New method for the protection of gas turbines against overload and overspeed
Gas turbine generator sets, in particular those with aero-derivative gas turbines, are sensitive to electrical fault events that can damage the gas turbine. In many cases, mechanical protection, e.g. using a torque limiting coupling, is not possible because of overspeeding of the turbine after release of the coupling. A new method for the protection of gas turbines against overtorques and overspeed has been developed. The overspeed limitation is achieved through the incorporation of a hydrodynamic coupling in the drive train; this acts as a brake and reduces overspeeding. When a gas turbine is disconnected mechanically from the workload and inertia of the generator, its speed will increase momentarily. The magnitude of the speed increase is controlled by the residual energy in the system, i.e. by the amount of fuel available and how this flames out. Overspeeding is controlled also by the residual inertia or energy. Thus the place in the drive train where the mechanical disconnection takes place has a significant influence. The braking torque transmitted over the hydrodynamic coupling has an approximately linear relationship with the speed difference, thereby producing a growing braking effect as the turbine overspeeds. Experiments have been carried out with powers up to 1 MW and accelerations up to 6000 rpm/s. Simulations of failure events for certain LM6000 configurations have been made.