HT instruments MPP300 - Manual (Page 27)

MPP300
EN - 25
9.2. NOTES ON MPPT (MAXIMUM POWER POINT TRACKER)
Solar irradiation on a surface such as the surface of a photovoltaic system has extremely
variable characteristics, since it depends on the position of the sun with respect to the
surface and on atmospheric conditions (typically, on the presence of clouds). A
photovoltaic module presents, for different solar irradiation values, and for different
temperature values, a range of characteristic curves of the type shown in the following
figure. In particular, the figure shows three I-V curves (in bold) which correspond to three
values (1000, 800, 600W/m
2
) of solar irradiation
On each characteristic curve there is one single point in which the power transfer towards
a hypothetical charge supplied by the photovoltaic module is maximized. The maximum
power point corresponds to the voltage-current pair for which the product V*I is maximum,
where V is the value of voltage at the module’s terminals and I is the current which runs in
the circuit obtained by closing the module on a hypothetical charge
With reference to the figure above, the product V*I is represented, for the three solar
irradiation values mentioned above, through the three curves in thinner lines
The figure shows that, as stated above, these curves only have one single maximum point.
For example, for 1000W/m
2
, the maximum power point corresponds to a voltage value of
approx. 36V and to a current value of approx. 5.5A. Obviously, if the power provided by
the system is maximized, it is possible to make the most of the system, both in case the
system is connected to mains, and in case it is stand-alone
MPPT is an inbuilt device in the inverters. It typically reads the voltage and current values
at any instant, calculates their product (i.e. the power in Watts) and, by causing small
variations in the conversion parameters (duty cycle), it is capable of determining, by
comparison, if the photovoltaic module is working in maximum power conditions or not.
According to the result, it operates again on the circuit in order to bring the system to an
optimal condition The reason why MPPTs are used is simple: a photovoltaic system
without MPPTs may operate anyway; however, with the same solar irradiation, it provides
less energy.