Study On Static Synchronous Series Compensator Control Strategy And Subsynchronous Resonance Mitigation

Static Synchronous Series Compensator(SSSC or S~3C)is a series FACTS device based on Voltage Source Converter(VSC),which is applied to control power system characteristic parameters.It can improve transmission line capacity,optimize network power flow and enhance power system stability.In this paper,the control course of SSSC was divided into 3 levels which were composed of top system level,middle device level and bottom switch level.And then according to characteristic of each level,the control object was specified.As the main conteng,the math model and control strategy of both device and system level were anlyzed thoroughly in this paper.As the important connection between top level and bottom level,device level control has three operation modes which were analyzed in detailr.The state space math model and reason of coupling between output AC voltage and DC capacitor voltage dynamic process were obtained.Two methods were applied to realize the decoupling between two control approach.For the system level control,the sovling of Phillips-Heffron model of single machine infinite bus system and multimachine system including SSSC was described in detail.Based on this model,the small signal idea of design method for damping low frequency oscillation by SSSC,difference of Phillips-Heffron model when SSSC device level operate at different mode and the damping torque provide by SSSC were analysed. For overcoming the error caused by small singnal approach,the differential geometry approach was applied to solving system level control which can improve system stability greatly.Based on the Phillips-Heffron model,the mutual affection between SSSC and excitor was analysed in this paper and then Extend State Observer(ESO)and EFL were combined to obtain coordinate control.The design of parameters of nonlinear function in ESO was also discussed.The fast response characteristic and immunity for SSR of SSSC make it an ideal device to surppress SSR.When SSSC operate in intead of part of capacity,this hybrid compensation system can not only remain series compensation capacity but also have the flexible capability of control the power flow.Besides,the SSR that is excited by fixed capacitor compensation can be damped by SSSC efficiently.The complex torque coefficient method and time domain simulation method were used to analyse the effection of hybrid series compensation on the SSR and the control approach.Finally, Real Time Digital Simulator is used for establishing a digital-physical closed loop control test platform,and a simple and practical method of establishing simulation model of large power system was proposed;based on the first project in which SVC is used for damping SSR which is excited by Fixed Series Compensation in China,the damping performance and the impact to the power system during the SSR damping process were analysed in the platform for the first time.The performance difference of SSR mitigation between SSSC and SVC was studied.