Improving turbine downtime through fail-safes

As a countermeasure against climate change, net zero greenhouse gas emission is a global issue, and the shift to renewable energy is of the utmost importance.

Wind power is expected to play an important part in renewable energy, and is about to be deployed worldwide, onshore and offshore. For onshore deployments, there will be an increase in the development of wind turbines in areas with severe wind conditions, such as mountainous areas and strong wind areas, while offshore, there will be floating wind turbine developments in long-distance and deep-sea areas, as well as fixed-bottom wind turbines in coastal areas. If it is not possible to operate stably and safely even in such places, it will be difficult to deploy them on a large scale.

This article will focus on the safety of wind turbine yaw devices, and reports on failure cause analysis, countermeasures, and verification of effectiveness based on field tests conducted in Japanese wind farms where the wind conditions are very severe.

The function of a yaw device

The yaw device, which points the blades of a wind turbine squarely in the direction of the wind, plays an important role in following changes in wind direction and maintaining the turbine’s attitude.

This mechanism consists of a yaw ring gear directly connected to the tower and multiple yaw drives installed on the nacelle that mesh with the ring gear. The driving force is controlled by an electric motor installed at the input area on the upper part of the yaw drive, and braking force is controlled by an electromagnetic brake attached to the electric motor and a rotation brake installed on the ring gear.

When a wind turbine is not generating power, turbine safety is ensured by levelling the blade angles to prevent the blades from rotating under wind load, and by performing yaw rotations.

Yaw device failure modes and costs

According to a survey of mechanical failures by the number of years in operation for yaw devices in Japan from 2011 – 2019,¹ a total of 24% of all failures were due to sudden premature failure in less than five years of operation, with the remainder due to fatigue failures that occurred within a short period of time. While wind turbines are designed with a safety factor in accordance with design guidelines established by DNV-GL² and other organisations to withstand 20 years of operation,² fatigue failures have occurred in a short period of times, either soon after installation or within just a few years. Yaw drive failure accounted for 48% of failure occurrences, ring gear failure for 9%, and others for 43%.

The repair cost for failure of the ring gear came to ¥3.17 million (€21 100), ¥4.95 million (€33 000) for the drive unit, and ¥2.31 million for others (€15 400). It has also been confirmed that there were large differences in ring gear repair costs depending on how repairs were carried out. In most cases covered by this set of data, repairs were made by welding following penetrant inspections. However, in some cases, the ring gear needed to be replaced, which required a large special work crane, incurring a cost of more than ¥100 million (€670 000).

Causes of yaw device failure

Regarding yaw drive, damage to power transmission components of the output shaft reduction mechanism, and in the case of ring gear, damage to parts that contact the pinion, are cited. Both cases were one-shot breakdowns due to large loads or fatigue breakdowns that occurred within a short period of time.

Figure 1 shows the result of factorial explication for clarifying the causes of failure. Apart from the characteristics of the wind turbine itself and temporary control problems, two major factors were involved: the large rotational moment of the nacelle caused by large deviations in wind direction due to turbulence and upward-blowing wind, and the generation of concentrated and excessive loads due to unequal load sharing between multiple yaw drives.

Large deviations in wind direction

Figure 2 shows the relationship between wind speed and yaw loads as measured on an actual unit. The figure clearly shows that yaw load does not increase in proportion to wind speed.

Figure 3 shows a comparison of wind direction and nacelle azimuth in the same timeframe. Variations in wind direction are seen to be larger than variations in nacelle azimuth. Therefore, the reason why the aforementioned rotation load is not proportional to wind speed is likely that the nacelle azimuth is not adapting to changes in wind direction, which results in a large rotational moment, albeit momentarily, to the nacelle.

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