Job Sheet 2 Aerodynamics Power Control Power Control Power control is an important feature of a wind turbine. It regulates the speed of rotation of the rotor assembly when wind is present. For stand-alone or off-grid direct current (DC) systems, wind speed. For on-grid or alternate current (AC) grid-tie systems, power control can aid in maintaining a constant sinusoidal output to match the phase of the AC distribution network to which the turbine is coupled. Grid-tie turbines can also overspeed if there is a loss of load on the generator (i.e., if the connectivity to the grid is broken). The reduction in generator load reduces the resistance on the turbine hub. This, in turn, can cause a rapid acceleration of the rotor. Power controls can be used to prevent or control this acceleration in order to avoid turbine damage. The need for reliability is common to all control system designs. Many forms of power servicing are preferred. Wind turbine power control is accomplished through a system of passive control, active control, or a combination of both. Passive Power Control Passive control systems that are often found in smaller wind turbines utilize methods such as centrifugal force-controlled tip braking or unique blade designs to reduce the risk of turbine overspeed. They do not require monitoring or a control system to enable power regulation. speed occurs (Figure 2-1). These are often mechanically or electromechanically controlled. They automatically deploy and retract as the blade speed increases and decreases. Often, a sophisticated control system is not required for these systems to operate. Festo Didactic Inc. 88186-20 19
Rotary Bearing Tip Shaft Screw Air Flow Direction of Blade Rotation Tip Brake Deployed Air Flow Compressed Spring Blade Pitch Angle Figure 2-1. Tip Brake Designs. An alternate method of passive braking is utilized in passive stall-control turbines. The wind moment the wind speed becomes too high. This stall prevents the lifting force of the rotor blade from acting on the rotor. It is the result of turbulence on the side of the rotor blade that is not facing the wind (Figure 2-2). Air Flow Figure 2-2. Blade Stall. of passive stall control include the lack of moving parts in the rotor itself and the absence of structural dynamics of the turbine, leading to stall-induced vibrations. Active Power Control Active power control systems are commonly found in larger turbines and are employed in the hub trainer. Active power control systems monitor the wind through instrumentation, and then mechanically adjust the entire blade pitch or tip pitch to respond to variations in wind speed. 20 Festo Didactic Inc. 88186-20
rotor hub (Figure 2-3). This allows the blades to pivot into and away from the wind, adjusting the angle of attack. In nearly all large-scale commercial wind turbines, mechanical control systems that employ either hydraulic pistons or electrical servomotors drive the blade pitch to Pitch Angle Plane of Rotation Pitch Angle Plane of Rotation Hydraulic Piston Hub Bearing Servo Motor Hub Bearing Figure 2-3. Pitch Control System. During normal grid-tie in-service operation, the electronic controller checks the power output of the turbine many times per second. This includes the monitoring of shaft speed, pitch angle, wind velocity, and generator output. When the power output becomes too high, the controller signals the blade pitch drive mechanism to turn the rotor blades slightly out of the wind (Figure 2-4). Generator Output Wind Speed V 2 Shaft Speed V 1 Supervisor Control Angle Pitch Control Figure 2-4. Pitch Control Block Diagram. Festo Didactic Inc. 88186-20 21
Conversely, the blades are turned back into the wind whenever the wind drops. During normal operation, the blades pitch a fraction of a degree at a time to keep the rotor turning at a nearconstant speed. If a pitch control system or total wind turbine failure occurs, the hydraulic system is spring-loaded to automatically return the blades to a feathered position. Electronic pitch controls use a backup power supply to feather the blades during a failure. An increasing number of larger wind turbines use a combination of pitch and stall-control systems called active stall control. Unlike the standard pitch control that reduces pitch angle to slow the turbine, an active stall-control turbine increases the angle of attack of the rotor blades and keeps the rotor speed closer to constant. Active stall controls the output power of a turbine rated power at the beginning of a gust of wind. Active stall-control turbines can be run almost go into their designed deeper stall. In a tip pitch power control system, only the tip angle is regulated to control wind speed. This is called partial-span blade pitch control when the tip-to-blade junction is placed closer to the hub. Tip pitch and partial-span pitch control at the blade tip is limited by the reduced tip space, which makes it incorporated bearings and actuators. Although it is less effective than tip control, a partial-span pitch control section allows more space for the incorporated bearings and actuators (Figure 2-5). Therefore, it is easier to access and maintain. Tip pitch power control or partial-span pitch power control turbine are designed for stall control and require a rotor hub. Figure 2-5. Partial-span Blade Pitch Control. 22 Festo Didactic Inc. 88186-20
OBJECTIVE In this job, you will determine the basic principles and terminology of power control. You will operate the electrical pitch hub trainer, and observe how the pitch control system responds to changes in wind and power. EQUIPMENT REQUIRED 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. Instructor initials: PROCEDURE Setup Begin Then, apply power to the training system by turning on the Main power switch (Figure 2-6). Festo Didactic Inc. 88186-20 23
Figure 2-6. ESS and Main Power Switch in On Positions. Figure 2-7. HMI Main Screen. 24 Festo Didactic Inc. 88186-20
Clear any alarms that display by pressing the green System Reset button located below the Main power switch on the On-Off panel (Figure 2-8). Figure 2-8. On-Off Panel (Figure 2-9). Figure 2-9. HMI Alarms Screen. Return to the Main screen and press the Start Trainer button. Festo Didactic Inc. 88186-20 25
wind conditions. Figure 2-10. HMI Simulation Screen. proceed to the Service screen and set the system to SI units before proceding. Return to the Simul screen and set the wind simulation for each step using the values provided in Table 2-1. Use the left and right step control arrows, and tap each parameter indicator to open and Step Duration (sec) Wind Direction (degrees) Wind Speed (m/s) 1 45 35 2 45 35 3 45 35 4 45 35 5 45 35 6 45 35 45 35 16.5 8 45 35 18.5 9 45 35 14.5 45 35 Table 2-1. Wind Simulation Parameters. 26 Festo Didactic Inc. 88186-20
Return the step indicator to Step # 1. Press the Start button to start the simulation. Navigate to the Main screen and press the Start Trainer button, if necessary. Then, press the Start Auto button (Figure 2-11). Figure 2-11. Start Trainer and Start Auto Buttons. Main screen. Allow the trainer to complete the auto initialization and step into the Auto Active Control IP phase of its programming (about 1 to 2 minutes). Automatic Pitch Control Once control begins, observe the changes in physical pitch angle relative to speed changes in the wind indicator. NOTE: If the trainer halts shortly after beginning pitch control, return to step 8 to reset and restart. Festo Didactic Inc. 88186-20 27
display and record the pitch angle displayed in Table 2-2. Allow the trainer to cycle through its program to acquire any missed values and to Wind Speed (m/s) Pitch Angle (degrees) 16.5 18.5 14.5 Table 2-2. Pitch Angle Vs. Wind Speed. Plot the values you recorded in Table 2-2 in Graph 2-1. 28 Festo Didactic Inc. 88186-20
Wind Speed vs. Pitch Angle Pitch Angle (degrees) Wind Speed (m/s) Graph 2-1. Wind Speed Vs. Pitch Angle Graph. Based on your graph, what happens to the pitch angle as the wind speed increases? The pitch angle is measured from the plane of blade rotation. As the More Less Use the Main power switch to turn off all system power. Festo Didactic Inc. 88186-20 29
Lockout/Tagout Procedure Review Questions 1. What is the basic function of a power control system? 2. Why is it important to regulate turbine speed on a grid-tie wind turbine when it is connected to the grid? 3. What is the difference between active and passive power control? 30 Festo Didactic Inc. 88186-20
4. What are the two primary pitch control systems used in large commercial wind turbines? 5. What is meant by active stall control? Festo Didactic Inc. 88186-20 31
Name: Date: Instructor approval: 32 Festo Didactic Inc. 88186-20