Job Sheet 6 Pitch Control

Transcription

1 Job Sheet 6 Pitch Control Not all wind can be captured as energy by the wind turbine. Some wind has to pass by the blades for the rotor system to function properly. There are limits to how fast motors and rotors optimized to produce maximum power output at the most probable wind speeds around 15 m/s. In stronger winds, it is necessary to waste excess wind energy in order to avoid damaging the wind turbine. Therefore, all wind turbines are designed with a basic power control that allows the rotor blade pitch angle to be adjusted. The rotor blade pitch angle is adjusted for two important reasons: safety and control. The conditions. The safety function performs as an aerodynamic brake and rotates the turbine blades to a non-rotating (feather or stall) condition in wind speeds that exceed safe rated speeds, or in other fault conditions. The controllers in these turbines perform aerodynamic braking by individually controlling the pitch of each blade. Applying the full braking effect to the rotor requires pitching all three blades into the wind at a 90-degree position. These systems have inherent redundancy. This means that if an individual blade pitch unit fails, the other two rotor blades complete the braking process safely. Each of the autonomous pitch systems include independent back-up battery packs or spring units that are used to provide power to each individual blade pitch system to feather the blades, or shut down the turbine in the event of a total power failure. The battery packs of the emergency power supply are also used for maintenance purposes. The pitch system is central to the turbine s safety features, and is highly dependent on an effective energy storage system for both operation and emergency power. The pitch system is located in the rotating central hub of the wind turbine. The power supply and control signals for the pitch systems can be transferred by a slip ring from the non-rotating part of the nacelle, or from the stationary enclosed pivot behind the hub. The slip ring is connected to a central control unit and control signals for the three individual blade drive units. In older wind turbines, some of these controls are performed mechanically; in recent designs, they are performed hydraulically or electrically using stepped up motors. Power control systems Passive Stall-controlled Wind Turbines Stalling works by increasing the angle at which the relative wind strikes the blades (angle of Festo Didactic Inc

2 wind speed becomes too high, turbulence is created on the side of the rotor blade, which is not facing the wind. This stall prevents the lifting force of the rotor blade from acting on the rotor. Rotor blades on a stall-controlled wind turbine are twisted slightly to ensure that the rotor blade stalls gradually rather than abruptly when the wind speed reaches its critical value. The basic advantage of a stall control is that it avoids moving parts in the rotor and it does not deal with a complex pitch-control system. On the other hand, it requires a very complex aerodynamic and engineering design to avoid stall-induced vibrations. Active Stall-controlled Wind Turbines mechanism. Technically, active-stall wind turbines resemble pitch-controlled turbines because their blades can be pitched. In order to get a reasonably large torque (turning force) at low wind speeds, these turbines are programmed to pitch their blades much like a pitch-controlled system at low overloaded, the control system pitches its blades in the opposite direction. This means that it increases the angle of attack of the rotor blades in order to make the blades go into a deeper stall; thus wasting the excess energy in the wind. One of the advantages of active stall is that it can control the power output more accurately than a passive stall system, avoiding the exceeding rated power of the turbine at the beginning of a wind gust. Another advantage is that the turbine can be run almost exactly at rated power at all high wind speeds. Passive stall-controlled wind turbines usually experience a drop in electrical power output for higher wind speeds because the rotor blades go into deeper stall. Mechanical Pitch Control There are creative methods of controlling the pitch or the angle of attack that a rotor airfoil presents to the wind stream. In a spring-operated mechanism, the higher rotational speed of the rotor generates a centrifugal force on a regulating balancing weight which compresses a spring. The force of the weight rotates the blade around a pivot, decreasing the angle of attack of the airfoil to the wind stream and reducing its rotational speed. The compressed spring tends to restore the airfoil to its original angle of attack once the wind speed decreases. 88 Festo Didactic Inc

3 Advanced Pitch Controls On a pitch-controlled wind turbine, the turbine s electronic controller checks the power output order to the blade pitch mechanism, which immediately pitches (turns) the rotor blades slightly out of the wind and protects the system from excessive stress. The controller generally pitches the blades a few degrees every time the wind changes in order to keep the rotor blades at the optimum angle in order to maximize output for all wind speeds. the wind drops again. This sophisticated control scheme allows the turbine to operate with reduced maintenance and longer turbine life. Hybrid Pitch Control turbine s blades are turned electrically and the fail-safe, which prevents damage to the blades, runs hydraulically. Potential risk of leaking oil is mitigated as the pitch control relies mostly on rely on a hydraulic system for fail-safe power, the possibility of a lost fail-safe battery charge is not a consideration. Variable Speed vs. Fixed Speed up sequence, the rotor may be held parked (stopped) and once the brakes are released, it to the electricity grid is made and the grid (through the generator) holds the speed constant. regulated by stall or by pitching the blades. The application of the variable speed technology allows the rotor and wind speed to be rotor is connected to the grid at low speeds in very light winds and speeds up in proportion to wind speed. As the rated power is reached, the rotor reverts to nearly constant speed operation, with the blades being pitched as necessary to regulate power. Other Power Control Methods The following power control methods include a combination of pitch and stall control: Constant speed turbines (stall-pitch control) Combined stall-pitch control is used on constant-speed turbines. At low and medium wind speeds, the blade pitch setting is slowly Festo Didactic Inc

4 speed is reached, the blades are adjusted to a more negative pitch setting, leading to an aerodynamic stall and releasing the excess power. At higher wind speeds, the pitch angle Variable speed turbines (pitch-stall regulation) Combined pitch-stall control is used on variable speed turbines. At low and medium wind speeds, the blade pitch setting is slowly speed is reached, the blades are adjusted to a more positive pitch setting, reducing the aerodynamic forces and maintaining the power level programmed into the turbine controller. At Flap power control in an aircraft. In this design, the geometry of the wing airfoil is altered to provide increased or decreased air lift. Rotor Monitoring As turbines become larger, operational and maintenance issues become much more important. Rotor monitoring systems (RMS) allow early detection of several issues, including: Presence of ice: ice build-up on a turbine blade. Blade damage: damage impacting the structural or aerodynamic performance of the blade. Rotor imbalance: mass and aerodynamic imbalances that result in non-optimal energy generation, or excessive vibration and wear. Electronic Control Systems Modern wind turbines feature electronic control systems with variable speed operation of the generator and active pitching of the blade for power regulation. Both systems are controlled by sophisticated power electronics. The control system monitors the generator speed and torque: torque is adjusted by controlling the current in the rotor winding in order to maintain turbine operation on the optimal speed-torque curve for maximum energy capture. At speeds above rated, the blade pitch is regulated to try to maintain constant shaft speed, while the torque is maintained at rated torque. This maintains constant power operation. 90 Festo Didactic Inc

5 Collective Pitch Systems settings of all blades are substantially equal and controlled simultaneously. The collective pitch is executed mechanically through revolute joints on each blade connected to a hydraulically actuated cam. Collective pitch systems may also use separate electromechanical drives on each blade. The pitch bearings on the turbine are geared on the inside. The pitch angle of each blade is controlled by a geared AC motor driving a pinion meshed with the bearing gear. In collective pitch systems, all three blades are maintained at approximately the same pitch setting at all times. Most advanced control and monitoring systems are designed to help: improve energy capture. reduce extreme or cyclic load fatigue, and damage accumulation of components. reduce reactive maintenance. reduce component failures. limit other factors that can result in reduced turbine life. Control schemes may include the following: Coupling blade pitch and generator torque control Tracking tower vibration feedback control Tracking independent blade pitch control for asymmetric load control Tracking load-limiting control Tracking alternative yaw control strategies Tracking damage monitoring and feedback control Tracking drivetrain damping control Electrical Pitch Control System The rotor may utilize three (one for each blade) independent electrical pitch motors and controllers to provide adjustment of the blade pitch angle during normal operation. The blade pitch angle is adjusted by an electric drive that is mounted inside the rotor hub and is coupled to a ring gear mounted to the inner race of the blade pitch bearing. In order to minimize the asymmetric loading on blades, an individual pitch control (IPC) is used. An IPC is a system that dynamically adjusts the pitch of each blade many times per rotation. Festo Didactic Inc

6 This system optimizes the loading on the rotor and eliminates bending movements that may damage the blades, drivetrain, and tower. In modern systems, blade pitch control actuators are driven by AC brushless servomotors. Controlled fail-safe operation reduces stress on blades in case of extreme wind conditions. by allowing the blade to spill excess aerodynamic lift. Also, when energy from wind gusts of below-rated wind speed is captured, the active-pitch controller allows the rotor to speed up and transform gust energy into kinetic, which may then be extracted from the rotor. Hydraulic Pitch Control System Hydraulic control systems can provide power for pitch control and other functions, such as yaw drive or brake systems operation. A hydraulic pitch system acts as a main brake in severe weather conditions or extreme wind speeds, helping to minimize the risk of structural failure and/or accidents. The hydraulic power unit is the heart of the hydraulic system, supplying the power necessary to operate pitch, yaw, and braking systems. Customized hydraulic manifold blocks simplify installation and servicing. These can include in cartridge valves offer many advantages over traditional hydraulic valves. Besides being compact, reliable, and economical, multiple cartridge valves can be combined in a common manifold block together with pilot pistons and power control electronics to provide an accurate control of the blade pitch. Hydraulic actuators installed inside the hub interact with transducers that work with the communication interface to control and reduce piston speed when approaching full extraction during the blade pitch. Hydraulic accumulators play a vital role in the hydraulic system. They provide backup pressure to keep the system pressure constant. Hydraulic pitch systems may use power packs installed inside the hub. Power packs can be mounted on bearings; thus, the hydraulic tank doesn t move as blades rotate. This means all high-tension power cables are stationary, reducing most of the slip rings and the size of the drill-hole through the gearbox, as well as eliminating the need for pipes running through the shaft. 92 Festo Didactic Inc

7 Pitch Actuator Pitch Accumulator Pitch Power Pack Figure 6-1. Hydraulic Pitch Control System. Electrical and Hydraulic Pitch Control Systems: Advantages and Disadvantages systems have no environmental issues, which can arise with a system relying on oil under high pressure. There is also lower consumption, or energy waste. Electrical pitch control systems tend to consume less power than hydraulic ones because the latter require a pump running at all times. That pump draws energy to keep the system s oil at high pressure and to turn rotor blades when necessary. In electrical pitch control systems, however, the fail-safe batteries or capacitors are a weakness. Another advantage of electrical pitch control systems is that they are better for colder climates. The oil in hydraulic systems loses viscosity as the temperature decreases. short start-up time after an emergency stop. The wind turbine is ready to go into production again once the emergency accumulators have been recharged. Oil cooler is integrated in the power unit when the wind turbine is working in a high-temperature environment. that relate to the use of hydraulic systems, including individual power backup via emergency accumulators to each pitch cylinder. In addition, hydraulic pitch systems are not sensitive to hot climate conditions and it is possible to heat accumulators with heating mats under cold climate conditions. Festo Didactic Inc

8 Table 4-1 presents a clear comparison of the advantages and disadvantages of both hydraulic and electrical pitch control systems. Design/Composition Hydraulic One hydraulic power unit (HPU) in nacelle Three actuators with control valves and accumulators in the hub Electrical Three sets of motors/gears, drives, controllers, and energy storage unit. Three to eight switchgear cabinets depending on what functions are assembled in one cabinet. Strengths High forces Low energy consumption Weakness Gearless No backlash Fail-safe powered by accumulator possible oil leakage and oil replacement and oil replacement Fluid rotary unit required Accumulator pressure loss Quiet operation Maintenance of batteries Backlash Increased probability of failure due to higher number of components (energy storage unit, motor/gear, controller) Cost Lower initial cost Higher initial cost Maintenance Working environment Best known as Higher running cost due to higher maintenance cost Cylinder seals needed to be replaced every 7 to 10 years Noisy due to pump Risk of oil leakage Highly reliable due to proven fail-safe functionality Lower running cost due to lower maintenance Largely maintenance-free except battery change Little room for movement in hub Environmentally friendly system Table 4-1. Electrical and Hydraulic Three-Blade Pitch Control Comparison Table. Limit Switches Limit switches are real-world condition sensors that are widely used in automated electrical pitch control systems (Figure 6-2). It is an electrical device that operates (opens or closes) when a physical position or limit is reached. In a wind turbine pitch control system, this occurs when the rotor blade pitch angle has reached its maximum rotation limit. 94 Festo Didactic Inc

9 Figure 6-2. Limit Switch. Limit switches are often used on machines that have moving tables so that, when the table travels to a certain point, the limit switch is activated and the programmable logic controller (PLC) knows that the table must stop or be reversed. In the case of a wind turbine pitch control, the limit switch tells the PLC when the rotation of the blade has reached a certain limit. In a hub and an adjustable stop is positioned to trigger the limit switch when the end of the desired travel is reached. Hydraulic pitch systems may incorporate a pitch position sensor in the actuating cylinder. They typically include a linear displacement transducer (LDT) that consists of a specialized transformer (Figure 6-3). It outputs an AC voltage level that is proportional to the linear position of the acuator on which it is attached. The LDT is located on the back of the hydraulic cylinder. The LDT sends signals to the PLC via an analog input I/O module to indicate the speed and position of the linear actuator. Figure 6-3. Cylinder Linear Displacement Transducer (LDT). Fail-Safe The term fail-safe is used to describe a system designed so that it is capable of compensating automatically and safely for a failure such as that of a mechanism or power source. A key component of a wind turbine fail-safe system is its pitch control system and supporting power supply. During extreme conditions, such as periods of excessive wind, the PLC control system detects and initiates a fail-safe procedure to protect the turbine from the damaging effects of high winds. Overspeed of the turbine can damage the blades due to the excessive stresses caused by high-speed rotation, and can damage either the generator or gearbox by leading to excessive vibration. Festo Didactic Inc

10 the amount of interaction with the wind and slows the turbine s speed. A high-speed shaft overspeed condition may also be monitored to provide backup for a failing wind speed indicator. If the high-speed shaft exceeds a given set point, the shutdown can also be initiated. Depending on the program, the wind turbine may also simply feather the blades to a full 90º position, and shutdown until a reset or a restart command is given. Another fail-safe system employed on an electrical pitch control system is an uninterruptible power supply (UPS) electrical source backup system. If the grid power feeding the pitch control system during operation is lost, the UPS can provide the electrical energy feed long enough for the blades to be returned to a safe, feathered 90º angle. Hydraulic Unit Fail-Safe The hydraulic pump provides pressure to the system and assures the accumulator is permanently charged. In the event of an electrical power failure or a hydraulic pump stop, providing the required pressure for a smooth fail-safe valve operation (de-energizing), and the consequent homing of the cylinder piston rod. In normal conditions, the actuator receives the signal to open from the closed limit and the bleed solenoid valves are energized. The hydraulic pump is started under no-load condition as a result of a delay in energizing the a spring opposed piston to drive the cylinder rod in the open direction. valve is de-energized, followed by the hydraulic pump, unless a new signal to open is given. The bleed solenoid valves remain energized in order to maintain the system pressure and hold the actuator position. But in the event the actuator receives the signal to close or an ESD (Emergency Shutdown) signal a fail-safe procedure (Figure 6-4) is activated. The by-pass solenoid valve, the bleed solenoid valves and the hydraulic pump are de-energized. Pressure is released and the attached LDT (Linear Differential Transformer), the cylinder piston rod retracts to return to the predetermined safe position, and will be ready to operate on the next received signal when the ESD signal is reinstated. 96 Festo Didactic Inc

11 Start Electric Power Supply Suspended or Hydraulic Pump Stopped? No Yes Stop Signal and Emergency Shutdown are Activated. Normal Blade Pitch Procedure PLC Send Signal to Set Pitch Angle to 90º Accumulator Provides Backup Pressure Cylinder Actuating Dir. Control Valves De-energized Current Bade Angle Determined by Cylinder Position Sensor Cylinder Rod Retracts Blade Pitch Angle= 90º No Yes Hydraulic Unit Shutdown Figure 6-4. Hydraulic Unit Fail-Safe Procedure in a Hydraulic Pitch System. Festo Didactic Inc

12 a hardware circuit by-passing the internal processor. This processor circuit monitors the position and internal hydraulic pressure and provides alarm monitoring, and the ESD signal, independent to other control signals, allows securing the valves in the safe position. 98 Festo Didactic Inc

13 OBJECTIVES In this job, you will force a fail-safe situation on the hydraulic pitch hub trainer and you will become familiar with the components involved in the fail-safe procedure. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required for this job. SAFETY PROCEDURES Before proceeding with this job, complete the following checklist. You are wearing safety glasses. You are wearing safety shoes. You are not wearing anything that might get caught such as a tie, jewelry, or loose clothes. If your hair is long, tie it out of the way. The working area is clean and free of oil. Your sleeves are rolled up. PROCEDURE Setup Perform the Basic Setup procedure described in Appendix D. Perform the Lockout/Tagout procedure described in Appendix D. Begin Perform the Energizing procedure described in Appendix D. Apply power to the training system by turning on the Main power switch. Press the green Start button. Festo Didactic Inc

14 Ensure the Emergency Stop switch is on (pulled outward). On the main screen of the hydraulic pitch hub trainer HMI, verify that there are no alarms triggered (in System Status box and Alarm button). On the Main section window, tap the Start Trainer button. The hydraulic unit starts. Tap the Manual button (to the right, below the wind vane indicator) to enable manual operation of the trainer functions. Tap the Manual menu button (on bottom of the screen). Tap the Move to Position button to expand the cylinder piston rod to its maximum length. On the Manual Blade Control screen, rotate the blade clockwise by tapping the Jog + button until you reach the 30º position. Read and record the pressure measurements on the hydraulic unit (gauge GP2 on the right): (psi). It should match the pressure displayed on the screen for PSP2. Verify this value. If necessary, adjust the reading recorded in procedure step 13. Read and record the pressure measurements on the hydraulic unit (gauge GP1 on the left): (psi). According to these pressure measurements, is the accumulator charged? Yes No Shut down the trainer. 100 Festo Didactic Inc

15 Did the piston rod retract to its home position? Yes No Explain why. If the answer to procedure step 19 is no, manually turn the SV2 valve clockwise to its unlocked position. Read and record the pressure measurements on the hydraulic unit (gauge GP2 on the right): (psi) Manually turn the SV2 valve counterclockwise to its locked position. Read and record the pressure measurements on the hydraulic unit (gauge GP2 on the right): (psi) Read and record the pressure measurements on the hydraulic unit (gauge GP1 on the left): (psi) Festo Didactic Inc

16 Compare the pressure recorded in procedure steps 22, 24, and 25. Explain these readings: Perform the Lockout/Tagout procedure (see Appendix D). Review Questions 1. Pitch control allows the turbine to a. b. c. reduce excessive loads on the drivetrain. d. Both B and C are correct. 2. In a wind turbine pitch control, the limit switch a. tells the PLC when the rotation of the blade has reached a certain limit. b. c. is mounted in an adjustable position relative to the blades. d. 3. Compared to electrical pitch control systems, hydraulic pitch control systems a. offer a shorter start-up time after an emergency stop. b. are not sensitive to hot climate conditions. c. perform better in colder climates. d. Both A and B are correct. 102 Festo Didactic Inc

17 4. In the event the cylinder actuator receives an emergency shutdown (ESD) signal from the PLC, a. a fail-safe procedure is activated. b. the hydraulic pump is de-energized. c. the cylinder piston rod retracts to the predetermined safe position. d. All of the above are correct. 5. Hydraulic pitch systems use a a. linear displacement transducer (LDT) located on a variable point relative to the hub and the actuating cylinder. b. limit switch on the back of the actuating cylinder. c. linear displacement transducer (LDT) attached to the back of the actuating cylinder. d. limit switch located in a variable position relative to the hub and the actuating cylinder. Name: Date: Instructor approval: Festo Didactic Inc