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A scheme that revitalised an abandoned 250-kW hydro scheme on the Onekaka River can thank a small group of enthusiasts who battled today’s environmental red tape nightmare to become investors in valuable renewable electricity. By consulting engineer Bryan Leyland.
Jim Baird, a local resident of Golden Bay, knew of a 250kW hydro scheme that was built to supply electricity to the Onekaka Ironworks in the 1920s. After the Ironworks shut down the old power station kept running for a few years but in the 1950s it was abandoned. Power shortages in 2001 and 2003 brought higher electricity prices to the region and Jim, with myself and two others, formed a partnership to redesign the Onekaka scheme using the concrete dam originally built for the Ironworks scheme, the same penstock route and a powerhouse site a bit further downstream – with an output of 940 kW and 3.8 GWh pa. The major construction problem was the penstock line itself. The route led up a steep ridge to a surge pipe. From there about 400 metres of 600mm pipe had to traverse a steep and unstable slope all the way to the dam. The pipe was buried for the whole of its route. The bench formed for the pipeline gave us access to the dam so that we could dig out the silt and debris that had completely filled it. We had to dig this out and cart it away at considerable expense. An ‘A’ frame design was chosen for the power house. It had curved beams to minimise the floor area and improve the appearance. The turbines
The original manufacturer claimed a turbine efficiency of about 82 percent but it was probably nearer to 80 percent. Mhylab in Switzerland, and hydraulic laboratory specializing in small hydropower schemes, designed a new runner for us. It provided 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. In this way, we are absolutely sure that the bucket shape is exactly as it should be and that the efficiency will match the 89.2 percent verified by model tests. The casting, machining and assembly of the runner was carried out by a New Zealand firm who did an excellent job. The individual buckets were weighed and assembled to balance out the difference in individual weights and the turbine runs smoother than the unit with the original runner. An inspection after three years of operation showed that the new bronze buckets were in perfect condition. We did not replace the runner in the second machine because it only operates between 20 and 30 percent of the time.
In spite of their age the generators were in good condition and only needed to be cleaned and have the windings varnished. The original excitation system was replaced with solid state excitation on the grounds of reduced maintenance and better overall efficiency. The DC generators were scrapped. The electrical system was designed to be as simple as possible. The two generators are switched at 400V using air circuit breakers. They are direct connected to the step up transformer. Second-hand cables and 1MVA transformer were purchased from an old paper mill and cost less than half the price of new equipment. A voltage transformer connected to detect neutral displacement was connected to the line to detect earth faults on the 33kV line. The control system used PLC equipment from Unitronics in Israel. This proved to be an excellent choice as the equipment combined unusually low cost ($8000) with excellent versatility. A feature of the equipment was the ability to send and receive text messages over the GSM cellular phone system. System control
The metering equipment operates at 400 volts and transmits half hour readings of active and reactive generation over the GSM system every month. The station operates on water level control from the head pond. A radio transmitter at the head pond sends the pond water level to the station controller at regular intervals. The station operating programme has the pond full by about six o’clock in the morning. At 7am, the output increases by a preset value to provide extra power during the morning peak period. At the end of the peak period, the control system 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, to send it instructions to change the peaking duration and output and to shut it down remotely in an emergency. As far as we know, controlling a power station like this by a cell phone was a ‘world first’. Power supply
In winter, the solar cell was unable to supply the 6W standing load of the PLC and radio transmitter. The solar cell has been supplemented by a micro hydro unit provided by a small firm (www.ecoinn.co.nz). The unit comprises a tiny two jet Pelton driving a generator made from the permanent magnet low speed drive motor from a “Smart Drive” washing machine. It cost much less than the solar cell installation and gives a steady output of more than 90W. CommissioningThe new Onekaka facility began operating in November 2003 and commissioning took several weeks. Getting the machines to synchronize successfully was quite difficult because the control system was unable to position the needle with sufficient accuracy. This was solved by opening the needle by a small amount and then using the jet deflector to control speed by moving it either ‘in’ or ‘out’. This means that the unit speed cycles above and below synchronous speed and the auto synchronizer eventually finds a situation when the speed and phase matching allow synchronizing. The process is rather interesting to watch but it never fails to synchronize successfully. The station was originally connected to the local 11kV distribution system via a three kilometre line from the main road to the power station.
We operated in this mode for about five months until the lines company completed a conversion to 33 kV. From then on we could operate up to full power without restriction. Tests showed that the new turbine had an output of 520kW when operating on its own whereas the unit with the old runner could only achieve about 430 kW. Maximum station output is 940 kW. Since the conversion to 33 kV, the station has run very reliably and there have been few problems. Water inflowAt the intake a simple chain type screen cleaner was installed to remove leaves and the like from the screen. We designed the cleaner and had it built by a local engineering company. It is driven by the drive mechanism from a 12V winch of the type used on recreational vehicles.
Water outflowFor environmental reasons, we release a nominal 20l/s from the dam to maintain the flow in the one mile section of the river between the dam and the powerhouse. There is no suitable measuring site in the rapids and waterfalls just downstream of the dam so we have installed a flow monitoring system just upstream of the power station. The flow is transmitted to the station by radio and the control system sends a signal to the dam where four valves are opened and closed in sequence to ensure that the flow is maintained at the agreed 30l/s. We now release more water than before during a dry period and less flow when the tributary flows just downstream of the dam are high. If the station trips off, all four valves open automatically to increase the flow downstream of the station. The costAs with most projects of this type, the final cost was well above the original estimate. As already mentioned, the penstock installation and civil works were the main source of the additional costs. Nevertheless, I am sure that if we had followed the now fashionable route of a turnkey design and build contract, the cost would have been much higher. Based on my experience with other turnkey hydropower developments, there would have been disputes and the legal costs would have been very high.
The RMA consent also involved a safety report for a dam that was built 70 years ago and, but for us, would still be holding back hundreds of tonnes of silt and debris and with no one responsible for it. Renewing our water right for 35 years involved lawyers, environmental scientists and consultants and cost us something like $80,000. On top of this various delays and complications in the construction process, probably cost us $100,000 overall, and there are ongoing costs that amount to more than $15,000 per year. During construction, very strict requirements were imposed on us. These included the number of trees we could to chop down for access during penstock installation. Half a dozen saplings, no more than 100mm in diameter, collectively cost us in excess of $10,000 by making it very difficult to excavate and lay the penstock. Without this requirement, we probably would have laid the penstock above ground. Overall, the above requirements probably added $300,000 to the cost of the project. In my view, maybe 10 percent of the sum actually provided a real benefit to the environment. If instead, we had put $50,000 towards fencing off the river downstream from cattle and providing cattle crossings, I am sure the environment would have been far better off.
ReflectionsThe station has now been in operation for seven years, selling electricity into the ‘spot market’ without any subsidies, tax credits or other assistance that is common overseas. The average price we receive is about 7.5c/kWh. If we could do it again, what would we do differently? Firstly, we would have a single new vertical four-jet Pelton turbine instead of the two, old single jet horizontal units. Although the two second-hand units cost us less than US$15,000, their downstream costs were unexpectedly high. Having two units instead of one led to a much larger powerhouse, additional switchgear, and additional complexity in the control system. The lowest cost option would have been to get the complete turbine design from Mhylab in Switzerland and make it all in New Zealand. As already mentioned, we would also probably have been better off if we had not buried the penstock. Supporting the penstock above ground as though it were a steam or oil pipe instead of the conventional civil engineering solution of expansion joints and massive anchor blocks would have saved a lot of money.
Energy NZ Vol.4 No.3 May-June 2010 |
In the 1970s there was a wave of interest in small hydro driven by oil price rises. In those days, obtaining the necessary approvals and then designing and building a scheme was relatively straightforward and something like 300MW of small hydro power was developed. When oil prices dropped, the enthusiasm waned and now consenting has pushed up the cost of such projects.
Two 500kW sets were salvaged from another power station where they initially provided the construction power when the station was being built in the 1920s and then provided the station auxiliary power until it was decided that they were no longer needed. Each nominally 500kW Pelton turbine drove a direct current generator and a 400V AC generator on the same bedplate.
The turbines originally had belt driven governors and low-pressure hydraulic servo motors driving the jet deflector and the needle. The inlet valve was manually operated. A new high-pressure hydraulic power pack was purchased together with hydraulic rams to operate the jet deflector, needle and inlet valve. This has proved to be quite successful but, on hindsight, it might have been better to spend more effort investigating the lower cost alternative of electrical actuators that would have made position control easier and would have eliminated the potential problems of having about 60 litres of oil in the powerhouse.
The design philosophy was that the station would shut down if the PLC failed. This was a very important decision because it meant that we could use the PLC for protection as well as for monitoring and control. Consistent with this philosophy was the use of the man-machine interface incorporated in the PLC to provide soft push buttons for all the control functions. The only two manually operated switches in the station are the emergency stop buttons on the main switchgear.
Power supply at the dam was originally provided by a 120W solar cell and a 24V battery at a cost of about $1/kWh.
The connection point is about 10 kilometres from a 66 kV substation. When the first unit went on line and we increased the output to about 400 kW, we discovered that we were pushing the local 11 kV voltage from about 10.7 kV to above 11.5 kV. Over the next few days we discovered that we could export about 400kW when the farmers in the district were milking their cows but we were restricted to about 250 kW for the rest of the time. To limit the voltage rise we did everything we could in the way of running under-excited - to the extent that the unit often pole slipped and tripped on overcurrent. Not something that I had experienced before! The lesson here is that even small distributed generators can upset a typical rural distribution system.
A small PLC at the head pond monitors the water level upstream and downstream of the intake screen and starts the screen cleaner if the differential is above about 200mm. If the differential is large, it sends an alarm. If it is excessive, it closes the penstock guard valve and trips the station in case the cause of the high differential is a burst penstock. The same controller also opens a motor driven scour valve every time there is a large flood so as to pass silty water through the dam rather than having it accumulate in the head pond.
A major and unforeseen cost was that of compliance with requirements imposed on us under New Zealand’s Resource Management Act. These included an ‘heritage inventory’ that scheduled and sketched every one of the rusty old riveted penstocks that were lying around in the bush; an historical report on the history of the Ironworks; and a number of studies into the stream life - which have to be repeated every year. 
The public have benefited from the scheme because we have provided safe and easy access to the dam – a very pretty spot that is now often visited. Our access road also makes it easier to shoot feral goats and lay traps for stoats and weasels that kill native birds. We even had a rare Blue Duck visit our head pond. For all this, we get no credit.