Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 260

nating current (AC) of 230 V and a frequency of 50 Hz (in some cases 120 V at
60 Hz, as in the US, for instance). Even, for stand-alone photovoltaic systems
without grid-connection, frequently inverters are used to convert direct current
(DC) into the appropriate alternating current (AC) power required by commer-
cially available appliances. However, inverters are by all means required to trans-
fer the properties of the electrical energy produced by grid-connected photovoltaic
generators into properties similar to these of the grid. A special type are the so-
called pump inverters that convert the supplied direct current (DC) from the
photovoltaic generator into alternating current (AC) of adjustable voltage and
frequency, which is suitable for the rotation-variable operation of water pumps
/6-29/, /6-30/.
The power range of inverters used in conjunction with photovoltaic appliances
stretches from about 100 W to several 100 kW, applying a great variety of cir-
cuitry topologies and components. A continuously growing market, as well as
new findings lead to new inverter concepts and new products /6-9/, so that in the
following, only the basic principles of inverter designs and the main requirements
are outlined.

Island inverters. While the low voltage grid supplies sine-shaped electrical energy
of 230 V/50 Hz in Europe, island inverters are subdivided into three groups ac-
cording to their voltage shape: rectangular, trapezoid, and sine inverters
(Fig. 6.17). Within the small power range, e.g. for the local supply of individual
AC consumers within a DC grid, frequently rectangular or trapezoid inverters are
used. For larger systems (more than 1 kW), by contrast, sine-wave inverters are
most commonly applied. Recently there is also a tendency to use sine-wave in-
verters for small scale applications.

Time Time Time

Rectangle Trapezoid Sine

Fig. 6.17 Typical output voltage curves of rectangular, trapezoid, and sine inverters

The rectangular inverter is characterised by a very simple structure. In the ex-
ample illustrated in Fig. 6.18 a battery voltage of 12 or 24 V is applied to the pri-
mary side of the transformer at a rhythm of 50 Hz and with alternating polarities
passing through the bridge circuit consisting of switches S1 to S4. The switches
S1 and S2, usually bipolar transistors or MOS field effect transistors, are closed
during the first phase. The same applies to S3 and S4 during the second phase.
This "cut up" direct voltage is subsequently transformed to the required output
voltage by the transformer /6-29/.