Overview of different wind generator systems and their comparison
- 1. Class-6: Overview of different wind generator systems and their comparisons Course: Distribution Generation and Smart Grid Prof. (Dr.) Pravat Kumar Rout Subhasis Panda (Research Scholar) Department of EEE, ITER, Siksha ‘O’Anusandhan (Deemed to be University), Bhubaneswar, Odisha, India
- 2. Principle of operation Kinetic energy present in the wind speed to electrical energy conversion. Wind power consists of converting the energy produced by the movement of wind turbine blades driven by the wind into electrical energy.
- 6. Wind Energy in Indian
- 7. 0 5000 10000 15000 20000 25000 30000 35000 40000 Wind Power Generation(MW) Wind Power Generation(MW) Wind power generation scenario in India (MW)
- 8. Answer… Wind energy is the world’s fastest growing renewable energy source. The average annual growth rate of wind turbine installation is around 30% during last 10 years. The overall capacity of all wind turbines installed worldwide by the end of 2018 reached 600 Gigawatt, according to preliminary statistics published by WorldWind Energy Association (WWEA) Wind energy benefit Renewable energy Inexhaustible Not pollutant Reduces the use of fossil fuels Reduces energy imports Creates wealth and local employment Contributes to sustainable development
- 9. Wind turbine generator types Referring to the rotational speed, wind turbine concepts can be classified into fixed speed, limited variable speed and variable speed. For variable speed wind turbines, based on the rating of power converter related to the generator capacity, they can be further classified into wind generator systems with a partial scale and full scale power electronics converter. Considering the drive train components, the wind turbine concepts can be classified into geared drive and direct drive wind turbines. In geared drive wind turbines, one, one conventional configuration is a multistage gear with a high speed generator; the other one is the multi-brid concept which has a single stage gear and a low speed generator
- 10. Site Selection for Wind Power Plant The WECS should be located where high average wind velocities available are in the range of 6 m/s to 30 m/s throughout the year. TheWECS must be located far away from cities and forests since buildings and forests offer resistance to wind. Wind velocities must be measured at several heights as the velocity of wind increases with height. Tower design must be adequate to withstand maximum wind speeds observed in the last few years in the area of installation.
- 11. Advantages of wind power plant It is a free and un-exhaustible source of energy. It is a clean and non-polluting source of energy. It has a low maintenance cost. It has a low cost of power generation (about Rs. 2.25/kWH).
- 12. Disadvantages of wind power plant At present capital cost of wind power plants is high. It is about Rs. 3.5 crores/MW. Wind energy is very fluctuating in nature. It is very difficult to design a wind energy system due to these fluctuations.This problem also requires the provision of a suitable storage device to ensure continuous power supply. Large variations in wind speed during storms may cause damage to windmills. The efficiency of the system is in the range of 35 to 44%. Windmill causes sound pollution. A large unit can be heard few kilometers away.
- 13. Type-1: Fixed speed concept The generator operating slip changes slightly as the operating power level changes and the rotating speed therefore not entirely constant. However, because the operating slip variation is generally less than 1% , this type of wind generator is normally referred to as fixed speed.
- 14. Continue… Have been used with a multiple stage gearbox (used to match the speed difference). Squirrel cage induction generator (SCIG) directly connected to the grid through a transformer. The SCIG operates only in a narrow range around the synchronous speed. SCIG always draws reactive power from the grid and so accompanied with the capacitor bank for reactive power compensation. Smoother grid connection was also achieved by incorporating a soft starter .
- 15. Continue… The function of the soft starter unit is to built up the magnetic flux slowly and so minimizing transient currents during energization of the generator or system start-up condition. Pole changeable SCIG has been used which leads two rotation speeds only.
- 16. Advantages…. Robust, easy and relatively cheap for mass production. It enables stall-regulated machines to operate at a constant speed when it is connected to a large grid, which provides a stable control frequency. Although the stall control method is usually used in combination with the fixed speed SCIG for power control, the active stall control or pitch control have also been applied.
- 17. Limitations The speed is not controllable and variable only over a very narrow range, in which only speeds higher than the synchronous speed are possible for generator operation. Wind speed fluctuations are directly translated into electromechanical torrque variations, causing high mechanical and fatigue stresses on the system. This may result also swing oscillations between turbine and generator shaft. Turbine speed cannot be adjusted with the wind speed to optimize the aerodynamic efficiency
- 18. Continue... Three stage gearbox makes the nacelle more burden and also a large fraction of the investment cost. Capacitors are connected in parallel to the generator for the reactive power compensation. It is necessary to obtain the excitation current from the stator terminal of SCIG . This makes it impossible to support grid voltage control.
- 19. Type-2: Limited variable speed concept
- 20. Continue… Uses a wound rotor induction generator (WRIG) with variable rotor resistance by means of a power electronics converter and pitch control method. The stator is directly connected to the grid. The rotor winding is connected in series with a controlled resistor. Variable speed operation can be achieved by controlling the energy extracted from the WRIG rotor.
- 21. Continue... Dynamic speed control range depends on the size of the variable rotor resistance, and the energy extracted from the external resistor. Slip rings can be avoided by power converter and resistors. The control signals are transmitted to the rotating electronics by an optical coupling. Reactive power and soft starter are also required
- 22. Type-3: Variable speed concept with a partial scale power converter
- 23. Continue… This corresponds to a variable speed wind turbine with a WRIG and a partial-scale power converter on the rotor circuit. The stator is directly connected to the grid. The rotor is connected through a power electronics converter The power electronics converter controls the rotor frequency and thus the rotor speed. The variable speed range is +/- 30% around the synchronous speed. The ratings of the power electronics converter is only 20-30% of the generator capacity
- 24. Continue… The power converter system can perform reactive power compensation and smooth grid connection Grid side converter can control can control its reactive power, independently of the generator operation; this allows the performance of voltage support towards the grid.
- 25. Limitations… A multistage gearbox is till necessary in the drive train because the range for DFIG is far from a common turbine speed of 10-25 rpm. A gearbox is inevitable to have some drawbacks, such as heat dissipation from friction, regular maintenance and audible noise. The corresponding control strategies are comparatively complicated. Under grid fault conditions, the both stator and rotor currents are very high and the protection is necessary particularly for converters Large stator peak currents under abnormal conditions may cause high torque loads on the drive train of wind turbines
- 26. Limitations …. The slip ring is used to transfer the rotor power by means of a partial scale converter which requires a regular maintenance, and may be result in machine failures and electrical losses
- 27. Type-4:Variable speed direct-driven concept with a full scale power converter
- 28. A variable speed wind turbine with a direct-drive generator connected to the grid through a full scale power converter. The direct-drive generator rotates at a low speed, because the generator rotor is directly connected on the hub of the turbine rotor. The low speed and high torque operation requires multi- poles design. So direct-drive generators are usually designed with a large diameter and small pole pitch. The full scale power converter can perform smooth grid connected over the entire speed range. Continue...
- 29. Case-1: Electrically Excited Synchronous Generator
- 30. Continue …. Rotor carrying field system provided by DC excitation. Stator carries a three phase winding similar to that of the induction machine Rotor may have salient poles or cylindrical. The amplitude and frequency of the voltage can be fully controlled by the power electronics at the generator side, so that the generator speed is fully controllable over a wide range, even to very low speeds Excitation current can be controlled by means of the power converter in the rotor side
- 31. Disadvantages In order to arrange space for excitation windings and pole shoes, the pole pitch has to be large enough for the large diameter-specific design, so a larger number of parts and windings properly make it a heavy weight and expensive solution. It is necessary to excite the rotor winding with DC, using slip rings and brushes, or brushless exciter, employing a rotating rectifier, and the field losses are inevitable.
- 32. Case-2: PM Synchronous Generator
- 33. Continue… The advantages of PM machines over electrically excited machines are higher efficiency and energy yield no additional power supply for the magnet field excitation improvement in the thermal characteristics of the PM machine due to the absence of the field losses higher reliability due to the absence of mechanical components such as slip rings lighter and there for higher power to weight ratio
- 34. Continue… The limitations are High cost of PM materials Difficulties to handle in manufacture Demagnetization of PM at high temperature
- 35. Type-5:Variable Speed single stage geared concept with a full scale power converter
- 36. Continue… In this scheme, a variable speed pitch control wind turbine is connected to a single stage planetary gearbox that increases the speed by a factor of roughly 10 and a low speed permanent magnet generator.
- 37. Type-6:Variable Speed multi-stage geared concept with a full scale power converter Case-1: PMSG system
- 38. Continue… With a multiple gearbox is used in order to reduce the generators volume and improve the generator efficiency in variable speed wind turbine concepts with a full scale power converter. Compared with the DFIG system the advantages are The generator has a better efficiency the generator can be brushless the grid-fault ride-through capability is less complex
- 39. Continue… The limitations are large, more expensive (100% of the rated power instead of 30%) The losses in the converter are higher because all powers are processed by the power electronics converter
- 40. Variable Speed multi-stage geared concept with a full scale power converter Case-2: SCIG system A variable speed multistage geared SCIG Full scale converter
- 41. Type-7: Other Potential Generator types for different wind turbine concepts Linear Induction Generators Switched Reluctance Generators Claw-pole Generators Brushless DFIGs (BDFIG)
- 42. Brushless DFIGs The output of the induction generator (in case of BDFIG) is directly connected to the grid, and thus the generator output frequency must be equal to the grid frequency It does not need slip ring ( in case of BDFIG); however it requires double stator windings, with different number of poles in both stator layers
- 43. Continue.. The second stator layer generally has lower copper mass, because only a part of the generator nominal current flows in the second winding The second stator winding is connected through a power electronics converter, which is related at only a fraction of the wind turbine rating It (BDFIG) has the capability of realizing the variable speed operation and independently controls the stator active and reactive power. The machine operation principle and its assembly are relatively complex.
- 44. Advantages of permanent magnet synchronous generator Flexibility in design allows for smaller and lighter designs Higher output level may be achieved without the need to increase generator size Lower maintenance cost and operating costs, bearings last longer No significant losses generated in the rotor Generator speed can be regulated without the need for gears or gearbox Very high torque can be achieved at low speeds Eliminates the need for separate excitation or cooling systems
- 45. Disadvantages of Permanent magnet synchronous Generator Higher initial cost due to high price of magnets used Permanent magnet costs restricts production of such generators for large scale grid connected turbine designs High temperatures and severe overloading and short circuit conditions can demagnetize permanent magnets Use of diode rectifier in initial stage of power conversion reduces the controllability of overall system
- 46. Advantages of Asynchronous Generator Lower capital cost for construction of the generator Known as rugged machines that have a very simple design Higher availability especially for large scale grid connected designs Excellent damping of torque pulsation caused by sudden wind gusts Relatively low contribution to system fault levels
- 47. Disadvantages of Asynchronous Generator Increased converter cost since converter must be rated at the full system power Results in increased losses through converter due to large converter size needed for IG Generator requires reactive power and therefore increases cost of initial AC-DC conversion stage of converter Many experience a large in-rush current when first connected to the grid Increased control complexity due to increased number of switches in converter
- 48. Advantages of Doubly fed Induction Generator Reduced converter cost, converter rating is typically 25% of total system power Improved efficiency due to reduced losses in the power electronics converter Suitable for high power applications including recent advances in offshore installation Allows converter to generate or absorb reactive power due to DFIG used Control may be applied at a lower cost due to reduced converter power rating
- 49. Disadvantages of Doubly fed Induction Generator Increased control complexity due to increased number of switches in converter Stator winding is directly connected to the grid and susceptible to grid disturbances Increased capital cost and need for periodic slip ring maintenance Increased slip ring sensitivity and maintenance in offshore installations Is not direct drive and therefore requires a maintenance intensive gearbox for connection to wind turbine
- 50. Advantages of Wound field synchronous generator Minimum mechanical wear due to slow machine rotation Direct drive applicable further reducing cost since gearbox not needed Allow for reactive power control as they are self excited machines that don't require reactive power injection Readily accepted by electrically isolated systems for grid connection Allow for independent control of both real and reactive power
- 51. Disadvantages of Wound field synchronous generator Typically have higher maintenance costs again in comparison to that of an IG Magnet used which is necessary for synchronization is expensive Magnet tends to become demagnetized while working in the powerful magnetic fields inside the generator Requires synchronizing relay in order to properly synchronize with the grid
- 52. Modern Power Electronics (Key Self Commutated devices) Insulated gate bipolar transistor (IGBT) MOSFET Integrated Gate commutated Thyristors (IGCT) MOS-gate thyristors Silicon Carbide FETs Note: Self commutated systems normally adopt pulse width modulated (PWM) control methods
- 53. References Li, H., & Chen, Z. (2008). Overview of different wind generator systems and their comparisons. IET Renewable Power Generation, 2(2), 123-138.
- 54. Key Questions …. Compare the performance of various wind turbines used for wind power generation? Enumerates the need of converter for wind generators? Enumerates the need of capacitors for wind generators? Classify the wind generators according to the speed of rotation, gear train used and converter used? How the converters enhance the performance and help in integration of the turbine with grid?