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THE NEW SUPPLY SCHEME
The site
selected for the 100 kW turbine is one kilometer to the east of the first
turbine site, and slightly beyond the eastern boundary of the distribution
system. The site is at the summit of a relatively smoothly rounded hill,
with open aspects to the prevailing south westerly winds. It is much further
from the high cliffs on the, western shore which affect the first site, but
it does have cliffs and steep slopes nearby on the eastern shore. The
wind
speed monitoring
that had been carried out suggested that it was a better site, with smoother
winds, than the first site. The approach to the site from the nearest hard
road was more gradual, and negotiable
by
tractors over dry turf.
A new dwelling and workshop had been
built, and another dwelling restored, near to the new site, and a new
95 sq.mm
aluminium cable laid to provide additional service capacity from the diesel
house at the same time. In anticipation of the new system, this cable had
been extended to the new wind
turbine site and capped. The heating network distribution cable laid in 1982
was also a 95
sq.mm
aluminium cable and ran to Busta,
the nearest dwelling some 210m from the site. A control
room and store was built by extending a local single storey outbuilding at
Busta.
The existence in place of these cables, plus a twenty pair
telephone cable, also laid in the anticipation of the new scheme alongside
the new service cable, allowed us to arrive at a new supply `scheme shown in
Fig.2. This scheme is clearly more complex than the original, but a little
study will show that it still embodies the features of the original scheme
but with more operating options.
The
diesel
generating plant may
feed the service distribution network in isolation, provided contactors E
and
J are not energised. The
service network is split into two main sections, those to the north and
those to the south of the generator house. There is some further
sub-division of the service network, not
shown,
which energises the various
cables in
a timed sequence, to load
the diesels progressively at start-up. When the service is provided by
diesel, each wind turbine
may
provide
power
to its
portion of the heating distribution network, which is also divided into two.
The 60 kW turbine feeds the north heating cables and the 100 kW turbine
feeds the south heating cables.
Since both
wind turbines are each equipped to operate as the only generator on the
system, it follows that either may, when the other
is
not generating, first feed
both sections of the heating network and then, if the diesels are shut down
(out of guaranteed hours of service supply), and there is sufficient power
from
that
turbine, the supply from the turbine may be allowed to feed the service
network also. In considering the logic required for automatic control of the
networks, it became clear that this was a very complex problem and there
were high risks of uncontrolled coupling, from a sequence interlock failure,
and attendant fault damage to plant. A simplifying strategy was needed.
Simplification, to a degree,
was obtained by using relays to
identify the first turbine to signal that it was able to deliver power. This
turbine then became the lead turbine and,: as such, was the only turbine
allowed to power the service network. The second turbine to come on line was
locked out from connection to the service cables. Once a turbine has been
selected to lead, it retains this status until it is failed by loading
beyond its current capability. This is detected by an under-frequency relay:
The second turbine serving part of the heating load
will be at, or above, nominal
frequency when the lead turbine is failed, and the lead turbine status will
be automatically transferred to it. Once more, if there is sufficient power,
the service network may be energised, but this time
from the new lead turbine;
This method
of using the power from the turbines was relatively simple, and the loads
to
the north
were in proportion to the 60 kW turbine when compared with the loads in the
south on the 100 kW turbine. However, when the lead turbine was also
supplying the service power, it could only provide a reduced output to its
area of the heating network. It was thus obvious that a way was needed to
couple the two turbines so that the distribution of the generated power
could be more fairly distributed. There were two basic methods of doing this
with simple rugged equipment, and several more complex and more costly
methods.
The first
was to couple the second turbine to the lead turbine using an induction
coupler, which consisted of two identical induction motors with their shafts
coupled together. With the appropriate phase rotation applied to the
windings, the motor connected to the faster second turbine will act as a
motor, and the motor connected to the slower, more heavily loaded lead
turbine will act as an induction generator. The shafts will be constrained
to run at a speed corresponding to the mean of the two frequencies, and the
power transferred from the higher frequency supply to the lower will be
proportional to the "slip" or half the difference between the frequencies.
The second
method was to arrive at a method of synchronising the two turbines. These
machines are of the fixed pitch, stall regulated type, and have no means of
control which will regulate the energy capture to control the speed. Speed
is
governed by matching the
connected heating network load to balance the energy captured by the
turbine. There is no simple way of remotely adjusting the settings of the
distributed load controllers which regulate the switching of the domestic
heating units. These are the governing loads in control when the turbine is
operating on the network. One possibility was
to
modify the dump load
controller to allow the set frequencies to be varied on command. This, on
reflection, was not considered wise in view of the need to keep close
matching of the dump load profiles, as discussed earlier, and because of the
dual safety functions of ultimate speed governor and fast acting
electrodynamic
overspeed
brake.
Coupling
techniques, using power electronics
to
transfer power between the
wind turbine busbars,
were considered
briefly but thought to be vulnerable to damage by the numerous switching
operations on
the
system in service.
The final
choice
for synchronising used
a
phase matching technique, subject to a
limit on
differential speed,
operating in conjunction with an induction coupler. This choice had the
added attraction that, should the synchronising method fail, we had the fall
back position of operating induction coupled.
The scheme
as it
is
shown in
Fig.2 does
not
remind us
that the 100 kW turbine is one kilometer distant from the diesel generator
house where the network control panel must be located.
Protective
multiple earthing is used throughout both distribution networks, and
neutrals are bonded to cable armour and local earths at each consumer cut
out. In addition, continuous earth tape bonds the first wind turbine
foundation earth to the diesel generating plant earth, which is laid in a
permanently wet meadow adjacent. The new wind turbine site is on a
well-drained hilltop with sparse turf cover, and the local tower earthing
tape runs 200m away
from
the cable
lines to the only permanently wet ground in the vicinity. This led to
concern about possible damage in the event of a lightning strike on the 100
kW turbine.
To reduce
the risk of damage to the control system and the telephone cable to the
diesel house, by the ground wave following a lightning strike, it was
decided to provide galvanic separation, between the 100 kW turbine control
system and the telephone link, of
up
to 2 kV. This precaution
increased the complexity of command and control, and set a restriction
on
the
information that could be sent over the telephone cable.
The contactors shown
as J and K in
Fig.2 which connect
the 100 kW turbine to the south service cable and the south heating cable
respectively are housed, with the associated circuit breakers, in an
isolating switch panel mounted beside the turbine control panel in the 100
kW control room at
Busta.
Coil power is drawn from the turbine but is under the command
of signal isolating relays operated over the telephone cable from the
network controller in the diesel house.
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