High-wind hysteresis losses explained

For anyone who has been presented with energy-estimate reports, a little entry called 'high-wind hysteresis' in the losses table is often brushed over. I've seen values from 0-4% depending on the site, so what actually is it?

In simple terms, it's the turbine's control-system lag between shutting down in high wind speeds, and starting up again. Most modern pitch-controlled turbines will shut down at 10-minute average wind speeds above 20-25m/s, with 3s-gust speeds a little higher than this (typically around 5m/s higher); to prevent the turbine from starting up again during the high wind (just to shut down again), a re-start speed of around 5 m/s less than this value is specified. Physically, this is controlled through discrete parameters in the turbine controller that specify this number.

An example of this is shown below in some historic 10-minute average hub-height wind data:

 

If we assume a modern turbine (such as the V90) was running here, the turbine would shut down at 25m/s and remain shutdown until the extreme event had passed and decreased below 20m/s. However from an energy estimate perspective, if we are running this data through a power curve, the turbine is assumed to be producing energy whenever the wind speed is below 25m/s and therefore doesn't consider this data. It is this value that we are trying to estimate.

This analysis should be conducted on a representative time series of at least a year to be meaningful. To provide a more detailed analysis, the 3s-gust speeds should also be used, as the turbines will often have a higher 3s-gust shutdown or startup speed; so both scenarios can be considered together, and a more accurate hysteresis loss estimated.

Empirically, the higher the wind speeds and the 'gustier' the sites (particularly inland mountainous sites), the higher the hysteresis loss that can be expected.