Stability Study On Low Frequency Oscillation Of Power System Including Asynchronized Turbogenerators

In the modern power system with the conventional synchronous generator as major power generation equipment, the rapid excitation system of generators has weakened the damping of the electrical and mechanical oscillation mode, so the electrical and mechanical oscillation mode showed weak or even negative damping that leading to unstable power system oscillations change issues in the increasingly prominent. The asynchronized turbogenerator as a new generator, owing to the rotor structure and excitation control, goes beyond the conventional synchronous generator with the excellent performance and damping characteristics. In this paper mainly aiming at the low-frequency oscillations of power system, based on the damping characteristics of the asynchronized turbogenerator, the low-frequency oscillations of the power system have been deeply researched when the asynchronized turbogenerator partly replacing the conventional synchronous generator in power system.Firstly the steady-state and transient-state operation of the asynchronized turbogenerator was studied in the one-machine infinite-bus power system. The results showed that the asynchronized turbogenerator under the dual-channel excitation control strategy has the fine steady operating performances beyond the conventional synchronous generator such as independent active and reactive power regulation, in-depth absorption into reactive power and the rapid response reactive power(voltage) regulation, as well as the transient process of achieving optimal excitation magnetic field orientation, maintaining larger electromagnetic torque and good transient voltage stability. The asynchronized turbogenerator has supplied a new way to solve the problems of continuous over voltage on extra-high voltage transmission lines due to the surplus of reactive power and the stability of power systems with long distance transmission line.The causes has been analysed that producing the low-frequency oscillation of modern power system from the perspective of damping. The results showed that the rapid excitation control system of conventional synchronous generator with voltage as feedback may weaken the damping of the electrical and mechanical oscillation mode, and the electrical and mechanical oscillation mode shows weak or even negative damping, that resulting the low frequency oscillation phenomenon in the power system, and the results pointed out that the most fundamental measure restraining low frequency oscillation is to enhance the damping of the electrical and mechanical oscillation mode, offset the negative damping effect by the rapid excitation system.The damping characteristics of the asynchronized turbogenerator has been detailed analysed, particularly the closed-loop damping characteristics of the asynchronized turbogenerator under dual-channel excitation control strategy was in-depth researched, and was compared with the damping characteristics of the conventional synchronous generator. The results showed that the damping characteristics of the asynchronized turbogenerator is superior to conventional synchronous generator, such as in the one-machine infinite-bus power system synchronous torque coefficient and damping torque coefficient is independent of the operational status of generators, but is only dependent on the excitation control parameters, the generator parameters and the transmission line parameters, through the regulation of excitation control parameters can improve synchronous torque coefficient and damping torque coefficient and the power system has a good stability and adequate damping.In this paper the linear mathematical model of the multi-machine power system was established including the asynchronized turbogenerator with the damping windings. Based on the damping characteristics of the asynchronized turbogenerator, the theory and methods of effectively restraining low frequency oscillation that the asynchronized turbogenerator partly replaces the conventional synchronous generator has been proposed, and the simulation results validate the theory and methods.The effect of restraining low-frequency oscillations was compared between the asynchronized turbogenerator and power system stabilizer (PSS), the rsults showed that the asynchronized turbogenerator not only can effectively restrain low-frequency oscillations of the power system, but also can improve transient stability, absorb surplus reactive and so on, thus the asynchronized turbogenerator can play a greater role on improving the stability of the power system.In this paper the focus on researching the asynchronized turbogenerator is to the multi-machine power system, and not to the generator individual. The research results have important theoretical value to the practical application of the asynchronized turbogenerator.