Fair Isle panorama from Buness
FAIR ISLE

Thursday March 18, 2010

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Fair Isle Renewed
W. M.
Somerville M.Sc.
Newcastle upon Tyne


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.

 


Text and photographs 2008 Dave Wheeler except where otherwise credited. (Logo picture courtesy of Sumburgh SAR)
If you would like to use photographs from this site please contact dave.wheeler@fairisle.org.uk
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