Investigation On Small-Signal Stability Of Hybrid LCC-MMC HVDC System

Line Commutated Converter based High Voltage Direct Current(LCC-HVDC)adopts semi-controlled thyristor as switching element,which has been widely used in recent years,and the technology is becoming more and more mature.However,LCC-HVDC presents some inherent drawbacks such as the risk of commutation failure(CF)issues at inverter station and cascaded CFs in multi-infeed HVDC system where inverter stations are located in close proximity.The Modular Multilevel Converter(MMC)uses fully-controlled devices as commutation components and has the advantages of no commutation failure.However,compared with LCC-HVDC with the same voltage and capacity level,MMC-HVDC has some shortcomings,such as power loss and high equipment cost.The hybrid LCC-MMC HVDC system with LCC on rectifier side and MMC on inverter side combines the respective advantages of LCC-HVDC and MMC-HVDC.Therefore,the hybrid LCC-MMC HVDC system will have a broad prospect of engineering application.Firstly,in order to provide a theoretical basis for this paper,the small-signal model of LCC-MMC HVDC system is established.The model consists of the following parts:AC system,LCC converter station,DC system,MMC converter station and control system.The LCC station adopts a constant DC current control and phase-locked loop and the MMC converter station adopts the vector current control,phase-locked loop control and circulation current suppression control.Compared with the model of PSCAD/EMTDC,the correctness of the established model is verified.Then,based on the derived model of hybrid LCC-MMC HVDC system,the oscillation modes of the hybrid HVDC system are analyzed by eigenvalue method.The weak damping modes which affect the stability of the system are identified,and then the main factors which affect the weak damping modes are locked by participating factor analysis.Based on the root locus method,the influences of the main circuit parameters and controller parameters on the small signal stability of hybrid HVDC system are studied and the feasible region of the system parameters is obtained.Finally,a simple and effective additional control strategy is proposed to solve the small signal instability of hybrid LCC-MMC HVDC system connected to a weak receiving AC grid,improving the small signal stability of the hybrid HVDC system connected with weakly receiving AC system.The influence of the proposed additional control strategy on the small signal stability and dynamic characteristic of the hybrid HVDC system is studied.