The Long-term Inflow and Structural Test (LIST) (reports) is a joint Sandia and NREL program collecting experimental inflow and turbine response data to validate design and analysis methods for inflow, turbine dynamics and fatigue. At a recent workshop, large structural events were noted as a major contributor to the damage incurred on a wind turbine during normal operation. The participants noted that these “extreme events” occur at all wind speeds. To date, the analysis of these damaging events has been centered on the use of curve fitting techniques to define the load spectrum on the turbine. To characterize these low-occurrence events requires a long-term, time-synchronized database that characterizes both the structural responses of the wind turbine and the inflow for at least a wind season.

The LIST program is obtaining this database using operating wind turbines of various sizes and configurations at various sites. Each database consists of time-synchronized inflow and structural data that is being used to identify and characterize inflow events that produce large structural loads and to determine the long-term distributions of fatigue loads. The database also provides a resource for validating the ability of the various models for analyzing inflow and turbine behavior to predict these load cases.

A major technical challenge of this activity was the development of a light-weight data acquisition and analysis system suitable for wind turbine applications. The new system, Accurate Time-Linked data Acquisition System (ATLAS), has been developed and installed on several turbines at various sites. Simultaneous data acquisition by rotor and ground units is accomplished by utilizing Global Positioning Satellite (GPS) receivers, in conjunction with custom-built programmable logic devices (PLDs). These receivers maintain time synchronization with Universal Time Coordinated (UTC) time to within one microsecond over indefinite time periods. LabVIEW software has been modified to acquire, store, display, and summarize the data from the ATLAS hardware. Successful time synchronization of the GPS/PLD subsystem with UTC time has been demonstrated at rotational speeds up to 350 rpm.

The initial experimental campaign under the auspices of the LIST program was on a relative small turbine: A Micon 65/13, located in Bushland, TX, a site that exposes it to a wind regime representative of a Great Plains commercial site.

Although a large database was obtained in the inital test campaign, the test was terminated early because of premature failure of the blade strain gauges. In addition, the ATLAS experienced several component failure, preventing continuous data acquisition. To correct these problems and to complete the demonstration of a reliable data system that is capable of taking data 24 hours a day, 7 days a week, the ATLAS was upgraded, and the strain gauges reworked. The second measurement campaign began in early February 2003 and was concluded in early May. During this period, the data system had an availability of 95.6 percent. The new data set contains over 1630 hours of data.

With the demonstrated capability to take and archive continuous data from an operating wind turbine, the LIST program has begun the instrumentation and testing of a GE 1.5 MW turbine at a Great Plains location. Data includes direct physical measurements of atmospheric parameters and turbine structural response as well as the remote sensing of atmospheric conditions known to produce organized turbulence that has substantial operational impacts. The data acquisition system will be based on a combination of the ATLAS (demonstrated in Bushland) and the NREL tall tower inflow monitoring experiment (demonstrated in Lamar, CO). The measurement campaign is a joint public/private partnership formulated under a CRADA involving NREL, Sandia, and GE personnel.