Wind power technology - Wind Power Integration: Connection and System Operational Aspects - page 70

3

10 R r

2

8 R r

6 R r

4 R r

1

2 R r

Generating
region

R r

0

Motoring
region

-1

-2

-3

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Slip (pu)

Figure 3.15 Torque-slip curves showing the effect of rotor circuit resistance

3.5.2 Fixed-speed induction generator-based wind turbine
In a fixed-speed wind turbine the induction generator, operating typically at 690 V,
transmits power via vertical pendant cables to a switchboard and local transformer,
usually located in the tower base (see Figure 3.16). Switched power factor cor-
rection capacitors are used to improve the power factor of the induction generator
while an anti-parallel thyristor soft-start unit is used to energise the generator once
its operating speed is reached. The function of the soft-start unit is to build up the
magnetic flux slowly and so minimise transient currents during energisation of the
generator. Also, applying the network voltage slowly to the generator, once ener-
gised, brings the drive train slowly to its operating rotational speed.
A pitch-regulated rotor is able to control the generator speed during this
starting period but a fixed pitch, stall-regulated turbine is allowed to run up, driven
by the wind, and the generator connected at slightly below synchronous speed.
Large steam-turbine generators supplying national electrical power systems all
use synchronous machines. Their advantages include high efficiency and the ability
to control independently the real output power ( P ) through adjusting the torque on
the shaft by a mechanical governor and the reactive output power ( Q ) by varying
the rotor field current. However, wind turbine aerodynamic rotors develop a sig-
nificant torque pulsation at the blade passing frequency, caused by tower shadow