Suppressing Transient Shaft Stresses of Turbine-Generators Using a Static Phase-Shifter

In this paper the technical feasibility of a thyristor controlled phase-shifters for damping the transient torques of the shaft segments of steam turbine-generators is presented. The system studied comprises a multimass turbine-generator unit, connected to an infinite bus, through two parallel transmission lines. The Electro-Magnetic Transients Program (EMTP) developed by the Bonneville Power Administration (BPA), is used for a detailed digital simulation study of the power system and the control circuitry, including the magnetic saturation of the generator and the transformers. The data used for the turbine-generator is that of the first IEEE benchmark model for subsynchronous oscillation studies. Positive damping for the shaft mechanical stresses is obtained by modulating the generator output power, using a static phase-shifter to inject a controlled magnitude of leadlag quadrature phase voltage in the system. The rotor speed deviation from the synchronous speed, as a result of torsional oscillations, is used as the control signal for the static phase-shifter. The injection of quadrature phase voltages at the generator high voltage terminals, changes the effective phase angle between the generator and the network voltage. This results in power modulation which is utilized for suppressing the shaft transient torsional stresses. Each phase of the static phase-shifter comprises an excitation transformer, a boosting transformer and sub-converters. The excitation transformer provides the required quadrature phase voltage and has four secondary windings. The boosting transformer injects the controlled magnitude of voltage in the system.