Wind power hydraulic system
A wind turbine has many rotating parts. The nacelle rotates horizontally, following the wind. The rotor rotates along a horizontal axis to generate power. In a variable pitch wind turbine, the blades that make up the rotor rotate around a central axis at their root to adapt to different wind conditions. When the turbine stops, the blade tips are thrown out to create damping. The hydraulic system is used to adjust the blade pitch, damping, and for stopping and braking.
1. Drive system
Wind turbines utilize two drive systems: a braking system (including the yaw and main shaft high-speed shaft rotation system) and a blade angle control and nacelle yaw rotation control system. The braking system is hydraulically controlled, while the blade and yaw control uses either hydraulic or electric drive. The debate over which drive system to use is no exception in wind turbine design. Whether to use hydraulic or electric power to control blade angle, output power, speed, or frequency response generally depends on the manufacturer's experience.
2. Pitch Control System
The design of the blade angle (pitch) control system should primarily consider the immediate shutdown of the wind turbine unit when encountering strong winds such as typhoons, thus interrupting power supply. In this situation, the blades need to be controlled to be parallel to the wind direction to ensure they no longer rotate. After the power interruption, the unit's energy storage system, such as a hydraulic accumulator or battery, will activate. For hydraulic control, a hydraulic linear actuator (hydraulic cylinder) is used; for electrical control, an electro-rotary actuator is employed. The hydraulic linear actuator, mounted inside the main shaft, and the accumulator used during shutdown are also mounted inside the shaft.
Foreign hydraulic linear actuators are hydraulic linear drive devices (Electro-hydraulic system), which integrate the advantages of hydraulics, electronics, and electricity. This type of system is energy-saving and worth promoting.
This integrated electro-hydraulic servo drive system, composed of hydraulic cylinders, hydraulic pumps, AC motors, accumulators, solenoid valves, sensors, and a power source, boasts advantages such as excellent dynamic performance, high output power, and easy electrical installation and maintenance. It reduces the drawbacks of hydraulic systems, such as oil leakage and contamination, significantly improving reliability. Furthermore, when power is interrupted, the advantages of hydraulic transmission are fully demonstrated: the hydraulic cylinders connected in series with the accumulator receive oil from the accumulator, aligning the impeller blades with the windward side and stopping the impeller's rotation. The hydraulic system is directly supplied with oil by hydraulic cylinders equipped with position sensors and a bidirectional gear pump, eliminating intermediate valves and reducing pressure loss and leakage points. This system is over 40% more energy-efficient than servo control systems.
Besides the aforementioned hybrid systems, linear electro-hydraulic servo proportional cylinders and rotary hydraulic proportional servo drive motors are also used abroad for blade angle control and yaw rotation. These systems have advantages such as good dynamic and static performance and long service life, but they are inferior to hybrid systems in terms of energy saving and oil contamination.
Currently, major global companies widely adopt proportional servo closed-loop control systems for wind power hydraulic systems. Parker Corporation (AAA) provides various hydraulic components and complete wind power systems (including control systems for braking, yaws, and blade angles) for wind power generation. The angle control system consists of specially designed hydraulic cylinders installed inside the wind turbine hub. Position sensors are installed within the cylinders, and the necessary hydraulic valves are integrated. Each wind turbine has two or three independent angle control systems (one per blade). This system offers high reliability and safety, good dynamic and static performance, convenient maintenance, and low leakage. The system uses high-performance proportional servo control and can be controlled by analog or digital signals. The valve assemblies provided by Parker undergo rigorous pre-testing, reducing installation and commissioning time, lowering costs, and saving on operation and maintenance expenses. The hydraulic power source is provided by a separate hydraulic station with excellent filtration performance. The yaw rotation system effectively maintains the blades correctly aligned with the wind direction, ensuring excellent wind power generation performance. Parker offers both electro-hydraulic and hydraulic control systems. The hydraulic system enables more compact direct drive and features excellent overload protection to prevent component damage. It employs closed-loop proportional servo control, providing superior dynamic and static performance. To complement these three systems, Parker also provides an independent, well-filtered hydraulic power source powered by an accumulator during power outages, ensuring safe shutdown and unit safety.
Eaton Corporation of the United States has done a lot of research on hydraulic control systems for wind power generation. The closed-loop proportional control system for wind turbine blade angles that it provides can withstand high and low temperature working conditions, and the system has good dynamic and static performance and high positioning accuracy.
Bosch Rexroth, a German company, is a supplier of hydraulic and electrical systems for wind power generation in Europe. They can provide complete sets of gearboxes, braking systems, rotor blade control systems, and yaw control systems for wind turbine units. Electrical control systems and hydraulic proportional servo closed-loop systems can be provided according to customer needs. Hydraulic drive systems are widely used in large wind turbine units.
Example of a pitch control system:
Z-40 hydraulic pitch control mechanism from Zond Corporation, USA
This hydraulic pitch control mechanism is an electro-hydraulic servo system. The typical principle of a pitch control hydraulic actuator is shown in the diagram above. The blades are connected to the hydraulic cylinder via a mechanical linkage mechanism. The pitch angle change is basically proportional to the hydraulic cylinder displacement. When the hydraulic cylinder piston rod moves to its maximum position to the left, the pitch angle is 88°, while when the piston moves to its maximum position to the right, the pitch angle is -5°. During normal system operation, the two-position three-way solenoid directional valves a, b, and c are all energized, the hydraulic check valve opens, and the displacement of the hydraulic cylinder is precisely controlled by the electro-hydraulic proportional directional valve. When the wind speed is lower than the rated wind speed, regardless of wind speed changes, the electro-hydraulic proportional directional valve maintains the blade pitch angle at 3°. Considering cylinder leakage, the electro-hydraulic proportional directional valve is fine-tuned to keep the pitch angle constant. When the wind speed is higher than the rated wind speed, the output flow rate is precisely changed using the electro-hydraulic proportional directional valve according to the output power, thereby controlling the blade pitch angle and keeping the output power constant.
3. Hydraulic braking system
The nacelle and main shaft high-speed shaft rotation system employs a hydraulic disc brake. This system is used for braking blades with diameters of 60–100 μm. Sudden braking can cause severe vibrations in the blades and the rotation system, generating significant loads. Therefore, feedback on the shaft speed is necessary, employing a method of adjusting the braking pressure by changing the amplitude (soft braking), which can reduce the load by several times.
Parker, Eaton, and Rexroth also manufacture disc-blade braking systems, which can withstand harsh conditions and offer high safety. They feature low leakage, small size, space saving, and are powered by a separate hydraulic station.
Example of a hydraulic braking system: Denmark's BONUS-150KW wind turbine brake hydraulic system
1) Start-up: When the control system issues a start command (automatic or manual), the motor starts immediately. Pressure enters the valve block through port "P". The left half of the valve block supplies pressure to the blade tip; the right half supplies pressure to the disc brake. Simultaneously with motor start-up, solenoid valves 10# and 11# are energized and change from on to off. Pressure oil can only enter the right half through check valve 6#-2. When the pressure reaches 10.3 MPa set by pressure switch 7#, valve 10# opens, and pressure begins to enter the blade tip, causing the blade damping plate to retract. At the same time, solenoid valve 12# opens and solenoid valve 13# closes, relieving pressure on the disc brake and preparing for start-up. After the blade tip retracts, the disc brake is also released. When the pressure at pressure switch 15# reaches 7 MPa, the motor stops rotating. Units 17 and 18 are accumulators that use compressed gas to store the energy in the pressurized oil, in order to compensate for the leakage caused by the blade tip resistance plate and the circular gate disc during operation, and reduce the frequency of motor starting.
2) Braking and stopping: When the wind power control system issues the stop command, solenoid valve 10# is immediately energized and solenoid valve 11# is de-energized. Solenoid valve 10# is closed and solenoid valve 11# is opened. Then solenoid valves 12# and 13# are de-energized, i.e., solenoid valve 13# is opened and solenoid valve 12# is closed. As a result, after the blade tip damping plate is ejected, the disc brake also acts to stop the wind turbine smoothly.
3) Performance characteristics
From a design perspective, the braking torque of this wind turbine comes from two sources: tip damping braking and disc brake braking. Both braking torques are applied to the low-speed shaft, thus minimizing the impact on the gearbox during braking. In addition, it has the following characteristics:
a) The braking process is smooth with little vibration. This is because the speed is reduced first by the damping poles of the three evenly distributed blades, and then by the disc brake. This makes the braking process relatively smooth.
b) Independent braking mechanisms: The two braking mechanisms of this braking system are independent of each other, meaning that the failure of one braking system will not cause the other to lose its working ability. For example, when the hydraulic system fails and the pressure cannot be built up, and the disc brake cannot brake normally, the blade tip damping plate is popped out due to the loss of pressure, which plays a damping braking role, thus enhancing the reliability of the braking system.
c) Equipped with a stall protection mechanism: The braking system has a centrifugal pressure cylinder installed at the root of the blade. When the wind turbine loses control and runs away, the centrifugal force causes the pressure to rise to the set value of pressure switch 14#. Switch 14# is activated and opened, the runaway safety valve 16# is connected, the pressure in the blade tip damping plate is released, and the damping plate pops out, thus playing a protective role.
