Research On Subsynchronous Oscillation Suppression Devices And Their Control Strategies

Series capacitor compensation of ac transmission lines and high voltage direct current (HVDC) transmission systems have been recognized as useful and economical means to increase long distance power transfer capability and to improve system stability. However, both of the two methods may lead to subsynchronous oscillation (SSO) problems and cause a great damage to the turbine-generator system. During the National 12th Five-Year Plan period, with the rapid growth of large energy bases in Western China, more series compensated transmission lines and HVDC transmission lines are needed in order to send large amount of electric power from West to East. The risks of SSO will be more severe than ever. In such circumstances, this dissertation studies the suppression devices and their control strategies of subsynchronous oscillation. The main works are organized as follows.(1) The basic principle of test signal method and its realization is introduced. Electrical damping and mechanical damping are calculated separately, and the results are used to indicate the system SSO stability. Based on the IEEE SSR first benchmark model (FBM), the analysis results of Prony modal identification and test signal method are compared in order to validate the effectiveness of the test signal method. The test signal method is then extended to multi-machine system. Different working conditions of a plant with three generators are analyzed and some rules are summarized.(2) The thyristor controlled series capacitor (TCSC) is adopted to suppress SSO. The basic structure and operating principles of the TCSC are studied. The electrical damping torque analysis shows in a hybrid system which includes both fixed series compensator and TCSC, disturbances may still excite torsional mode oscillations. A supplementary damping controller of the TCSC is then designed to solve this problem. The design method is based on phase compensation principle and the procedure for the syntheses of the controller is presented in detail. The effectiveness of the controller has been validated using modified IEEE SSR FBM in frequency domain and time domain simulations. The simulation results show that the proposed controller is effective to mitigate SSO.(3) The application of a static synchronous series compensator (SSSC) for damping subsynchronous resonance in series compensated system is examined. The modeling and basic operating principles of the SSSC are analyzed. A case study based on modified IEEE FBM is adopted to investigate the SSSC damping characteristics with varied SSSC rating. Special band-pass filters having low passband phase shift are designed. With these filters, a multimode controller design method is developed to suppress multiple unstable torsional frequencies. Simulation results based on IEEE FBM show the proposed controller can provide positive electrical damping around each torsional frequency and is effective to mitigate the torsional interaction between the electrical network and the turbine-generator shafts.(4) A series-connected FACTS device named gate-controlled series capacitor (GCSC) is investigated to mitigate subsynchronous oscillation. The fundamental structure and basic control method of the GCSC is firstly introduced, and the steady state characteristics of the GCSC are also analyzed. The GCSC with different asymmetric compensation structures are proposed. The ability of asymmetric structure to reduce the SSO destabilization is studied by test signal method. A single phase GCSC subsynchronous oscillation damping controller is then designed to suppress SSO and to reduce the rating and cost of the GCSC.(5) A new scheme to solve SSO problem with a combination of a supplementary excitation damping controller (SEDC) and an additional induction machine damping unit (1MDU) is proposed. By phase shifting and amplifying of the feedback control signal, the SEDC can provide effective damping under steady state and small disturbance operations. The IMDU is mechanically coupled to the generator shaft, acting like an active damping controller to mitigate the transient torque amplification during transients. Firstly, the design method of the SEDC is analyzed. The configuration of the IMDU is then introduced, and the influences of IMDU mechanical and electrical parameters are also discussed. Time domain simulation results based on IEEE SSR first benchmark indicate that, the SEDC may fail to damp SSO under some large disturbances, while the combination control scheme can eliminate SSO instabilities in both steady and transient states of power systems.