Study On The Subsynchronous Resonance Characteristic Of Large-Scale Thermal Power Group And Its Damping Control Strategy

According to the construction plan of the north-channel of the power transmission project from west to east, a series of big thermal power plants will be built in north-west China, west inner Mongolia, Shanxi province and other areas, which will lead to the formation of large-scale thermal power group sending out systems. It is an economical scheme to adopt the transmission mode of point to network through UHV and EHV lines with series compensation in view of the far distance between the power plant in the west and the load center in the middle and east, while the installation of series compensation capacitors may lead to stability problems associated with subsynchronous resonance (SSR) and threaten the safe operation of generator and the transmission system. Therefore, study on the SSR problem of large-scale thermal power group is of distinct value to guarantee the easy send out of the western thermal power. This dissertation mainly focuses on the study of SSR characteristic of large-scale thermal power group and its control strategy. The main contents include:(1) The phenomenon of common-anti mode and share damping produced by the torsional interaction in multi-machine system is analyzed based on a dual-machine system. For the differences existed in torsional oscillation behavior of parallel identical multi-machines, studies have been performed in a parallel dual-machine system to check its relationship with output differences of each unit, the series compensation level and strength of the access system. A model that simulates the thermal power groups collects their output together through series compensated lines and then send out collectly has been built and used as a foundation to study the multi-machine and multi-modal characteristic of the SSR in a thermal power group system.(2) Modal separated SEDC controller with its parameter adjusting strategy is designed to damp SSR. The IEEE first benchmark system is considered for the study. The effectivity of the damping control strategy is verified by eigenvalue analysis, test signal method and time domain simulation. For the system with multi operation modes, an index of comprehensive damping coefficient is proposed to help find the phase compensation degree with which SEDC can have the best comprehensive SSR damping effect under all the possible operation modes. Finally, analysis of the advantage and disadvantage of using SEDC to damp SSR is carried out.(3) Application of STATCOM with supplementary damping controller for damping SSR is studied. The IEEE first benchmark system is considered for the study. Test signal method and time domain simulation results indicate that STATCOM with supplementary damping controller can damp SSR in a single-machine system effectively. With reference to the experience of damping SSR using unbalanced series compensation scheme, an exploration research is carried out to investigate the feasibility of damping SSR using single-phase STATCOM. Further studies over adaptability problems including STATCOM capacity requirment, influence of STATCOM supplementary damping controller on the local electric signal of the output, influence of the steady control mode of STATCOM on the SSR damping effect, influence of the location of STATCOM and strength of the access system on the SSR damping effect is carried out by means of time domain simulation. Advantage and disadvantage of using STATCOM to damp SSR is also analyzed at last.(4) A simulation model of damping SSR in a multi-machine system using only STATCOM is built based on the extention of the single-machine system damping control strategy. Analysis result indicates that only the common-mode torsional oscillation can be effectively damped by STATCOM, while the anti-mode torsional oscillation only decays under the effect of mechanical damping. The combination scheme of STATCOM and SEDC can combine the advantage of each SSR damping measure and damp the SSR in a multi-machine system rapidly and completely. For the system that have series compensated lines between STATCOM and the turbine-generators, SSR may not be effectively damped due to the potential risk that generators may suffer from a unstable torsional oscillation between each generator group. Conclusion of the study may provide reference for the design of SSR damping control strategy of large-scale thermal power groups.