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The original Onekaka scheme was built in Golden Bay in the 1930s to provide power to the Onekaka Ironworks, but was abandoned a few years after the closure of the ironworks in the 1940s. The dam was restored and the new station started generating in late 2003, re-using the existing concrete dam and the penstock route. The scheme has a maximum output of 930kW and generates more than 10 percent of the electricity used in Golden Bay, and reduces the losses in lines supplying the settlements of Takaka and Collingwood and, in a small way, alleviates the severe transmission problems in Christchurch and the Blenheim-Nelson region. HistoryJim Baird, a local resident and small hydro enthusiast, knew of the old Onekaka Ironworks scheme and was the inspiration behind restoring it. He and a partner, Brian Kidson, spent something like 20 years measuring flows of the Onekaka stream, obtaining water rights, purchasing land and buying two, redundant 500kW generating sets that had been for built in the 1920s for the Tuai power station at Waikaremoa. These units almost exactly matched the 500 I/s flow and 210 metre head available at Onekaka. The damThe new scheme uses the concrete dam originally built in the 1930s after 70 years of silt and debris was dug out and carted away. After looking at alternative routes, it was decided that the original penstock route was still the best. It traverses a steep and unstable slope from the dam to a surge pipe, and then plunges down a steep ridge. The new penstock pipe, which has been buried for the whole of its route, was bought second hand from the Manapouri power station where it was used during the excavation of the the tailrace tunnel. The power stationAn ‘A’ frame design was chosen for the power station with curved beams to minimise the floor area. Had the developers realized how expensive it was to build in a remote area during what was construction boom at the time, they say more effort would have been made to reduce the overall size and cost of the powerhouse. The generating setsFirst used to generate power for the construction of the Waikaremoana scheme in the 1920s and later as the auxiliary generators for Tuai Power Station, the generators were extensively refurbished and are expected to last for another 100 years or so. At Tuai, the nominally 500kW Pelton turbines drove a direct current (dc) generator and a 400V alternating current (ac) generator on the same bedplate. The two sets provided the construction power when the station was being built in the 1920s. One provided dc power for the station crane and the other provided station ac services. The original manufacturer claimed a turbine efficiency of about 82 percent, but it was probably nearer to 80 percent. Calculations showed that the original runner diameter was too small for the available head and Mhylab in Switzerland, an hydraulic laboratory specialising in small hydropower schemes, designed a new runner – providing all the drawings and data files needed to have the buckets cast in New Zealand from high tensile bronze and then machined under computer control. The casting, machining and assembly of the runner was carried out by ADC in Wanganui. The upgraded turbine runs more smoothly than the unit with the original runner. The runner in the second turbine was not replaced because it only operates between 20 and 30 percent of the time. The turbines originally had belt driven governors and low pressure hydraulic servo motors driving the jet deflector and the needle. The gate type inlet valve was manually operated. A new hydraulic power pack was purchased together with hydraulic rams operating at high pressure. These operate the jet deflector, needle and inlet valve. Tests showed that the new turbine has an output of 520kW when operating on its own whereas the unit with the old runner can only achieve about 430kW. Maximum station output is 930kW. Operation and controlThe station operates primarily on headpond water level control. A radio transmitter at the head pond sends the pond water level to the station controller at regular intervals. A flow gauge and another radio transmitter monitors the environmental flow released at the dam. It controls the four bypass valves at the dam to release the flow required by the operator’s water rights. If the station is shut down, the environmental flow is increased. A small PLC at the headpond monitors the water level above and below the intake screen and starts the screen cleaner if the differential is above about 300mm. If the differential is large, it closes the penstock guard valve and trips the station in case the cause of the high differential is a burst penstock. It also opens the scour valve during a flood to pass incoming sediment on down the river. The normal station operating programme has the pond full by about 6am. At 7am, the output increases to provide extra power during the morning peak period. At the end of the peak period, the station holds the water level constant until the evening peak when it again increases output and draws the head pond level down further. It then operates to hold the water level constant until about 1am when the output is backed off to allow the pond to refill before 6am. By the use of text messaging it is possible to monitor the status of the station from anywhere in the world, and send it instructions to change the peaking duration and output, and to shut it down remotely in an emergency. The system also sends alarms to the operator if the station has problems. At the damPower supply at the dam was originally provided by a 120W solar cell and a 24V battery. In winter the solar cell was unable to supply the 6W standing load of the PLC and radio transmitter. The inadequate solar power cost us more than $1 .5/kWh and was supplemented by a micro hydro unit provided by Eco Inn in Taranaki. The unit is made up of a tiny two jet Pelton turbine driving a generator made from a ‘Smart Drive’ washing machine motor. It cost much less than the solar cell installation and gives a steady output of more than 80W. Screen cleaning is a challenge. When the station was originally constructed the developers elected not to install an automatic screen cleaner because experience had shown that it is important to understand what sort of debris will block the screen before designing a screen cleaner. The intake is well underwater so most screen cleaning problems occur only when it begins to rain and the fresh wash brings down dead leaves that have accumulated in the river bed. A low cost chain type screen cleaner was installed and performed even better than expected, say the operators. DistributionThe station was originally connected to the local 11kV distribution system via a three kilometre line from the power station to the main road. It is then about 10 kilometres to the 66 kV substation at Motupipi (Takaka). When the first unit went on line the output was increased to about 400kW, pushing the local 11kV voltage from about 10.7kV to above 11.4kV. Over the following few days it was discovered that the scheme could export about 400kW when local farmers were milking, but was restricted to about 250kW for the rest of the time. The project was operated in this mode for about five months until a conversion to 33kV was completed and since then has operated up to full power without restriction.
Energy NZ Vol.4 No.3 May-June 2010 |