Variable-Speed Constant-Frequency Double Output Induction Generator (Vscf-Doig).

Abstract

This dissertation investigates the Variable-Speed Constant-Frequency Double Output Induction Generator (VSCF-DOIG) which is capable of taking the mechanical energy directly from the shaft of a variable speed prime mover and converting it into ac energy at a constant frequency. The steady-state performance of the (VSCG-DOIG) is investigated using a control strategy that keeps the stator current constant and equal to its rated value while the generator delivers rated power output from the stator side and an amount of power from the rotor side that is proportional to the rotor speed. The analysis is carried out for two operating modes: first, by inserting an external resistance in the rotor circuit, and then, by replacing the external resistance with a dc link converter which acts as a variable resistance when the inverter firing angle is changed. Included is the practical realization of experimental three phase inverter firing circuits used. The analyses are based on the equivalent circuit and the d-q model technique. The overlap angle which is the duration of the time required to transfer the current from one phase to another is investigated for the case when the stator flux changes were neglected and then when these changes were included. In the latter case, the analyses are carried out in two ways: first, using the d-q model, and then, using the direct three-phase model. The analyses show that the overlap angle decreases as the slip increases. After finding the overlap angle, the waveforms of the stator current, the inverter current, the rotor current, the rotor voltage, and the torque are constructed, and the harmonic contents of these waveforms are found using Fourier analysis. Because this generator is well-suited for wind energy conversion systems, the interaction between the generator and a fixed pitch angle type of wind turbine is investigated by proposing two methods of operation: either by using a constant current control or by operating the wind turbine in a constant optimum power coefficient mode.