Page 198 - Applied Photovoltaics

The standoff or rack mounts would be used for retrofitting and, although their costs
are likely to be higher than for integral systems, allow more air flow around the
modules and offer the opportunity for optimal tilting. Direct mounts are secured onto
the roofing material but are likely to suffer from overheating owing to limited air flow
behind the modules.

10.2.2 The inverter
As for stand-alone PV systems, an inverter, or power conditioning unit, is needed,
since photovoltaic arrays generate DC power at low voltage. Two main types of
inverters can be used to achieve AC power at the voltage used in the main grid. These
are:

1. Line-commutated—where the grid signal is used to synchronise the inverter
with the grid.

2. Self-commutated—where the inverter’s intrinsic electronics lock the inverter
signal with that of the grid.
An alternative division of the available products is by application:

1. Central inverters are designed to convert the output of all the parallel strings
of modules in large arrays, with total power in the range 20–400 kW. Self-
commutated designs based on insulated gate bipolar transistors (IGBTs) or
field effect transistors (FETs) are now dominant.
2. String inverters accept power only from a single string, with total power in
the range 1–3 kW.
3. Multi-string inverters include various independent DC-to-DC converters,
which feed their outputs to a common inverter. These allow the acceptance of
power from module strings with different configurations or orientations, each
able to operate at its own maximum power point.

4. AC module inverters sit behind individual modules, resulting in an integrated
AC module.
An Australian (Standards Australia, 2002b) and various international standards
(Appendix E) apply to grid-connected inverters. Issues to be considered when
selecting an inverter include (Florida Solar Energy Centre, 1987; Bower, 2000;
Abella & Chenlo, 2004; Standards Australia, 2002b; Schmid & Schmidt, 2003;
Krampitz, 2004):
x Efficiency—An improvement of 1% can result in 10% more power output
over a year. Some designs pay particular attention to partial-load efficiency.
Inverters with line-frequency transformers can achieve a power conversion
efficiency of 92%, whereas those with a high-frequency transformer can yield
94%, although in general higher efficiency is possible if the transformer can
be avoided. In addition to operating efficiency, standby power losses during
periods of negligible load need to be assessed.
x Safety (particularly via disconnect modes)—Run-on or ‘islanding’, for
instance, can result in the grid being energised, even when disconnected (see
Section 10.5). Isolation transformers are therefore commonly used. Similarly,
protection is required against over-currents, surges, under- or over-frequency,

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