266
M. Pravettoni
depending on the power generated from the PV modules. In order to minimize the
efficiency losses in operating conditions, inverters are dimensioned to have maximum
efficiency at the maximum power available from the modules, which is rather lower
than the rated power at STC. (This rated power is rarely reached during operating
conditions, as we have already noticed). As an example, the Kaco Powador 6600 of
Fig. 10.9 is designed for 6.6 kW p power plants, but has a maximum output of 6 kW,
above which the power is cut off. It is also designed to have maximum efficiency
of slightly more than 96% at 2 kW and 350 V (where the inverter efficiency loss is
about 3%). But if the power plant of this example generates more than 6 kW, the
inverter is capped at that value, and we can call the surplus power loss a high power
loss. But for the same reason, on the opposite side there are also low power losses,
when the power delivered to the inverter from the PV modules is too low to activate
the inverter, and no power is generated at all.
Similar losses from the inverter occur also when the voltage from the modules to
the inverter is above or below certain “cut-on” and “cut-off” thresholds, where the
inverter works: we refer to these power losses as high input voltage and low input
voltage losses, respectively.
While the inverter efficiency losses are unavoidable, the power and input voltage
losses at the inverter are indeed avoidable, if the inverter is well dimensioned.
10.4.2 Overview of Module Failure Modes
So far in this chapter, we have analysed in detail how the energy production of a
PV power plant can be calculated, depending on the irradiance, the geometry of the
installation, the module technology and the temperature of the modules. We have
also analysed in the last section what are the losses of power between the modules
and the grid that may cause less energy production than predicted. But most often
all this is still not enough, because other factors may cause more or less unexpected
energy losses from the power plant: it is often a hard exercise to inspect and discover
the failure modes that may cause a severe loss of investment for the owner of the
power plant (see [6]).
It is therefore important to provide a description of the failure modes, at least of
those known nowadays. Luckily enough, the PV system failure modes are sometimes
either fully or partially recoverable, when the initial power of the PV plant can be
restored (fully or in part) without substituting any element of the system. When
the initial power of the PV plant cannot be restored without re-placing parts of
the installation, then we refer to them as unrecoverable failure modes, as they are
intrinsically related to module manufacturing.
8
8 As certain partially recoverable failure modes are special cases of analogous unrecoverable failure
modes, it will be more convenient to start with the latter (Sect. 10.4.4) and describe the partially
and fully recoverable failure modes afterwards (Sects. 10.4.5 and 10.4.6).
M. Pravettoni
depending on the power generated from the PV modules. In order to minimize the
efficiency losses in operating conditions, inverters are dimensioned to have maximum
efficiency at the maximum power available from the modules, which is rather lower
than the rated power at STC. (This rated power is rarely reached during operating
conditions, as we have already noticed). As an example, the Kaco Powador 6600 of
Fig. 10.9 is designed for 6.6 kW p power plants, but has a maximum output of 6 kW,
above which the power is cut off. It is also designed to have maximum efficiency
of slightly more than 96% at 2 kW and 350 V (where the inverter efficiency loss is
about 3%). But if the power plant of this example generates more than 6 kW, the
inverter is capped at that value, and we can call the surplus power loss a high power
loss. But for the same reason, on the opposite side there are also low power losses,
when the power delivered to the inverter from the PV modules is too low to activate
the inverter, and no power is generated at all.
Similar losses from the inverter occur also when the voltage from the modules to
the inverter is above or below certain “cut-on” and “cut-off” thresholds, where the
inverter works: we refer to these power losses as high input voltage and low input
voltage losses, respectively.
While the inverter efficiency losses are unavoidable, the power and input voltage
losses at the inverter are indeed avoidable, if the inverter is well dimensioned.
10.4.2 Overview of Module Failure Modes
So far in this chapter, we have analysed in detail how the energy production of a
PV power plant can be calculated, depending on the irradiance, the geometry of the
installation, the module technology and the temperature of the modules. We have
also analysed in the last section what are the losses of power between the modules
and the grid that may cause less energy production than predicted. But most often
all this is still not enough, because other factors may cause more or less unexpected
energy losses from the power plant: it is often a hard exercise to inspect and discover
the failure modes that may cause a severe loss of investment for the owner of the
power plant (see [6]).
It is therefore important to provide a description of the failure modes, at least of
those known nowadays. Luckily enough, the PV system failure modes are sometimes
either fully or partially recoverable, when the initial power of the PV plant can be
restored (fully or in part) without substituting any element of the system. When
the initial power of the PV plant cannot be restored without re-placing parts of
the installation, then we refer to them as unrecoverable failure modes, as they are
intrinsically related to module manufacturing.
8
8 As certain partially recoverable failure modes are special cases of analogous unrecoverable failure
modes, it will be more convenient to start with the latter (Sect. 10.4.4) and describe the partially
and fully recoverable failure modes afterwards (Sects. 10.4.5 and 10.4.6).