Rotor blade “fingerprint” prevents false alarms – a case history

Hoarfrost, ice, and storms are frequent winter events in the Sternwald Forest at a thousand meters above mean sea level and on the border between Austria and the Czech Republic. Despite this, seven wind turbines faithfully work there without unnecessary downtime. Credit for this success goes to a BLADEcontrol system, built by the IGUS ITS company, Dresden. (The firm was purchased by Bosch Rexroth AG in 2009). This monitoring system detects damage and ice. Sensors, installed inside the rotor blades measure the blades’ natural resonances. “To know whether or not ice has formed on the blades, measurements must be made at the turbine,” says Dr. John Reimers, responsible for marketing and sales.
The natural resonances of the rotor blades are well suited for analysis because e blade resonates along a characteristic frequency curve. These resonances are used to create something like fingerprints during normal operations and in various malfunction modes. “Structural damage always shows itself in the lower frequency range while minor damage is seen only at the high end,” Reimers explains. Ice formation causes certain frequency peaks to shift. The rotor blade becomes heavier and thus resonates more slowly. “The system actually provides protection against false alarms.”

The ice-recognition system has to react rapidly and precisely so the turbine can be shut down quickly. “If you don’t shut down the turbine, the rotor blade can throw ice all over the place.” The system previously used in the turbines sometimes shut down too late. Conversely, rotors were often stopped at low temperatures, even though blades were ice-free. And once the turbine had stopped, a technician had to check things out before it could be restarted.
That is no longer necessary. The wind turbine can restart itself– without being inspected beforehand. This prevents unnecessary downtimes and the revenue losses they involve.
In this pilot project the wind farm also made a contribution to the great quality of the measurements made by the system. Sternwind GmbH is owned in large part by the residents of nearby communities that had decided to use power from renewable sources.

Bosch Rexroth
www.Igus-its.de

Soft brake for turbines

Before engineers access a nacelle and hub for maintenance, the rotor must be stopped and secured in a particular position. Today, an electrohydraulic brake control can provide a soft stop to the required position. Then hydraulic controls can engage a rotor lock.

Stopping a turbine by the drive train brake alone puts high torque on the gearbox. Doing so for service work generates undue strains and wear. Hence, Rexroth distinguishes between emergency and service brakes, so slowing a rotor to position requires a soft brake. Locking the rotor allows releasing the soft brake, eliminating loads otherwise acting on the drivetrain, a task that lengthens the service life of the wind turbine.

In addition, when wind direction changes, lowering the clamping pressure exerted by the soft yaw brake lets the yaw drive work. After reaching a new position, controls restore pressure to hold the nacelle in position.

Bosch Rexroth
www.boschrexroth.com