Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 264

ble power factor. For motor start-up (e.g. of refrigerators, washing machines) it
should be able to withstand 2 to 3-fold overload for short periods.
− Coverage of the entire voltage range. The inverter entry side should cover the
entire voltage range of the battery energy storage of -10 to +30 % of the nomi-
nal voltage. In the event of falling short of a certain minimum input voltage it
should be disconnected, either automatically or via a control input to protect
the battery from deep discharge.
In the future, island inverters will almost exclusively be designed as sine-wave
inverters and an increasing number of devices will meet the discussed require-
ments. Additionally there are further cost saving potentials due to a large-scale
production and the use of modern semiconductor components.
For larger stand-alone hybrid systems increasingly bi-directional inverters are
used. They allow battery charging from additional generators, such as e.g. wind
generators, diesel generators and hydropower without additional charging equip-
ment. Although the system is simplified by the above-mentioned measures, high
requirements are placed on the inverter /6-9/.
Grid-connected inverters. To feed solar power into the grid generally an inverter
is required to convert the direct current (DC) power generated by the photovoltaic
system into alternating current (AC) power compatible with the mains /6-29/.
Unlike island inverters, which are mainly provided with direct current by a bat-
tery, grid-connected inverters are directly connected to the photovoltaic system
without additional storage systems.
Most of the grid-connected inverters of the 1980's were modified thyristor in-
verters, which had been applied numerously for electrical drives. However, these
devices, optimally operated at nominal load, are often inappropriate for partial
load operation typical for photovoltaic generators. Due to this they only achieve
very low operational efficiencies.
Modern semiconductor components, such as MOS-FET's (Metal Oxide Semi-
conductor - field effect transistors) or IGBT's (Insulated Gate Bipolar Transis-
tors), in conjunction with optimised circuit typologies, triggered the development
of special solar inverters with significantly improved properties at the early nine-
ties of the last century. Especially, the system's self-consumption had been tre-
mendously reduced, so that the required efficiency of over 90 % could be reached
at a nominal output power of 10 %. While island inverters typically convert the
major energy share at approximately 20 % of the nominal power, grid-connected
inverters under Central European irradiation conditions have a relative evenly
distributed loading over the entire power range. Besides the low self-
consumption, grid-connected inverters are also characterised by high efficiencies
at rated power. In the meantime, a wide range of devices within the power range
of 10 W up to several 100 kW has become available on the market. Usually, de-
vices within the MW-range consist of several inverters (e.g. 300 kW devices op-
erated in master/slave mode (often with a rotating master)). For this purpose, a
wide range of functional principles is applied. Some are discussed below.