Wind turbine technology Fixed speed: Variable speed: Fixed-speed induction generator (FSIG) Doubly fed induction generator (DFIG) Variable-speed wind turbines have more control flexibility and improve system efficiency and power quality Fully-rated converter wind turbine (FRC)

Induction machine construction The rotor of an induction machine may be one of two types: Squirrel cage The rotor of a squirrel-cage machine carries a winding consisting of a series set of bars in the rotor slots which are short circuited by end rings at each end of the rotor. For analysis purposes a cage rotor may be treated as a symmetrical, short circuited star-connected three-phase winding. No access to rotor ‘windings’. Wound rotor The rotor of a wound rotor machine carries a three-phase distributed winding with the same number of poles as the stator. This winding is usually connected in star with the ends of the winding brought out to three slip rings, enabling external circuits to be added to the rotor for control purposes.

Typical torque-speed characteristic At standstill the speed is zero and the slip is equal to 1 per unit (pu). Between zero and synchronous speed, the machine performs as a motor. Beyond synchronous speed the machine performs as a generator

FSIG configurations for wind generation Fixed-Speed Induction Generator (FSIG)-based wind turbines employ a squirrel-cage induction generator directly connected to the network. The slip (and hence the rotor speed) varies with the amount of power generated. In these turbines the rotor speed variations are very small (1 or 2%). The induction generator consumes reactive power so capacitor banks are used to provide the reactive power and improve the power factor. An anti-parallel thyristor soft-start unit is used to energise the generator and once its operating speed is reached it is bypassed. Avoids overcurrents at start-up. Power control is typically exercised through pitch control.

Two-speed operation Wind turbine rotors develop their peak efficiency at one particular tip speed ratio. But the FSIG rotates at one speed that does not vary. Energy capture can be increased by varying the rotational speed with the wind speed so that the turbine is always running at optimum tip speed ratio (DFIG and FRC machines), or alternatively a slightly reduced improvement can be obtained by running the turbine at one of two fixed speeds so that the tip speed ratio is closer to the optimum than with a single fixed speed. Two-speed operation is relatively expensive to implement if separate generators are used for each speed of turbine rotation. Either generators of differing number of poles may be connected to gearbox output shafts rotating at the same speed, or generators with the same number of poles are connected to output shafts rotating at different speeds. The rating of the generator for low-speed operation would normally be much less than the turbine rating.

Configuration for variable-slip operation Induction generator with a variable resistor in series with the rotor circuit, controlled by a high frequency semiconductor switch. Below rated wind speed and power, this acts just like a conventional fixed-speed induction generator Above rated, however, control of the resistance effectively allows the airgap torque to be controlled and the slip speed to vary, so that behaviour is then similar to a variable-speed system A speed range of about 10 percent is typical with a consequent energy loss of 10% in the additional resistor

Reactive power equipment Reactive power compensation for a wind turbine generator must be designed to manage power factor and voltage regulation requirements at the point of common coupling with the local grid under all normal operating conditions. Typically, reactive power compensation is performed at each individual wind turbine in the form of switched shunt capacitor banks, dependent on the reactive power requirements and characteristics of the generator. The power factor correction capacitors are generally switched in stages to provide a greater degree of control of reactive power and also to limit capacitive switching currents. Small reactors may be connected in series with the capacitors to reduce the inrush current.

FSIG dynamic performance Connection of an FSIG-based wind turbine to an infinite bus. Three-phase earth fault occurs at point A.

Step change in infinite bus voltage System stable System unstable – generator runs away

Performance during network faults FSIG performance during a sustained fault – machine runs away

Wind turbine technology Fixed speed: Variable speed: Fixed-speed induction generator (FSIG) Doubly fed induction generator (DFIG) Variable-speed wind turbines have more control flexibility and improve system efficiency and power quality Fully-rated converter wind turbine (FRC)

FRC-based wind turbine This wind turbine uses either an induction generator or a synchronous generator (it can either be an electrically excited synchronous generator or a permanent magnet machine). The aerodynamic rotor and generator shafts may be coupled directly, or they can couple through a gear box. To enable variable-speed operation, the synchronous generator is connected to the network through a variable frequency converter, which completely decouples the generator from the network. The electrical frequency of the generator may vary as the wind speed changes, while the network frequency remains unchanged. The rating of the power converter in this wind turbine corresponds to the rated power of the generator.

Types of systems FRC-based wind turbine Both induction and synchronous generators are being used To grid

FRC-based wind turbine To grid IGBT-based Voltage Source Converter Thyristor-based Phase-controlled rectifier Diode-based recifier Generator-side converter configurations

FRC-based wind turbine Power control methods Method 1: The VSC controls generator power and the inverter looks after the dc-link

FRC-based wind turbine Method 2: The inverter controls generator power and the VSC looks after the dc-link voltage

FRC-based wind turbine FRC with permanent magnet synchronous generator and diode rectifier

FRC-based wind turbine FRC with permanent magnet synchronous generator and two back-to-back voltage source converters C1 C2 C1 maintains the generator at the maximum power extraction curve. C2 maintains the DC-link voltage at a specified reference value by exporting active and reactive power with the grid. C1 can be implemented using either load angle or vector control technique.

Wind turbine technology Fixed speed: Variable speed: Fixed-speed induction generator (FSIG) Doubly fed induction generator (DFIG) Variable-speed wind turbines have more control flexibility and improve system efficiency and power quality Fully-rated converter wind turbine (FRC)

Typical DFIG wind turbine Windmill Wound rotor induction generator Gearbox DFIG PWM Converters Power Network C1 C2 Crowbar Network operator CONTROL SYSTEM

Typical DFIG wind turbine Doubly-Fed Induction Generator (DFIG)-based wind turbines employ a wound rotor induction generator with slip rings to take current into or out of the rotor. Variable-speed operation is obtained by injecting a controllable voltage into the rotor at slip frequency. The rotor winding is fed through a variable frequency power converter. The power converter decouples the network electrical frequency from the rotor mechanical frequency enabling the variable-speed operation of the wind turbine. The generator and converters are protected by voltage limits and an over-current ‘crowbar’.

DFIG power electronic converters Converter C1 Converter C2 DC-link Machine Side (rotor) Grid side (Machine stator) Back-to-back voltage source converters (VSCs) Graetz bridge (two-level VSC) IGBT-based Pulse Width Modulated (Sinusoidal, Space Vector PWM) Typical switching frequencies above 2 kHz Trade-off between switching frequency (losses) and harmonics

P P r > s r < s DFIG power relationships A DFIG system can deliver power to the grid through the stator and rotor, while the rotor can also absorb power. This is dependent upon the rotational speed of the generator P r > s Super synchronous operation (slip negative) P r < s Sub synchronous operation (slip positive)

Relationship of Ps, Pr, and Pe

DFIG power relationships Mechanical Input Electrical Output Stator losses Rotor losses Slip Power through the slip rings : Mechanical power delivered to the generator : Power delivered by the rotor : Power at the generator’s air gap : Power delivered by the stator

Summary Basics of conversion of mechanical energy to electrical energy Introduction to synchronous and induction machines Introduction to basic large-scale wind generator technologies (FSIG, DFIG, FRC). Advantages and disadvantages of each system.