20.4.1
the installation contains two or more areas with different shading and irradiance, for
example on two different sides of a roof. An alternative option is to connect different
strings to different inverters, which would mean that the system uses several independent
inverters.
As for N S , a maximum exists since each inverter type has a maximum allowed input
voltage. The maximum voltage the PV array can generate is the maximum open circuit
voltage of the array, occurring when the inverter is not operating. The maximum open
circuit voltage is determined by the number of modules in series N S and the maximum
open-circuit voltage of an individual module. Because of the negative value of the
temperature coefficient, the maximum open circuit voltage occurs in the coldest period of
the year. There is also a minimum value for N S . Each inverter has an input voltage
window, where the maximum power point tracker operates. If the array maximum power
point voltage falls below this voltage window, the maximum power point tracker does not
operate any more. The lowest array maximum power point voltage occurs in the hottest
period of the year.
In a conservative assumption, the power on the DC side at STC now is given as
The inverter must be chosen such that its maximal Power
is above the maximal PV
output,
Further, the nominal DC power of the inverter should be approximately equal to the PV
power at STC,
In practice, the nominal DC power of the inverter is selected slightly below the PV power
at STC, up to 10%, depending on the climate zone, because of the different irradiance
distributions. Also, for P DC0 < 5 kWp, single-phase inverters are used while for P DC0 > 5
three-phase inverters are advised.
The inverter efficiency is dependent on the input power and voltage. A model
discussing the inverter efficiency is presented below.
Inverter efficiency
As we already discussed in Chapter 19, modern inverters fulfil two major functions: irst,
maximum power point tracking (MPPT), and secondly, the actual inverter function, i.e.
converting the incoming direct current (DC) to alternating current (AC) that can be fed
the installation contains two or more areas with different shading and irradiance, for
example on two different sides of a roof. An alternative option is to connect different
strings to different inverters, which would mean that the system uses several independent
inverters.
As for N S , a maximum exists since each inverter type has a maximum allowed input
voltage. The maximum voltage the PV array can generate is the maximum open circuit
voltage of the array, occurring when the inverter is not operating. The maximum open
circuit voltage is determined by the number of modules in series N S and the maximum
open-circuit voltage of an individual module. Because of the negative value of the
temperature coefficient, the maximum open circuit voltage occurs in the coldest period of
the year. There is also a minimum value for N S . Each inverter has an input voltage
window, where the maximum power point tracker operates. If the array maximum power
point voltage falls below this voltage window, the maximum power point tracker does not
operate any more. The lowest array maximum power point voltage occurs in the hottest
period of the year.
In a conservative assumption, the power on the DC side at STC now is given as
The inverter must be chosen such that its maximal Power
is above the maximal PV
output,
Further, the nominal DC power of the inverter should be approximately equal to the PV
power at STC,
In practice, the nominal DC power of the inverter is selected slightly below the PV power
at STC, up to 10%, depending on the climate zone, because of the different irradiance
distributions. Also, for P DC0 < 5 kWp, single-phase inverters are used while for P DC0 > 5
three-phase inverters are advised.
The inverter efficiency is dependent on the input power and voltage. A model
discussing the inverter efficiency is presented below.
Inverter efficiency
As we already discussed in Chapter 19, modern inverters fulfil two major functions: irst,
maximum power point tracking (MPPT), and secondly, the actual inverter function, i.e.
converting the incoming direct current (DC) to alternating current (AC) that can be fed