panels; sometimes they are also directly integrated into the PV panels (so called ‘AC PV
panels’). One of the most distinguishing features of this system is the ‘plug and play’
characteristic, which allows a complete (and readily expandable) PV system to be built at
a low investment cost. Another advantage of these inverters is the minimisation of
mismatch losses that can occur because of non-optimal MPPT.
All these advantages come at a price. Because these inverters are mounted on a PV
module, they must operate in a harsh environment such as high temperatures and large
daily and seasonal temperature variations. Further, these inverters often accept only a very
narrow range of input voltages. Therefore it is not possible to use bypass diodes within the
modules that bypass one- or two-thirds of the module. Another disadvantage is that the PV
module voltage is much lower than the output AC voltage, therefore the DC-DC
conversion has to boost the voltage a lot. This has a detrimental effect on the inverter
efficiency. Also, the specific costs are the highest of all the inverter topologies. Many
topologies for micro inverters have been proposed, with some of them being already
implemented in commercially available inverters.
String inverters
String inverters, as illustrated in Figure 19.6 (c), combine the advantages of central and
module integrated inverter concepts with little trade-offs. A number of PV modules that
are connected in series form a PV string with a power rating of up to 5-6 kW p in 1-phase
configurations and up to 20-30 kW p in 3-phase configuration.
The open circuit voltages are up to 1 kV, which already makes one disadvantage of
this topology apparent: these high DC voltages require special consideration, as was the
case for the central inverter architecture. Here, this issue is even more important because
string inverters are usually being installed in households or on office buildings without
designated support structures or increased safety requirements. The protection of the
system also requires special consideration, with emphasis on proper DC cabling.
Although partial shading of the string will influence the overall efficiency of the
system, each string can independently be operated at its MPP, if each string has its own
MPPT. Also, because no strings are connected in parallel, there is no need for series
diodes as in the case of PV arrays with multiple parallel strings. This reduces losses
associated with these diodes. However, it is still a risk that within a string, hot-spots will
occur because of unequal current and power sharing inside the string.
Central inverter with optimizers
This architecture is a hybrid between a central inverter and micro inverters. An optimizer
box is attached to every module, which contains a MPP tracker and a DC-DC converter as
panels’). One of the most distinguishing features of this system is the ‘plug and play’
characteristic, which allows a complete (and readily expandable) PV system to be built at
a low investment cost. Another advantage of these inverters is the minimisation of
mismatch losses that can occur because of non-optimal MPPT.
All these advantages come at a price. Because these inverters are mounted on a PV
module, they must operate in a harsh environment such as high temperatures and large
daily and seasonal temperature variations. Further, these inverters often accept only a very
narrow range of input voltages. Therefore it is not possible to use bypass diodes within the
modules that bypass one- or two-thirds of the module. Another disadvantage is that the PV
module voltage is much lower than the output AC voltage, therefore the DC-DC
conversion has to boost the voltage a lot. This has a detrimental effect on the inverter
efficiency. Also, the specific costs are the highest of all the inverter topologies. Many
topologies for micro inverters have been proposed, with some of them being already
implemented in commercially available inverters.
String inverters
String inverters, as illustrated in Figure 19.6 (c), combine the advantages of central and
module integrated inverter concepts with little trade-offs. A number of PV modules that
are connected in series form a PV string with a power rating of up to 5-6 kW p in 1-phase
configurations and up to 20-30 kW p in 3-phase configuration.
The open circuit voltages are up to 1 kV, which already makes one disadvantage of
this topology apparent: these high DC voltages require special consideration, as was the
case for the central inverter architecture. Here, this issue is even more important because
string inverters are usually being installed in households or on office buildings without
designated support structures or increased safety requirements. The protection of the
system also requires special consideration, with emphasis on proper DC cabling.
Although partial shading of the string will influence the overall efficiency of the
system, each string can independently be operated at its MPP, if each string has its own
MPPT. Also, because no strings are connected in parallel, there is no need for series
diodes as in the case of PV arrays with multiple parallel strings. This reduces losses
associated with these diodes. However, it is still a risk that within a string, hot-spots will
occur because of unequal current and power sharing inside the string.
Central inverter with optimizers
This architecture is a hybrid between a central inverter and micro inverters. An optimizer
box is attached to every module, which contains a MPP tracker and a DC-DC converter as