A Study of Torsional Reversals Caused Through Wind Events and Operating Conditions
Although wind turbines have been around for decades, recent research has been focused on what occurs with a wind turbine under various wind conditions. It is understood anecdotally that high gusts and turbulent winds can add to the chance of breakdown of wind turbine equipment and lead to an increase in O&M and capital costs. Ridgeline and downwind turbines see higher O&M costs. Most of the earlier focus was on the effects on blades and tower structures. New data shows how the entire drive-train sees an impact from these transient events. Drive-train torque monitoring on various turbine models has shown that an asymmetrical torque control device reduces the damaging loads and helps extend turbine life.
By Doug Herr, General Manager, AeroTorque Corp., USA
Types of Transient Wind Conditions Faced by Wind Turbines
Wind turbines see a broader range of dynamic loads than other rotating equipment. They experience variation from the grid/generator (in the form of curtailments, grid loss, voltage changes etc) and also see very frequent wind changes. Storms, gusting conditions and even a sudden wind loss can cause significant variability in drive-train loads. These common events all contribute to the reduction in the expected life of drive-train components.
Extreme wind events have been defined for a long time. However, the transient torque reversals (TTRs) that these events impart in the drive system have only recently been identified, measured and recognised as a cause of increased O&M costs. Extreme wind events of relatively short duration can be caused by wind shear and turbulence resulting from topography and local weather phenomena, but the design of the wind farm itself can also cause problems due to the downwind wake effects of moving rotor blades (see Figures 1 and 2).
What is a Torque Reversal and Why is It a Concern?
The magnitude of the dynamic torsional loading of wind turbine drive systems during extreme transient wind conditions has not been well understood. A wind turbine has two large inertial masses, one at each end of the drive system and rotating in sync with one another. The blades typically have 80 to 90% of the relative inertia, with the generator rotor having most of the remaining 10 to 20%. A torsional reversal is a rapid torsional unloading of the drive-train and loading up in the opposite direction. While the direction of rotation of the shafts do not change, the direction of the bearing load zone in the gearbox shifts up to 180 degrees very rapidly. Once this shifting begins, the system can wind-up back and forth many times causing multiple load zone shifts until the torsional energy is dissipated. These rapid reversals can cause substantial impact loads on rollers and races in bearings. It is believed that a significant number of gearbox failures are caused by the impact loads from these reversals, evidenced by the axial cracking observed in turbine gearbox bearing race analysis. These torque reversals can occur any time there is a significant transient event on either end of the system (i.e. significant turbulence or shear winds, e-stops, curtailments etc).
How Extreme Wind Conditions Can Cause Torque Reversals
Modern wind turbines use blade pitch control to both moderate the power that is delivered to the generator and to provide the primary means of braking to safely decelerate the turbine. Depending on the blade pitch angle, the blades can be driving or decelerating the generator. From cut-in to rated turbine power, the blades are pitched to extract the maximum power possible. At higher wind speeds, the blades are precisely pitched to safely deflect the excess wind power. Further pitching beyond the optimum feathering or stalling angle will cause the blades to try to decelerate the generator and can reverse the torque in the system. During an emergency stop at wind speeds close to cut-out speed, rapid blade pitching can reverse the torque in the system in less than one second.
A sudden change in wind direction can result in momentary torque reversals that can cause damage to the turbine drive system. Blade control systems cannot react fast enough to always mitigate these fractional second loads, especially during rapid stopping during high winds.
Blade Length and Wind Conditions
The industry trend is towards increased blade length on new installations, in order to optimise the power production. An increase in length of only 15 feet (4.6 metres) per blade can generate 7% more power per year, according to one turbine manufacturer. This is a significant increase in the economics of lower wind sites. Figure 3 shows the decrease in cut-in speed, speed for rated power and increase in power potential with increased rotor length.
However, there can be a significant downside to this increase in blade swept area. As wind speed increases beyond rated power, more of the potential energy in the wind needs to be safely deflected via pitching of the blades. Even with precise pitching the risk of torque reversals caused by wind gusts as winds approach cut-out speed will increase dramatically.
Turbulent Conditions and Their Load Effects – The Field Data
In standard wind conditions, the normal running of a turbine experiences small normal torque variations (Figure 4)
The torque during normal operation sees small changes in torque loads. This variation is minimal and is linked to slight wind changes, control system operations and other similar events and is readily handled by the control system and by the power converter.
In a more dynamic wind environment, the torque oscillations can be significantly higher (Figure 5)
The graph shows a non-stopping event can still cause a torque load above 200% of nominal torque. As seen in the plot, the turbine has significant load variation, caused by turbulence.
Aggregate data (Figure 6)
The data, when reviewed in aggregate, is even more concerning. In standard operation (no stopping events) over a 30-day span, swirling winds and turbulence are causing significant drive-train torque variances (up to 250% of nominal) and lead to potential damage to bearings and gearing.
During an event (Figure 7)
The plot shows what happens when a fault occurs in the same turbine. There is a large forward spike, followed by repeated torsional reversals. These are not the worst-case events, and, potentially, can be increased in magnitude at higher wind speeds.
An Asymmetric Approach to Torque
A standard torque limiter design has one setting, forward and reverse. This means that it is likely to provide little or no protection against a torsional reversal. With the variability inherent in the wind, the best approach would be to take an asymmetrical approach to torque control.
Asymmetric torsional control (ATC) means having different torsional characteristics in reverse versus forward. In the normal forward mode, ATC provides exactly the same power production and torsional profile. It is only during a transient torque reversal that it reduces rapid high magnitude torque reversal load spikes.
Wind Turbine Controls Can Only Do So Much
In order to maintain safe operations in low-wind optimised turbines, the control system must maintain a higher level of control than their counterparts with shorter blades. Unfortunately, in shear winds and in turbulent flow, these pitched blades may actually catch more wind than desired. Most control systems are designed to react to data inputs captured over multiple seconds or minutes, not the fractions of seconds needed to protect against a sudden transient reversal. Even with optimal response time in the controls, the reaction of a mechanical system takes time. Also, if the turbine reacts too quickly, it can actually increase the risk of reverse torsional events. Regardless of the control’s reaction, the additional energy and loading will have to be absorbed somewhere in the drive-train of the turbine.
Figure 8 is a torque trace from a fairly basic stopping event in a common 1.6MW turbine. As shown, forward loading oscillations have significant magnitude, leading to fatigue in drive components. The torque reversal at the end is an impact load on the bearings.
With an asymmetric torque control in the system (Figure 9), the damaging loads are much more manageable. The magnitude and the frequency of the shock load has been reduced and the reversal has been eliminated completely.
Figure 10 overlays both controls and shows that an asymmetric mechanical torque control is a major improvement in the reduction of drive-train loads. This is for a relatively minor stopping mode. In a full e-stop or other major event, these loads would be significantly higher.
Conclusion
As rotors grow, turbines will see a significant growth in transient loading due to the larger swept area. Current control systems cannot react fast enough to mitigate sudden torque reversals. Even with greater control sophistication, the response time of the mechanical or aerodynamic systems cannot mitigate all the damaging loads.
Asymmetric torque control mitigates sudden reverse loads. As a torsional shock absorber in the system, an asymmetric approach can quickly reduce the transients. The wind-up of the system can be reduced, thus preventing high damaging loads.
Reducing reverse transient loading is crucial to reducing costly O&M expenses during the life of the turbine. A holistic approach to turbine health must be a part of industry practices, including controls, gearbox and bearing design, aerodynamics and asymmetric load controls. Only when all factors are addressed will we see turbines optimising their investment.
