Analysis Of Subsynchronous Oscillations In HVDC System And Study Of Suppression Strategy

With the development of society and economy,the existence of electricity is becoming more and more important.With the development of modern industry and intelligent life,the demand of electricity for People's Daily life is becoming more and more indispensable.However,there is a problem of uneven power load supply in China.For example,the power load in northwest China and parts of southwest China is far lower than that in the east and north China,but there are abundant coal and other resources.Therefore,"west-east power supply" is particularly important.High Voltage Direct Current(HVDC)is closely related to our life and has important value and significance in long-distance transmission.But while HVDC brings great convenience to people's economy and life,it also brings some problems,such as the Subsynchronous Oscillation(SSO)problem,The problem is mainly caused by the rapid control of the HVDC system's constant current controller.The occurrence of SSO results in torsional vibration of shafting of some ac generator sets in the system.If continuous oscillation is not suppressed,it may even lead to shafting fracture in serious cases.It is self-evident that the operation condition and stability of power system will be seriously affected.In this paper,this SSO phenomenon,based on HVDC system to this phenomenon and the solution of in-depth analysis and research,research contents include the following:1.First of all,in this thesis,introduction was made for research background and significance of the project,and careful elaboration was made for SSO research status and its related analysis method at home and abroad.After that,careful introduction was made for suppression principle of method of SSO,the SSO mechanism generated by HVDC system was analyzed,and the transmission characteristics of sub-synchronous component in alternating and direct current system was derived via formula.Wherein steam turbine generator unit in the alternating current part was employed with shafting spring centralized mass block model,and the six mass blocks were provided with careful analysis and research.2.The HVDC control system is analyzed and studied,which mainly analyzes the core pole control stage,including the constant turn-off Angle control,constant current control and constant voltage control.Based on the PSCAD/EMTDC simulation platform,based on the typical CIGRE HVDC HVDC transmission model,and by referring to the IEEE first standard test model,the multi-mass block model of the machine end shafting is introduced to build a complete HVDC transmission system time domain simulation model.The operating conditions of the system were simulated by the time domain simulation method,and SSO phenomenon was simulated in the case of failure.After the frequency spectrum analysis of the generator speed,the oscillation frequency and amplitude of SSO modes were obtained,which laid an important foundation for the design and implementation of the suppression measures in the following thesis.3.For the modes with the most serious oscillations,an Supplementary Subsynchronous Damping Controller with wideband is designed at the network side,The suppression mechanism,design idea,input signal and parameter determination of each link are studied in detail.The designed SSDC is attached to the constantcurrent controller and its suppression effect is verified by time domain simulation.The SSO of the system is significantly improved and the oscillation of each shafting is convergent in a short time.4.An Supplementary Excitation Damping Controller with separate modes is designed on the machine side to compensate for the failure of SSDC to suppress SSO effectively when there is a small disturbance in the system,the suppression effect of SEDC acting alone was verified by time-domain simulation.The lateral SEDC on the machine and the network side SSDC design is completed,the machine side and the network side joint suppression,the simulation results show that when SEDC or SSDC role in separate suppress SSO effect is not as good as the joint inhibition effect is good,also the design presented in this thesis machine net boycott damping controller is feasible,and in the PSCAD/EMTDC simulation platform to verify this measures the effectiveness of the control.