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OPERATING HISTORY 1982 -1996
The wind turbine suffered an early gearbox
failure, after six months of operation, and again, after some 22,000 hours,
at which point a different make of gearbox was fitted. Another major outage
was caused by insulation failure in the alternator stator. Finally, the site
chosen for the turbine, relatively close to the diesel generator house, was
prone to oscillating wind direction between the north and the south of
Malcolm's head in westerly winds. This caused excessive wear on
a 30 degree sector of the original
yaw gear plate. The damaged plate was replaced with a new plate, in a
tougher material and of greater face width, during a three day outage. The
details of this scheme and the early operating history has been covered more
fully in previous papers. (2), (3)
Despite this run of disasters in the early
years, relatively little generating time was lost, and the system has
provided an excellent service to the island community, giving longer hours
of supply at a much reduced cost. Also, as surplus wind energy was, when
available, being distributed for heating, household expenditure on coal and
heating oil imports was reduced. The heating appliances used were mainly of
the heat storage type. These were charged in the order of the priority set
by the operators on the domestic programmable
load controllers. This form of
heating tended to smooth out the delivery of heat, and was thus less
susceptible to rapid variations in wind strength.
The island
has a vigorous wind regime, as
indicated by the figures in Table 1., and exceeds 30
m/s many times each
year. On one such occasion, when the wind strength had increased suddenly,
it was considered advisable to stop the turbine, as winds to 50
m/s were
expected to follow. It was discovered that the remote stop had been disabled
at the tower panel, and by then the winds were too strong to be safe on the
hill top. The turbine spoilers were up and the turbine was delivering close
to maximum power. Uncertain what to do, the operators reasoned that an
overload shutdown would occur if the power increased, and they left the
turbine running. The turbine ran all night through the storm without
shutting down, and it was noted at the meteorological station nearby that a
gust of 142mph has been recorded. One feature of the wind on the island is a
low frequency oscillation in the wind strength, with a period of some 12 to
20 seconds. This is not a problem in good wind strengths as the system is
well able to handle such changes. However, in reduced wind strengths, when
the mean power output from the turbine is reduced to near to the level of
service demand, these variations in the wind speed periodically reduce
output to less than the demand. During these lulls the turbine is retarded
and can disconnect the service due to the under-frequency trip. This can
happen even when the mean output is significantly above the demand.
An interesting trial was made using a
"Kinetic Energy Storage System" with a flywheel as the energy store. The
energy stored was sufficient to bridge the low energy portion of the wind
cycle. This system, operating in conjunction with the wind turbine, allowed
the turbine to sustain the service demand almost to the point where the mean
output was equal to the demand. The net result was
a saving
in diesel fuel and longer hours of
service, as the turbine could sustain the service demand
in mean wind speeds 1 - 1.3
m/s
less than it could without the storage system. This trial, by
BP. Engineers, was discontinued when BP decided to withdraw from this field
of activity.
During this period of operation a number
of improvements in the system were
made by the island, as circumstances permitted, including strengthening the
service distribution network with heavier cables when new . dwellings and
workshops came onto the system. The old diesel generators were replaced
in 1992 by two new matched sets,
each rated at 32 kW which could, if required, be operated in parallel. There
was also an ongoing programme by
the National Trust to refurbish the housing stock on the island. This
included several new houses as well as the reconstruction of some old
dwellings. These were all wired to a much higher standard of provision than
before. Inevitably, changes in the island load have taken place, with new
consumers, several new businesses and an inexorable increase
in basic service demand. These,
despite excellent wind turbine performance, resulted in increasing use of
diesel generation, and, hence, increased costs. The wind turbine was also
showing its age and was in need of a
major service. The generation and utilisation of electrical energy
during this period is summarised in
Table 2. The system records run from 1
st
of May to 30th April. Thus, the turbine
only operated for some nine months in 82 - 83. The turbine suffered
a gearbox, main shaft, oil seal
failure and had to be shut down to avoid serious damage in February 1996,
and lost two months of generation in that year.
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