extreme cyclonic weather

Based on some recent experiences, I thought I'd recap on extreme cyclonic weather and how it influences wind-turbine site analysis.

Cyclonic weather relates to weather systems that rotate clockwise around low-pressure systems in the Southern hemisphere, and anti-clockwise in the Northern hemisphere. Once they exceed a set level (and there are various classification systems) then they are called: typhoons in the Pacific, Hurricanes in the Atlantic, and tornadoes when they are land based. Tornadoes can be spawned from typhoons/cyclones/hurricanes when they hit land.

Research in the field is fairly limited, however they seem to be characterised quite well by the Holland model: a warm centre surrounded by colder air with no fronts. Extreme winds occur in the outer eye wall. As the cyclone rotates, centripetal acceleration is balanced by the pressure gradient (suction internally), therefore the lower the eye pressure the higher the wind speeds.

example cyclone passage over a weather station

So, how do we deal with them? In my opinion they affect wind turbines in three ways:

• extreme wind speeds
• extreme changes in direction as the eye passes
• grid-outages, and the turbines ability to 'center' with no external power

On the extreme wind side, the Vref (10-minute average) and Ve50 (3s gust) should be investigated closely, difficult to get data however any cyclone tracking station data should have estimates of these speeds. These are typicaly at 10m height, so you have to extrapolate them to hub height, a wind shear of around 1.2 is typically representative. However how do you estiamte the one-in-fifty cyclone strength? To do this, I did a Gumble analysis on peak cyclone wind speeds in the area. Failing this, the local building codes may also give you an idea of the extreme wind speeds (however not recommended!).

Extreme wind direction changes can be estimated by looking at the cyclone's speed along the path, and estimating the angle change over time. The peak rate of change can be compared to the IEC standards.

Grid outages are typically coincidental with cyclone passages, as they knock down power lines in the area. Although a turbine might be rated to class I extreme winds (DLC 6.1) the load case with loss of power (DLC 6.2) doesn't use safety factors as it's an accidental load case - therefore the loads are significantly lower. For maximum survivability then, it's worth investigating a UPS (a diesel backup generator for example) to kick in when a cyclone is coming to keep the turbine centering into the wind. These are really 'soft systems', where the turbines are manually stopped switched over to the UPS, and not put back into run again until the cyclone has passed.

For further reading, I strongly recommend checking out Risoe report RIS-R-1544.