Magnetostrictive sensors for wind turbine generators
I. Hydraulic variable propeller pitch system's requirement for displacement sensor
Wind turbine generator generators needs to confirm paddle angle according to wind speed. Through changing paddle angle, then to change rotating speed and power of paddle rotor, then get stable output power. The rotating angle of paddle is between 0° to 90°. When paddle is at working position, almost all the largest area of paddle is towards wind direction, the wind area is the largest. When use paddle for braking, the front end of paddle is towards wind, wind area is the minimum, and with rotating resistance, then to reduce wind turbine generator rotating speed gradually until stop. Paddle angle adjusting actuator has motor driving and hydraulic driving. When use motor driving, DC motor and a gear box work cooperatively, driving paddle adjusting bearing, change paddle angle. When use hydraulic driving, using three independent hydraulic oil cylinders to control pitch angle of three paddles separately. Both driving methods need sensors to monitor pitch angle at real time. When use motor driving, through measure angular displacement to measure pitch angle indirectly. When use hydraulic driving, through measure linear displacement to measure pitch angle.
Currently, some wind turbine generator manufacturers use motor driving method, while wind turbine generator manufacturers in Denmark normally use hydraulic driving method. Its hydraulic variable propeller pitch technology has over 25 year's history, almost all Denmark manufacturers benefit from hydraulic variable propeller pitch technology. Compare these 2 variable propeller pitch methods from technology stand, the wind turbine generator using motor variable propeller pitch needs regular gear lubricant oil, maintenance cost is high, especially for the wind turbine generator mounted on the sea. Hydraulic variable propeller pitch has better advantage at environment adaptability and maintenance cost. It can work well under minus 30 degrees, especially suitable for harsh environment including plateau, cold weather etc. Besides, wind turbine generator with hydraulic variable propeller pitch has leading edge for performance, including larger torque output, shorter start up time etc.
In hydraulic variable propeller pitch system, linear displacement sensor which measuring hydraulic oil cylinder piston stroke plays an important role, while magnetostrictive sensor becomes the best among all stroke measuring displacement sensors in different industry, relying on its measuring accuracy, reliability, environment adaptation etc. Magnetostrictive displacement sensors are widely used in wind power generation field.
II. Mechanical disc brake requirement on displacement sensor
There are two braking methods for wind turbine generator, one is pneumatic braking, and the other is mechanical braking. Normally wind turbine generator will use both above mentioned braking methods. The combination of both braking device could guarantee wind turbine generator working well under different conditions. Even under emergency condition, it can avoid the wind turbine generator suffering from damage. Pneumatic braking is achieved by pitch angle adjusting system of wind turbine generator. Under normal stop condition, variable propeller pitch system drives blade to idling position, then paddle blade stop rotating gradually. Under emergency condition, every blade has an independent storage battery through variable torque controller to supply power to braking. As aid measure for pneumatic braking, add mechanical disc braking system, it's mounted in the high rotating speed section of wind turbine generator. When it's shut down for maintenance, pneumatic braking stops wind turbine generator, use the mechanical disc braking to brake the wind turbine generator. Pneumatic braking will generate large braking torsion on generator gear box under braking , If require it for keeping braking condition when wind turbine generator shut down for maintenance, that will generate extra stress and unnecessary friction loss, because of this, must separate system emergency braking operation and maintenance braking operation. When rotor stops at preset position, rotor is fixed by clamp pin. At this time gearing chain brake could be open. Then, brake could release load, otherwise load will work on the drive unit, and affect the whole wind turbine generator life time. Clamp pin operation is reliable or not, should be monitored through displacement sensor. Therefore, most clamp pins in mechanical disc brake system should be mounted with position sensor. Using magnetostrictive linear displacement sensor to apply for clamp pin piston stroke monitoring, greatly improve the clamp pin safety.
Clamp pin mounted with magnetostrictive sensor:
Hydraulic disc braking system mounted with clamp pin and magnetostrictive displacement sensor: