Study On The Modeling And Mechanism Analysis Of Subsynchronous Oscillation In AC/DC Power Systems

With the carrying out of West-East electricity transmission project, large capacity and long distance electricity transmission have been the characteristic of Chinese power grids. The series compensation capacitor and HVDC are widely used in Chinese power grids to increase the transmission capability of long distance lines, which might cause subsynchronous oscillation (SSO) risks to the security and stability of power systems. Simulation modeling and mechanism analysis of subsynchronous oscillations in DC/AC power systems are studied and proposed in this dissertation.(1) An on-line measurement approach of generators'torsional mechanical damping coefficients for subsynchronous oscillation analysis is developed in the dissertation. The phenomonen of in-phase mode (IPM) and anti-phase mode (APM) of parallel connected identical turbine-generators is revealed and the torque coupling relationship is analyzed. Based on the above analysis, an active stimulating method for APM is presented and a practical scheme for on-line torsional mechanical damping coefficient measurement is designed. This approach takes advantage of the features of APM and decouples the generators from grids. Moreover, the proposed approach has been implemented in field to realize the on-line measurement of the torsional mechanical damping coefficient.(2) The reason for SSO induced by AC faults at inverter station is expounded, which extends the comprehension that only the rectifier station may cause SSO. The process of AC fault at inverter station leading to commutation failure, and then SSO adjacent to rectifier side is analyzed. The SSO in field is duplicated by electromagnetic transient simulation, which verifies the correctness of the theoretical analysis.(3) In this dissertation, the mechanism and characteristic of SSO in multi-machine systems with close torsional frequencies are analyzed. The generator-grid mode and the generator-generator mode are revealed and the damping characteristics of the two modes are contrasted. Moreover, the key factor for the torsional risk caused by generator-generator mode, unit combination, is analyzed. Basing on the proposed analysis, the formation mechanism of the SSO phenomenon occurred in actual power plant is expounded clearly.(4) The modeling method for generator shaft multi-mass model with alterable mechanical damping for subsynchronous oscillation simulation is developed, by wich the torsional mechanical damping coefficients are treated as variables. The proposed modeling method is implemented at RTDS simulator. A modeling method for generator rotor speed impulse output in real time simulation is developed. By this means, the rotor impulse signals have the same dynamic response characteristics to that generated by the tooth wheel fixed on the turbine generator shafts. Furthermore, a detailed simuation model of an actual DC/AC power transmission system is developed and the mechanism and characteristic analysis of SSO is conducted, which verify the theoretical analysis.