cannot remain trapped in the neighborhood of a suboptimal
operating point, as it would happen, in mismatching operating conditions, if stan-
dard CMPPT techniques (P&O and/or INC techniques) were adopted. The HMPPTF
technique exploits the FEMPV algorithm. To describe in detail the working of such
an algorithm, a preliminary discussion related to the modeling of LSCPVUs in terms
of IeV and PeVoutput characteristics is needed and it is the subject of subsections
“Boost-based LSCPVUs”, “Buckeboost based LSCPVUs,” and “Buck-based
LSCPVUs”.
3.2.1 Exact and Approximate IeV and PeV Characteristics of LSCPVUs
In the following the few, simple guidelines allowing to obtain the IeVand the PeV
output static characteristics of LSCPVUs will be provided. The knowledge of such
characteristics represents the starting point to develop the IeV and PeV character-
istics of strings composed by arbitrary numbers of LSCPVUs with series-connected
output ports. In fact, once the IeV characteristics of the N LSCPVUs belonging to a
given string are known, the IeV equivalent characteristic of the whole string is ob-
tained by summing, for each value of string current I b , the corresponding values of
the N LSCPVUs’ output voltages. From the IeV string characteristic, it is then easy
to obtain the corresponding PeV characteristic. Examples of PeV characteristics of
strings of LSCPVUs have been already shown in Fig. 5.3. It is worth noting, however
that, in practice, as it will be clearer in the following, what is really needed to obtain
the efficient working of a PV system equipped with microconverters is the knowl-
edge of the features of the optimal operating range R b (amplitude, location, and cor-
responding value of the total power), rather than the detailed PeV characteristic of
the whole string of LSCPVUs. Therefore, to get enough accurate information con-
cerning R b , it is not necessary to deal with the exact IeV characteristics of
LSCPVUs, but a proper approximate version of such characteristics is enough.