Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 266

terminal block (module-oriented inverters). These concepts offer a series of bene-
fits.
− Each module or module group is optimally operated in its maximum power
point (MPP).
− Losses due to different characteristic curves of the modules (mismatch losses)
are reduced.
− Partial shadings only affect individual modules or the module groups.
− Inverter failure only impairs the corresponding generator component.
− Cabling of modules or module groups among each other is only performed on
the alternating current (AC) side, thus lowering the hazard potential of a con-
ventional technique (direct-current (DC) arc).
− Mass production of numerous small identical units permits cost reduction.
However, besides the benefits mentioned above also some disadvantages must be
expected that have to be compensated by a more sophisticated design of the indi-
vidual devices.
− Especially module-integrated inverters are subject to high thermal stress that
requires the use of appropriately designed components. For some modules war-
ranties of up to 25 years are granted. However, regarding the technical lifetime
of module-integrated inverters and the corresponding modules still a big gap
needs to be closed.
− The replacement of module-integrated inverters, e.g. in facades, is extremely
expensive.
− The function of any small inverter has to be controllable from a central place.
This requirement can, for instance, be fulfilled by data transfer over the power
line without any additional wiring, but requires special efforts and investment
into the communication interfaces.
− At decreasing nominal power of energy converters it is increasingly difficult to
obtain an appropriate efficiency curve for photovoltaic appliances, since the
own energy consumption cannot be equally reduced. Hence, for very small
units a decreased overall efficiency needs to be expected.
Especially, for the large-scale plants currently installed, a tendency towards larger
inverter units, ranging from some 5 kW up to several 100 kW, can be observed.
Large inverter units permit to considerably reduce inverter costs, both initial in-
vestment costs as well as running expenses for maintenance, supervision and re-
pair.
On the whole, grid-connected inverters should meet the following requirements
/6-9/, /6-29/.
− The output current is synchronous to the mains. Unlike island inverters sup-
posed to supply basically constant output voltages (voltage source), grid-
connected inverters act as power source whose amperage depends on the cur-
rent input power level.
− The output current should be of sine shape. Distortions and thus the level of
harmonics must not exceed the prescribed limits (VDE 0838, EN 60555).