Solar energy i

Solar energy is a pollution-free and renewable energy source, which
can effectively alleviate the pressures on power industries caused by
energy crisis and environmental issues. In recent years, photovoltaic
systems (PVs) are rapidly developing worldwide. It is reported by
the European Photovoltaic Industry Association that the world’s
cumulative PV capacity is over 138.9 GW by the end of 2013,
which can save as many as 73 million tons of CO2 each year [1].
However, since the grid-coected PV capacity is not uniformly
distributed, some regions in the world, for example, Germany, are
already experiencing a very high local PV penetration of more
than 200 kWp/km2 [2].
Considering that the cost of installation and maintenance of PVs is
still relatively high and the ‘fuel’ is free [3], PVs nowadays mostly
operate in maximum power point tracking (MPPT) mode to
maximise the income. In such a mode, because power output of
PVs directly depends on the operational conditions (e.g. solar
irradiance and temperature), PVs caot regulate their power
output to provide ancillary services (e.g. peak shaving and
frequency regulation) to the power grid. Therefore synchronous
generators should retain their capacity to perform the functions
and thus caot be retired [4, 5], which is not cost-effective.
Moreover, as the penetration of distributed energy resources
(DERs) increases (especially in microgrids), several impacts on the
operation of the power system will appear: (1) The power output
of PVs reaches the peak at noon, which coincides with the
off-peak loading period and leads to power imbalance in the
electric network [6]; (2) The variability and unpredictability of
power output of DERs can lead to unstable operation of electric
networks, such as voltage variation and frequency fluctuation [7];

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