Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 265

− Grid-commutated inverters. Due to their design, grid-commutated inverters
need a strong electrical power grid to function. Thyristors, characterised by ro-
bustness and cost-efficiency, serve as basic electronic system elements. How-
ever such inverters generally grossly distort (overtones) the supplied output
voltage and cause phase shifts with regard to the mains voltage. Since this dis-
advantage requires additional filter and compensation measures, other concepts
within the low power range result as more cost-efficient. But thyristor inverters
are still applied for the power range of several 100 kW and above.
− Self-commutated inverters. Self-commutated inverters are based on disengage-
able power switch and thus do not require an external grid for normal opera-
tion. A widely used function principle is pulse-width modulation offering a
wide range of circuit topologies. In comparison to island inverters, grid-
connected inverters need to be synchronised with the grid; a continuous opera-
tion of the inverters in case of a power failure within the grid must by all means
be prevented for safety reasons. To minimise the risk of creating isolated grids
while the overall mains is disconnected, e.g. German regulations prescribe sin-
gle-phase grid supervision for solar inverters, abbreviated ENS. The supervi-
sion aims at preventing that areas of the mains considered to be disconnected
remain live due to the inverters, so to ensure that maintenance work could be
performed at the grid under safe conditions. Many other countries lack such
regulations.
Provided that the input voltage is sufficiently high, inverters can feed directly into
the power grid and do not require any transformer for voltage adjustment. Besides
reducing costs, weight, and volume, direct feeding into the grid markedly reduces
the inverter's self-consumption. The latter, in turn, contributes to an improved
efficiency in the partial-load range. There is thus a tendency towards concepts
without transformers.
Devices provided with transformers are further distinguished into systems
equipped with a 50 Hz transformer and systems provided with high-frequency
transformers. The latter offers weight and volume advantages on the one hand, but
is also prone to higher losses and failure probability due to a more complex over-
all circuit layout. With regard to devices, there is a tendency towards more simple
and sound concepts, whereby a favourable efficiency curve is obtained by apply-
ing top quality components, especially for the 50 Hz transformer /6-29/.
The first generation of grid-connected devices had been provided with central
inverters. Therefore the photovoltaic modules were first connected in series
(strings) to obtain the required voltage. To guarantee the required power, the
strings are subsequently operated in parallel. The so interconnected photovoltaic
generator then feed an individual inverter. Occasionally, the respective power is
distributed among several central inverters operating in master/slave mode.
Decentralised modular inverters increasingly gain importance. Such devices are
further distinguished into inverters for small strings (string concept) and inverters
that are integrated with individual modules or even directly incorporated into their