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THE NETWORK CONTROL SYSTEM
The network control system, as the name
implies, handles the information provided by transducers about the power
available from all three sources, and automatically routes this power
through both of the island distribution networks to maximise the energy
available to the consumers. When the demand for service power is less than
that provided by the lead wind turbine, the system is set up to monitor this
status for a preset time interval, to ensure that it is not a transient
condition. Then, if the operators have elected to select "auto changeover",
the system will switch the service network to wind power from diesel power.
The unloaded diesel continues to run on standby for a set time,
and if not switched back into service in this interval, the engine is shut
down to save fuel, wear and tear and maintenance costs.
The control system, as mentioned earlier,
will sense the first of the turbines to signal that it has power to deliver,
and mark it as the lead turbine after blocking the second. This prevents any
simultaneous attempts to switch the turbines to the service network. The
lead turbine, when first on line, is automatically connected to the heating
distribution network for the whole island. When the second turbine signals
that it too is able to deliver power, the heating network is split in two
and the second turbine connected to the de-energised portion. This
arrangement minimises the loss of wind generated power to dump. At this
stage of a start-up sequence, each wind turbine is operating independently
of the other, feeding its allocated section of the heating load.
Power transducers in each turbine control
panel measure the total power produced by the turbine. A third transducer
has been fitted in the existing diesel control panel to signal the total
power being delivered to the service network by the diesel generators. This
signal gives the service demand when the diesel
plant is running. All transducers
provide galvanic isolation from
the supplies that they monitor, and all are precalibrated
to deliver
a signal of one volt per twenty
kilowatts. These signals are compared in a monitoring rack, which is, for
convenience, mounted below the dump load controller rack in the display
window of the 60kW turbine control panel.
The monitoring rack consists of three
control amplifier boards, one for each power source, three indicating meters
showing the instantaneous power levels and a stabilised voltage source with
a series of set output levels within a range 0 - 1 volts. This latter
facility is for the selection of the margin of power above the actual diesel
load at which the system may prepare for switching the service network from
diesel to wind generation. Each control amplifier board has three switched
outputs. The first is driven by a comparator amplifier which changes state
according to whichever of the two inputs is more negative. The second and
third outputs change state when the incoming signal is more negative than an
internal, adjustable, preset, negative reference voltage. The second output
is used to set the power level for auto changeover when the diesels are off,
and the third is a back-up overpower trip signal.
In operation, the output of the lead
turbine is compared with the load on the service network, signaled by the
diesel plant transducer, plus a selected margin. When the turbine output
exceeds this sum, a timer is started and will, after the set time,
(typically 8 minutes), initiate changeover. If the wind power falls
momentarily below the required level the timer is reset to zero and must
start again when the power recovers. This arrangement reduces the likelihood
of power drop-out after changeover. The margin chosen is determined by the
variability of the wind and operators' experience. The lower the margin
selected, the greater the fuel saving achieved.
When operating the system outside
guaranteed hours of supply, the diesel plant is stopped and there is no
power signal for comparison. Operation of the diesel time clock resets the
monitor rack logic, and the lead turbine power is
compared with an adjustable preset reference controlling the second output
channel of the lead turbine amplifier board. When the wind power signal
exceeds the set reference, a different timer
is
started. When this set time, typically
two minutes, has elapsed without interruption, the lead turbine is switched
to the service network. The setting of the reference
is
a matter of experience,
and would typically he between 28 and 36 kW. The low setting gives more
hours of supply but with occasional drop outs, and the higher setting
reduces the hours of supply but with less risk of drop-out.
In light
winds the lead turbine may not produce enough power to initiate changeover
of the service network, and the facility exists to soft couple the lead
turbine through an induction coupler which was described earlier. This
is
to provide some power to
support a single diesel in operation, and avoid calling up the second
diesel. When this facility is enabled, the induction coupler is started by a
star-delta starter from the lead turbine heating
busbar,
and when up to speed the
induction generator is switched onto the service
busbars.
This form of operation has
been tested briefly in service, but suitably stable light winds for
successful unattended operation are rare
on
Fair Isle. In stronger
winds the wind turbine can so easily overpower the diesel set increasing its
speed up to and through the
overspeed
trip. This
causes the diesel to shut down and lock out.
Once the
lead turbine has taken the service load, the second wind turbine may, if the
option has been selected, soft couple to the service
busbars.
If
the 60 kW turbine is the second turbine, only one 30 kW induction coupler is
normally needed. However, when the 100 kW turbine is second, both 30 kW
induction couplers are used, starting in sequence.
The final operating option is to bring the
two turbines into synchronism and couple them directly to each other. This
mode of coupling is, when selected, dependent on soft coupling already being
established through the induction couplers to reduce the relative speed
between the turbines. The synchronising sequence is automatic, provided the
slip frequency is less than six cycles per minute, and will close the
synchronising switch within a few degrees of phase matching. After the
turbines are synchronised the induction coupler/s are stopped, as they are
then no longer needed. Once synchronised, the turbines run together sharing
the combined island load in proportion to each turbine's energy capture.
Synchronised operation is interrupted automatically if either machine ceases
to deliver power. During commissioning it was found to be necessary to
disable synchronised and coupled operation on the instant either machine is
shut down by fault or command. Another feature added to the system during
the commissioning work allows, when selected, a second contactor to close
after the synchronising contactor, in order to close the ring formed by the
service and the heating distribution cables. This has the effect of
paralleling the cables and reduces voltage drop in the system.
If the
demand on the service network increases,
or the wind strength
diminishes, the system will automatically reduce the heating load until it
is
zero. At this
point
the system is in unstable equilibrium,
and any further increase in load
or
reduction in wind strength will cause the turbine/s to slow down. When the
frequency falls to 45 Hz. they will uncouple and disconnect the service
network. Any wind power remaining will be fed to the heating network as soon
as the turbines recover to the "turn on" frequencies.
In these
conditions, if it is a period of guaranteed supply, failure of the service
supply " will initiate diesel start, and the service network will then be
provided by the diesel plant. Outside the period of guaranteed
supply
the service power
will
fail, the lead turbine
status is cancelled and must be established once more. If the wind strength
is sufficient to hold the new lead turbine above the set changeover limit
for the set time (2 minutes) this turbine will attempt to take and maintain
the service load and, provided the demand on the service has been reduced in
the meantime, it is possible for the service to resume. If the demand
remains high, the service will again fail and the lead turbine status will
be transferred to the other turbine and, if this can sustain power above the
changeover limit for the set time, another attempt
will
be made to
take the service load.
The system will continue to attempt to
provide a service supply in this way until successful, or until the turbine
is unable to maintain the set level of power for changeover, or until a
period of guaranteed supply begins and the diesel plant is brought into
service. This scheme of repeated attempts to take the service load may, at
first, seem crude, but it is simple and, in practical terms, it is rare that
more than one or, two attempts will be made in any period, as the wind is
rarely sufficiently constant to allow the necessary conditions to persist.
It is more likely that a failing wind will continue to diminish. Similarly,
a wind freshening from calm tends to strengthen. Further, in a freshening
wind, the wind strength which is able to start a turbine from rest is
generally able to support load by the time the turbine is at operating
speed, or shortly thereafter. The island consumers adapted to this scheme
with alacrity, and it is remarkable how sharply the service demand falls
after a drop-out.
To assist the operators, a system set-up
panel was provided at the network control panel. This has a plant selector
switch giving four choices:- 1. Diesel generators' only. 2. Diesel plus the
60kW turbine. 3. Diesel plus the l00kW turbine and 4. Diesel plus both wind
turbines. Four other switches select auto control or inhibit:- A. Auto
changeover. B. Soft couple to diesel. C. Soft couple wind turbines. and D. Synchronise wind turbines. Remote start and stop pushbuttons allow control
of the IOOkW turbine from this panel, and its state is indicated by three
LEDs for stopped, available and generating. Guarantee period, diesel start
command and diesel plant voltage balance trip indication is also provided.
A live mimic diagram of the system was
also needed, and is viewed through the door panel of the network control
panel. The mimic shows which sections of the network are live, and the
status of all the contactor switches. This mimic proved to be invaluable
during commissioning, as it was quite impossible to follow sequences by
observing the contactors.
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