Wind Power - Understanding Renewable Energy Systems - page 225

The power factor cos ϕ is 0.88, far from the ideal value cos ϕ = 1. During part-

load operation, cos is even worse and reaches values around 0.5 at 25 per

cent part load. This causes a high amount of reactive power that must be
reduced by compensation circuits. A capacitor bank for instance can make this
compensation and achieve cos

ϕ

ϕ factors of about 0.99.

E LECTRICAL S YSTEM C ONCEPTS

Asynchronous generator with direct mains coupling
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 R R 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 P N 600 kW Reactive power Q at full load 324 kvar

ϕ Rated voltage V N 690 V cos at full load 0.88

1515-1650 min -1

Mains frequency f 1 50 Hz Speed range n

1575 min -1 Rated current I N 571 A Nominal speed n N

Pole number 2· p 4 Slip range s -0.1-0

η N Winding connection Star connection Nominal efficiency 95.2%

Source: Vestas, 1997