Modeling, Simulation & Control of Induction Generators Used in Wind Energy Conversion

Transcription

1 Chapter-3 Principles of Electrical Energy Conversion 3. 1 Introduction Several forms of energy can be converted into electrical energy basically by two methods known as direct or indirect conversion. In direct conversion which is usually static, primary energy is converted directly into electricity. Examples of direct conversion are photovoltaic and thermoelectric. Indirect conversions use one or more intermediary processes like converting hydro, thermal and wind power to electrical energy. The principle of a rotating electrical generator, which is employed in wind power generation, is based on the relative movement between a coil and a magnetic core. In a wind farm, as it is very important to make maximum use of available energy, output characteristics of turbine generator power should match in the most reasonable possible way and the gearbox is so designed that generator is driven at its correct rated speed. If it is too low, the high speed of wind will be wasted and if it is too high, the power factor will be harmed Wind Energy Conversion Wind energy is basically the kinetic energy of air moving over the earth s surface. When the Sun strikes the earth, it heats the soil near the surface that in turn warms the air lying above it. Warm air is less dense than the cool air, therefore warm air rises in the atmosphere and cool air flows in to take its place and is itself heated. The rising warm air eventually cools and falls back to earth [59]. This cycle is continuous. Normally winds are stronger along the shores of large lake, along the coasts and mountain valley because of differential heating. It is very important to make maximum use of available energy resources.

2 A wind turbine is a machine that converts kinetic energy of the moving air into mechanical energy which in turns is converted into electrical energy by a generator. Wind passes over the blades, generating lift and exerting a turning force. The rotating blades turn a shaft inside the nacelle. It extracts the energy from the wind by transferring the thrusting force of the air passing through the turbine rotor into the rotor blades. All the wind turbines work on two (or three) physical principles by which energy is extracted from the wind. These principles are either: (i) Drag force or (ii) Lift force or (iii) Combination of the two forces [16, 31] Drag Principle Drag is a mechanical force. It is generated by the interaction and contact of a solid body with a fluid (liquid or gas). It is the component of aerodynamic force parallel to the relative wind. One of the sources of drag is the skin friction between the molecules of the air and the solid surface of the blade. Because the skin friction is an interaction between a solid and a gas, the magnitude of the skin friction depends on properties of both shape of the blade and air flow [49, 50]. Drag devices are simple wind machines that use flat, curved or cup-shaped (unlike aerodynamic shapes of the lift devices) blades to run the rotor. When the wind blows and drag devices comes in its way, the wind pushes away the angularly curved blades out of its way to force its way through the blades and forces the blades and the horizontal shaft to rotate [47] Lift Principle The WPP blades operate on the principle of lift. The longer path theory is based on the Bernoulli s principle states that as the speed of a fluid flow increase, its pressure decreases. Lift devices are generally more efficient than drag devices. Unlike the drag devices, the WPP blades are shaped similar to the aerofoil wings of an aeroplane. The rotor consists of the rotor blades and the hub placed upward of the tower and the nacelle in most of the wind turbines. This is primarily done because the air current behind the tower is very irregular (turbulent). The rotor blades are aerofoils that act on the principle of lift

3 similar to the wings of an aircraft. The top surface of a blade aerofoil is more curved than the bottom surface. A cross section of a rotor wing is shown in Figure 3.1. When the air particles approach the leading edge of the blade, it travels either over or under the blade. Hence, nearby particles split up at the leading edge, and then come back together at the trailing edge of the blade. The air sliding along the upper surface of the wing moves faster than on the lower surface. Since the particle travelling over the top goes a longer distance in the same amount of time, it must be travelling faster. This means that the pressure will be lowest on the upper surface. This differential pressure creates a thrust force. This lifting force is perpendicular to the direction of the wind and is converted into a mechanical torque. This torque in turn makes the turbine rotor to rotate. It is then transformed to mechanical and electrical forms. Therefore, the shape of the aerodynamic profile of the blade is the most crucial component of the WPP performance. Figure 3. 1 Rotor cross-section of rotor WPP Rotor Characteristics Several technical characteristics of WPP rotor are as follows: i. Tip-Speed ratio and Optimum WPP Rotor Speed The speed of the rotor tip is called the tip speed. The blade tip of the modern WPPs can have speeds about 10 times faster than the wind speed. For a 2-bladed rotor, the maximum power extracted from the wind occurs at a TSR of about 6, for a 3-bladed rotor it will be 4 and for a 4-bladed rotor, it will be about 3. With careful design, the optimum TSR may be about 30 % above these values.

4 ii. WPP power Coefficient The rotor blades convert some of the kinetic energy in the wind to mechanical energy which is transmitted to the rotor shaft. The proportion of the power in the wind that a WPP rotor can extract is termed the Power Coefficient Cp is a non-dimensional quality. According to German physicists Betz, the maximum power that can be extracted is not more than 59.3% [16]. Mass in motion carries a certain amount of energy. This kinetic energy air with mass m and moving with a velocity Kinetic energy per second is: E ke of a stream of V w is given by a well known equation (3.1). 1 2 E ke = mv w (3.1) 2 If a wind turbine rotor of cross sectional area A is exposed to this wind, kinetic energy available can be expressed as shown by equation (3.2). E 1 = (3.2) 2 2 ke ρvv w where ρ = Density of air (kg/m 3 ), v = Volume of air parcel available to rotor (m 3 ), = wind velocity (m/s). The air parcel interacting with the rotor per unit time has a cross-sectional area equal to that of the rotor and thickness equal to wind velocity. Therefore, the power can be expressed (energy per unit time) by equation (3.3). Vw 1 P ρav w 2 3 = (3.3) 2 where P = Power generated (watt), A = Cross sectional area of the rotor ( m ) The density of air depends on factors like temperature, atmospheric pressure, elevation and constituents in air. A wind turbine cannot extract power completely from the wind. A part of the kinetic energy is transferred to the rotor and remaining energy is carried away by the air leaving the turbine. Therefore equation (3.3) can now be expressed as shown by equation (3.4).

5 P 1 C p AV w 2 3 = ρ (3.4) where Cp = Co-efficient of power The co-efficient of power is given by the ratio of power extracted to the power available. All the energy present in wind cannot be converted into useful energy at the rotor shaft. Its theoretical maximum limit is 59% set by Betz s law which means that under ideal conditions only 59% of the wind power could be extracted [14]. Power co-efficient (Cp ) is a function of the tip speed ratio ( λ t ) of the wind turbine and depends on the blade dynamics. The tip speed ratio is given by equation (3.5). ω r λv r r t w λ t = or ω r = (3.5) Vw rr where ω r = Rotational speed of rotor (rpm) at any given speed, r r = Radius of the rotor blade (m). The aerodynamic torque, which is ratio of rotor power and rotor speed, is given by equation (3.6). T = Power ω r 1 πr = 2 2 r V 3 w ω r ρc p T = πrrvw ρct (3.6) 2 where, C t = Torque co-efficient and is related to the power co-efficient byc t Cp = λ t The relations between power, power co-efficient and tip speed ratio indicate that the mechanical power available from the wind is the maximum for a specified wind speed, which corresponds to optimalc. Below the cut-in speed (minimum speed needed for the p turbine to generate power), there is no generation of power and when the speed limit is reached, a constant speed characteristic up to the rated power is adopted [31]. For a Vesta-90 turbine, a PSCAD simulation circuit based on equation (3.6) is developed as shown in Figure 3.2. The diameter of the blade is 90 m with swept area of 6362 m 2.

6 Figure 3. 2 Model to obtain the relation between wind speed and power co-efficient A relation between wind velocity, torque and power co-efficient is simulated as shown in Figure 3.3. It can be observed that the cut-in speed is 3.5 m/s and cut-out speed is at about 25 m/s [57]. The maximum power co-efficient is obtained at about wind velocity of 12 m/s. The power curve gives the steady-state electrical power output as a function of the wind speed at the hub height. Figure 3. 3 Wind velocity versus torque and power co-efficient

7 The power curve has three key points on the velocity scale: i. Cut-in wind speed: It is the minimum wind speed (in order of 3-5 m/s) at which machine will deliver useful power. ii. Rated wind speed: It is the speed at which rated power is delivered (between 11m/s and 16 m/s). iii. Cut-out wind speed: When the wind speed becomes very high, the energy contained in the airflow and the structural loads on the turbine becomes too high and the turbine is taken out of service (between 17 and 30 m/s). It is the maximum wind speed at which the turbine is allowed to deliver power and is normally limited by loads and safety constraints. Wind turbine remains shut down below the cut-in speed because the speed of the wind is too low for the useful energy production. The power output increases following a broadly cubic relationship with wind speed until rated wind speed is reached. Above rated wind speed the aerodynamic rotor is arranged to limit the mechanical power extracted from the wind and so reduce the mechanical loads on the drive train. At very high wind speed, the turbine is shut down. The torque developed determines the size of the gearbox and must be matched by whatever generator is being driven by the rotor. Modern high speed turbines that generate electricity are designed to have a very low starting torque thus reducing the gearbox costs. From above discussion, three basic rules are applicable [31]. i. The speed of the blade tips is ideally proportional to the speed of wind. ii. The maximum torque is proportional to the square of wind speed. iii. The maximum power is proportional to the cube of wind speed. However annual average power produced by wind turbine will depend on local wind speeds. Normally there is not enough energy in the wind to overcome the mechanical losses at 5 m/s known as cut-in wind speed. If the wind speed is below the cut-in wind speed, the turbine is not allowed to rotate and brakes are applied. Also the output power is not actually in cubic relationship with wind speed because the co-efficient of power Cp varies with wind speed.

8 3. 3 Power Control by Turbine Manipulation The main function of the wind turbine is to extract the maximum amount of energy at a wind speeds around meters per second under allowable weather conditions and provide protection to the rotor and power electronic equipments at high wind speeds in case of storms or an emergency. At high wind speed, the rotor speed can no longer be controlled by increasing the generated power because this would overload the generator and power electronic equipments. In case of stronger winds it is necessary to waste a part of the excess energy of the wind in order to avoid damaging the wind turbines. This is achieved by proper design of blade and control systems. There are three main techniques for regulating the power of a wind turbine as discussed below Yaw Orientation Control This approach is used in small wind turbines. In this arrangement yawing or tilting the plane of rotation in the direction of wind pressure limits power extraction. This offers the reduction in the effective flow cross-section of the rotor and the flow incident on each blade is considerably modified [19]. Figure 3.4 shows the drastic drop in performance co-efficient when wind turbine is turning out of the wind. In a horizontal-axis wind turbine a yaw system turns the rotor in such a way that the wind direction is perpendicular to the swept rotor area. In case of very high wind speed, more than the cut-out speed, the rotor axis is turned perpendicular to the wind direction. This action is carried out by an active yaw-control system. Figure 3. 4 Rotor performance co-efficient under skewed air flow

9 Stall Regulated Turbines In this arrangement, rotor blades are bolted onto the hub at a fixed angle. The geometry of the blade profile however has been aerodynamically designed to ensure that the moment the wind speed becomes too high; it creates turbulence on the side of the rotor blades which is not facing the wind [59]. This stall the aerodynamic design of the rotor blade regulates the power of a wind turbine and it does not require any moving parts. The blade is rigidly fixed to hub at a fixed angle and the pitch angle distribution along the blade is constant for all wind speeds. The wind turbine produces the maximum power within a certain wind speed limit. When the wind speed exceeds certain level, the design of rotor aerodynamics causes the rotor to stall. It is simple, robust and most economical but suffers from difficulties of over speeding of wind turbine in case of lose of grid connection and high mechanical stress caused by wind gusts. A normal stall controlled wind turbine will usually have a drop in the electrical power output for higher wind speeds, as the rotor blades gives more stall Pitch Regulated Turbines In this arrangement the blades are physically rotated about their radial axis in case of change of wind velocity to reduce the angle of attack so that it operates at a maximum Cp and obtain maximum power output and hydraulics or stepper motors are usually used to provide pitch mechanism [6]. It is therefore possible to have an almost optimum pitch angle at all wind speeds and a relatively low cut-in wind speed. The speed of the rotor is continuously adjusted such that the tip speed ratio remains constant at a value to give the maximum Cp and efficiency of the turbine. Positive pitching increases the pitch angle so as to decrease the angle of incidence whereas negative pitching increases the angle of incidence. In case of storm or an emergency, the pitch angle increases to a large value thus increasing the angle of attack and forces the turbine to stall to prevent it from damage. The drawback with this method is that it involves the extra complexity due to the pitch mechanism and its controller.

10 Variable speed pitch regulated wind turbines operate at fixed pitch with a variable rotor speed to maintain an optimum tip speed ratio. When the rated power is reached, the generator torque is used to control the electrical power output while the pitch control is used to maintain the rotor speed within acceptable limits. A conventional control system for a pitch-controlled turbine is shown in Figure 3.5. First the power generated by the wind turbine is measured and is compared with the predefined power value. The error signal then is passed to the controller which finally controls the actuator to rotate the blades. Figure 3. 5 Pitch regulation control system 3. 4 Wind Turbine Generator Concepts A Wind Turbine Generator (WTG) is a device that extracts kinetic energy from the wind and drives the generator to produce electricity. There are different ways to classify the wind turbines; they are either horizontal axis or vertical axis, depending on their axis of rotation. In modern wind turbines, the most common design is the horizontal axis wind turbine wherein the axis of rotation is parallel to the ground. The important components of a modern wind generator plant are the tower, the rotor and the nacelle. The nacelle, which is mounted on the top of the tower, houses transmission mechanism, generator, transformer, switching system, protection systems and other accessories [49]. In

11 horizontal-axis wind turbine generator, the nacelle also accommodates yaw systems for steering in response to changes in wind direction and velocity. Modern wind turbines use three-bladed upwind rotors. The wind turbine with less number of blades rotor has to operate at a higher rotational speed in order to extract the wind energy. Wind generators are classified according to their operation and control. In present-day practice, on the basis of the speed of the prime mover i.e. wind turbine, there are three common generating schemes that are used to harness wind energy [8]. They are; i. Fixed-speed wind generator. ii. Variable speed wind generator employing Doubly-Fed Induction Generator (DFIG). iii. Multi-pole synchronous machine used in direct drive technology. NEG Micon, G E Wind Energy, Nordex, Enercon, Suzlon and Vestas are some of the leading manufacturers of large wind generators Fixed-Speed Wind Generator This scheme employs Squirrel Cage Induction Generator (SCIG) shown in Figure 3.6. The stator and rotor of the induction machine are equipped with three-phase windings. In squirrel cage induction machine, the rotor windings are directly short-circuited onto the rotor. The stator is directly connected to the grid and the rotor speed is constant for the whole range of the wind speed. In fact the rotor speed of the fixed-speed wind turbine in principle is determined by a gearbox and the pole-pair number of the generator. The fixed speed wind turbine system often has two fixed speeds and is designed to provide maximum efficiency at one particular speed [8, 24]. The generator may have two windings with different ratings and pole pairs which result in increased aerodynamic capture as well as reduced magnetizing losses at low speeds. One is used at low wind speeds and the other at medium and high wind speeds. SCIG is directly connected to the grid with a bank of capacitors to reduce the reactive power requirement. The TSR in this category varies over a wide range due to the continuously changing wind speed which results in poor efficiency. Therefore, proper gearbox ratio should be chosen to get optimal C p. These wind turbines are simple in construction, rugged and economical, but suffer

12 with limited power quality, need a reactive magnetizing current and more mechanical stress. It is not possible to store the turbulence of the wind in the form of rotational energy because the speed is fixed to the grid frequency and all the fluctuations in wind speed are transmitted into the electrical output, resulting in large voltage fluctuations particularly in case of weak grids [52]. Another limitation is that the fluctuations form the wind turbine is directly translated as torque pulsations on to the gear box resulting in more failures. Moreover it is difficult to control active and reactive power in Fixed Speed Wind Generators. Figure 3. 6 Fixed Speed Wind Generator Variable-Speed Wind Generator Variable-speed wind turbines are more popular nowadays and are designed to provide maximum efficiency over a wide range of wind speeds. These are equipped with a Wound Rotor Induction Machine (WRIM) and are controlled with electronic converters, which make it possible to control the power and rotor speed. In wound rotor induction machine rotor windings are not permanently shorted onto the rotor as it is done in squirrel cage induction machine, but the ends of the rotor windings are taken outside through slip rings. The stator is connected to the grid and rotor windings are connected by bidirectional frequency converter as shown in Figure 3.7 that allows electrical energy flow in both directions; to the rotor if generator is operating with sub-synchronous speed and to grid when operating speed is super-synchronous [32, 45]. It has been discussed in detail in Section 3.6. This configuration is also known as Doubly-Fed Induction Generator (DFIG). DFIG can supply power at constant voltage constant frequency while the rotor speed varies. The bidirectional AC/DC/AC converters in the rotor circuit control the speed

13 range above the synchronous speed and power in that case will be generated both from the stator and rotor [42, 43]. Under normal condition it has the advantage of relatively small size of the converters, which deal with only the rotor power. As the generator and its controllers are designed for the variable speed mode operation, it keeps the torque fairly constant and is possible to maintain the tip speed ratio constant at a predefined value to achieve the maximum co-efficient of power [39]. The variable speed operation will maintain a constant TSR which results in more energy capture. Figure 3. 7 Variable-Speed Wind Generator Multi-pole Synchronous Wind Generator This configuration employs electrically excited synchronous generator or a Permanent Magnet Synchronous Machine (PMSM) which operates under variable speed. The rotor of the AC synchronous machine carries an either salient pole PM rotor or a smooth-core electromagnets. If electromagnet is used in the excitation system, then the required exciting energy is supplied via slip rings. In some machine with brushless excitation arrangement, slip rings are no more required but an auxiliary machine is used which is attached to the shaft and supplies the necessary exciting energy. In this scheme the generator is connected to the grid via back-to-back converters that handle total power. It provides the only path between the stator and power system. Moreover this converter decouples system frequency from the frequency at the stator of the synchronous generator; hence there will be no changes in electromagnetic torque because any change in system frequency will not be seen at the stator of the machine. In case of change in system frequency, this configuration will not provide an inertial response [7]. In this scheme

14 permanent magnets of alternating polarity are fixed to the rotor to create an excitation field. This system may or may not have gearbox and a direct driven multi-pole generator with a large diameter is used if the gearbox is not used. The schematic diagram of multipole synchronous generator is shown in Figure 3.8. The multi-pole synchronous generators are very much useful for low speed WPPs applications. Figure 3. 8 Multi-pole Synchronous Machine 3. 5 Comparisons of various WPPs The Table 3.1 gives the comparison of the three main types of WPPs [19, 50]. Constant Speed Doubly-fed Direct Drive Mechanical stress Less mechanical stress Less mechanical stress Noisy Less noisy Less noisy Aerodynamically Less Aerodynamically efficient Aerodynamically efficient efficient Soft starter is used for Small rating of converter Large rating of converter smooth connection grid Flickering is important, particularly in weak grids Flickering is not important because rotor acts as an energy buffer Flickering is unimportant because rotor acts as an energy buffer Compensation possible in node voltages changes with additional equipments Compensation possible in node voltages changes but dependent on converter rating Compensation possible in node voltages changes but dependent on converter rating Electrically efficient Electrically less efficient Electrically less efficient

15 Gear box required Gear box required No gearbox Less Expensive Expensive Expensive It can contribute to fault current It can contribute to fault current No, converter cannot contribute to fault current Table 3. 1 Comparison between types of WPPs 3. 6 Components of a WPP The various important parts of a WPP are described in this section [50] Tower The commonly used tower is tubular tower which has a door at the bottom. The door can be used to enter into the tower for operation and maintenance. The door is usually provided with locking arrangement to prevent unauthorized entry of outsiders. All the towers are provided with ladder to climb up to the top so as to do the maintenance. In these towers at certain intervals, platforms are fitted inside the tower for inspection of joints of the tower section Blades The turbines are provided with three blades. The blades are self supporting in nature made up of Fibre Reinforced Polyester. The blades are mounted on the hub. The blades are built with aerodynamic brakes that prevent over speed of the wing Nacelle The Nacelle is the one which contains all the major parts of a turbine. The nacelle is made up of thick rugged steel and mounted on a heavy slewing ring. Under normal operating conditions the nacelle would be facing the upstream wind direction. The nacelle is fitted with a cover to protect the parts from climatic damages.

16 Hub The Hub is intermediate assembly between the wing and the main shaft of the turbine. Inside the hub, a system to actuate the aerodynamic brake is fitted. The hub is covered with nose cone Main Shaft The shaft is to connect the gear box and the hub. Solid high carbon steel bars or cylinders are used as main shaft. The shaft is supported by two bearings Main Bearing The main bearings are placed inside housing. The main bearings are usually roller bearings. The bearing house has the provision to fit temperature sensors and has the facility to apply lubrication Gear Box The gearbox is used to increase the speed ratio so that the rotor speed is increased to the rated generator speed. The helical gears are used in the gearbox. Oil cooling is employed to control the heating of the gearbox. Gearboxes are mounted over dampers to minimise vibration Brake Brake is employed in the wind turbines to stop the turbine mainly for maintenance check. Brakes are also applied during over speed conditions of the turbine. The brakes are placed on the high speed shaft. There are different types of brakes used in wind turbines. But Hydro-spring pressure disc brakes are widely used. During the operation of the turbine the brake system is active and keeps the brakes in released condition. For this purpose, electrical power is used to pressurize the hydraulic oil or hold the spring in compressed

17 state. Once the power falls, the brakes are applied. Hence during braking, the control system cuts off the electrical continuity to brake the system Yaw System The yaw ring is fitted at the top of the tower over which the nacelle is seated. The nacelle is able to rotate around the yaw ring. The yaw ring is fabricated in such a way that it allows the nacelle to rotate freely but prevents lifting up from the tower. The yaw gear is in contact with yaw ring. The rotation of the yaw gear turns the nacelle to the desired direction. The yaw gear is coupled with an electric motor called yaw motor. Single or double yaw gear is used to operate the yaw ring. Yaw brakes are used to keep the nacelle in a steady position when it is not yawing. During yawing the nacelle is able to rotate smoothly. The yaw brake has brake liners loaded by springs, which act as brakes. While yawing, the yaw gear overcomes the braking force of the yaw brake Coupling Couplings are fabricated with rubber elements over metallic shaft to reduce the strain and vibration due to error in the alignments Generator The generator invariably uses induction type of generator depending upon the RPM selection. For better efficiency two generators with different cut-in wind speed requirements are mounted on the same shaft. The generators are provided with monitoring sensors in each phase winding to prevent damage to the generators Sensor Different sensors are used in turbines for monitoring. They are as listed below: 1. Anemometer - to measure the wind speed. 2. Wind Vane - to sense the wind direction.

18 3. Tacho rotor sensor - to measure the rotor RPM. 4. Tacho gen. sensor - to measure the generator RPM. 5. Yaw sensor - to measure the yawing angle of the nacelle. 6. Vibration sensor - to sense the vibration of the tower. 7. Temperature sensors - to measure the temperature of main bearing, gear oil, generation winding etc Power Panel The power panel is equipped with number of contractors, auxiliary contactors, thermo relays, soft components, phase compensation batteries and circuit breaker etc. The contactors are used to connect and disconnect the generator, yaw motor, hydraulic pump motor and brake magnetic coil, etc. The auxiliary contacts used to feed information to the control system about the status of different contactors and components. Thermo relays are used as protection to the motors used in the turbines. The circuit breakers used at the incoming side of the panel is to protect the electrical systems of the turbines from damage on fault conditions and also used to shut down and attend servicing on the electrical system Control Panel Microcontroller/Micro processor based control circuits are provided in this panel. It receives input signals from the sensors and auxiliary contacts of the power panel and sends output signals to different contactors to on/off the related systems as per the programme fed into the controller. It has display modules, which displays instantaneous records of power produced, wind speed, grid instantaneous records of power produced, wind speed, grid voltage/current, fault status and so on. A programmable key board facility is available in the micro processor system to feed the set parameters and see different data logged in the control processor.

19 3. 7 Summary This chapter emphasises on the working principles, the broad classification of WPPs and basic terms associated with the wind energy extraction. It is concluded that all the generators can be grouped in two categories namely, Constant speed electrical generators and Variable speed electrical generators. Various components of a wind turbine are discussed herewith.