IMPACT OF HVDC INVERTER STATION FAULTS ON TURBINE-GENERATOR SHAFT TORSIONAL TORQUES
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The phenomenon of the interaction between turbine-generator shaft systems and the electrical network disturbances has been extensively investigated during the last two decades. Those investigations have shown that some of these disturbances can inflict severe torsional stressing on the shafts of the turbine-generators. The induced high stresses reduce the life expectancy of the turbine-generators and, in severe cases, may cause shaft damages. Besides the electrical network disturbances, turbine-generator shaft torsional systems can interact also with other power system components. These potential sources include power system stabilizers, staticvar compensators, HVDC systems, high-speed governor controls and variable speed drive converters. In most of the reported studies concerned with the interaction of HVDC systems with turbine-generator shafts, attention has been focused on the effect of the HVDC controllers on this phenomenon.Fewer studies have, however, investigated the impact of HVDC converter station and line faults on turbine generator shaft torsional torques. This thesis explores the impact of HVDC inverter station faults, namely short circuit across bridge terminals, short circuitacross a non-conducting valve and commutation failureon the torsional torques induced in turbine-generator shafts. The results of these investigations are presented in the form of typical time responses as well as parametric studies. Time domain simulations are carried out using the EMTP. The results of these investigations have demonstrated that inverter station faults including commutation failures excite torsional torques in the turbine-generator shafts. The magnitudes of these torques depend on the initial operating conditions as well as on the fault incidence instant.