Besides overall drive train efficiency, the effectiveness of capturing wind power dramatically influences the total power production. To achieve this goal, the blades’ optimal rotational speed must be adjusted by the drive train to the actual wind speed, again, as shown in Figure 1.

The turbine’s rotational speed can be adjusted by varying the displacement of the hydraulic motors. Because the generators operate at grid frequency, their rotational speed must be constant. Moreover, the motors’ swashplate angle can define the system’s flow. On top of that, the pumps have a constant displacement, leading to a proportional correlation of flow rate and rotational speed.

When opting for this strategy, the controller would have to use the actual wind speed and specify the optimal rotational  speed according to the characteristics of the blades. The motors would then adjust for the desired rotational speed.

However, two major challenges arise with this approach. First, the actual wind speed is difficult to measure on a real wind turbine. The blades cover a large area in which the wind varies in speed and direction. A single wind-speed sensor mounted on top of the nacelle can hardly detect all these effects, so it will deliver only approximate values. The second challenge concerns the large inertia of the turbine itself, which makes it difficult to change the rotational speed in response to fluctuating wind speed and direction. Each short gust of wind would lead to a short acceleration of the turbine initiated by the controller. In this phase, all of the incoming power would be stored in the rotor’s inertia, leading to interrupted power production. After the gust, the controller would decelerate the rotor again and have an increased power output for a short time.

These fast load changes are possible with the hydrostatic transmission, but it complicates adapting the system to the actual load. When using the switching strategy presented in Figure 4, each harsh change of the braking torque involves the switching of several components. This type of operation must be avoided in order to maintain system reliability.

Control of transmission load torque

The wind turbine’s rotational speed also can be adjusted by controlling the braking torque of the transmission. If the rotational speed is too high, a defined amount of additional braking torque could be added to decelerate the turbine. However, the controller needs information about the actual wind speed to define the optimal rotation speed.

One way to avoid wind-speed measurement is to determine the wind situation according to the turbine’s acceleration. Such a method is used in a control strategy where the controller of the hydrostatic transmission applies a braking torque to the turbine based on the actual rotational speed (assuming an optimal point of operation). Figure 5 shows the torque equilibrium on the inertia of the rotor. The braking torque applied by the transmission on the right is independent of wind speed. When wind speed is higher than assumed, it captures more torque than discharged by the transmission, which accelerates the turbine.

The left side of Figure 6 represents turbine operation, which was accomplished by maximizing both plots in Figure 5. A groove between the two surfaces occurs in which the turbine will operate in steady state. The marked arrows in the plot show the reaction on a gust of wind. Whenever a fast gust occurs, the captured torque will accelerate the turbine. As rotation speed increases, the transmission also increases the braking torque of the drive train. Once the gust fades, the braking torque becomes too high, which decelerates the turbine to the groove again. By varying the parameter of the torque curve deposited in the controller this groove can be adjusted to the optimal point of operation in which the overall efficiency of turbine and drivetrain is optimal.

The two plots on the right in Figure 6 show a simulation result of the controller. The upper curve describes the applied wind speeds on the simulated turbine, leading to the points of operation marked below. All points accumulate around a straight line defined by the groove shown on the left.

Unlike the motor-displacement-based control strategy, the torque-based control strategy makes power production smoother because the turbine’s inertia is used as flywheel to store peak power from the wind. No problems are encountered when switching between different transmission configurations since the braking torque can only change as fast as the rotation speed. Most importantly, the strategy is based on the rotation speed of the turbine, which can be measured precisely and cost effectively.

Validation of dynamic behavior

To verify the controller’s simulation results on the test bench, the tests must include the inertia of the turbine. Of course, it’s impossible to install such a huge flywheel directly on the test bench; therefore, it’s done virtually in a real-time simulation. Figure 7 shows how the simulated inertia is coupled to the test bench. The initial point is a torque signal that’s applied on the model of a flywheel. The inertia’s rotation speed is sent to the test-bench drive, where a rotation-speed controller sets the same rotation speed on the test bench.

The braking torque of the hydrostatic transmission is measured at the slow-turning shaft and transmitted into the simulation to be applied on the inertia as load torque. In the measurements described below, all units of the hydrostatic transmission will be in operation. However, two different setups are used: The first has all motors set to a constant displacement; the second activates a controller for the braking torque (discussed previously).