While turbine technologies change, the need for precise current measurement and control remains constant. The quality and performance of current transducers - whether they are controlling the position of the rotor and nacelle or the power to the grid - can have a direct impact on the overall efficiency of the wind turbine.
One key area in which transducers are used is to adjust with electrical drives the position of the nacelle and the rotor blades in relation to wind direction, to ensure maximum power generation or - in the case of too-high wind speeds - allow safe 'switch off' of the wind turbine. In this application, closed-loop current transducers with a low current rating offer the optimum combination of high bandwidth, short response time and good linearity, which is essential for high accuracy.
Current transducers are also essential for controlling power into the grid, whether the turbine is based on a conventional or modified 'Danish concept' with an asynchronous generator, or uses modern synchronous generator designs.
Conventional Danish-concept turbines combine a stallcontrolled rotor with three rotor blades; a gearbox to adapt the rotor revolutions to the generator speed; and a poleswitched asynchronous generator with squirrel-cage rotor. These turbines are coupled directly to the mains grid to produce a system with a virtually constant operating speed in the super-synchronous slip area. A slip control is used to adjust the rotor speed within narrow limits, while switching the poles of the generator enables wider adjustments. Power from the grid is used to build the rotating field, with soft-start between the generator and the grid to limit the inrush currents during start-up.
The majority of today's large wind turbines, however, use a modified Danish-concept design, which features a double-fed asynchronous generator with a slip-ring rotor coupled to the grid by an inverter. This creates a variable speed system, from sub-synchronous up to supersynchronous, and uses two identical pulse-controlled IGBT inverters with a voltage DC link as the converter. In this style of turbine, closed-loop current transducers with a medium current rating are used to provide the precise and fast current detection that is needed to control power for the grid and the DC link voltage.
Other modern turbines use a synchronous generator to provide a variable-speed system. In these designs, the gearbox, with its mechanical losses and intensive maintenance, is replaced by a multipoled ring generator that adapts the rotor revolutions to the generator speed, while pulse converters are used for the mains grid coupling. In this application, dynamic closed-loop current transducers are used in the rectifier and inverter stages.
The choice of closed-loop transducer for each type of turbine technology and application will directly affect overall system performance in accordance with grid code requirements. Current transducers, such as LEM's standard LF xx10 series or custom-designed versions, can be used as ready-to-use solutions in virtually any wind turbine design, where they offer good common-mode behaviour as well as an accuracy of 0.2-0.6% of the rated value from -40 to +85°C.
Their fast response time also provides short-circuit protection for the power semiconductor in the inverters. This, combined with optional encapsulation, offers an invaluable advantage for wind turbines in offshore areas where maintenance is difficult and expensive.