July/August 2020

42 n RENEWABLE ENERGY July/August 2020 www.drivesncontrols.com AC drives give a novel spin to vertical turbine A German wind turbine developer is using a standard AC inverter to convert the variable output voltage and frequency of a new vertical-axis wind turbine to a stable output suitable to feed into a grid. Elegant Energy has used some of the inverter’s standard functions – in particular, its abilities to avoid specific frequencies and perform flying starts – to enhance the performance of its Marlene turbine. When embarking on the development, Elegant’s engineers quickly rejected horizontal-axis designs because these need more space to install. However, the engineers also knew that, despite their quiet operation and unobtrusive appearance, vertical-axis turbines had a reputation for poor performance and efficiency. They addressed these issues with a novel, high- efficiency rotor design, and by using an 8kW induction generator rather than a permanent magnet generator. To convert the turbine’s variable voltage and frequency output to a stable supply, they needed an inverter, often known as a grid converter. They opted to use a standard AC inverter rather than a customgrid converter, thus cutting development costs and ensuring proven performance and reliability. Elegant Energy chose converters from the Danfoss Drives’ NX family. These are available in ratings up to several megawatts, ensuring scalability of the Marlene design to produce uprated versions for areas with higher wind speeds. Another attraction of these converters was that they come with pre-programmed functions can be useful in wind turbine applications. One of these is a flying-start function. The Marlene turbine is designed to rotate even when there is not enough wind to generate electricity. As soon as the wind reaches a critical level, the generator is connected to the converter and can start to produce electricity immediately. The flying-start option allows the generator to handle the sudden connection to an aerodynamic rotor that is already spinning. A second useful function is known as“prohibited frequency”. Resonance is an issue that complicates the design of wind turbines. Over a limited range of rotor speeds, potentially damaging resonant vibrations can build up in the turbine structure. One answer is to addmore structural bracing, but this adds costs and increases the weight of the turbine. An alternative chosen by Elegant is to use the converter’s prohibited-frequency function to sense when the rotor speed is approaching the lower end of the resonance range. It then loads the generator to prevent further speed increases until the wind speed is high enough to drive the rotor at speeds above the resonant range. At this point, it restores normal operation. The function works in a similar way to prevent operation in the resonant range when the wind speed is falling. “The prohibited-frequency function has greatly simplified the design of Marlene,”says Elegant Energy’s co-owner, Kim van Wagtendonk.“It meant we could eliminate resonance problems without incurring any additional costs, and it was exceptionally easy to use. All we had to do was to specify the upper and lower limits of the rotor speed band where resonance could occur – just a fewminutes work to solve a potentially difficult and expensive problem!” The NX converters also support the connection of energy storage in the form of a secondary battery. This will allow the Marlene turbine to be supplied as a hybrid system, with batteries that are charged when the wind is blowing, and will continue to supply power if the wind speed drops too low. The mast’s base can house all of the electronics and batteries. A Beckhoff industrial PC is providing the turbine’s control and emergency functions, such as applying an electromagnetic brake automatically in the event of an equipment failure. The first Marlene vertical-axis wind turbine has now been operating for several months and Elegant Energy is expecting to start series production this month. It has received its first orders and international interest is said to be even higher than expected. n The costs of a new vertical-axis wind turbine have been cut by using standard AC drive technology to stabilise its output. Several of the drives’ built-in functions have been used to enhance the performance of the machine. Erecting Elegant Energy’s Marlene vertical-axis wind turbine