Computation And Application Of Electric Torque Coefficients In Multi-machine Power Systems

Low-frequency oscillation which happened occasionally in the modern interconnected power systems has become a threat to the security and stability of power systems. It is of great fundamental significance to analyze the low-frequency oscillation problem. The negative damping mechanism of low-frequency oscillation has gained widespread approval with its clear physical concept. Usually the modal analysis method based on mathematical theory is applied to discuss the damping characteristics in multi-machine power systems. For the coupling between generators in multi-machine systems the electric torque coefficients defined by single machine infinite bus system is difficult to calculate with unclear physical concept. Time-domain data is used to identify the coefficients by some algorithms. The calculation and application of electric torque coefficients are studied in this paper for electric torque analysis in multi-machine power systems.Firstly, the relationship between the electro-mechanical eigenvalues and the electric torque coefficients in single machine infinite bus system is expanded to multi-machine systems. The coefficients of multi-machine systems are defined according to different modes. The results of modal analysis and the frequency-domain method are adopted to calculate the electric torque coefficients. The effects of the gain of excitation system, line reactance and operation condition are also studied. Then Prony algorithm is used to separate different modes in time-domain data to calculate the coefficients of different generators. The coefficients gained by time-domain data through Prony algorithm correspond with the results by theoretic calculation and are not affected by time windows and perturbation methods. Finally, a fast estimation method for electro-mechanical eigenvalues is given based on the damping torque coefficients of multi-machine power systems, which can supply good initial values for other precise eigenvalue algorithm, increase computational efficiency and avoid missing values. A quantitative assessment of generator damping of multi-machine system is also given through calculating the contribution of different generators to the real part of the eigenvalue of an oscillation mode. It’s useful to find the generator that is the negative damping source and can help to study low-frequency oscillation. It can also be applied to locate and design controllers.