Modeling and Analysis with Induction Generators, 3rd Edition

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13

Applications of Induction

Generators in Alternative

Sources of Energy

13.1 SCOPE OF THIS CHAPTER

This chapter deals with the most ordinary applications of induction generators. Such

applications are usually based on the variable speed features of the induction genera-

tor, which enables the control of a wide set of variables such as frequency, speed,

output power, slip factor, voltage tolerance, reactive power, and others. In its variable

speed operation, an asynchronous link (dc link) is usually interposed between the

generator and the load or network to which the power is being delivered. The sim-

plest application of the induction generator is its direct connection to a grid, which

is sufcient to guarantee the electrical rotation above the synchronous to control a

scheduled power ow. Stand-alone applications are also common in cases of low

levels of power generation. In all these cases, a well-designed speed control or speed

limiter is mandatory for good results.

The primary energy, turbine type, and number of poles in the generator deter-

mine the speed, commonly stated in rotations per minute (rpm). Most electrical

loads demand that generators must be driven at a speed that will generate a steady

power ow at a frequency of 50/60Hz. The number of poles will dene the neces-

sary shaft speed of the turbine. For US 60Hz frequency applications, a two-pole

generator demands speeds as high as 3600 rpm, making it too high for practical use

with small wind power or hydropower plants. For hydropower systems, the 1800 rpm

four-pole generator is the most commonly used. For small wind power systems, the

900 rpm eight-pole is often encountered in eld applications. In many cases, it is

necessary to use speed multipliers, making the whole system heavier, more main-

tenance demanding, and relatively less efcient, so the cost of the generation unit

is more or less inversely proportional to the turbine speed and the type of primary

energy. The lower the speed, the larger the frame size needs to be for equivalent

output power.

Induction generators are generally appropriate for smaller systems. They have the

advantage of supplying an almost constant output frequency at variable speeds. The

power ow in this case will depend on the slip factor. In addition, they are lighter,

rugged, and cheaper than synchronous generators.

As discussed in Chapter 6, the induction generator is a standard three-phase

induction motor, wired to operate as a generator. Self-excitation capacitors are used