Automated pumps for irrigation in NZ

Glenroy Irrigation Scheme is located 70 km south of Christchurch to irrigate around 1100 Ha of land. Water flows from the Rakaia River into three separate tanks containing three lift pumps to lift the water to the suction of the main pumps. The project is owned by a cooperative and the user is billed on water usage. In total, there are 24 irrigators including six centre pivots.

The system runs at a pressure of 11-12 bars to enable the water to flow up to the irrigators. The flow requirements can vary from 42 to 540 L/s.

The challenge was to install a completely automatic system that meets hydraulic demands while offering the optimum use of water and energy resources. To make things even more difficult, the pump station is located around 20 km away from the sub-station, bringing down the short-circuit rating of the transformer to 500 A.

The total load of the pump station, 1200 kW of AC drives, on the single 1500 kVA transformer could potentially create large distortions on the supply mains and make the pump shed vulnerable to voltage spikes. Calculation on VLT Motion Control Tool MCT 31 harmonic simulation software showed the distortion level of more than 15% on 11 kV side of the transformer without introducing harmonic attenuation techniques.

After discussing various options, Nairn Electrical decided to go with VLT Harmonic Filter AHF 010 matched to the VLT 8000 Aqua variable speed drives.

Various combinations of these pumps are managed and modulated by the VLT 8000 Aqua drives' pump management system. The cascade controller optimises pump performance while maintaining system pressure across the 10:1 flow turn down. Most of the other system management is also done using standard VLT Aqua drive features. This eliminates the need for a sophisticated PLC and custom programming.

On the main pumps the impellers were trimmed to a diameter of 602 mm. The power requirement at 50 Hz delivering a design flow of 320 L/s was only 424 kW.

Two VLT 8652 Aqua drives are used on 500 kW pumps, together with one VLT 8072 and a VLT 8052 on smaller pumps.

The system is running in closed loop pressure control, maintaining a constant pressure at varying flow.

Additional benefits include: pipe fill mode - filling the long pipe lines with pressure ramp; pump sleep mode - using power calculations to stop the pumps in case of a no-flow situation; built-in dry run protection - optimally designed for variable speed pumps; and two setpoint PID which reduce pump speed in case of falling suction pressure.

According to Gary Roxburgh, the owner of Nairn Electrical, "The Danfoss VLT 8000 Aqua offered the best protection against weak power supplies and as standard included very high-grade radio frequency interference and harmonic filtration".