Observer-based excitation control of turbine-generator shaft oscillation

During disturbances in a power system, turbine-generator shafts are subjected to mechanical stress caused by severe torsional torques. Shaft torque is proportional to the shaft twist, which is caused by angle difference between both ends. However, observing the angle difference in an actual generation system is difficult technically. On the other hand, measuring rotational speed of the shaft ends is relatively easy. Then, if the shaft torque could be estimated effectively, it would be used in order to control the rotational speed for shaft torque reduction. This paper proposes a shaft torque excitation control for rotating machine by making use of an observer for the shaft torque. Firstly, the relationship between the observation signals and the accuracy of the estimation are discussed, and secondly a shaft torque reduction control system developed by using the excitation control based on the estimate by an identity observer. An in-house 7-mass system comprised of joints, shear-pins, and an exciter is used for the test model and Park's equation for the synchronous machine is employed. The controller is established by feeding back the error between the target output and the observer output. Linear quadratic regulator theory (LQR) is employed to design the observer gain, which provides acceptable estimation.