published_or_final_version / abstract / Electrical and Electronic Engineering / Master / Master of Philosophy

The Brushless Doubly-Fed Machine (BDFM) has recently become an important research topic in the field of variable-speed AC drives. In recent studies, the BDFM has shown significant potential for improving the reliability and performance of AC drive systems, as well as reducing total system cost. While the BDFM offers several advantages over existing AC drives in steady-state operation, it suffers from dynamic instabilities and slow response times, and a feedback control system is necessary. The mathematics of the BDFM are much more complicated than those of a singly-fed machine, and thus traditional control methods can't be applied. In this thesis, a control method known as "Direct Torque Control" has been adapted from that of a singly-fed induction machine and successfully applied to the BDFM. The thesis begins by discussing the background of the BDFM, its open-loop operating characteristics, and some of the control considerations. The reduced-order system differential equations are introduced, and it is noted that they are coupled and nonlinear. Furthermore, all state variables are time-varying (but periodic), even in steady-state operation. In the controller development, it is found that a linear relationship exists between the desired torque/flux-level change and the d-q voltages to be applied to the control winding of the machine via the power-electronic converter. This linear relationship, together with a one-step-ahead predictor to compensate for computational delay, is successfully used to control the speed and efficiency of the machine, for a wide range of speeds and load torques. Numerous open- vs. closed-loop simulations are compared and summarized, and it is found that the performance of the BDFM is greatly improved in the closed-loop, with faster response and reduced oscillation. Further simulations investigating the robustness of the controller are summarized, and it is found that the controller is reasonably insensitive to errors in most of the the static machine parameters. Hardware implementation is briefly discussed but is not complete; laboratory results are not yet available but should be soon. Future controller considerations are then discussed; included among the recommendations are an on-line parameter estimator for use in adaptive control, and a controller for generator applications of the BDFM. / Graduation date: 1993

The loads on the electrical systems of automobiles are projected to increase significantly in the near future. This will result in a requirement for improved efficiency over the present-day car alternators. An alternative scheme proposed at Oregon State University employs a Brushless Doubly-Fed Machine (BDFM) as an alternator. This thesis begins with a study and characterization of the existing car alternator system. The configuration of the proposed scheme is discussed. In the proposed configuration, the power winding of the machine generates the bulk of the power and the control winding provides the excitation. The power winding feeds a power rectifier, which in turn charges the battery in an automobile. The control winding is supplied through an inverter. Issues related to inverter and rectifier design are discussed. A 3-phase pulse width modulated inverter and a bridge rectifier were developed and tested for performance. A PSPICE simulation model for the rectifier was developed and results are compared with laboratory tests. A Voltage Regulator Circuit (VRC) and an Efficiency Maximizer Unit (EMU) for the system are designed and developed. A prototype alternator system is tested and the principle of efficiency maximization is verified. Finally. the comparative performance of the the existing and the proposed system is discussed and some recommendations for further improvements in the prototype system are made. / Graduation date: 1992

This research report is submitted as a partial fulfilment of the requirements for a Master of Science degree in Electrical Engineering (Power). / Thyristor Controlled Series Capacitors (TCSCs) are FACTS devices which incorporate power-electronic-based and other static controllers to enhance controllability and increase power transfer capability. This research investigated the possibility of applying TCSCs on the South African transmission network, in particular application on long transmission lines connecting bulk thermal generators in the northern part of South Africa to load centres located hundreds of kilometres elsewhere in the country. The investigation, conducted using PSS/E (a power system analysis software) demonstrated that application of TCSCs on this part of South African transmission network results in improved transient stability margins. The resulting improvement in transient stability is comparable to other transient stability enhancement options like addition of transmission lines, thus the use of TCSCs can be considered as an alternative. Further studies would be vital to understand the interaction between Power System Stabilizers (PSSs) and TCSCs in order to ensure proper tuning and interaction amongst the devices. / MT2017

Modern Control Theory is applied to analyze an existing aluminum mill drive, which was originally designed by Classical Methods. The system is studied with Optimal Control and z-transform zero-pole locus tools. The results and also the original design are compared with the performance of simulation. The models are built with the transition matrix and iterative procedures. Conclusions about the appropriated design methods and improvements to the control scheme result from the investigation. / Master of Science / incomplete_metadata