Page 126 - Applied Photovoltaics

systems or higher voltages for larger or grid-connected systems. Inverters in stand-
alone systems are required to supply constant voltage and frequency, despite varying
load conditions, and need to supply or absorb reactive power in the case of reactive
loads. Most inverters for stand-alone PV systems include isolation transformers that
separate the DC and AC circuits (Standards Australia, 2005). The inverters most
commonly used in stand-alone PV systems are discussed below (Watts et al., 1984;
Bower, 2000; Schmid & Schmidt, 2003; Ross, 2003).
1. Light duty inverters—typically 100–10,000 W continuous output, with or
without frequency control. These are suitable for powering appliances such as
computers and television sets, but can be relatively inefficient and can
generate both electrical and audible noise.
2. Medium duty inverters—typically 500–20,000 W continuous output, some
with ‘load-demand-start’ (automatic start-up and shut-down when a load is
turned on or off). These are suitable for use with a wide range of small
appliances and power tools, but may not have sufficient surge capacity to
operate larger AC induction motors.
3. Heavy duty inverters—typically 10,000–60,000 W continuous output, but
able to accommodate AC induction motor start-up surge loads of 30,000–
200,000 W.
Most of the inverters discussed above have efficiencies of 80–85% with loads in the
range 25–100% of the inverter rating, but efficiencies can be very low for smaller
loads. Some of the newer inverters are able to accommodate low loads better, offering
efficiencies of around 80% (and 90% or more for higher loads) (see Kobayashi &
Takigawa, 1993).
Two additional aspects that need to be considered when selecting an inverter are
wave shape and standby power draw. The preferred wave shape for AC loads is a sine
wave, as is supplied by the electricity grid. However, many small inverters produce
square waves, or approximations to sine waves, which can lead to motor starting
problems or even appliance burnout, owing to the energy content in the high
frequency harmonics. Sine wave inverters are becoming increasingly available, even
in small sizes, so that this problem will reduce with time.
If left on, inverters can continue to draw significant power, even with no load. This
can rapidly run down solar-charged batteries, so that systems with an inverter must
incorporate some means of inverter control, if this is not built-in. Circuit topologies
are discussed by Schmid and Schmidt (2003).
Preferred requirements for inverters in stand-alone PV systems are (Preiser, 2003):
x large input voltage range
x voltage waveform close to sinusoidal
x tight control of output voltage (r8%) and frequency (r2%)
x high efficiency for low loads (>90% at 10% load)
x tolerance of short overloads, particularly for motor starting
x good behaviour with reactive loads
x tolerance of loads that use half-wave rectification
x tolerance of short circuits.

113