Power from thin air

Having just provided gearless permanent magnet generator systems for a set of 3 MW wind turbines in Norway, the head of Arctic Wind Power Consortium gives the low-down on the latest hi-tech practice that is taking the large-scale turbine industry by storm.

Arctic Wind Power, Finland

aking large amounts of energy from the wind is not an easy task and the challenge becomes greater still when moving up to large scale turbines. In 2004 the Finnish consortium Arctic Wind Power was contracted by the Norwegian wind turbine manufacturer and system integrator ScanWind AS to supply gearless permanent magnet generators for nine 3 MW turbines. The first 3 MW system was shipped from Turku, Finland on April 15, 2005, and was taken into use in mid-October 2005 in a wind park developed by Norway’s Nord-Trondelag Energi (NTE), located 200 km north of Trondheim in northern Norway.

Gearless permanent magnet generators were selected over conventional technology as the companies involved decided that at higher megawatt levels the new alternative could provide better reliability and performance than gear-based systems. Seeking to lead the gearless revolution, experts from Rotatek Finland Oy, Verteco Oy and Vaasa Engineering Oy, worked under the Arctic Wind Power Consortium to develop an optimal solution.

“We are truly proud of our ability to coordinate the efforts of so many talents and experts and deliver the gearless system to the customer in just one year. It normally takes two to three years to deliver a project of this magnitude. During 2006, we will supply a total of nine 3.5 MW systems to ScanWind,” says Jarmo Saaranen, head of Arctic Wind Power Consortium and CEO of Rotatek Finland Oy.

No gears, no fears

Direct-drive permanent magnet machines are becoming increasingly common in several power application areas. The development of NdFeB-based (Neodymium, iron and boron) permanent magnets has made it possible to manufacture high-efficiency, high-power factor synchronous AC machines. The NdFeB-based permanent magnets have a high flux density, a high coercive force, a high performance/cost ratio and are high energy producers.

As the gear is normally the equipment that is most vulnerable to mechanical damage, it is subject to considerable service costs. Gearless technology therefore eliminates this particular problem, which according to Saaranen, is a benefit magnified in remote areas: “Our technology brings major advantages, especially for hard-to-reach wind turbine installations. And as we know, the trend for very large installations is to move them away from populated areas into more remote regions, with an increase in offshore installations in particular.”

Figure 1. A 3 MW ScanWind turbine featuring technology supplied by Arctic Wind Power consortium and Vacon
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Wind turbine manufacturers, like all involved in the power industry, are seeking ways to improve the efficiency of their products. When comparing many other power generation systems to wind turbines the main difference in wind power systems is the consistent ‘fuel’ fluctuation. Modern power electronics with right control algorithms give full wind range operation with variable speed. As the system was been designed from the beginning to be redundant, the MTBF (Mean Time Between Failure) and MTTR (Mean Time To Repair) figures can be drastically improved. In conventional systems all the drive-train main components such as the generator, the gearbox and the inverter are, from the reliability point of view, connected in series. Thus a failure in one of the components stops the electricity generation. The system supplied in this case is designed with parallel modules.

Power converting

“The megawatt powers are generated by using a rotational speed of 16 r/min. Our technology brings many important benefits to the customer that have previously not been available,” explains Verteco’s director of marketing and sales, Marko Kristola. “First and foremost, our system is extremely redundant, or fault-tolerant, which means that it has been constructed so that it will continue operating even in if one or even several parts of the system have been damaged, for instance due to a direct hit by lightning.”

One generator consists of 12 three-phase linear generator segments. Four linear segments are symetrically coupled into a single 1.2 MW inverter package. Therefore one 3 MW turbine system consists of three 1.2 MW packages. So, if one inverter package is put out of order, or in the unlikely case of two packages being damaged, the system can still keep running. The power output to the network will of course decrease but the voltage level will remain the same. Naturally, servicing activities can be carried out without any interference to the rest of the running system. The converters can run in a four-quadrant drive configuration, which also gives attractive features during installation and service operations.

Figure 2. The power converter includes generator inverter and filter (right), cooling and intelligent control units (centre) and the grid inverter and filter (left)
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For example, a 3 MW wind turbine can be constructed out of three 1.2 MW power converter modules. These can be designed to run in parallel so that each power converter module contributes independently to the overall load. Such a design makes it possible to run the wind turbine with any combination of the three modules so the output power can range from 33 per cent via 67 per cent to 100 per cent.

At low wind speeds the modular design increases the system efficiency by reducing energy losses, as only the required number of modules need to be active for the ambient wind conditions. It is also possible to work on a converter module for routine maintenance or repair when the other modules are supplying energy to the grid.

Liquid cooled

The power converter system consists of two inverters connected to a common DC bus. The generator-side inverter brakes the generator with the optimal torque and the braking energy increases the DC-bus voltage. From this point the line-side inverter then transfers the energy to the electricity grid with the required voltage and frequency.

A generator inverter can be connected to one or up to four generator segments. If a winding failure happens the faulty generator segment may be isolated from the system and replaced with a spare segment in the nacelle. In this design, each segment gives 1/12 of the total power and the system continues to generate electricity when any number of segments are isolated or removed. This means that the redundancy is improved from 1:3 to 1:12 to further increase reliability and enable maintenance to be scheduled more efficiently.

The converter modules can each run independently, with any combination of one, two or three modules in operation depending on the power need and maintenance requirements. For example, with low winds it may be necessary for only one or two to be connected. This modular approach gives improved efficiency, as converter losses are lower than with all three modules running.

If low wind conditions persist leading to prolonged running on one or two modules, the hot modules can be rotated in use to give extended lifetime and improved reliability. If there is a fault in one of the modules, the faulty one can be disconnected while the remaining units remain connected to the grid and the turbine continues to produce electricity at partial power. Although the output power is reduced the reliability and overall efficiency are improved.

Figure 3. Permanent magnet directly driven generator consists of 12 three-phase linear generators.
Click here to enlarge image

As the gearless wind turbine is based on using full-size inverter technology instead of double-fed generators, which are still the most commonly used technology for controlling wind turbines today. The new inverter technology makes older conventional solutions obsolete, since it can utilize all available wind speeds, from very weak to very strong. The Vacon CH64 liquid-cooled NXP inverters, coupled to permanent magnet generators, put less mechanical stress on the system and provide high-quality 690 V electricity that is supplied via a transformer to a 20 kV network.

Wind turbines using these modular liquid-cooled inverters out-perform those that the use standard double-fed generators, according to Vacon’s Jari Perkiomaki, who says: “With conventional solutions, the speed range is à‚±15 per cent of the nominal r/min (on a 3 MW system, the nominal speed is 16 r/min). However, with our solution, the speed range can vary from 2 to 22 r/min. This means that we can generate power during the very weak wind conditions as well.”

Stopping cogging

Arctic Wind Power’s design improvements give operators a great deal of flexibility allowing maintenance to be easily scheduled for the most appropriate times. The permanent magnet excitation optimizes the efficiency of the 12-segment design. As an added advantage, the 12-segment design reduces the cogging phenomena to give improved operational performance.

Cogging may happen when a permanent magnet is moving with respect to the stator permanence variations producing torque spikes and sags and is a normal occurrence in all permanent magnet generators or motors. However, the effects of cogging are harmful for wind turbine applications because the stepping generator is located in the nacelle at the top of the tower. Cogging will make the complete wind turbine vibrate and increases the mechanical stress on bearings and other mechanical elements.

The vibration formed in the turbine can be amplified, in extreme cases, by the tower and the blades, with a high probability of resonance at specific speeds. Resonances must be avoided at all costs in wind turbines, especially if they occur in the operational speed range. If not, the efficiency and especially the reliability of the wind turbine will drop away markedly. One of the core competencies in Rotatek is electro-magnetic design and due to its high torque quality the generators supplied are able to run very smoothly. This requires comprehensive simulation of electro-magnetic circuits to find the correct magnet size, shapes and magnet module arrangements.

While demand for wind turbines is high the manufacturing world is struggling to keep up, placing more emphasis on large-scale developments. Estimating that the amount of wind power generated worldwide will have increased tenfold by the year 2020 to 500 000 MW, Arctic Wind Power is confident about the future of gearless technology within the market.

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