Research On Strategies Of Circulating Current Suppression For MMC-HVDC

Since most of China's large-scale new energy power generation bases are concentrated in the economically underdeveloped "Three North" areas,and far from the power load centers in central and eastern China.What's more,the local power consumption capacity is insufficient.Therefore,the development of high/ultra voltage direct current(HVDC/UHVDC)transmission is the key to the large-scale long-distance transmission of new energy in China.However,the capacity of voltage withstand and current conduction of a single power switch tube is insufficient,which makes it difficult for the traditional three-phase voltage source converter(VSC)to meet the needs of HVDC transmission projects.The modular multilevel converter(MMC)has the advantages of high modularity,easy system expansion,and redundant control so on.It is an effective way to solve the shortage of switching device capacity in HVDC systems.Nevertheless,the special circuit topology of the MMC is also easy to lead to more serious circulating current problems than the common VSC,which leads to serious system component loss and operation stability problems.In addition,when the three-phase unbalance fault occurs in the system,it will also cause serious circulating current problems.Therefore,the circulating current suppression of MMC-HVDCs is an urgent challenging problem to be solved.This thesis takes the MMC-HVDC system as the research object,focusing on the circuit topology,the structure and working principle of MMC sub-modules,the control strategy and modeling of MMC systems,the generation mechanism and suppression strategies of circulating current in MMC systems circulating current suppression,and the small signal modeling and stability analysis of MMC-HVDCs.The main tasks completed are as follows:(1)Research on the topology,control strategy and modeling of MMC-HVDCsThis thesis introduces the topology of MMCs at sub-module-level and system-level respectively,and deduces the mathematical model of MMC-HVDCs.On this basis,the double closed loop control control strategy of outer voltage loop and inner current loop is designed.And the electromagnetic transient model simulation of MMC-HVDC system is completed to ensure the stable operation of the system.(2)Analysises of generating mechanism of circulating current in the MMC system.According to the principle of instantaneous power conservation of AC and DC sides in converters,the mathematical model of internal circulating current is derived by formulas,and the negative sequence and second harmonic characteristic of circulating current is obtained.The mechanism of circulating current caused by unbalanced voltage drop of MMCs is revealed,which lays a foundation for circulating current suppression.(3)Design and verification of circulating current suppression strategies.In this thesis,the corresponding circulating current suppression strategies are designed respectively for the circulating current problems under different operating conditions.The effectiveness of the circulating current suppression strategy proposed in this thesis is verified by simulating unbalanced faults such as the different number of upper and lower arm sub-modules,the three-phase imbalance load,and single-phase or multi-phase grounding faults,and the small signal modeling and the stability analysis of MMC-HVDCs are finished too.Due to the dynamic characteristics of DC bus in MMC-HVDCs,there is frequency coupling between the AC and DC sides of the converter.In addition,because the existence of phase to phase circulating current problem of MMCs makes it more likely to result in a new type of subsynchronous oscillation/resonance(SSO/SSR)when the system is suffered from the external disturbance.Therefore,the frequency coupling characteristics of the converter are studied,and a small-signal sequence impedance model of the MMC-HVDC considering frequency coupling is established.And its impedance characteristics are analyzed too.This thesis builds the MMC-HVDC system in MATLAB/Simulink for simulation.The results show that the proposed circulating current suppression strategy has good inhibition effect under different operating conditions.Besides,the characteristic curves of the impedance model can more accurately predict the potential resonance points of the MMC-HVDC,providing a theoretical reference for resonance mitigation.