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Wind Turbine Session 4. Fixed-speed wind turbines.
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Fixed-speed wind turbines • In the early 1990s the standard installed wind turbines operated at fixed speed. That means that regardless of the wind speed, the wind turbine’s rotor speed is fixed and determined by the frequency of the supply grid, the gear ratio and the generator design.
Fixed-speed Wind Turbines The generator in fixed-speed wind turbines is of the induction type connected directly to the grid. Synchronous generators have been used in some early prototypes. but the induction machine has been more widely adopted because of lower cost and a superior mechanical compatibility with rapid wind variations.
Fixed-speed Wind Turbines • The generator together with a gearbox are placed in a nacelle on the top of the tower. The function of the gearbox is to change the low rotational speed of the turbine to a high rotational speed on the generator side.
Fixed-speed Wind Turbines • The rotational speed of an induction generator is typically 1000 or 1500 rpm. The turbine speed is dependent on the rotor diameter, for example a 200 kW turbine has a rotational speed of approximately 50 rpm, while the rotational speed of a 1000 kW turbine is approximately 30 rpm.
Fixed-speed Wind Turbines • A fixed-speed wind turbine is designed to obtain maximum efficiency at one wind speed that will give the optimum tip speed to wind speed ratio for the rotor airfoil.
Fixed-speed Wind Turbines • In order to capture more energy, some fixed-speed wind turbines have two different rotational speeds. This can be achieved either by two generators or by one generator with two windings.
Fixed-speed wind turbines It is characteristic of fixed-speed wind turbines that they are equipped with an induction generator (squirrel cage or wound rotor) that is directly connected to the grid, with a soft-starter and a capacitor bank for reducing reactive power compensation.
Fixed-speed wind turbines • The advantages: • being simple • robust and reliable and well-proven • the cost of its electrical parts is low.
Fixed-speed wind turbines • The disadvantages: • an uncontrollable reactive power consumption, • mechanical stress • limited power quality control.
Fixed-speed wind turbines • Owing to its fixed-speed operation, all fluctuations in the wind speed are further transmitted as fluctuations in the mechanical torque and then as fluctuations in the electrical power on the grid.
Fixed-speed wind turbines • In the case of weak grids, the power fluctuations can also lead to large voltage fluctuations, which, in turn, will result in significant line losses.
Variable-speed wind turbines • Variable-speed wind turbines are designed to achieve maximum aerodynamic efficiency over a wide range of wind speeds.
Variable-speed wind turbines • With a variable-speed operation it has become possible continuously to adapt (accelerate or decelerate) the rotational speed ω of the wind turbine to the wind speed v.
Variable-speed wind turbines • This way, the tip speed ratio is kept constant at a predefined value that corresponds to the maximum power coefficient.
Variable-speed wind turbines • Contrary to a fixed-speed system, a variable-speed system keeps the generator torque fairly constant and the variations in wind are absorbed by changes in the generator speed.
Variable-speed wind turbines • The electrical system of a variable-speed wind turbine is more complicated than that of a fixed-speed wind turbine.
Variable-speed wind turbines • It is typically equipped with an induction or synchronous generator and connected to the grid through a power converter.
Variable-speed wind turbines • The power converter controls the generator speed; that is, the power fluctuations caused by wind variations are absorbed mainly by changes in the rotor generator speed and consequently in the wind turbine rotor speed.
Variable-speed wind turbines The advantages: • increased energy capture • improved power quality • reduced mechanical stress on the wind turbine.
Variable-speed wind turbines The disadvantages are: • losses in power electronics • the use of more components • the increased cost of equipment because of the power electronics.
Types of Turbine • Type A: constant Speed • Stall Controlled • Pitch controlled • Type B: Variable Speed (Rotor Resistance) • Type C: Variable Speed Doubly Fed • Type D: Variable Speed Full power Converter
Type A: fixed speed • This configuration denotes the fixed-speed wind turbine with an asynchronous squirrel cage induction generator (SCIG) directly connected to the grid via a transformer. Since the SCIG always draws reactive power from the grid, this configuration uses a capacitor bank for reactive power compensation. A smoother grid connection is achieved by using a soft-starter.
Type A: fixed speed • Type A are used in the wind turbine industry, and they can be characterised as follows: • Type A0: stall control • Type A1: pitch control • Type A2: active stall control
Type B: limited variable speed • It uses a wound rotor induction generator (WRIG) and has been used by the Danish manufacturer Vestas since the mid-1990s. The generator is directly connected to the grid. A capacitor bank performs the reactive power compensation.
Type B: limited variable speed • The unique feature of this concept is that it has a variable additional rotor resistance, which can be changed by an optically controlled converter mounted on the rotor shaft. Thus, the total rotor resistance is controllable.
Type B: limited variable speed • This optical coupling eliminates the need for costly slip rings that need brushes and maintenance. The rotor resistance can be changed and thus controls the slip. This way, the power output in the system is controlled.
Type C: variable speed with partial scale frequency converter • This configuration, known as the doubly fed induction generator (DFIG) concept, corresponds to the limited variable speed wind turbine with a wound rotor induction generator (WRIG) and partial scale frequency converter (rated at approximately 30% of nominal generator power) on the rotor circuit
Type C • The converter compensates the difference between the mechanical and electrical frequency by injecting a rotor current with a variable frequency.
Type C • The concept of the DFIG is an interesting option with a growing market. The DFIG consists of a WRIG with the stator windings directly connected to the constant-frequency three-phase grid and with the rotor windings mounted to a bidirectional back-to-back voltage source converter.
Type C • The term ‘doubly fed’ refers to the fact that the voltage on the stator is applied from the grid and the voltage on the rotor is induced by the power converter. This system allows a variable-speed operation over a large, but restricted, range.
Type C • Both during normal operation and faults the behavior of the generator is thus governed by the power converter and its controllers.
Type C • The power converter consists of two converters, the rotor-side converter and grid-side converter, which are controlled independently of each other.
Type C • The main idea is that the rotor-side converter controls the active and reactive power by controlling the rotor current components, while the line-side converter controls the DC-link voltage and ensures a converter operation at unity power factor (i.e. zero reactive power).
Type C • Depending on the operating condition of the drive, power is fed into or out of the rotor: in an over synchronous situation, it flows from the rotor via the converter to the grid, whereas it flows in the opposite direction in a sub synchronous situation. In both cases – sub synchronous and over synchronous – the stator feeds energy into the grid.
Type C • The DFIG has several advantages. It has the ability to control reactive power and to decouple active and reactive power control by independently controlling the rotor excitation current.
Type C • The DFIG has not necessarily to be magnetised from the power grid, it can be magnetised from the rotor circuit, too.
Type C • It is also capable of generating reactive power that can be delivered to the stator by the grid-side converter. However, the grid-side converter normally operates at unity power factor and is not involved in the reactive power exchange between the turbine and the grid.
Type C • In the case of a weak grid, where the voltage may fluctuate, the DFIG may be ordered to produce or absorb an amount of reactive power to or from the grid, with the purpose of voltage control.
Type C • The size of the converter is not related to the total generator power but to the selected speed range and hence to the slip power. Thus the cost of the converter increases when the speed range around the synchronous speed becomes wider.
Type C • The selection of the speed range is therefore based on the economic optimization of investment costs and on increased efficiency. A drawback of the DFIG is the inevitable need for slip rings.
Type C • The partial scale frequency converter performs the reactive power compensation and the smoother grid connection. It has a wider range of dynamic speed control compared with the OptiSlip, depending on the size of the frequency converter.
Type D: variable speed with full-scale frequency converter • This configuration corresponds to the full variable speed wind turbine, with the generator connected to the grid through a full-scale frequency converter. The frequency converter performs the reactive power compensation and the smoother grid connection.
Type D: variable speed with full-scale frequency converter • The generator can be excited electrically [wound rotor synchronous generator (WRSG) or WRIG) or by a permanent magnet [permanent magnet synchronous generator (PMSG)].
Type D: variable speed with full-scale frequency converter • Some full variable-speed wind turbine systems have no gearbox . In these cases, a direct driven multipole generator with a • large diameter is used. The wind turbine companies Enercon, Made and Lagerwey are examples of manufacturers using this configuration.
