Large thermal turbine-generator technique is a crucial part of important equipment in our nation. The shafting of high-power unit takes advantages of lighter, softer, more support, longer span, higher section power density. The higher material utilized coefficient in generator, the higher section power density in shafting. Additionally, the length of shafting is increased, which results in a lower spring constant, a higher fixed shafting spectrum density and a lower energy threshold of oscillation. Furthermore, series capacitor will be widely applied in the future grid to support super high voltage transmission and larger coverage.
Subsynchronous oscillation (SSO) could be caused by series capacitor compensation in the transmission line, HVDC, improper installation of the power system stabilizer (PSS), the feedback action of generator excitation system, silicon controlled system, electro-hydraulic control system and so on. The rotors of the turbine generator have big inertia, and are more sensitive to torsional oscillation modes and thus assume the forced state of low cycle fatigue and high stress. When an electromechanical disturbance occurs, balance between the turbine driving torque and the generator magnetic braking torque is broken, torsional stress acting on the shafting is also changed, the fatigue of the rotor will be increased, and the useful life will be decreased. When the torsional stress is great to a certain level, the shafting will be damaged or even ruptured.