Self-excited induction generator : a study based on nonlinear dynamic methods

Abstract

An induction generator offers advantages in terms of its low cost, simplicity, robust construction, nature protection against short circuits and ease of maintenance in today’s renewable energy industry. However, the need for an external supply of reactive power (to produce a rotating magnetic flux wave) limits the application of an induction machine as a standalone generator. It is possible for an induction machine to operate as a Self-excited Induction Generator (SEIG) if capacitors are connected to the stator terminals in order to supply the necessary reactive power to achieve generating electrical energy in remote areas. Poor voltage and frequency regulation is the main drawback of a SEIG as the system is highly dynamic under variable load conditions. The regulation of speed and voltage does not result in a satisfactory level although many studies have been focused on this topic in the past. Therefore, the aim of the thesis is to provide a better understanding of the behaviour of a smooth airgap, selfexcited, squirrel cage induction generator as a nonlinear dynamic system when operating under a variety of load conditions, which would hopefully contribute to the development of a better regulated/controlled, viable SEIG system. Allowing for the cross-saturation nonlinear effect, a mathematical Simulink, d -q axis model of the SEIG system utilising currents as state space variables is developed and verified by both the experimental results and numerical analysis. The SEIG computer model is constructed and tested using Matlab/Simulink R2010b throughout the thesis. The self-autonomous system is shown to exhibit a transition from a stable periodic orbit to a quasi-periodic orbit (leading to likely chaotic motion) through a Neimark bifurcation, as a result of small changes in the values of system parameters (such as load resistance, load inductance, rotational speed and self-excitation capacitance). This characteristic dynamic behaviour of the SEIG system is firstly identified in this work and is verified experimentally using a laboratory test rig. The stability of the periodic and quasi-periodic orbits exhibited by the SEIG system when feeding an inductive load ( RL) is numerically analysed and the movement of the eigenvalues of the system’s characteristic matrix when changing a system parameter is presented to verify the qualitative change in system behaviour from a stable period-one orbit to unstable quasi-periodicity. Eigenvalue technique is successfully applied to assess the stability of the period-one and quasi-periodic orbits of the SEIG when feeding variable load conditions.