Drives help extract maximum energy

Better inverters improve the efficiency of green electric power generation as well as electric motor usage. Tom Shelley reports

New and improved electronic inverters are crucial to the efficiency of both wind turbines and low-head water turbines, such as those used to extract power from river and coastal currents. They achieve benefits because a constant output voltage can be achieved even when wind speed and water currents vary. Changing the blade angle to maintain constant speed also achieves this goal, but at the price of wasting some of the energy which might have been extracted from high winds and flows. All advanced wind turbines, even small ones, now use some kind of electronic process to extract maximum energy from the generator and deliver a usable output voltage under a wide range of conditions. ABB has developed a range of permanent magnet generators with AC inverters to match specially designed for wind turbines rated at up to 5MW. The generators do not have to produce AC at grid frequency – since the inverters take whatever power is being produced, even if the turbine is rotating at low speed – and convert it to grid frequency and voltage. As a user, particularly interesting comments were made by Paul Schraven of the Dutch company, Lagerway. He said: "The total efficiency of a wind turbine involves many aspects. We distinguish the aerodynamic efficiency, mechanical efficiency and electrical efficiency." The aerodynamic efficiency is determined by the chosen blade profile, control principle (stall, pitch, etc) and the way of this principle is adopted (ie stall versus active stall and blade pitch versus variable speed combined with blade pitch). The mechanical efficiency is determined by the drive train, with gearbox efficiency and bearing efficiency having an effect. Electrical efficiency is determined by the generator design: synchronous versus a-synchronous, active excitation versus permanent magnet direct coupled to the grid versus converter. He expands: "Lagerwey has chosen to eliminate the gearbox. A good gearbox is characterised by an efficiency of 98% at full load and less efficiency at partial load. Big gearboxes are equipped with auxiliary equipment: oil pumps, oil coolers, and ventilators that may use many kilowatts. Our design uses a permanent magnet generator for the 1.5 – 2.0MW wind turbine. By doing so, no excitation power is required – saving up to 40 to 50kW. The design also incorporates a converter running at 3,000 to 4,000V. Higher voltage results in less current. Consequently converter losses are reduced compared to operating at 690V. Based upon calculations and comparisons of drive trains and electro-technical configuration of the wind turbines, we found that the efficiency of these components together gives an improvement of about 4%." Inverters allow the extraction of the maximum amount of energy at usable voltage from wind turbines and low-head water turbines working under a wide range of conditions Further improvement in efficiency may be obtained by eliminating gearboxes and using permanent magnet generators working at higher voltages