Turbine-generator shaft torsional oscillations is an interdisciplinary power system dynamic problem as it involves mechanical and electrical engineering. Torsional oscillations is a well-recognized power system operational problem from the standpoint of power utilities and a major design problem from the standpoint of the machine manufacturers. Torsional oscillations occur in the mechanical systems represented by turbine-generator shaft segments and long low-pressure steam turbine blades for electrical grid induced reasons such as fault, fault clearing and autoreclosure. The autoreclosure is an indispensable protective and control scheme that originally designed for power system stability improvement and system security enhancement. Various autoreclosure schemes have been widely used in all over the world for decades as fault statistics indicate that over 80% of overhead transmission line (OHTL) faults have a temporary nature. The reclosure process is either successful in case of transient faults or unsuccessful in case of sustained faults. Due to the inherent weak damping associated with the torsional oscillations, the unsuccessful reclosure imposes devastating torsional oscillations in the nearby turbine-generators with very slow decaying rates. The torsional oscillations cause fatigue life expenditure of the turbine-generator which could lead to shaft cracks or even complete shaft fracture. A reported well-known 952 MW generator shaft fracture due to torsional oscillations occurred in Maanshan nuclear power plant in Taiwan in 1985 with the complete unit sabotage. Due to the devastating effects of unsuccessful reclosure on the fatigue life of the shaft metal, recommendations have been made by pioneer turbine-generator manufacturers that the autoreclosure should be eliminated near power plants despite the its considerable merits. Therefore, there have been great motivations to mitigate the shaft torsional oscillations particularly when unrestricted high speed reclosure (HSR) is utilized on the OHTLs emanating from a power plant. The mitigation of torsional oscillations makes a compromise between the overwhelming benefits of HSR utilization and preserving shaft fatigue life of the involved turbine-generators. Several mitigation methods and techniques are found in the literature. The dynamic braking resistor (BR) is a very effective candidate mitigation method. It was utilized for the purpose of enhancement of electric power system transient stability in the first place. It acts as an extra load capable of dissipating surplus generation in case of severe faults in hydro dominated power systems. Therefore, by consuming the excess power, it prevents generator pole slipping, or out of step, conditions and thereby, enhancing system transient stability. A novel BR model, namely rectifier controlled braking resistor (RCBR) model, controlled via fuzzy logic controller (FLC) is proposed in this work for torsional oscillations mitigation. RCBR is basically a single BR unit connected to the generator terminals via six pulse full wave rectifier bridge. |
