THREE PHASE INDUCTION GENERATORS
In Chapter 7 we alluded to the induction generator mode both in stand-alone (capacitor excited) and grid-connected situations. In essence for the cage-rotor generator mode, the slip S is negative (S < 0). As the IM with a cage rotor is not capable of producing, reactive power, the energy for the machine magnetization has to be provided from an external means, either from the power grid or from constant (or electronically controlled) capacitors. The generator mode is currently used for braking advanced PWM converter fed drives for industrial and traction purposes. Induction generators-grid connected or isolated (capacitor excited)–are used for constant or variable speed and constant or variable voltage/frequency, in small hydro power plants, wind energy systems, emergency power supplies, etc. . Both cage and wound rotor configurations are in use. For a summary of these possibilities, see Table 19.1. Cogeneration of electric power in industry at the grid (constant voltage and frequency) for small range variable speed and motor/generator operation in pump-back hydropower plants are all typical applications for wound rotor IMs. In what follows, we will treat the main performance issues of IGs first in stand-alone configurations and then at power grid. Though changes in performance owing to variable speed are a key issue here, we will not deal with power electronics or control issues. We choose to do so as IG systems now constitute a mature technology whose in-depth (useful) treatment could be the subject matter of an entire book [25,26].