Electrical System Concepts Asynchronous generator with direct mains coupling

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A very simple wind generator concept is the so-called Danish concept. It is mainly used for small- and medium-sized wind generators that have been developed in Denmark. These systems connect a stall-regulated asynchronous generator directly to the mains (see Figure 5.33). A gearbox adjusts the speed of the rotor blades to the generator speed. This system concept appeals with its simplicity. The asynchronous generator need not be synchronized with the grid as with a synchronous generator. It reaches its operating speed without further control. However, very large generators can cause high starting currents when they are connected to the mains. Controlled-torque starting circuits can be used to limit these currents.

The stall regulation of the rotor limits the power at high wind speeds. The generator can cushion rapid fluctuations in the wind speed while it changes its speed through the slip s. Asynchronous wind generators allow changes in the speed of the order of about 10 per cent. However, losses get higher and efficiencies worse if the slip rises. Therefore, modern concepts use asynchronous machines with variable slip. These generators have no cage rotor with short-circuited ends of the windings, but controllable resistances Rr in the rotor circuit instead. The rotor windings are either connected via slip rings to controllable resistances outside the machine or to controllable resistances rotating with the rotor. Figure 5.34 shows the change of the speed-torque characteristics for asynchronous machines if resistances are connected to the rotor circuit. The breakdown torque moves to higher slip values for higher rotor resistances. Since the power is proportional to the torque, the speed increases at higher powers and cushions power fluctuations.

A speed-power diagram shows how well suited an asynchronous generator is for connection to the mains.

Table 5.8 Technical Data for a

600-kW Asynchronous Wind Generator

Nominal power PN

600 kW

Reactive power Q at full load

324 kvar

Rated voltage VN

690 V

cos^ at full load

0.88

Mains frequency f1

50 Hz

Speed range n

1515-1650 min-1

Rated current /N

571 A

Nominal speed nN

1575 min-1

Pole number 2p

4

Slip range s

-0.1-0

Winding connection

Star connection

Nominal efficiency nN

95.2%

Source: Vestas, 1997

Rotor

Gearbox

Generator

Figure 5.33 Asynchronous Generator with Direct Mains Coupling

"n >, the approximation for cp of a certain wind generator is:

cP = 0.00068 ■ 2: - 0.0297 • X2 + 0.3531 -1-0.7905 (see also Equation 5.45)

the dependence of rotor power P on the speed n for a constant wind speed v can be calculated (see Figure 5.35). However, the asynchronous generator allows only relatively small fluctuations in speed. The stator frequency and the gearbox define the rotor speed, and this can only be varied by the slip that increases with the power. Asynchronous generators with variable slip can increase the slip at higher powers. This can cushion high power fluctuations and high strains to the mains.

Figure 5.35 shows also that the rotor speed has an important influence on the usable wind energy. If the rotor speed is nearly constant, the wind generator

Mains

Slater

Mains

Slater

<otor

<otor

yifK=o

Slip S

Slip S

Figure 5.34 Torque Characteristics as a Function of Slip s with Variation of the Rotor Resistance R

. ^Variable v = 13 m/s

slip /

Asynchronous

/ \

generator

/ V = 12 m/s \

/ _____11 m/s \ \

_______ m/s \ \

____om/r^N. \ \

\ \ \

\ \ \

\ \ \ \ \

Rotor speed n

Rotor speed n

Figure 5.35 Operating Points for a Wind Turbine with Asynchronous Generator that is Directly Coupled to the Mains does not use the optimal power at all wind speeds. For this example, no power can be taken from the wind at wind speeds below 4 m/s because the rotor speed is too high. Maximum power usage is achieved at wind speeds of about 8 m/s. The relative share of the power that can be taken from the wind decreases with rising wind speeds.

System concepts with two different rotational speeds achieve higher energy gains. One concept uses two asynchronous generators that can be coupled to the rotor one after the other. Another concept applies one asynchronous change-pole generator. The stator of this generator has two separate windings with different numbers of poles. When switching from one winding to the other the stator speed also changes because the rotor speed directly depends on the pole pair number. Hence, systems that work over two speed ranges can be designed. Figure 5.36 shows that the ranges for taking the optimum power from the wind can be extended by two different generator speeds. The first asynchronous generator of this example operates at wind speeds between 3 m/s and 7 m/s, the second at wind speeds above 7 m/s.

The main disadvantage of the asynchronous machine is the demand for reactive current. If the asynchronous machine is connected to the mains, the mains can provide this reactive current. However, electric utilities usually want high compensatory payments so that it is more economical to install reactive power compensators. This can be, for instance, a capacitor bank as shown in Figure 5.33. This capacitor bank can usually be designed for some operating points only, so that the mains must still provide the remaining reactive power. Modern power electronics can better compensate the reactive power.

The conditions for island systems are totally different. Since the reactive power demand rises with increasing power, it must be controlled within the island system. Power electronic units or a synchronous machine, which operates as a phase modifier, can provide the respective reactive power demand.

ASG

' ' V = 13 im/$

M = 12 m/s Ny

--.11 m/s N. \

///

—_10 m/s \ \

Asynchronous

generator 1

\ \ \

g m/s ^^ \ \ \

fl m/s N. \ \ \ \

_____

m/s \ \ \ \ \

--

\ \

Rotor speed n

Figure 5.36 Operating Points for a Wind Turbine with Two Asynchronous Generators with Different Speeds

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Renewable Energy 101

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