Page 208 - Applied Photovoltaics

levels are rare now, they may become more common in future, and active protection
methods may be necessary, since passive methods are not effective under perfectly
balanced load conditions. Problems could also potentially arise if large numbers of
inverters on a section of grid interfere with each other’s sensing of grid conditions.

Standards Australia (2002b) specifies anti-islanding requirements in Australia and it
should be consulted for all installations. A grid disconnection device incorporating an
electromechanical switch is to be provided unless there is galvanic insulation (e.g. a
transformer) and the inverter is unable to continue supplying power in the absence of
an otherwise energised grid. Semiconductor switches are acceptable in cases of
galvanic isolation. Both passive and active anti-islanding protection are required to
prevent the situation where islanding may occur by multiple inverters providing a
frequency and voltage reference for one another. Permitted methods of active
protection include frequency shift, frequency instability, power variation and current
injection. Passive protection devices sense both frequency and voltage. Disconnection
must occur within two seconds of the specified islanding conditions beginning.

10.6 THE VALUE OF PV-GENERATED ELECTRICITY

The ‘value’ of PV generated power can be viewed from several perspectives
including:
x global—taking into account such issues as the use of capital, environmental
impact, including climate change, access to power
x societal—local impacts, manufacturing, employment, cost of power, security
of energy supplies, balance of trade, infrastructure
x individual—initial cost, increased house value, reduction in utility bills,
energy independence
x utility—PV output in relation to demand profiles, impact on capital works,
‘green power’ or mandatory renewable energy requirements, maintenance etc.
The following discussion is taken from a utility perspective.

10.6.1 Energy credit
The value to the grid of PV-generated electricity depends largely on the time of day
when the grid experiences peak demand. Electricity supplied during this peak can be
worth 3–4 times that generated ‘off-peak’. Hence, PV is well suited to ‘summer
peaking’ grids. The trend in Australia is towards summer rather than winter peaks,
which is also happening in some US states and elsewhere. The value of PV during
summer peaks was recognised in the 2004 Australian Energy White Paper and
underlies the ‘Solar Cities’ funding program (Commonwealth of Australia, 2004).
Partly due to increased popularity of air conditioning, summer peak loads have grown
rapidly and the efficiency of use of the grid infrastructure has consequently fallen. In
one area in Adelaide, the upper 50% of the distribution feeder capacity was used for
only 5% of the time (Watt et al., 2003). PV systems being assessed for use as peaking
stations would be competing with such options as load management, combustion
turbines, cycled coal plants, pumped hydro and perhaps, in future, compressed air or
ice storage, all with target costs equivalent to retail electricity tariffs (Iannucci &
Shugar, 1991).

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