Runaway Speed and Acceleration Profile

Abstract

Under normal operating conditions, the Pelton turbine is loaded by the electric generator which provides a mechanical counter-torque on the turbine shaft and in so doing ensures the dynamic balance of the rotating system. If this counter-torque falls out by load shedding of the generator, the rotation of the Pelton wheel under the full load of water jet begins to accelerate. The maximum speed that can be reached at the Pelton wheel is called the runaway speed, at which the jet almost completely bypasses the Pelton wheel, without transferring its energy to the rotating buckets. The runaway speed is about twice the nominal speed of the Pelton wheel. Regarding the mechanical safety and the construction costs primarily of the generator and occasionally of the gearbox, the runaway speed is an important factor that must be taken into account already in the design phase of a Pelton turbine.