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Extreme wind speed: risk and mitigation

This short GE Energy document explains the risk in the event of extreme wind conditions including hurricane or tornado and any mitigation. Note, the document acknowledges the risk of blade throws and tower collapse.

Introduction

Certain wind project sites may experience extreme wind speeds caused by a severe weather situation, such as a hurricane or tornado. Since extreme wind events may result in mechanical load levels that can lead to damage or failure of wind turbine components, the purpose of this document is to inform customers about risk from extreme wind events and suggest risk mitigation actions that are based on recognized industry practices.

GE's wind turbines are designed to withstand a certain level of loading caused by an extreme wind event. As defined in the IEC 61400-1 wind turbine design/safety standard, the largest wind speed to be considered is called "Ve50," which is the maximum gust over a 50-year return period for a 3-second averaging time.

In a Ve50 situation, the control system of the wind turbine is assumed to be able to pitch the blades in a feathered position, resulting in minimal rotor torque. Table 1 lists the Ve50 limits for different GE wind turbines for the site conditions specified in the IEC 61400-1standard.

Actual Ve50 limits can vary based on site-specific conditions, and the Ve50 limits in Table 1 assume the following site conditions [1]:

• Maximum flow inclination angle: 8 degrees
• Air density: 1.225 kg/m3 (sea level)
• Vertical wind shear exponent: 0.11

The Ve50 limits in Table 1 apply as long as the site-specific conditions are within those specified by the IEC standard.[1] If any or several of the site conditions in terms of flow inclination angle, air density, and vertical wind shear exceed those specified in the IEC 61400-1 standard, the actual Ve50 limit of the wind turbine of interest may be lower than that listed in Table 1 and GE should review these conditions. Also, if one or several blades should fail to pitch to a feathered position, the maximum wind speed the wind turbine can sustain may be lower than the values listed in Table 1, for given site conditions.

Risk

Wind turbine component damage or failure can occur when extreme wind produces forces on the wind turbine plant buildings/machines above the Ve50 design limit. Failures may not only prohibit the operation of the wind turbine, but could also lead to third party risk. Natural disasters such as hurricanes and tornadoes are well documented and the areas they affect are well defined, but their occurrence and behavior are not well anticipated.[2]

Furthermore, other natural storming wind producers such as-but not limited to-squall lines, microburst, or extra-tropical cyclones can occur at anytime, regardless of the location on the globe. With today's meteorological knowledge, predicting the maximum wind speed from a storm is unrealistic in most cases.[3]

The mode of failure of a wind turbine due to an extreme wind event cannot be generalized and depends on the turbine type and configuration, as well as the specifics of the extreme wind event and site conditions. Examples of possible failure scenarios include blade failure or a tower buckling or overturning. When winds are above the cut-out speed, the wind turbine should have its blades idling in a position creating minimal torque on the rotor. This is the only safety mechanism other than the yaw control. If a grid failure were to occur in conjunction with an extreme wind event-which is a likely scenario-the yaw control will become inactive. The loss of yaw control could increase the likelihood of damage/failure in the case of an extreme wind event. Also, the grid components/structures could also be part of the potential windborne debris. At this time, GE has no modeling capability in place that can predict the impact made to a wind plant if an extreme wind event occurs.

Ger4277_extremewind_thumb
Ger4277 Extreme Wind

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NOV 15 2007
http://www.windaction.org/posts/13098-extreme-wind-speed-risk-and-mitigation
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