MODELING AND PARAMETER EXTRACTION OF A214 MVA TURBO-GENERATOR DRIVEN DY DC MOTORS DURING OFF-LINE SHORT-CIRCUIT
This thesis aims to extract d-axis machine parameters of a 214 MVA high-speed turbo-generator using well-established off-line techniques. MATLAB/SIMULINK tool has been utilized to model the generator and perform sudden short-circuit (SSC) tests. Because constant speed operation of the generator is vital consideration for the accuracy of SSC tests for parameter extraction, this thesis has employed an arrangement of 2 DC motors operating alongside a drive system, which is configured to couple the combined motor to drive the shaft of the generator to maintain synchronous speed throughout the test. Simulating the different drive configurations, the SSC oscillographs were captured and transformed into symmetrical envelope waveforms which were then used to extract the operational impedances and time constants of the generator. From the simulation tests, the influence of speed departures on shaft torques and current waveforms were presented and analysed. The results obtained for the variable-speed simulation using the drive system were then validated with those calculated from standard equations, as well as those of the constant-speed mode. The test results confirm that SSC tests can be performed on synchronous machines under rated conditions if the couplings can be appropriately designed to contain the dangerous values of electromagnetic torque that are developed during such tests. In addition, an electronic switch was designed to manage the abnormal field current which results from the introduction of SSC currents. In the end, the switching contraption is seen to reduce the power losses in the machine’s field winding during SSC.