Modeling and Analysis with Induction Generators, 3rd Edition

17

2

Steady-State Model of

Induction Generators

2.1 SCOPE OF THIS CHAPTER

The induction machine offers advantages for hydro- and wind power plants because

of its easy operation either as a motor or as a generator, robust construction, natural

protection against short circuits, and low cost compared with other generators. Only

the case of connection of the induction generator to the innite bus—the point of the

distribution network with no voltage drops across the generator terminals for any

load situation (ideal voltage source)—will be considered in this chapter.

Many commercial power plants in developed countries were designed to operate

in parallel with large power systems, usually supplying the maximum amount of

available primary energy for conversion in their surroundings (wind, solar, or hydro).

This solution is very convenient because the public network controls voltage and fre-

quency while static and reactive compensating capacitors can be used for correction

of the power factor and harmonic reduction. Aspects related to voltage regulation,

stand-alone generation, and output power are further discussed.

2.1.1 InterconnectIon and dIsconnectIonoftheelectrIc

dIstrIbutIon network

When interconnected to the distribution network, the induction machine should have

its speed increased until equal to the synchronous speed. The absorbed power of the

distribution network in these conditions is necessary to overcome the iron losses. The

energy absorbed by the shaft to maintain itself in synchronous rotation is necessary to

overcome mechanical friction and air resistance. If the speed is increased, a regenera-

tive action happens, but without supplying energy to the distribution network. This hap-

pens when the demagnetizing effect on the current of the rotor is balanced by a stator

component capable of supplying core losses. In this situation, the generator is supplying

its own iron losses. From this point on, the generator begins to supply power to the load.

The electrical frequency is the number of times per second a rotor pole passes in

front of a certain stator pole. If p is the number of poles, the synchronous speed is

determined by

n

f

p

s

s

=

120

(2.1)

where

n

s

is the synchronous mechanical speed in rpm

f

s

is the synchronous frequency in Hz