Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 263

power. However, an efficiency of 85 to 90 % at rated power is sufficient since
the inverter is only operated at rated power for a small fraction of its opera-
tional time. Out of the two efficiency curves represented in Fig. 6.20 the curve
"ideal efficiency curve" represents the more appropriate efficiency curve for
photovoltaic appliances with the higher annual efficiency.
− Low self-consumption. Energy losses due to high self-consumption can be
reduced if the inverter is only switched on when needed, as its stand-by con-
sumption during the remaining time (sleeping mode) is much lower. However,
it has to be ensured that even small loads, such as compact fluorescent lamps,
are always securely detected and the inverter is turned on. The master / slave
process represents such a possibility for a securely load detection: a small in-
verter (master) ensures permanent power supply whereas the slave is only
switched on and operated if additional power is required.

Good efficiency characteristics

1.0

0.9

0.8

0.7

Bad efficiency characteristics

0.6

0.5

0.4

0.3

0.2

0.1

0.0 0.0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Share of the power loss relating to the nominal power

Fig. 6.20 Favourable and unfavourable characteristic efficiency curves of island inverters
(see /6-29/)

− Stable operation behaviour. The output voltage of island inverters should be as
stable as possible regarding frequency and amplitude (voltage source). This ap-
plies in particular to larger scale plants, if many consumers are served simulta-
neously. During the start-up of larger consumers (e.g. washing machines, re-
frigerators) the voltage level should not fail and cause, for instance, a simulta-
neously operated computer to crash.
− Sine-shaped output voltage without direct current bias. The output voltage
should be sine-shaped (i.e. few overtones or distortions). An indicator is the
distortion factor, which should be below 5 %. In addition, the output voltage
should not have a direct current bias, since it may pre-magnetise and deterio-
rate trans-formers and electric motors. Furthermore, the inverter should be ca-
pable of operating inductive as well as capacitive loads (e.g. fluorescent lamps,
alternating current motors) and asymmetric loads such as e.g. hair dryers where
only one half of the sine wave is used. This feature is described by the admissi-