Fossil fuels have long been the preferred energy source for power generation. However, with growing concerns about the sustainability of fossil fuel supplies and greenhouse gas production, along with agreements to reduce CO2 emissions from combustion, many initiatives are moving towards renewable energy. It is only possible to manufacture modern wind turbines (WETs) with rated capacities up to 5 MW (currently in the experimental stage) by using modern materials to meet mechanical requirements and by employing modern electronic and power electronic components to efficiently supply power to the main grid. Various types of current sensors are essential components in every wind turbine for optimal converter control. Wind energy has been used as an energy source since early human development. Windmills convert the energy contained in the wind into mechanically usable energy that can be used for grinding or pumping water. In the first half of the 20th century, many of the physical and design theoretical foundations of modern wind turbines were discovered. German engineer Albert Betz, in his 1926 publication, calculated the maximum theoretical efficiency of an ideal wind turbine to be approximately 59.3%. In the 1940s, Ulrich Hütter developed the theoretical basis for the design of all modern free-running and high-speed wind turbines with two or three rotor blades (derived from his excellent aeronautical knowledge)[1]. However, it was not until the early 1990s, when political structures changed, that many countries provided government assistance for renewable energy. This government action spurred the intensive commercial development of wind turbines (WET). More and more wind turbines and wind farms were installed and built; the first 4.55 MW wind turbines are now in the testing phase. Germany leads the world with 14,609 MW of the 39,151 MW of wind power capacity, ahead of the United States, Spain and Denmark[2]. Power control of wind turbines Wind is the result of air mass exchange, mainly caused by local or even large-area temperature differences formed by the effects of solar radiation. Obstacles such as forests, mountains and buildings can create turbulence that affects the persistent changes in wind speed. The rotor of a wind turbine converts the energy contained in the wind into rotational (kinetic) energy, which drives a generator to produce electricity. Wind energy, and the amount available for use, is proportional to the cube of the wind speed. There is also a simple correlation between the rotor area, calculated from the rotor diameter, and the energy generated from the wind flowing through that area. When the wind speed exceeds a fixed limit, wind turbines must be equipped with power controllers to avoid mechanical and/or electrical overload. Generally, the generator's rated power is a threshold level that must be considered. [align=center]Figure (3) Figure (4)[/align] There is another equally important reason for power control. To provide a continuous supply of electricity to the grid, it is necessary to keep the generator operating at its optimal state, even though wind speeds change every second. Turbines use various power controls. The degree of control can be achieved passively or actively through the rotor blades. Passive limiting can be achieved through a single rotor blade with a special shape. At a certain wind speed, the airflow that causes the rotor to rotate suddenly disappears (so-called stall), and the rotor stops rotating (stall control). Modern large wind turbines typically employ active power control systems to regulate the rotor blades within their longitudinal axis (pitch control). By adjusting the blade angles relative to the rotor plane, it is possible to control more than just generator power. At higher wind speeds, rotor blades can twist in a way that allows the rotor to stop rapidly. Small-power electric drives are commonly used for this purpose. Small, PCB-mounted current sensors are widely used within some inverters. These sensors are part of the converter's closed-loop control and therefore react quickly. When used in conjunction with intelligent power control of the generator, they ensure continuous power supply to the grid over a wide wind speed range after the wind turbine (WET) starts up, until the turbine stops at the upper wind speed limit. Yaw Control It is important that the rotor is always perpendicular to the wind direction. There are two reasons for this: first, it ensures that the wind flows through the maximum rotor area, thus maximizing energy extraction from the wind; second, it avoids uneven loading of the rotor blades by ensuring that the rotor blades do not extend and retract during each rotation. Large commercial wind turbines are often called upwind turbines, meaning the rotor faces the wind in front of the tower, but this is an unstable state. Therefore, the fairing and rotor must be actively oriented to the wind direction by the action of the electric motor. Additionally, brakes are used to ensure that the fairing does not twist due to short-term changes in wind direction. To achieve optimal driver positioning, sensors within each converter continuously measure the current. The quality and response time of the circuit controller are ultimately determined by the design and performance of the current sensor. This is why closed-loop current sensors with small current ratings are used in this application. In addition to excellent linearity and therefore excellent accuracy, closed-loop current sensors themselves have advantages such as high bandwidth and fast response time. The principle of the closed-loop current sensor is described in [3]. The next problem is to obtain electrical energy from the wind and feed it into the grid. Wind turbine manufacturers have developed competitive systems for this purpose. In practice, each wind turbine is equipped with either an asynchronous generator or a synchronous generator. Asynchronous generator and grid coupling A typical “Danish concept” describes a wind turbine that includes a stall-controlled rotor with three rotor blades, a gearbox, a pole-switching asynchronous generator with a squirrel-cage rotor, and a direct grid coupler. The direct grid coupler produces a “constant speed” system with a near-constant operating speed in the supersynchronous slip region. The rotor speed can be adjusted in a narrow range by slip control or in a wider range by switching the polarity of the generator. The gearbox adapts the rotor rotation to the generator speed. The equipment requires power from the grid to gradually generate the rotating magnetic field. To limit the inrush current generated when the generator is coupled to the grid, a soft starter is used between the generator and the grid during startup. This direct grid coupling method is no longer used for large wind turbines due to certain technical drawbacks (such as compensation processes at the grid connection via switching actions for power regulation). Doubly Fed Induction Generator Most wind turbines now use a modified "Danish concept," in which a doubly fed induction generator acts as the generator. [align=center] (Figure 5)[/align] The stator frequency and voltage are tightly coupled to the main grid. The slip-ring rotor is coupled to the grid via a special inverter that must be able to transfer energy to both the machine and the grid. This inverter only needs to specify the slip power, which is typically only 20% of the generator's rated power. A wind turbine designed in this way is a variable-speed system ranging from subsynchronous to supersynchronous. Two identical pulse-controlled IGBT inverters with DC links are used as converters. Regardless of the energy transmission direction, one converter will function as a rectifier while the other functions as an inverter, and vice versa. To control grid power, precise and rapid current sensing is required in addition to DC link voltage. LEM offers closed-loop current sensors with medium current ratings perfectly suited for this application. These sensors are small and available in various mounting configurations. Furthermore, LEM voltage sensors can be used to monitor and/or control DC link voltage. Synchronous Generator and Grid Coupling Both concepts described above use a gearbox to adapt the relatively slow rotor rotation to the generator speed. A different concept that has been successful in the market uses a synchronous generator to power a variable-speed wind turbine. Due to the mechanical losses inherent in the gearbox and the lack of extensive maintenance, the adaptation of rotor rotation to generator speed can only be achieved at low rotor speeds. Therefore, a so-called ring generator device with multiple poles is used. [align=center] (Figure 6)[/align] A crucial advantage of synchronous generators is their ability to provide inductive or capacitive reactive power (even zero) depending on the control of the magnetic field/excitation controller. Main grid coupling is achieved by specifying a pulse converter for delivering total power. For these applications, LEM’s dynamic closed-loop current sensors can be used in rectifiers and inverters. Packaged sensors are also available for harsh environments. All LF series current sensors [4] available for the above applications have good common-mode characteristics and 0.3% accuracy (for rated values) at ambient room temperature. [align=center] (Figure 7)[/align] By applying the closed-loop principle, a fast-response sensor can be achieved, providing short-circuit protection for power semiconductor devices within the inverter – an invaluable advantage for offshore wind turbines, which are difficult and expensive to maintain. Summary Current sensors are an indispensable component of modern wind turbines. LEM, with over 30 years of experience in sensor development, has contributed extensive expertise. For many of these applications, LEM offers ready-to-use solutions; others can be developed and customized to meet the needs of customers and applications. References [1] Heiner Dörner, Institut für Flugzeugbau at the University of Stuttgart – http://www.ifb.uni-stuttgart.de/～doerner/doerner.html [2] Bundesverband Windenergie – http://www.wind-energie.de [3] LEM Technical Information CH22102 E/US - LF Series for Nominal Current Measurement from 200A to 2000A [4] LEM Application Notes CH24101 E/US - Isolated Current and Voltage Sensors (Features – Applications – Calculations) Figure 1: Windmill in Rhodes, Greece – The cloth used to make the rotor blades has been rolled up Figure 2: Wind-powered coastal pumping station on Rügen Island, Germany, in the Baltic Sea Figure 3: Gran Canaria, Spain Figure 4: Closed-loop current sensor circuit diagram for Canaria wind power plant. Figure 5: Doubly fed asynchronous generator circuit diagram. Figure 6: Synchronous generator circuit diagram. Figure 7: LF series includes current sensors from 20A to 2000A. About LEM : As a market leader in the sensor field, LEM provides customers with innovative technologies and high-quality electrical parameter testing solutions. Its core products are current and voltage sensors, which are widely used in industry, railway, energy and automation, and automotive sectors. As a global company, LEM has 1,000 employees worldwide and production centers in Geneva (Switzerland), Machida (Japan), and Beijing (China). Sales offices in various locations provide comprehensive services to customers globally. Beijing LEM Electronics Co., Ltd. is a wholly-owned subsidiary of LEM Electronics in China, with sales offices in various regions to provide seamless global services to Chinese customers. For more information, please visit the LEM China official website: www.lem.com.cn