Insatiable demand for renewable energy sources has led to major technological strides in wind-power development. Consequently, a vast number of new wind turbines installed worldwide over the past 20 years now offers fresh avenues of electrical power generation.

Today, two different types of turbines dominate the market, both employing a three-bladed rotor with a horizontal axle of rotation. The first uses a mechanical transmission to transfer the slow-turning shaft of the turbine rotor into a higher rotational speed to drive a generator. The second doesn’t require mechanical transmission — a huge generator directly uses the high torque and converts it to electrical energy. In both cases, the generator’s rotation — and, therefore, the frequency of the electricity produced — are coupled with the turbine.

A major issue with these designs is that the turbine’s variable rotation speed requires a frequency converter to connect each turbine to the power grid. Other issues include reliability problems with mechanical gearboxes and the weight of the directly driven generator due to increasing turbine sizes. 

A new concept — transferring power via a hydrostatic drivetrain — combines good efficiency and grid stability with high reliability and low costs. In a research project funded by the Federal Ministry for the Environment, Nature Conservation, and Nuclear Safety of Germany, the Institut für Angewandtes Stoffstrommanagement (IFAS), Hamburg, developed a prototype of a hydrostatic transmission for 1-MW power-class wind-energy plants that’s intended to replace the commonly used gearbox and the frequency converter.

The basic design employs a slow-turning pump connected to the turbine shaft to transfer the power into a high-pressure oil flow. Hydraulic motors then convert the oil flow back into mechanical power to drive a generator. The high transmission ratio needed in a turbine can easily be achieved by the displacement ratio of the pump and motor. Using a variable-displacement motor allows varying transmission ratio so that the generator runs at constant speed.

A key requirement when scaling a transmission is to have good efficiency at rated power as well as in partial load, which is where the turbine operates most of the time. The rotor also influences the total power output because energy captured from the wind can be optimized by adjusting the turbine’s rotational speed to the actual wind conditions. Figure 1 (over wind speed and over rotational speed) shows that a specific rotational speed maximizes the captured power at each wind speed. At the same time, these points of operation are not at maximum torque. To achieve peak power production of a wind turbine, the drive train should be designed for the optimum points of operation while using a control strategy to ensure operation in these points. Therefore, a compromise must be found between steady-state operation and controllability.

Transmission design

The initial point for dimensioning of the hydrostatic transmission was to select a wind turbine that provides a torque curve over wind and rotation speeds. Previous simulations proved that switching a hydrostatic transmission’s single pumps and motors increases the overall efficiency in partial load. Subsequently, different combinations of pumps and motors were analyzed, which led to the creation of a new transmission design, Figure 2.

Two radial-piston pumps, with a total displacement of 66 liter/rev, drive three variable-displacement motors and one constant-displacement hydraulic motor. The four motors are mounted to two generators. In partial load, the pump with 80% of the total displacement can be switched off by opening a valve to low pressure. Because a reduced flow rate is available to the motors, three of the motors are switched off at this point in operation.

Not all component data for the simulation was available for this project. Therefore, a test bench was built to measure and validate static and dynamic parameters of individual components as well as the entire transmission, Figure 3. A hydrostatic power feedback was installed to operate the test bench efficiently as well as avoid a 1.2-MW electric motor and 1-MW generator. It allows using the output power of the transmission to drive the slow-turning input shaft.

Two electric motors, powering two axial-piston pumps (A1 and A2), feed in losses of the transmission and the drive. A radial-piston motor (A3) represents the wind turbine and drives the slow-turning shaft. Output power from the transmission is fed back to the electric motors. In this way, the electric power consumption is only 400 kW (2 × 200), whereas 1 MW can be applied on the turbine shaft.

Rotational speed and torque at all transitions of power across the system boundaries of the transmission is measured to evaluate the overall efficiency. In steady-state measurements, rotational speed is controlled by the transmission’s hydraulic motors, and the test bench drive applies a specific torque based on the displacement of the two axial-piston pumps.

Efficiency results

Initially, the transmission’s overall efficiency was measured in the point of operation of the turbine marked with the line in Figure 1. However, due to the possibility of adapting the transmission to the transferred power, measurements also were performed in a different configuration.

Overall efficiency from the slow-turning shaft to the output of the hydraulic motors was plotted versus turbine rotation speed, Figure 4. When activating both pumps and all motors, overall efficiency at maximum rotation speed is 85%. A decreasing rotational speed also decreases the transferred power, and constant losses in the system reduce overall efficiency.

Figure 4 indicates that when turbine shaft speed drops below 16 rpm, it’s best to switch off one of the generators and use only two motors. At even lower speed and much lower torque, one pump can be set to idle mode, leading to an increased system pressure and improved higher efficiency.

Another measurement in the plot shows that when the smallest motor is in operation, and another one is still connected to the generator shaft, it causes a constant amount of drag losses. In this case, the overall efficiency is worse compared to activating the motor and using two motors at a much lower displacement. For component scaling, this means that the rated power of the generators should be adjusted to the size of the pumps. Thus, a pump and a generator can be activated simultaneously to avoid drag losses of idling motors.