Page 194 - Advances in Renewable Energies and Power Technologies

3. Necessity of Joint Adoption of Distributed Maximum Power Point 167

compensation network of Fig. 5.2 belongs to the wide-bandwidth current controller
inner loop, which performs the power factor correction on the AC side
[45,46,52,53]. The bulk voltage compensation network belongs instead to the outer
low-bandwidth loop, which regulates the low-frequency components of the energy
storage capacitor voltage v b and therefore ensures the balance between the average
DC input power and the average AC output power. The CMPPT controller of Fig. 5.2
provides the voltage reference value v b ref , which must be followed by v b to maxi-
mize the energetic performances of the whole PV system. In fact, in Refs. [54e56]
it has been shown that, to get full profit from DMPPT, it is necessary that the bulk
inverter voltage belongs to an optimal range whose position and amplitude are func-
tions of the following factors: the number of PV modules and dedicated DC/DC con-
verters in a string, the atmospheric operating conditions characterizing each PV
module (irradiance and temperature values), the voltage and current ratings of the
physical devices the DC/DC converters are made of, and the adopted DC/DC con-
verter topology. Therefore, to extract as much as energy as possible, not only the
PV module voltages but also the bulk inverter voltage v b must assume specific
optimal values that change, case by case, with the atmospheric operating conditions
of the PV system.

3. NECESSITY OF JOINT ADOPTION OF DISTRIBUTED
MAXIMUM POWER POINT TRACKING AND CENTRAL
MAXIMUM POWER POINT TRACKING: HYBRID MAXIMUM
POWER POINT TRACKING
The reason why the DMPPT technique alone is not able to lead to the working of
each PV module of the field in its MPP, whichever mismatching operating condition,
is linked to the limited voltage conversion ratio of the adopted DC/DC converters
and to the finite voltage and current ratings of devices of the power stage of
LSCPVUs [43,54,55]. In this chapter, the symbol V ds max will be used to identify
the maximum drain-to-source voltage of the microconverters’ switches when they
are in the OFF state and the symbol I ds max will be used to identify the maximum
drain current in the microconverters’ switches when they are in the ON state.
Because of the series connection of the output ports of N LSCPVUs, it is:

P pank v b
v outk ¼ ¼ $P pank (5.1)
N
I outk P
P pani
i¼1
where P pank is the power extracted from the k-th PV module; v outk and i outk , respec-
tively, are the output voltage and current of the k-th LSCPVU; and v b is the inverter
DC input voltage. From Eq. (5.1), it is clear that, in mismatching conditions, the
higher the output power of the LSCPVU, the higher its output voltage that, in certain
conditions, may become very large. The voltage across the output capacitor and the