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Drives aid irrigation down under01 July 2008The dry landscape of New Zealand’s South Island has put great demand on water resources, as farmers avoid booming property prices in the North Island and move south to buy cheaper land. Irrigation has thus become an important feature of the South Island landscape, but controlling these schemes has required the application of modern drives technology specific to widely distributed pumped water schemesThe Glenroy irrigation scheme is located 70km south of Christchurch and irrigates around 1,100hectares of land. Water flows from the Rakaia River into three separate tanks containing three lift pumps that feed water to the suction inlets of the main scheme pumps.
The project is owned by a cooperative and users are billed on water usage. In total there are 24 irrigators, including six centre pivots. The system runs at a pressure of between 11 and 12bar, which is sufficient to ensure adequate water flow up to the irrigators. Flow requirements of this scheme can vary from 42 litres/s up to 540 litres/s.
Need for automation
The challenge facing the Glenroy irrigation scheme cooperative was to install a completely automatic system that would be capable of meeting these variable demands while making optimum use of a scarce water resource in an energy efficient manner. The solution was to use variable speed drives on the pumps – in this case, the ‘Aqua’ application-specific VLT drive from Danfoss.
However, to make things even more challenging, the pumping station is located some 20km away from the sub-station. This would have the effect of increasing the harmonic distortion on the primary side of the transformer, as well as making systems within the pump house vulnerable to voltage spikes.
The total load on the 1,500kVA transformer had the potential of creating large distortions on the supply mains. The main transformer load was a total of 1,200kW (mostly ac drives), and a calculation using MCT 31 harmonic simulation software indicated that the distortion level was likely to exceed 15% on the 11kV side of the transformer. After discussing various options with the electricity supplier, the scheme’s contractor - agricultural electrical installation specialist, Nairn Electrical - decided on Danfoss AHF 010 filters for the VLT drives installation.
Aqua to the fore
Various combinations of pumps are brought online and modulated, based on demand, by utilising the features of the Danfoss VLT 8000 Aqua drive’s cascade control card. Most of the control functions are executed from within the drives, avoiding the need for a separate master PLC and its programming.
On the main pumps, the impellers were trimmed to a diameter of 602mm, and the power requirement (at 50 Hz) for a design flow of 320litres/s for each of the main pumps was calculated at 424kW. Two VLT 8652 Aqua drives are used on the 500kW main pumps, while one VLT 8302, two VLT 8072 and a VLT 8052 are deployed to control the smaller pumps. The system runs in closed loop pressure control in order to maintain a constant pressure regardless of the flow rate.
The Aqua drive is designed specifically for water pumping duties and has several advantages in applications of this kind. In addition to maintaining a pressure ramp in the long pipe lines of the Glenroy scheme, the drive has a built-in sleep mode - a big selling point as the pumps can be idled in the event of a no-flow situation. Aqua also features dry-run protection, optimally designed for variable speed pumps, and two PID loops that can be used to maintain the suction level while controlling the pressure.
Nairn selected the Danfoss drives for their features and benefits, confident in the company’s ability to provide local engineering support and expertise in variable speed pumping technology. And as senior Nairn electrician, Lincoln Teale observes, the drive is very easy to wire up with both power and motor cables thanks to the ease-of-access and ample space provided in the VLT 8000 Aqua design.
According to Nairn Electrical proprietor, Gary Roxburgh, the Danfoss VLT 8000 Aqua is “generations ahead” of any competitor both in terms of hardware design and software features. He also believes the Aqua offers the best protection against weak power supplies, high levels of radio frequency interference protection and very effective harmonic filtration.
Moreover, the complete package is offered in a factory tested metal enclosure reducing design and commissioning time to a great extent. The Danfoss harmonic filtering package chosen for this application worked extremely well, and while MCT 31 simulation was used prior to the installation, the results were actually slightly better than predicted. Contact Details and Archive...Related Articles...Most Viewed Articles... |
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