Wind Energy Systems - Wind Energy Systems for Electric Power Generation - page 89

a) b)

Fig. 5.12 Torque and power characteristics of the cascade system ( a ) Torque characteristics; ( b )
Power characteristics, example for k 2 = 0 , 1

5.3.3.3 Doubly-Fed Asynchronous Machine

The term doubly-fed induction machine applies to a system where both stator and
rotor winding of a slip-ring machine are supplied. With grid-connected primary
winding and a rotor-side converter adapting the slip power to the grid side the sys-
tem acts as a cascade. Use can be made of a cyclo-converter or a converter with
intermediate d.c. circuit as shown in Fig. 5.13. Both rotor-side and grid-side inverter
(MSI, GSI) are self-controled devices, allowing active power transfer in both direc-
tions and the adjustment of reactive power on both sides. Hence the cascade can,
different from the Kramer system in Fig. 5.9, also operate in supersynchronous mo-
tor and subsynchronous generator mode. Note that Fig. 5.13 contains a filter (F) and
a transformer (T) to adapt the rotor side voltage to the grid. In most cases a crowbar
is additionally provided at the rotor-side, by which in case of grid faults the rotor
is switched to an external resistor to protect the machine-side inverter from excess
current.
For wind energy system purposes the cascade is usually equipped with a VSI
type inverter. A common design realizes a speed interval of ratio 1:2, by means
of a control such that the range between lower and upper speed is approximately
2 / 3 < n / n syn < 4 / 3, where n syn is the machine synchronous speed at grid frequency.

Fig. 5.13 Doubly-fed induction generator with rotor-side converter