EET-216 W8 LAB # 7 - Wind Turbine PDF

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Electrical Engineering Electrical Engineering Technician AMAT/ SETAS Course: EET-216 DRAWINGS & INSTALLATION 3 Names: 1. _Deanne Aira P. Pimentel____________________________ 2. _________________________________________________________ 3. _________________________________________________________

Lab # 7 Title: Wind Turbine Make sure that you are wearing appropriate protective equipment when performing the tasks. You should never perform a task if you have any reason to think that a manipulation could be dangerous for you or your teammates.

Introduction The production of energy using renewable natural resources such as wind, sunlight, rain, tides, geothermal heat, etc., has gained much importance in recent years as it is an effective means of reducing greenhouse gas (GHG) emissions. The need for innovative technologies to make the grid smarter has recently emerged as a major trend, as the increase in electrical power demand observed worldwide makes it harder for the actual grid in many countries to keep up with demand. Furthermore, electric vehicles (from bicycles to cars) are developed and marketed with more and more success in many countries all over the world. This Introduction to Wind Power lab, covers how a permanent magnet wind turbine produces electricity from wind power, and how Maximum Power Point Tracking controllers are able to adjust the blade velocity of variable-speed fixed pitch wind turbines.

The Discussion of this exercise covers the following points: • • • • • • • • •

Air density Kinetic energy in the wind Calculating wind power Relationship between wind power and wind speed Relationship between torque, rotation speed, and rotational mechanical power Conversion of wind power into rotational mechanical power and electrical power Typical torque-versus-speed curve at the wind turbine rotor Variable-speed, fixed-pitch Wind turbines. Wind turbine generator efficiency

Air density The air density, symbolized by the Greek letter 𝜌 (rho), is an important parameter to know in wind power applications. Air density is the mass of air per unit volume: 𝜌 = 𝑚/𝑉 Where: ρ is the air density, in kilograms per cubic meter (kg/m3) 𝑚 is the mass of air, in kilograms (kg) 𝑉 is the volume, in cubic meters (m3) The air density varies with atmospheric pressure, temperature, humidity, and altitude. In SI units, ρ = 1.225 kg/m3 under standard sea level conditions, which are: a temperature of 15.5°C, an atmospheric pressure of 101.325 kPa, and a relative humidity of 36% Kinetic energy in the wind Any object or fluid in motion has kinetic energy. For example, wind, which is a mass of air in motion, has kinetic energy. The faster the speed of the wind, the higher the kinetic energy of the wind. The kinetic energy in a mass of air in motion can be calculated by using the following equation: 𝐸𝐾 = 𝑚V2 / 2 Where: 𝐸𝐾 is the kinetic energy, in joules (J) or [feet-pound force (ft∙lbf)]. 𝑚 is the mass of air, in kilograms (kg) V is the velocity of the mass of air, in meters per second (m/s) 2 is a constant. 𝐸𝑘 = 𝑚V2 / 2𝑔𝑐 imperial unit version of the above SI unit equation. Where 𝑔𝑐 is equal to 32.174 lbm/lbf∙s2. Calculating wind power The Figure 1 below shows wind of constant speed passing through a cross-sectional area 𝐴. This area could be, for example, the area swept by the blades of a wind turbine. Cross-sectional area A

Wind

Velocity V

Figure 1, Wind flowing through a cross-sectional area A

In SI units, the power of the wind passing through the cross-sectional area is: 𝑃𝑊 = 𝜌AV3 / 2 Where: 𝑃𝑊 is the power in the wind, in watts (W, or kg∙m2/s3). 𝜌 is the air density, in kilograms per cubic meter (kg/m3). 𝐴 is the cross-sectional area, in square meters (m2). V is the wind speed (m/s).

The observations below can be made from the equation used to calculate the power in the wind. • Any change in the temperature of the air, atmospheric pressure, or relative humidity causes the air density ρ to change, causing the wind power to change in the exact same way (for given wind speed and cross-sectional area). For instance, when the air density ρ increases by 5%, the wind power 𝑃𝑊 also increases by 5%. • When the cross-sectional area 𝐴 swept by the blades of a wind turbine rotor is increased, the wind power intercepted by the blades increases in direct proportion. • When the wind speed V increases, the wind power also increases.

Relationship between wind power and wind speed As already mentioned, the wind power increases when the wind speed increases. More precisely, the wind power 𝑃𝑊 varies with the cube (the third power) of the wind speed V, as Figure 2 shows. • When the wind speed doubles, the wind power increases eight times (23 = 8). • When the wind speed triples, the wind power increases 27 times (33 = 27). • When the wind speed quadruples, the wind power increases 64 times (43 = 64).

Relationship between torque, rotation speed, and rotational mechanical power When a force is applied to an object mounted on a rotation axis (such as the bladed rotor of a wind turbine), the object starts to rotate at a certain speed, as shown in Figure 3. The rotation speed 𝑛 is expressed in revolutions per minute (r/min). One revolution is equal to 360°, or 2π radians (rad).

F 𝑛 Pm = T ∙ 𝑛 / constant T

Figure 3. Torque, rotation speed, and rotational mechanical power.

The rotational mechanical power 𝑃m produced at the rotating axis of the object is the product of the torque T developed at the rotating axis and the rotation speed 𝑛, divided by a constant. The equation below allows the rotational mechanical power to be calculated in SI units. 𝑃m = T ∙ 𝑛 / 9.55 Where 𝑃 m is the rotational mechanical power, in watts (W). T is the torque, in newton meters (N∙m). 𝑛 is the rotation speed, in revolutions per minute (r/min). 9.55 is a constant Conversion of Wind Power into Rotational Mechanical Power and Electrical Power When wind hits the blades of a wind turbine rotor, the pressure of the air acting on the surface of the blades creates a force, which applies a torque onto the rotor of the turbine, as Figure 4 shows. When the wind is strong enough to produce a torque higher than the force (torque) opposing rotation, the wind turbine rotor starts to rotate at a certain speed. In this condition,

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The blades of the wind turbine convert a portion of the power contained in the wind they intercept (linear mechanical power) into rotational mechanical power that makes the wind turbine rotor turn.

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The rotational mechanical power produced at the wind turbine rotor drives an electric generator. The electric generator converts the rotational mechanical power into electrical power. Three-blade wind turbine rotor F Wind turbine generator

F T F Figure 4: A fraction of the power in the wind intercepted by the blades of the turbine is converted into rotational mechanical power to drive the electric generator of the turbine.

Wind, rotor, and rotor efficiency coefficient 𝐶𝑝 As already mentioned, the power contained in the wind passing through the area swept by the blades of a wind turbine rotor is: 𝑃𝑊 = 𝜌AV3 / 2 Where: 𝑃𝑊 is the power in the wind, in watts (W, or kg∙m2/s3). 𝜌 is the air density, in kilograms per cubic meter (kg/m3). 𝐴 is the cross-sectional area, in square meters (m2). V is the wind speed (m/s). Not all the power in the wind passing through the swept area is transferred to the wind turbine rotor. Only a fraction of the available wind power is extracted by the blades and transferred to the rotor. This fraction indicates the efficiency of the wind turbine rotor in converting linear mechanical power into rotational mechanical power. The fraction of wind power extracted by the blades and transferred to the rotor is called the rotor coefficient efficiency 𝐶𝑝. The rotor efficiency coefficient depends on the design (shape) of the rotor blades. The rotor efficiency coefficient is sometimes expressed as a percentage (rotor efficiency coefficient multiplied by 100%). The rotor efficiency coefficient 𝐶𝑝 is generally between 0.4 and 0.5 for most blade designs. The rotor efficiency coefficient 𝐶𝑝 must be taken into account to determine the fraction of wind power 𝑃𝑊 that is transferred to the wind turbine rotor. The formula used to calculate the mechanical power 𝑃𝑚 at the wind turbine rotor is therefore: 𝑃𝑚 = 𝑃𝑊 ∙ 𝐶𝑝 = (𝜌A𝑣3 / 2) ∙ 𝐶𝑝

The rotor efficiency coefficient 𝐶𝑝 of a wind turbine is virtually constant over the normal wind speed range of the turbine. Therefore, the mechanical power at the wind turbine rotor varies in the same way as wind power, i.e., with the cube (the third power) of the wind speed.

Typical torque-versus-speed curve at the wind turbine rotor

Rotor torque 𝑇

Figure 5 shows a typical torque-versus-speed curve at the rotor of a wind turbine obtained for a given wind speed.

Optimum torque

Optimum Velocity

Mechanical power at rotor 𝑃𝑀

Rotor speed 𝑛

Maximum power

Maximum power point (MPP)

Rotor speed 𝑛

Figure 5. Typical torque-versus-speed curve and mechanical powerversus-speed curve at the rotor of a wind turbine, for a given wind speed.

As the rotor speed increases, the torque produced at the rotor increases until a point is reached, beyond which the torque gradually decreases to zero. Consequently, the mechanical power produced at the rotor also increases up to a certain maximum value, and then gradually decreases to zero, as Figure 5 shows. The point at which the mechanical power is maximum is referred to as the maximum power point (MPP). A wind turbine must be operated as close as possible to the optimum speed to maximize the mechanical power developed at the rotor and thus obtain the maximum amount of electrical power. This is performed by setting the rotor torque to the optimum value, through adjustment of the current drawn by the electrical load at the wind turbine generator output. Note that the rotor speed at which the maximum amount of power is produced varies with the wind speed. Therefore, to operate the wind turbine at the maximum power point (MPP) and maximize the energy produced at any wind speed, the rotor speed must be continuously monitored and changed, by adjustment of the rotor torque. This is generally performed automatically by an MPPT controller in the wind turbine.

Variable-speed, fixed-pitch Wind turbines. Variable-speed wind turbines are generally characterized as having higher efficiency than fixed-speed wind turbines and hence are becoming more popular, particularly for small wind turbines. Typically, variable-speed wind turbines are controlled, usually by using power electronics, to regulate the torque and speed of the turbine in order to maximize the output power. Variable-pitch controlled wind turbines

are more costly and complex. Therefore, variable-speed fixed-pitch approach is becoming more popular for low cost construction and is the most common scheme for small wind turbines. In fixed-pitch variable-speed wind turbines, Wind-rotor performance is fixed, the rotor speed and torque are adjusted in order to keep the generator operating at the maximum power point (MPP). The MPPT controller varies the velocity of the blades by varying the current from the generator.

Wind turbine generator efficiency With a wind turbine generator (as well as any other generator), not all the mechanical power applied to the rotor shaft is converted into electrical power, due to power losses in the stator windings. Figure 8 shows that the actual electrical power produced by the generator is lower than the ideal power value over most of the rotor speed range. On the ideal curve, the maximum electrical power is 224 W, and it is reached when the rotor speed is 987 r/min. On the actual curve, the maximum electrical power is 177 W, and it is reached when the rotor speed is 1013 r/min. Therefore, the actual electrical power produced by the generator is lower than the ideal value by 47 W, which corresponds to a power conversion efficiency of about 79% (177 W ÷ 224 W).

1. OBJECTIVE Students will set up a circuit containing a solid magnet wind turbine, a prime mover (hand crank or coupled motor) three phase rectifier and variable resistive load. Then, with the measuring equipment, study the behavior of the wind turbine (frequency, output current, and generator torque) as the prime mover speed and load are varied.

2. MATERIALS REQUIRED Data Acquisition and Control Interface Prime mover Variable resistive load Three phase power rectifier Diodes and Bread Board Multi-function tester Clamp-on ammeter Miscellaneous wire and tools NOTE: Do not install any improper or damaged components. Have instructor review finished connections PRIOR to applying power.

High voltages are present in this laboratory exercise. Do not make or modify any banana jack connections with the power on unless otherwise specified.

3a. Effect of Velocity and Load on Wind turbine. 1. Set up the six diodes to form a three phase full wave bridge rectifier. 2. Make sure that the AC and DC power switches on the power supply are set to the O (off) position, then connect the Power Supply to the three-phase AC power outlet. Connect the Power Input of the Data Acquisition and Control Interface to a 24V AC power supply. Turn the 24V ac power supply on. 3. Connect the USB port of the Data Acquisition and Control interface to a USB port of the host computer. 4. Turn the host computer on, then start the LVDAC-EMS software. In the LVDAC-EMS Start-Up window, make sure the Data Acquisition and Control Interface is detected. Select the network voltage and frequency (60Hz, 120V) then click the OK button to close the LVDAC-EMS Start-Up window. 5. Connect the equipmnet as shown in the figure Below. Using you’re the rectifier that you constructed and the three phase resistor bank (all resistors in parallel)

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6. Have your instructor verify your connections.

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7. In the LVDAC-EMS, open the Metering window. Make the required setting in order to measure the output voltage and frequency from the generator, and rectified Voltage, Current supplied to the resistor bank. 8. In the LVDAC-EMS, open the Oscilloscope, then make the appropriate settings in order to observe the waveforms of the Generator. Before and after rectification (E1 and E2). Select the AC generator voltage (Input E1) as the trigger source of the Oscilloscope. 9. With all the resistors in the resistor bank disconnected, turn the turbine at a consistent lowmedium speed. Record frequency and amplitude of the AC turbine output in Table 1. 10. Observe the waveforms on the Oscilloscope, how are they behaving? (voltage, frequency) As the turbine was turned at a consistently low-medium speed, the amplitude of the voltage and the value of the frequency gradually increased until it remained constant at a close range of values. /2 Also, less ripple was observed. 11. Change the settings on the Resistor bank so that the resistance is 5 Ω. Observe the waveforms on the Oscilloscope, record frequency and amplitude of the AC turbine output in Table 1. 12. How does this change effect the counter torque produced by the turbine. (i.e. are the blades more difficult to move?) Compared with the disconnected resistor bank, the blades were more difficult to move at 5 Ω resistor load. In an open resistance bank, there is no counter torque produced, /2 making it easier to move the blades in an open load. 13. Change the resistor bank settings to 2.5 Ω. Record frequency and amplitude of the AC turbine output in Table 1. 14. How does this change effect the counter torque produced by the turbine. (Are the blades more difficult to move?) At 2.5 Ω, the blades were even more difficult to move compared to the 5 Ω resistor load. Since a decrease in the resistance value was implemented, an increase in the current /2 was therefore observed, resulting now to a higher counter torque. 15. Change the resistor bank settings to 1.67 Ω. Record frequency and amplitude of the AC turbine output in Table 1. 16. How does this change effect the counter torque produced by the turbine. (Are the blades more difficult to move?) At 1.67 Ω, the blades were even more difficult to move compared to the 2.5 Ω

resistor load. Since a decrease in the resistance value was implemented, an increase in the current was therefore observed, resulting now to a higher counter torque. 17. Disconnect the resistors. Turn the turbine at a consistent medium-high speed. Record frequency and amplitude of the AC turbine output in Table 2. 18. Change the settings on the Resistor bank so that the resistance is 5 Ω. Observe the waveforms on the Oscilloscope, record frequency and amplitude of the AC turbine output in Table 2. 19. How does this change effect the counter torque produced by the turbine. (i.e. are the blades more difficult to move?) The blades were more difficult to move at 5 Ω resistor load compared to an open resistor bank. /1 20. Change the resistor bank settings to 2.5Ω. Record frequency and amplitude of the AC turbine output in Table 2. 21. How does this change effect the counter torque produced by the turbine. (Are the blades more difficult to move?) At 2.5 Ω, the blades were even more difficult to move compared to the 5 Ω resistor load. 22. Change the resistor bank settings to 1.67 Ω. Record frequency and amplitude of the AC turbine output in Table 2. 23. How does this change effect the counter torque produced by the turbine. (Are the blades more difficult to move?) At 1.67 Ω, the blades were even more difficult to move compared to the 2.5 Ω resistor load. /1 24. Lab Equipment powered off and cleaned up. Instructor to verify _______ (initial)

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4. QUESTIONS 1. From your observations of the waveforms on the Oscilloscope, how does the turbine behave as the rotational speed and load are varied (output Voltage, Frequency, and counter torque produced by the generator)? From the data tabulated below: As the rotational speed increases (from 100bpm to 150bpm), the value of the output voltage, frequency, and the current increases. Consequently, when the load resistance increases (from 1.67 Ω to 5 Ω), the value of the output voltage, frequency, and current decreases. Moreover, when current increases, the counter torque also increases. This the reason why it is harder to move the turbine at 1.67 Ω (low resistance). /4 2. What can you conclude about the number of poles in the turbine stator? The number of poles in the turbine stator greatly influences its rotational speed. As the number of poles increases, the rotational speed (in RPM) decreases. Most wind turbines only use generators having four or six poles as these relatively high-speed generators are much more convenient in cost and size. /4 3. Can the synchronous generator (Fixed-pitch, Variable-speed Wind turbine) be synchronized directly to the to the AC power network using a synchronizer as we did in the previous lab? □ Yes

□ No

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If one was to try, what could the complications be? As the speed of the wind varies, the speed of the rotor varies, and so is the frequency of the voltage. This cannot be directly connected to the AC power network. Instead, the frequency must first be corrected such that it is constant. If not, there is no assurance that the frequency of the wind turbine will be the same as the frequency of the network. Thus, it my lead to damage to the power

system network and the generator itself. /4 4. How would a Maximum Power Point Tracking controler manage a fixed pitch variable-speed wind turbine such that the blade speed could be controlled with respect to wind velocity? Since the performance of the wind-rotor is fixed in fixed-pitch variable-speed wind turbines., the rotor speed and torque are adjusted to keep the operation of the generator at the maximum power point (MPP). The MPPT controller varies the velocity of the blades by changing the c...