Back to the programme printer.gif Print




Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Aerodynamics and rotor design' taking place on Wednesday, 12 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

Charles Plumley University of Strathclyde, United Kingdom
Co-authors:
Charles Plumley (1) F P William Leithead (1) Ervin Bossanyi (2) Michael Graham (3) Peter Jamieson (1)
(1) University of Strathclyde, Glasgow, United Kingdom (2) GLGH, Bristol, United Kingdom (3) Imperial College London, London, United Kingdom (4) Peter, Jamieson,

Printer friendly version: printer.gif Print

Abstract

Fault ride-through for a smart rotor DQ-axis controlled wind turbine with a jammed trailing edge flap

Introduction

A Smart Rotor wind turbine is able to reduce fatigue loads by deploying active aerodynamic devices along the span of the blades. This can lead to a reduced cost of energy via reduced material constraints. However, a major drawback is the complexity and reliability of the system. Faults can cause catastrophic damage and without compensation would require shutdown of the turbine, resulting in lost revenue. This is the first study to look at a fault ride-through solution to avoid shutdown of the turbine and lost revenue during a fault, while keeping damage to a minimum.

Approach

The Smart Rotor concept has the ability to reduce fatigue loads on traditional horizontal axis wind turbines [1]. This is done through active control of the local aerodynamic characteristics of the blade to the local inflow. These load reductions reduce the material requirements and are particularly effective on turbines with large swept areas, where the wind speed varies substantially across the rotor as a result of wind shear, tower shadow, downstream wake and natural turbulence.

For the Smart Rotor, micro-tabs, jets, vortex generators, plasma fields, active twist, inflatable structures and many other control surfaces are being considered, along with a variety of sensors and actuators [2]. However, concerns over the implementation of these more novel control surfaces have led the two demonstration plants in operation to opt for more traditional trailing edge flaps, much like that of an aileron on an aircraft wing. That option is therefore modelled here. Nevertheless, the conditions under which an aircraft and wind turbine operate are quite different. The regular maintenance and no-expense-spared safety requirements of aircraft are quite different to the repetitive continuous operation and cost-effectiveness requirements of devices on wind turbines. Reliability and maintenance are therefore a key issue; especially offshore where the Smart Rotor concept is most beneficial due to the high unit costs such as foundations, cabling and maintenance that help weigh optimal size analysis to fewer larger machines.

Fears though over the reliability of the devices have not yet been addressed. Shutdown should the Smart Rotor system fail is undesirable due to lost revenue and swift corrective maintenance is likely to be costly when considering the conditions offshore. A preferable solution is to continue to operate the wind turbine until maintenance can be conducted, while sustaining power output and not eliminating the benefits of the Smart Rotor through increased loadings. A fault ride through system has been developed that does exactly that.


Main body of abstract

A state-of-the-art controller has been implemented for the NREL 5MW conceptual wind turbine in Bladed based on the UpWind controller [3]. Flaps have then been added to each blade spanning 10m, with a 10% chord width. A dq-axis control system for the Smart Rotor control was then developed, similar to [4].

The dq-axis control strategy involves converting the rotating blade root bending moment of each blade to tilt and yaw moments in a stationary plane using the Coleman transform, PI controllers then act to minimise these tilt and yaw offsets, before the inverse Coleman transform is used to set the demand angle for each flap. This strategy has led to lifetime load reductions of 15% in the out-of-plane blade root bending moment.

It is judged that two main faults are likely: 1) a broken linkage, 2) a jammed actuator. Under the first condition, aerodynamic pressures on the flap will keep it close to the zero angle position. The second condition can cause far more destructive damage, and it is this second fault that is considered here.

If a single flap gets jammed cyclic loadings result due to the one blade experiencing different aerodynamic forces to the other two. For the blade with the jammed actuator the damage to the blade root is increased substantially, but the loads on the other two blades are also drastically increased. As an example, a +5 degree flap angle is applied to one of the three flaps, while the other two are allowed to operate as normal. Calculated from IEC standard power production runs for a Class II B wind turbine 1 Hz damage equivalent loads for the blade with jammed flap are seen to be over twice those the turbine would be under a collective pitch control strategy, and almost three times what it would be with correct Smart Rotor control operation under certain wind conditions, as shown in Figure 1. Indeed, if a turbine was to operate under this condition for more than 15 hours per year a collective pitch control would result in lower loads than a Smart Rotor control, Figure 2. Even onshore this time period is short when considering pitch system failures result in an average downtime of 75 hours [5] and offshore turbines are likely to be down for much longer due to weather constraints. This highlights the requirement to recognise when a fault has occurred and act quickly. Without any fault ride-through system, catastrophic failure may result.





Detection of a fault is possible through a number of methods: direct feedback from sensors measuring the angle of the flap, measurement of the hinge moment of the flap, or indirect measurements of the blade root bending moment, tower motion or high speed shaft, as demonstrated in Figure 3. A rapid automatic response is required not just to reduce loads, but also to identify the fault mode and avoid automatic shutdown due to excessive vibrations.



The fault ride through system developed removes the cyclic loadings by adjusting the other two flaps to balance the third in a simple and effective way: the operational flaps are set to the angle of the jammed flap. This does result in a system with increased loads compared to the case where the flaps are working; however, the improvement over the non-adjusted case is considerable. The loads are in effect reduced to those of the collective pitch control case. The energy capture is also maintained. Naturally, the longer the duration the flap is in the fault position, the lower the benefit the Smart Rotor control has for fatigue load reduction, but the load reduction is sustained even if corrective maintenance takes weeks before the weather conditions are practicable for offshore maintenance. A fault that is present for as much as 20% of the time still allows a load reduction of 10% over the collective pitch control case using this control technique.


Conclusion

The Smart Rotor has the ability to reduce loads on wind turbines, which is likely to be particularly important for the next generation of multi-MW offshore machines with large swept areas. However, one of the key concerns associated with the Smart Rotor concept is the reliability and maintenance of the system, which could lead to increased costs or lost revenue. Indeed, it is shown in this work that if a fault occurs and the wind turbine is allowed to continue to operate normally, the load reduction benefits are quickly eroded, ultimately requiring the wind turbine to be shutdown. In an offshore environment, where corrective maintenance will take time due distance, equipment and weather conditions, this is a serious problem, and could result in significant lost revenue. Fortunately, a solution has been found which is both simple and effective.

A fault ride through system has been implemented that responds rapidly to faults and allows operation of the wind turbine to continue with loads that are substantially less than that of the fault case. Operation under a fault condition has been shown to be viable even for extended periods of time, while still allowing load reductions due to the Smart Rotor system to be realisable. This conserves the benefits of the Smart Rotor, while the reliability and maintenance requirements are relaxed, as load reductions and close to optimum power output may still be achieved even in cases where a flap jams. This research then helps facilitate the deployment of the Smart Rotor on commercial wind turbines by recognising and eliminating one of the key concerns.



Learning objectives
Reliability and maintenance requirements for the Smart Rotor are much more lenient than one might expect, and the fears that faults could hinder deployment of the Smart Rotor are unsubstantiated.


References
[1] Andersen, P. B., Henriksen, L., & Gaunaa, M. (2010). Deformable trailing edge flaps for modern megawatt wind turbine controllers using strain gauge sensors. Wind Energy, 13(December 2009), 193–206. doi:10.1002/we
[2] Barlas, T. K., & van Kuik, G. a. M. (2010). Review of state of the art in smart rotor control research for wind turbines. Progress in Aerospace Sciences, 46(1), 1–27. doi:10.1016/j.paerosci.2009.08.002
[3] Bossanyi, E., Witcher, D., & Mercer, T. (2009). Project UpWind: Controller for 5MW reference turbine. Contract (pp. 1–18). Bristol.
[4] Lackner, M. a., & van Kuik, G. a. M. (2010). A comparison of smart rotor control approaches using trailing edge flaps and individual pitch control. Wind Energy, 13(July 2009), 117–134. doi:10.1002/we
[5] Spinato, F., Tavner, P. J., van Bussel, G. J. W., & Koutoulakos, E. (2009). Reliability of wind turbine subassemblies. IET Renewable Power Generation, 3(4), 387. doi:10.1049/iet-rpg.2008.0060