Modular drives chosen for power transmission system test rig

15 May 2012

One company has developed a solution to eliminate the mechanical gearbox of a wind turbine and replace it with a computer-controlled hydrostatic power transmission system that is said to be lighter, cheaper and more reliable than the gearbox it replaces.

Artemis Intelligent Power has already won two Carbon Trust Innovation Awards with its design for wind turbine power transmission based on ‘Digital Displacement hydraulic power’ technology. This has been developed using an advanced test rig, at the heart of which are drives supplied by Control Techniques.

One of the most vulnerable parts of a wind turbine is its gearbox, absorbing stresses and strains transmitted by the rotor blades as they are battered by sudden gusts and gales. Wind turbine gearbox failure or downtime can be a significant proportion of the total maintenance costs, particularly with regard to off-shore wind turbines.

Artemis Intelligent Power, whose technology originated from wave power research in Edinburgh University, was recently acquired by Mitsubishi Heavy Industries. The digital displacement pumps and motors which came out of the research extended the range of applications for hydrostatic transmissions to include multi megawatt wind turbine transmissions. In 2011, with backing from the Carbon Trust, the company completed a prototype hydraulic transmission for a 1.6 MW wind turbine.

“We required a test rig that would allow us to test our prototype digital displacement transmission to see how it would behave in a wind turbine,” said research and development engineer, Peter McCurry.

The hydrostatic transmission consists of a low speed high torque Digital Displacement pump which feeds two high speed Digital Displacement motors. To test it the final design of the test rig uses a 1.6MW induction motor, which drives through a reduction gearbox, simulating the low speed high torque wind-driven rotor. The high-speed hydraulic motors each drive inverter-controlled induction motors. -The rig can simulate any wind conditions, whereby either, or both, of the hydraulic motors can be operated.

“We were impressed by the Control Techniques solution from the point of view of price and technical input,” said McCurry. “Control Techniques were prepared to be flexible to allow us to tailor a system to fit both our requirements and our budget.” The solution comprises a 1.6MW prime mover – an inverter driven induction motor – and two 800kW generators, again inverter driven induction motors – 22 SPMD modules in all.

“We needed to be able to re-circulate electrical power as, when testing, we could not draw the full 1.6MW from the grid,” said. McCurry. “To achieve this, we used a common DC bus system connected to all the modular drives and this is fed by a 572kW active front end module that can either make up losses in the test rig, when it is running at full power, or supply power when the test rig is not in regenerating mode.”

The active front end, which minimises harmonic distortions, is made up of three Unidrive SPMD 1424 modular AC drives, whilet the main drive comprises 10 similar modular units to give a total power of 1620kW. This drives the 1.6MW pump that is the input end to the transmission. Two further motor/generator control units, each 972kW, comprise six Unidrive SPMD modules. This means that a total of 25 Unidrive SPMD 1424 modules – 4136kW – are all on one system connected to the common DC bus with regenerated power being used to reduce the demand from the grid. All drives are fitted with Ethernet modules for communications with the main computer controlling the rig. All drives, except for the active front end, which operates in regenerative mode, operate in closed loop vector mode, with encoder feedback to give the accuracies required.

The modular nature of the SPM’s power circuits minimise space requirements, while providing the full standard and optional features of the Unidrive SP range, and are integrated using the same software tools for commissioning and programming.

The outcome of the tests is the SeaAngel 7MW wind turbine which will use the Artemis Digital Displacement Technology.

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