Modeling and Analysis with Induction Generators, 3rd Edition

65

4

Self-Excited Induction

Generators

4.1 SCOPE OF THIS CHAPTER

As previously discussed in Chapter 2, the induction generator has a serious limita-

tion: an inherent need for reactive power. It consumes reactive power when connected

to the distribution network, and, actually, it needs an external reactive source perma-

nently connected to its stator windings to provide output voltage control. This source

of reactive power keeps the current going through the machine windings needed to

generate the variable magnetic eld. The movement of the rotor conductors causes

the voltage across the induction generator terminals. Another way of inducing this

voltage is to use the residual magnetism associated with an external capacitor that

generates current by rotor movement inside this magnetic eld, inducing voltage as

described in the following.

The advantages of the induction generator are enhanced under optimized condi-

tions of performance, as discussed in Chapter 10.

4.2 PERFORMANCE OF SELF-EXCITED

INDUCTION GENERATORS

Interaction among the operating states of the primary source of energy, the induction

generator, the self-excitation process, and the load state will dene the global perfor-

mance of the power plant.

1,2

Performance is greatly affected by the random character

of some of the variables involved related to the availability of primary energy and to

the way consumers use the load. Therefore, the performance of induction generators

depends on appropriate power plant specications at the design stage, in particular,

the following items:

• Parameters of the induction machine

• Operating voltage

• Rated power

• Rated frequency used in the parameter measurements

• Power factor of the machine

• Rotor speed

• Capacity for acceleration

• Isolation class

• Operating temperature

66 Modeling and Analysis with Induction Generators

• Carcass type

• Ventilation system

• Service factor

• Noise

• Load parameters

• Power factor

• Starting torque and current

• Maximum torque and current

• Generated harmonics

• Form of connection to the load: directly to the distribution network or

through converters

• Load type: resistive, inductive or capacitive, constant or variable, and

passive or active

• Evolution of the load over time

• Self-exciting process

• Degree of iron saturation of the generator caused by the choice of

capacitor

• Fixed or controlled self-excitation capacitor

• Speed control

• Type of primary source: hydro, wind, biomass, or combinations

Nonlinear loads like electronic power converters generate many harmonics,

and they can have a variable power factor (PF) if some control techniques are not

adopted. The harmonics are minimized with the installation of lters or the utiliza-

tion of supplementary signals. In general, the cost of passive lters is relatively small

with respect to the cost of the speed control in the conventional power plants exerted

by the electronic variation of frequency. The self-excitation capacitor in stand-alone

power plants or with electric or electronic control of the load contributes favorably

in these cases. Supplementary signals for harmonic minimization, in spite of being

a relatively recent technology and not very well established yet, seem to be more

promising techniques for such situations. The IEEE Std. 519 establishes the lim-

its of 2% of harmonic content for single- and three-phase induction motors (except

category N, that is, conventional) and 3% for high efciency (H and D). The other

standard is IEC1000-2-2.

In the case of stand-alone operation of power plants, the connection of a capac-

itor bank across the terminals of the induction generator is necessary, as dis-

played in Figure 4.1, to supply its need for reactive power. Notice that in practical

schemes it is advisable to connect each excitation capacitor across each motor

winding phase, in either Δ–Δ or Y–Y. The necessary capacitance of the bank will

depend on the primary energy and on the instantaneous load as discussed in the

following sections.

For a SEIG, the equivalent circuit in per unit values (p.u.) is shown in Figure4.2.

This circuit can be used to represent a more generic form of power plant.

2

The

frequency effect on the reactance should be considered if it is used at different

frequencies from the base frequency in Hertz f

b

at which the parameters of the