Modular Drives Chosen for Power Transmission System Test Rig

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. Every day’s downtime eats into the profitability of the project.

Now, one company has developed a solution to eliminate the mechanical gearbox and replace it with a computer-controlled hydrostatic power transmission system that is much lighter, cheaper and more reliable than the gearbox it replaces. Artemis Intelligent Power of Midlothian has already won two Carbon Trust Innovation Awards with its innovative 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 modular drives from Control Techniques.

Artemis Intelligent Power, a venture company established in 1994, 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 among others 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,” says Research and Development Engineer, Peter McCurry. “We started work on a concept for a test rig that would give us the most for our budget. Once we had a concept for the test rig, we contacted several inverter drives manufacturers to get a feeling for the price and suitability of their solutions.”

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 most impressed by the Control Techniques solution from the point of view of price and their technical input,” says Peter McCurry. “Control Techniques proved to be the easiest to work with and they were prepared to be flexible to allow us to tailor a system to fit both our requirements and our budget!”

Control Techniques’ 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,” he says. “To achieve this, we used a common DC bus system connected to all the Control Techniques 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, whilst the main drive comprises ten 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 Unidrive SPM’s power circuits minimise space requirements, yet provide the full standard and optional features of the well-proven and popular Unidrive SP range, and are integrated using the same software tools for commissioning and programming.

The Unidrive SPM is extremely versatile, configurable into open and closed loop modes, paralleled and load sharing, with active input and regeneration, in multi-pulse configuration (12, 18, 24 etc.) or fitted on a common bus for circulation of energy between drives. A wide range of modules give additional functionality, communications or connectivity with feedback devices.

The 1.6MW test rig has confirmed that the design offers high efficiency at all speeds and super-fast response at all load levels. “The project has progressed extremely well,” adds Peter McCurry, “and this is in no small way thanks to Control Techniques and Tom Donohue of Dalepark Motion Controls – their engineers put in some long days and nights to get us up and running. They were incredibly flexible and helpful when it came to meeting our tight time-line which required us to get the drives set up very quickly under challenging circumstances. As the test rig is a prototype, its commissioning was far from a straightforward task – with the transmission itself and the drives being commissioned in parallel.”

The outcome of the tests is the SeaAngel 7MW wind turbine which will use the Artemis Digital Displacement® Technology. This was unveiled in Amsterdam at EWEA 2011.