I. INTRODUCTION
OKINAWA Prefecture is one of Japan’s southern prefectures
with subtropical climate, and consists of hundreds
of the Ryukyu Islands in a chain over 1000 km long, which extends
southwest from Kyushu (the south westernmost of Japan’s
main four islands) to Taiwan [1]. Some of those Ryukyu islands
are very small, are far from the mainland of Okinawa, and
mostly depend on diesel generators for electric power supply.
The generation cost of electric power by using diesel generators
in isolated islands is expensive compared to a conventional generation.
Besides, as the mitigation of global warming and the reduction of CO2 gas emissions are of great interest worldwide,
the consumption of fossil fuels in these isolated islands must
be reduced and clean and renewable energy sources must be
introduced.
One of the most promising applications of renewable energy
technology is the installation of hybrid energy systems in remote
areas, where the cost of grid extension is prohibitive and
the price for fuel drastically increases with the remoteness of
the location. Renewable energy sources, such as photovoltaic
(PV), wind energy, or small-scale hydro, provide a realistic alternative
to engine-driven generators for electricity generation
in remote areas [2]–[4]. It has been demonstrated that hybrid energy
systems can significantly reduce the total life cycle cost of
stand-alone power supplies in many situations, while at the same
time providing a more reliable supply of electricity through the
combination of energy sources [5], [6].
This paper concentrates on the control and application of PV–
diesel hybrid energy systems, which account for the majority of
systems installed today. One of the inherent advantages of PV
electricity generation is the absence of any mechanical parts
(unless tracking of the sun is included). Professionally installed
PV arrays are characterized by a long service lifetime, exceeding
20 years, high reliability, and low maintenance requirements,
which are highly desirable for remote area power supplies. In
sunny locations, PV generators compare favorably with wind
generators, despite the higher investment cost for PV modules
per peakWatt [7].Wind generators require regular maintenance
and are susceptible to damage in strong winds.
The penetration of PV systems in Japan is rising. Two factors
have been boosting this: improved generation efficiency
of PV modules and governmental subsidies for the initial cost
of residential PV generation systems [8]. However, PV power
fluctuates depending on the weather conditions, season, and
geographic location, and may cause problems like voltage fluctuation
and large frequency deviation in electric power system
operation [9]–[11]. To date, it has not been necessary for small
PV generators to provide frequency-regulation services to the
isolated utility. In the future, with an increasing penetration
of PV generation, their impact upon the overall control of the
power system will become significant [12]. This will lead a situation,
where the PV generators will be required to share some
of the duties, such as frequency control. Therefore, for the large
penetration of PV system’s output power in the isolated utility,
suitable measures must be applied to the PV system’s side.