Method And Characteristics Analysis Of SSO Problem For Transmission Systems With Large Scale Power Plant

As the location of the energy resources and the power electric loads, large scale electrical power transimissions through long distance, i.e."West Power to East and North Power to South Projects", are widely developed in China. To enhance the transmission capability, series capacitor compensation and HVDC (High Voltage Direct Current) are widely used in the transmission systems for large scale power plant integration. Featured with multi-machine and AC-DC multiple transmission channels, transmission systems for large scale power plant integration are confronting with severe problem of SSO (Subsynchronous Oscillation) with very complex electromechanical coupling, and they can not be simply equivalent to single-machine-infinite-bus system. Additionally, with the widespread application of FACTS (Flexible AC Transmission Systems) as well as the development and integration of the large scale wind power in northwest of China, the SSO problem would be more complex and with new characteristics. Given the above-mentioned background, eigenvalue and complex torque coefficient methods for SSO analysis of multi-machine system with AC-DC channles are studied, and the SSO characteristics of the transmission systems with FACTS and wind power for large scale power plant integration are analyzed. The main work and contributions of this thesis are as follows.(1) The method to construct the network state-space for power system SSO analysis is studied. To cope with the problems of constructing the state-space model of the power network which is featured with improper network, e.g. selecting of the network state variables, the principles used to select the state variables of the network for SSO study are summarlized, the dynamic conductance matrix of capacitor branch is defined, and a novel method using branch-adding to construct the state-space model of the network is proposed. This method can select the state variables by the network configuration conveniently, and the algorithm can be easily developed. The effectiveness of the proposed method is verified by eigenvalue analysis and time domain simulation of a practical system.(2) The complex torque coefficient method using CPCM (Complex Frequency Domain Port-equivalence Conductance Matrix) is improved, and the corresponding software is developed. Method to model the power system components and network in complex frequency domain is studied, and the algorithm to interface the models of generator and network is improved in the complex torque coefficient method using CPCM. The improved method can take account of the interactions among the generator shafts to increase the accuracy of the calculation for electrical complex torque coefficient, and can analyze the effect of the SSO damping devices on the electrical complex torque coefficient. The effectiveness of this proposed method is verified by eigenvalue analysis and time domain simulation of a practical system. The software developed can be used for SSO studies in power system planning and operation.(3) Taking a practical system in China as the prototype system, the SSO characteristics of the AC-DC power system with TCSC (Thyristor Controlled Series Capacitor) are studied by the improved complex torque coefficient method using CPCM, and comparison studies are performed either the series compensated lines are equipped with FSC (Fixed Series Capacitor) or TCSC. It is concluded that the SSO risk of the transmission system with AC-DC channles is less than that of the system with only series capacitor compensation and that application of TCSC in series compensated system could reduce the risk of SSO, and the above-mentioned characteristics are valuable for the planning and operation of the transmission systems for large scale power plant integration.(4) There are dense high capacity thermal power and wind power in northwest of China need be transmitted to the load center, thus the wind farm and the thermal power bundled transmission system are to be realized in the future. For the new SSI (Subsynchronous Interaction) problem of the wind farm and the thermal power bundled transmission system with series compensation, the SSI coupling interactions reveal that the bundled system can relieve the SSI of the wind farm and the thermal power. The feasibility and method of using the supplemental control of DFIG (Doubly Fed Induction Generator) for the mitigatint of the SSO and SSI problems of the thermal power plant and the DFIG based wind farm are explored.