Now that we are back with more bits to fix things with, it is possible that there may be a few unexpected interruptions of the electricity supply. We will try to plan that these mostly occur out of guaranteed hours but beg your understanding if there is the odd whoops! other times.

The green light in the new box. Derek has been round you all now to connect these Tariff relays which are not confused when both turbines are working, as were the old boxes. The new box measures the frequency of the service supply very accurately, and when the frequency is above 50.1 cycles per second the light comes on and a relay operates to start the metre (which was the dear metre but is now the cheap metre) to. record the cheap kilowatt hours you are using.

When the frequency falls to below 50.1 cycles per second the green light goes out and the relay opens to stop this cheap metre. The diesel generators are governed to run at 49.9-50.0 cycles per second and when the diesels are supplying the service the light will be out. You may notice that the light will blink from time to time. This is caused by spikes which sometimes occur when switches are operated. The spike can fool the Tariff relay into thinking the frequency has risen above 50.1 Hz for 20 milliseconds, hence the blink. This is too fast to affect the metre.

When service power is supplied by one of the turbines the frequency will be above 50.1 Hz until the wind falls and the turbine power is less than the service demand. This causes the turbine frequency to fall and the light will go out shortly before the power fails. When the wind is variable you may see the light go on and off several times, which tells you that the service demand is higher than the available wind power. If demand is reduced you can prolong the service from the turbines. It is to your advantage to reduce load in marginal, variable wind conditions during guaranteed service hours since, once the supply has reverted to diesel, the wind power has to rise to at least 1OkW above the service demand for 8 minutes without failing before the wind can take over the service supply again to give reduced cost electricity.

Outside guaranteed supply times the wind turbine on service may drop the supply, and a quick changeover to the other turbine will take place, provided it has enough power to take the service. If it does not, the supply will be interrupted until one of the turbines has been producing steady power above the prospective demand for at least two minutes.

Those of you who use computers on the network, who do not have un-interruptable power units for the computer, should always remember to save what you have entered every few minutes, or every sentence if your programme does not have autosave built in. If you do have an autosave feature use it set to a short time interval such as one minute.

Murray

The metres are now reading accurately and the heating frequencies have been re-set. As from 1st March and for the rest of this quarter electricity used will be charged at the old rates. Unit charges have not increased in 8 years, the current financial review of the system will indicate what the new rates will be.

Barry

The Fair Isle Renewable Energy Electricity Scheme

Once one wind turbine has the service supply the second turbine will start up the induction couplers in the diesel house in order to share the load with the first. The induction coupler is there to provide load sharing between turbines and also to bring the different frequencies of the two turbines closer together.

When the turbines are close to the same frequency (within 0.2Hz) a special circuit prepares to match phases and closes a contactor switch to connect the two turbines together when the phases match exactly. This is called synchronising and once it has been done the power is shared directly between the generators at exactly the same frequency. The induction coupler now has nothing to do and switches off.

Synchronising is carried out on the service cables for technical reasons and, when selected, a second contactor switch closes at the same time to connect the heating cables to make a path to share heating load directly between generators.

The synchronised turbines will share load in proportion to the energy captured by each turbine but the effect on the service cable is as if only one generator is operating. If one turbine loses power in wind shadow or light wind it is automatically disconnected so that it can not drain power from the network. This turbine will automatically rejoin the system if the wind freshens or changes direction.

When operating synchronised there is no need for frequent changeover between turbines as you experienced when the two turbines were running independently so there will be less momentary blackouts. There will still be short disconnections when the wind fails, while the diesel starts up, so those of you with computers either have to accept loss of data or get a non interruptible power supply unit.

There will also be the possibility that the system operator may decide that power transfer is too severe for synchronised or even coupled operation when you will have to revert to a split system. Hopefully this will not be too frequent.

We hope that you will find this new system satisfactory with less interruptions of supply and that you will all benefit from the greater amount of power available for heating. Even so you must always remember to be careful about putting too much load on the service cables or even with. two turbines you will switch to diesel and high tariff. Keep an eye open for your green light.