Eigenvalue Analysis And Control Of Subsynchronous Oscillation In HVDC System

Subsynchronous Oscillation (SSO) is a kind of electromechanical interaction which usually occurs between the turbine generator shaft and HVDC transmission system. It may lead to generator rotor damage or even rupture, destroying the security and stability operation of the power system. With the development and promotion of HVDC transmission technology, SSO problems caused by the rapid control characteristic of HVDC system have received much attention. It is of great significance for theoretical research and engineering practice to study SSO mechanisms intensively and adopt effective measures for restraining. Based on eigenvalue analysis method, SSO caused by HVDC system are analyzed and studied intensively in this paper. The main contents include:(1) The transfer process and frequency conversion of subsynchronous frequency signals between different parts of the power system are analyzed, and the variation processes of state variables while SSO occurs are explained in detail, revealing the mechanisms and essence of SSO caused by HVDC.(2) Detailed linearized models are established which can be used in SSO analysis for HVDC system, including multimass shaft model, generator electromagnetism circuit model, excitation system model, HVDC and its control system model, AC network electromagnetic model, and turbine model. The dynamic responses and effects of power system stabiliser (PSS) and governor are also taken into consideration. With difficult modeling and good versatility, the models can accurately reflect SSO problems caused by HVDC.(3) Respectively for single-machine system and multi-machine system, SSO characteristics are analyzed using eigenvalue analysis method, and some of the obtained conclusions are verified through simulation. Further more, the influences of HVDC control system parameters on SSO characteristics of the generating sets are researched, and the torsional interactions between different generating sets as well as their influencing factors are analyzed in detail in multi-machine system.(4) According to a typical HVDC system, Supplemental Subsynchronous Damping Controller (SSDC) is designed and analyzed for its validity of effect. In addition, the influence rules of SSDC parameters on its depression effect are discussed, which are indispensable important parts in SSDC design and adjusting.