A Tale of Two Windfarms, Mt Barker vs Denmark
The key reason for locating Mt Barker where we did was the Mt Barker substation, just three and a half kilometres to the west. The powerline that supplies both Mt Barker and Kendenup from the substation runs east west along the southern boundary of the sheep farm. Being so close, we could simply just T bone into the grid. Western Power installed a single pole 10m north of pole QA32. They installed aerial cable to this pole, through a mechanical disconnector then ran the cabling down the pole and underground into our switchroom a further 10m north.
The nearest substation to Denmark is the Albany substation, some 70 kilometres of powerline away. Also, the line to the quarry from the Caravan Park entrance turned out to be inadequate and needed replacing. With one and a half kilometres of underground cable and a High Voltage kiosk connection to supply the Surf Life Saving Club and the Boating and Angling Clubrooms, it was extremely expensive. Studies determined that the output of the windfarm had to be limited to just over 1.44 MW due to voltage rise considerations, high output / low load - think of a front coming through very late at night.
A communication link is required between the windfarm switchrooms and the Western Power substations. UHF radios were used to send data on the voltage, power, reactive power, some status signals (customer main switch and the 48V battery that powers it) and a few alarms, every two seconds. If the link fails more than three times in a row, then the windfarm is automatically switched off. Authorised personnel then have to go to site to turn it back on. Such a link over 3.5 kilometres is not difficult and we used six element yagi antennae on 2m high J masts simply bolted on top of each building in Mt Barker.
In Denmark, the substation is 47 kilometres away as the crow flies with the 110m high ridge of the Nullaki Peninsula in between. This is on the edge of what this technology can do. Fortunately, Western Power had only just installed a 40m mast at the Albany substation and we were able to install a 16 element yagi at 35m. In Denmark, we installed a self supporting 12m mast with two 16 element yagis, in parallel, at the top.
Given that turbines are very tall structures located on high points in the landscape, getting hit by lightning is a question of when not if. So earthing is particularly important. In Mt Barker, there is about 15 cm of topsoil sitting on a cap of clay. This clay is quite conductive and resistance readings were low enough, and were easy to prove compliance. In Denmark, we have sand with occasional limestone 100m above sea level, so getting low enough readings was very difficult. A dozen 2 m long copper plated steel rods were driven into the sand around each foundation after the concrete pour and the formwork was removed but before backfilling and compacting. These rods and the stainless steel conductors embedded in the foundations are all connected by bare copper cabling that runs between the turbines and the switchroom.
A final point about the electrical connections; in Mt Barker, the total line losses to the actual point of connection were calculated to be around 9% but in Denmark it was over 45%. This means not only is there more wind in Denmark, but also the value of the electricity generated there is considerably more than in Mt Barker. This is the benefit of generating where the load is on such a ‘thin’ local network requiring such expensive connection works and a restricted output.
It means that despite having one less turbine, smaller blades and shorter towers, the income for the Denmark project is much the same as for Mt Barker. This is the simple, overriding value of the higher winds that exist right on the coast near to a town far from the nearest substation.
For all the planning, electrical and civil engineering difficulties in installing the Denmark Community Windfarm, compared to Mt Barker, the economics are still likely to be better.
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