Electrical Generators

The mechanical power of a wind turbine is converted into electric power by usually an alternating current (AC) generator. The AC generator can be either a synchronous machine or an induction machine.

The electrical machine works on the principle of action and reaction of electromagnetic induction. The resulting electromechanical energy conversion is reversible. The same machine can be used as a motor for converting electric power into mechanical power or as a generator for converting mechanical power into electric power.

Figure 5.1 depicts common construction features of electrical machines. Typically, there is an outer stationary member (stator) and an inner rotating member (rotor). The rotor is mounted on bearings fixed to the stator. Both the stator and the rotor carry cylindrical iron cores, which are separated by an air gap. The cores are made of magnetic iron of high permeability and have conductors embedded in slots distributed on the core surface. Alternatively, the conductors are wrapped in the coil form around salient magnetic poles. Figure 5.2 is a cross-sectional view of a rotating electrical machine with the stator made of salient poles and the rotor with distributed conductors. The magnetic flux, created by the excitation current in one of the two coils, passes from one core to the other in a combined magnetic circuit always forming a closed loop. Electromechanical energy conversion is accomplished by interaction of the magnetic flux produced by one coil with the electrical current in the other coil. The current may be externally supplied or electromagnetically induced. The induced current in a coil is proportional to the rate of change in the flux linkage of that coil.

The various types of machines differ fundamentally in the distribution of the conductors forming the windings, and by their elements: whether they have continuous slotted cores or salient poles. The electrical operation of any given machine depends on the nature of the voltage applied to its windings. The narrow annular air gap between the stator and the rotor is the critical region of the machine operation, and the theory of performance is mainly concerned with the conditions in or near the air gap.

Turbine Conversion Systems

The early wind turbines were what is today called windmills that were used by the Persians (Iranians) in 640s for mechanical power. The first wind turbines used for electrical power generation used DC generators in 1890s. Over the decades the wind turbine conversion systems developed in terms of aerodynamics and mechanics, generator and power electronics, and the control schemes. In its modern form, one of the early turbines used an induction machine connected directly to the grid, and referred to direct online (DOL) as schematically shown in Figure 5.3. An issue with the DOL conversion is the lack of a control by the generator and dependency on a variable

Common constructional features of rotating electrical machines

FIGURE 5.1 Common constructional features of rotating electrical machines.

Cross-section of the electrical machine stator and rotor

FIGURE 5.2 Cross-section of the electrical machine stator and rotor.

Direct online (DOL) turbine

FIGURE 5.3 Direct online (DOL) turbine.

speed gearbox that is mechanically controlled. This made these turbines unpopular. In addition, the reactive current for the induction generator in the DOL scheme is supplied by the grid. To improve this a switched capacitance bank may be connected to the terminals of DOL scheme, as shown in Figure 5.4, where the capacitor bank supplies the required induction generator reactive power.

Introducing power electronics to the wind turbine power system, the wind-to- electrical power conversion efficiency gets significantly improved. Figure 5.5 shows a wind power system with doubly fed induction generator (DFIG), and two power electronics converters, an AC/DC and a DC/AC converter. The AC/DC and DC/AC converters in Figure 5.5 are connected in back-to-back arrangement, where they decouple the variable-speed variable-frequency rotor of the DFIG from the fixed- frequency fixed-voltage 3-phase grid. The rotor and back-to-back converters process a portion of the power while the DFIG stator is directly connected to the grid. In the past decade, the wind turbines with DFIGs have been widely used in the wind farms. However, the DFIG-based systems are limited in the power and speed operation as the converters are not fully rated.

To obtain full operational speed, the generator is decoupled from the grid using fully rated power electronics converters as shown in Figure 5.6. In this scheme, the

Direct online (DOL) with a capacitor bank to supply the excitation power

FIGURE 5.4 Direct online (DOL) with a capacitor bank to supply the excitation power.

Turbine conversion system with doubly fed induction generator (DFIG)

FIGURE 5.5 Turbine conversion system with doubly fed induction generator (DFIG).

Fully rated wind turbine power conversion system with power electronics for maximum power extraction

FIGURE 5.6 Fully rated wind turbine power conversion system with power electronics for maximum power extraction.

generator outputs are rectified to DC using an AC/DC converter. A DC/AC converter is then used to convert the DC to a 3-phase AC output voltage with fixed magnitude and frequency. The two power electronics converters are arranged in back-to-back and usually use a voltage source converter (VSC) form. The generator may be permanent magnet generator, induction generator, or conventional synchronous generator.

As the wind velocity changes, the output voltage, frequency, and power of the generator vary. The AC/DC converter controls the generator to extract maximum power from the wind at each wind velocity, a concept referred to as maximum power point tracking (MPPT). The generator’s variable voltage and frequency output is then converted to DC. The DC/AC converter controls the DC-link around a nominal fixed value, which is essential for the correct operation of the AC/DC converter. The frequency and voltage at the output of DC/AC converters are fixed. As the conversion scheme in this scheme is rated at low voltage, a step-up transformer is used to increase the voltage suitable for transmission to a substation that collects the power from all turbines.

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