Modeling and Analysis with Induction Generators, 3rd Edition

249

11

Doubly Fed Induction

Generators

11.1 SCOPE OF THIS CHAPTER

There are two main types of induction machines: squirrel cage machines, with rotor

windings short-circuited, and wound rotor induction machines, with accessible slip

rings that can be either short-circuited or connected to an external circuit. Wound

rotor machines are used in applications where it is desirable to inuence the rotor

circuit, keeping the stator supply constant. Before the advent of semiconductors,

external resistors were used to control the slip, and, consequently, power was lost in

those resistors. By using power electronic converters, it is possible to recover such

slip power. Thus, a wound rotor induction generator (WRIG) with power electronic

converters, that is, fed on both stator and rotor, is called a doubly-fed induction gen-

erator (DFIG).

In this chapter, a theoretical and practical coverage of DFIG systems will pro-

vide a foundation for its application to high-power renewable energy systems. Since

the control is tied to the rotor, only the slip power is processed where the purpose

of thecontrol is to synchronize the rotor current with respect to stator reference.

Thus,the smaller the operating slip range, the smaller is the required power con-

verter. The size of the power converter may strongly affect the installation cost of

energy ($/kW). The utility side of the power converter of the DFIG can be con-

trolled differently from both sides of the power converter. The power factor can be

controlled either by the utility side or by the rotor side of the DFIG. The generator

operates in four quadrants around the synchronous speed. Therefore, it can easily

operate in motoring mode, for example, for pumped hydro applications.

11.2 FEATURES OF DFIG

The WRIG is not as rugged as the squirrel cage type, but the brushes have little wear-

ing and sparking when compared with dc machines and they are the only acceptable

alternatives for alternative energy conversion on the megawatt power range.

The fundamentals of slip recovery systems have been known since the beginning

of the twentieth century. Between 1907 and 1913, Kraemer and Scherbius proposed

cascaded connections to the rotor of induction machines using rotary machines.

The Kraemer system transformed the slip energy back into mechanical energy by

a second machine in tandem connection to the shaft, while Scherbius proposed a

second induction machine connected to the rotor to send the slip power back to

250 Modeling and Analysis with Induction Generators

the line. By using modern power electronic devices, it is possible to recover the

slip otherwise dissipated in resistances. Therefore, wound rotor generators use an

inverter system connected between the rotor and the grid, while the stator is directly

connected to the grid. When the mechanical speed is conned to ±20% around the

synchronous speed, the rotor converter is rated only for a portion of the rated stator

power, a clear advantage for very-high-power systems.

Figure 11.1 shows a model for a wound rotor induction machine.

Compared with the squirrel cage induction generator (SCIG), the DFIG operating

at variable speed has the following advantages:

• The SCIG requires a power converter with full power-processing capabil-

ity, whereas the DFIG requires a smaller power converter to process the

slip power. Thus, the smaller the range of the operating slip, the smaller

the required power converter. The size of the power converter may strongly

affect the cost of energy ($/kWh).

• The DFIG has a simpler control because the magnetizing current is practi-

cally constant regardless of the rotor frequency. The purpose of the control

is to synchronize the rotor current with respect to stator reference.

• The utility connection to the DFIG can be controlled from both machine

sides, and the power factor can be controlled either by the stator or by the

rotor side of the DFIG.

• The DFIG can be commanded by two techniques: either using (1) active

and reactive power from the stator side to control the rotor converter or

(2)setting up the stator voltage from the rotor side.

• If a DFIG system is capable of operating in four quadrants around the

synchronous speed, it can then operate in motor mode, for example, for

pumped hydro applications.

• The DFIG has increased power system dynamics and stability, permitting

the suppression of power system uctuations by quickly exchanging energy

from the electric system to the machine inertia without decreasing synchro-

nism as happens with synchronous machines. The system can also compen-

sate for quick reactive power needs, and for this reason, it can improve the

overall power system dynamics.

ψ

qds

ψ

qdr

+ –

i

qdm

R

r

L

ℓs

L

ℓr

jω

e

ψ

qds

j(ω

e

–ω

r

)ψ

qdr

R

s

i

qds

i

qdr

V

qds

V

qdr

=0

–+

L

m

FIGURE 11.1 Complex synchronous dq

s

equivalent circuit for wound rotor induction

machine.