Steam turbine-generator torsional response due to interaction with the electrical grid

The rotating part of a large steam turbinegenerator is typically composed of individual turbine rotors that support the turbine blades or a generator rotor that holds the generator windings. These individual parts are usually attached together by bolted couplings. Although this combined rotating part of the turbine-generator can be excited in torsion by oscillating torques applied to its various components, most of the torques that can damage the large steam turbine-driven machines that provide power to the grid is due to interaction with the electrical grid. Due to the increasing size of these turbine-generators, they can have between 20 to 30 torsional modes of vibration with frequencies in the range where they can be excited by the grid during normal operation. Since damping for torsional modes can be very low (in the order of 0.1 per cent of critical damping or less), vibratory response at resonance can exceed 500 times the response that would occur from an equivalent static load of the same magnitude. As a result, turbine-generators can be quite sensitive to torsional vibration stimulus from the electrical grid and they should not be allowed to be operated at near-resonance conditions. In addition the steam turbinegenerator must be capable of sustaining suddenly applied impulse torques that can be generated by electrical grid faults, mal-synchronization, turbine or generator trips, and switching of transmission lines. One way that torsional resonance can occur is if the turbine generator is allowed to operate with unbalanced load on the generator terminals while it has a torsional mode with a frequency close to twice the grid frequency. The unbalanced load can result in a steady oscillating negative sequence current torque at twice the grid frequency that can cause significant damage to the turbinegenerator if it has an excitable torsional mode with a frequency too close to that of the exciting torque. It is important to design the turbine generator to provide sufficient torsional frequency margin with respect to such steady oscillating torques at twice the grid frequency while also providing a sufficiently rugged design that can withstand the suddenly applied torques that occur due to transients, upsets, and faults that occur from time-to-time during operation.