Moog has presented important developments in control systems to ensure safe operation of wind turbines in adverse conditions at the European Wind Energy Association Event, EWEA 2014, in Barcelona.
Dr. Tobias Rösmann, Technology Development Manager, has presented the latest information on Moog’s self-sensing closed-loop pitch speed control for wind turbines. The system enables synchronous motors to be used for pitch systems, since it can keep them under closed-loop speed control even if there is a failure in the motor position feedback. This avoids high turbine loads that would otherwise result from non-synchronized pitch speeds in the turbine in traditional approaches.
This self-sensing closed-loop speed control feature is only one of the benefits built into the new AC Pitch Axis Servo (PAS) concept from Moog. The new interior permanent magnet synchronous (IPMSM) motor was specially designed for the pitch application. It ensures maximum efficiency at low speeds during pitch operation and a peak torque capability of up to 3.5 times nominal pitch torque, even at grid fault condition. The motion control concept guarantees a wide field weakening operation area in order to fully control the feathering speed, even under grid loss condition.
In order to guarantee that the performance requirements set out in the IEC 61400 and GL 2010 standards are fulfilled for each application, Moog developed a standardized performance qualification process for pitch actuators.
In his presentation at EWEA 2014, Rösmann will discuss the results of a case study for a 7 MW turbine carried out by Moog and Fraunhofer IWES in Kassel to demonstrate the influence of pitch position incoherence on the turbine structural loads during the feathering run.
A wind turbine pitch control system has to fulfil two essential functions: First, it performs as an actuator for the turbine speed and power control when the wind speed exceeds the turbine’s rated values and second, it acts as the actuator for the wind turbine braking system.
To stop the turbine, all three blades have to be moved into the feathering position, and to distribute and balance the loads on all structural parts during the procedure, all three pitch axes have to move out of the wind synchronously.
Various open-loop control schemes are already available for the pitch actuator feathering drive if the pitch axis position feedback signal is lost including: Hydraulic actuators, DC motor pitch actuators and AC induction pitch actuators. But all three have one common disadvantage: The speed of each individual axis depends heavily on the load applied by the blade. The new control scheme from Moog overcomes this disadvantage.
”The solution was developed to prevent blades from sticking in the event of an encoder failure—the worst-case load scenario for the turbine. Simulations show an additional benefit: The solution also ensures a synchronized feathering speed,” said Tobias Rösmann .
Compared to state-of-the-art self-sensing control schemes for AC induction or synchronous motors, the new closed-loop speed control scheme provides up to three times the rated torque from standstill in order to ensure a safe and high performance movement into the feathering position.
Press release, March 11, 2014; Image: Siemens