Digital simulation of a gas-turbine generating unit
Most existing synchronous machine stability studies are based on the assumption that the machine is operating on an infinite busbar of constant voltage and frequency. The present study however, is concerned Mainly with the dynamic stability of the type of gas-turbine generating unit commonly used in remote sites, such as off-shore oil rigs and desert areas, where the' grid is far from stiff and is liable to become unstable in the event of a severe system disturbance. Because of this, there is a pressing need for an accurate representation of the system, to enable investigations to be made into problems concerned with the system response and to consider any improvements that may be effected in the associated control scheme. The initial stage of the investigation involves the development of digital models for a synchronous machine, using both direct-phase co-ordinates and a stationary 2-axis representation. Symmetrical and unsymmetrical faults at the generator terminals are simulated and comparisons with test results are made. Despite being more restricted in its range of application than the direct-phase representation, the 2-axis model is used throughout the remainder of the investigations described in the thesis, Mainly because of its computational convenience. The increasing use of large induction motor drives makes it necessary to study their effect on system stability, especially in the case of small gas-turbine power systems. Various types of switching transients on both small and large squirrel-cage induction motors are therefore studied. The successful development of the synchronous and induction machine models is followed by a fault simulation of a composite system containing both types of machines. The machine investigation is followed by the simulation of a conventional steam turbo-alternator unit operating on an infinite grid system. Various types of system disturbances are simulated and the effect of generator saturation on the system response is considered. Following this study, a mathematical model of a gas-turbine generating unit is developed, with emphasis given to operation in an isolated grid situation. To validate this model, correlations between predicted and test results on a typical installation are presented.