Combined Cycle Power Plants (CCPP) in single shaft arrangements consist of a gas turbine, a generator and a steam turbine on one shaft line. In order to enhance the plant availability and operational flexibility, Siemens Fossil Power Generation introduces a switchable clutch between steam turbine and generator. The clutch is a synchronous self-shifting device that engages automatically at rated speed as soon as the steam turbine overruns the gas turbine-generator. It disengages automatically when the steam turbine speed drops below the speed of the gas turbine. A rather complicated mechanism consisting pawls and ratchets and a thread of helical splines including damper mechanisms is used to provide the required coupling functions. The primary reason for the clutch is to ensure independent gas turbine and steam turbine operation below steam turbine rated speed. The clutch is especially advantageous during startup and gas turbine simple-cycle operation. Next to these advantages, the clutch engaging processes could introduce significant impact loading to the shaft components which differ from other. Next to the normal engaging process fault cases like engaging processes after gas turbine trip at high acceleration values due to the gas turbine compressor losses must be sustained by all rotor train components. This paper documents a nonlinear torsional analysis of the single shaft arrangement to assess the impact loading due to clutch engaging processes. A dynamic three-mass-model of the clutch including nonlinear stiffness and damping functions is set up and applied for the simulations. The coupling of the translatory and the rotatory inertia effects of the main sliding component of the clutch has been taken into account. Different load case scenarios in different single shaft component arrangements respectively different inertia ranges of the steam turbine rotor train are investigated in detail by the transient analyses. Based on this procedure, it is ensured that the mechanical layout of the single shaft components is sufficiently designed to withstand all operational loads under normal and faulty operating conditions.

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