MICROPROCESSOR BASED VOLTAGE CONTROL OF STAND-ALONE INDUCTION GENERATOR

Abstract

This thesis covers the analysis, modelling and simulation of an isolated self excited induction generator (SEIG). A microprocessor-based closed-loop system has been developed for wind-driven stand-alone induction generators using a controlled rectifier to maintain a constant dc load voltage with varying rotor speeds. The configuration and implementation of the control scheme have been fully described. Results on a stand-alone induction generator demonstrate the satisfactory performance on software. A similar voltage build up is obtained when the isolated induction generator is excited using an inverter/rectifier system with a single DC capacitor on the DC link of the converter. In this type of excitation the voltage build up starts from a small DC voltage in the DC link and is implemented using vector control. The dynamic voltage, current, power and frequency developed by the induction generator have been analysed, simulated. To model the self excited induction generator accurate values of the parameters of the induction machine are required. The use of the variation in magnetizing inductance with voltage leads to an accurate prediction of whether or not self-excitation will occur in a SEIG for various capacitance values and speeds in both the loaded and unloaded cases. The characteristics of magnetizing inductance, Lm, with respect to the rms induced stator voltage or magnetizing current determines the regions of stable operation as well as the minimum generated voltage without loss of self-excitation. In the SEIG, the frequency of the generated voltage depends on the speed of the prime mover as well as the condition of the load. With the speed of the prime mover of an isolated SEIG constant, an increased load causes the magnitude of the generated voltage and frequency to decrease. This is due to a drop in the speed of the rotating magnetic field. When the-speed of the prime mover drops with load then the decrease in voltage and frequency will be greater than for the case where the speed is held constant. Dynamic simulation studies shows that increasing the capacitance value can compensate for the voltage drop due to loading, but the drop in. frequency can be compensated only by increasing the speed of the rotor.