Thesis (Ph. D.)--University of Nevada, Reno, 2005. / "August 2005." Includes bibliographical references (leaves 143-154). Online version available on the World Wide Web.

This thesis presents both an analysis and simulations of a passively controlled variable-speed generator system, which can be applied for renewable energy sources such as wind turbines. Parallel connected passive/external resistors and inductors are connected to the slip rings of a wound rotor induction machine to provide an acceptable speed operation range, while maintaining high efficiency of the generator system. Two generators, of 80 kW and 186 kW ratings, have been tested and compared to their simulations and good correlation has been obtained. It is shown by both modelling and by laboratory tests that the steady-state power characteristic is well suited to the application and the efficiency compares well with similar rated machines in which either a fixed speed is required or power electronic converters provide the adjustable speed control. Thus, the tested systems are comparable in energy capture while being lower in costs and being both more robust and more reliable. In consequence, it is a more practical solution than power electronics for remote locations. The dynamic results indicate that the generator is dynamically stable following three types of transient conditions: connections to the grid at non-synchronous speed; sudden decreases and increases in applied torque to simulate wind gusts; cyclic torque changes to demonstrate tower shadow effects. Unreasonable transients and undamped conditions have been neither predicted nor observed. Finally, it is proposed that the external elements could be developed to be linked to the rotor circuit without slip rings enabling a complete passive and brushless system. / Graduation date: 2000

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

Thesis (M.S. in electrical engineering)--Washington State University, December 2009. / Title from PDF title page (viewed on Jan. 28, 2010). "School of Electrical Engineering and Computer Science." Includes bibliographical references (p. 99-101).

This dissertation analyses the reliability of power transformers and its impact of failure on system performance. Eskom Distribution, Eastern Region is used as a practical case study, which has an installed transformer base of 6066MVA comprising of 428 transformers ranging from 1 MVA to 8OMVA with voltage levels of 6.6kV to I 32kV. The literature review illustrates the theory and principles of transformers, evolution and changes in design criteria, the function of cellulose and insulating oil, failure modes. operations and maintenance practices and factors affecting the distribution systems performance. This study included a conditional assessment and an oil analysis review of transformers at Eskom. A method to trend multiple oil samples was developed and illustrated. The research further investigates the reliability of series and parallel systems using actual component reliability values. A study was conducted to establish the degree of network firmness. Transformer failure data was analysed and were shown to be characteristic of a bathtub curve. Defects from on site inspections were analysed and identified oil leaks as a maintenance focus area. The Distribution Supply Loss Index was determined to be the major impact Key Performance Index due to transformer failures. Transformer failures using statistical methods, showed HV/LV winding to be the main component to fail. The cost of a transformer failure to Eskom and the customer was determined. International Benchmarking was investigated to establish the criteria for network reliability indices and to compare the network infrastructure and performance of international utilities and Eskom. The later part of the study involved the analysis of a risk ranking methodology to establish a risk ranking matrix. The transformers were ranked according to the matrix, identifying the high risk focus areas. Projects were raised within Eskom to replace the identified high risk transformers. This study has concluded that the reliability of transformers is impacted by the changes in transformer design, increased maintenance defects and inadequate transformer protection at substations. The reduced oil volume per kVA, increased hot spot and ambient temperature, and compact tank designs have resulted in the cellulose being overheated and fault gases being produced during normal operating conditions. The increase in load demand from the existing transformer fleet and a reduction in capital expenditure to maintain and build additional substations have also contributed to accelerated aging, since the transformers are forced to operate at 100 percent loading. There is an increase in transformer maintenance defects due to in sufficient operational staff, high staff turnover, reduced skills transfer. and insufficient network contingencies to allow for planned outages to clear the defects identified. The failure analysis showed that the main component to fail is HV/LV windings. The winding failures were traced to there being no or inadequate transformer protection at -20% of substations. / Thesis (M.Sc.)-University of KwaZulu-Natal, 2007.

The alternators in today's automobiles are of the claw-pole or Lundell construction, which is a readily manufactured, low-cost derivative of the conventional rotating dc field synchronous generator. The efficiency of the Lundell system is low due to a complicated magnetic circuit of predominantly solid steel and a high windage rotor structure. As the number of electrical devices in a car increases, so does the demand on the generator system. The Lundell alternator is not able to meet the demands and numerous alternative systems are under investigation. This led to the development of the brushless doubly-fed alternator system with the advantages of regulation over a wide speed range, competitive system cost based on inexpensive machine construction, low rating controller, diode rectifier and robust, low maintenance configuration. The conventional alternator has only one degree of control (de excitation), whereas the doubly-fed alternator has three control quantities: excitation magnitude, frequency and phase sequence. Excitation magnitude is used to regulate the output voltage, which leaves two control parameters to optimize efficiency over the alternator speed range. Simulation tools were developed for conducting design studies on the BDFM alternator system. Various stator and rotor configurations were studied through simulation and a few prototypes were built. A proof-of-concept prototype built in an existing induction machine frame achieved comparable efficiency characteristics to the Lundell System and exceeded the Lundell performance over part of the speed range. Significant performance improvements are expected for a new, optimized prototype which will not rely on the induction machine laminations, but will utilize custom components designed for this low voltage, high frequency application. Since the increase in automotive power demand is likely to be coupled with an increase in system voltage, a 24V, 2kW alternator system is investigated and simulation results are presented. / Graduation date: 1993

[Truncated Abstract] This thesis describes investigations carried out in the development of a wind powered generation system suitable for interconnection with a power grid ... The simplicity and flexibility exhibited by the induction machines in providing electromechanical energy conversion make it the most favoured choice for wind-powered systems operated in parallel with an existing powers system. Although not as common as the ‘squirrel cage’ induction machine, the slip-ring type has several attractive features in providing constant-frequency ac power when driven from a varaible-speed source such as a wind turbine. The fundamental energy conversion properties of the induction machine system are studied in order to determine the suitability of these systems for constant-frequency variable-speed electric power generation. Two distinct control policies are formulated: (i) optimising power from the wind when the generator is connected to an ‘infinite’ bus bar and, (ii), scheduling a constant output power irrespective of fluctuations in wind speed when the generator is operated in parallel with a ‘weak grid’.

Thesis (M.S.)--Brigham Young University. Dept. of Mechanical Engineering, 2006. / Includes bibliographical references (p. 81-83).

Thesis (M.S.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 105-106). Also available on the World Wide Web.

Thesis (M.Sc. (Hons)) -- University of Western Sydney, 2000. / "Thesis submitted to the University of Western Sydney, Nepean for the degree of Master of Engineering (Hons)" "School of Mechatronics, Computer and Electrical Engineering, December 2000" Includes bibliographical references (leaves 111-113).