A METHOD FOR REDUCING TRANSIENT TORSIONAL STRESSES OF TURBINE-GENERATOR SHAFT SEGMENTS

This paper presents a new method for reducing transient torsional stresses of steam turbine-generators in series capacitor compensated power systems. The fundamental concept of the proposed method is to reduce the coupling between the electrical network and the shaft system with respect to the troublesome torsional frequency. The electrical network and the rotating shaft system are coupled through the generator air-gap torque. This coupling can be attenuated by unbalancing the network with respect to the phenomenon of torsional oscillations. This is achieved by setting the protective voltage levels of the three phases of a three-phase series capacitor at unequal values. The nonlinear ZnO over-voltage protection schemes of series capacitors can readily perform this function. The technical feasibility of the proposed method is demonstrated on a test system, using the BPA's Electromagnetic Transients Program (EMTP) as the study tool. The results show that the impact of the proposed countermeasure on the first encountered peak torque is not significant; however, it can effectively attenuate the subsequent peak torsional stresses. One important advantage of the proposed method is that it utilizes the existing system components and does not require controllers, measurement devices and additional power components.