THE DOUBLY-EXCITED GENERATOR AND ITS UTILIZATION IN A VARIABLE-SPEED GENERATING SYSTEM

The doubly-excited machine can be used as a variable-speed generator or as a variable-speed-drive motor. In this thesis the focus is on the aspects associated with generating electric power (real and reactive) from erratic energy resources such as water or wind. In such cases, the power input to the prime mover will vary; so will the speed of turbine at which maximum efficiency could occur. One fundamental theory to be proved in this thesis is that by applying a doubly-excited generator in a variable-speed generating system it would be possible to electrically control and adjust the turbine speed to maintain maximum efficiency operation of the total system.; A mathematical model of the machine is presented from which the single-phase equivalent diagrams, the active and reactive power generation and distribution, and the steady-state characteristics are obtained. Detailed experimental work on the verification of a graphical method for revealing the essential steady-state characteristics of the machine for the purpose of determining the desirable target modes of operation of the generator is presented. Next, zero-speed synchronization with a utility grid and the excitation requirements of the doubly-excited machine for maximum efficiency operation of turbine are investigated. Finally, the maximum efficiency operation of a variable-speed generating system utilizing a doubly-excited machine is presented. All of these are verified by experiments in a laboratory system set-up.; The test results indicate that the application of a doubly-excited machine in a variable-speed generating system appears quite promising in terms of improved efficiency and potential for reactive power generation.