Studies Of MMC-HVDC Control Strategy

With the dwindling of fossil energy reserves and the increasing demand for clean and renewable energy,it is urgent to develop a more friendly transmission mode to meet the needs of renewable energy transmission.Due to the advantage of decoupled active and reactive power control,dynamic reactive power compensation ability,power supply to weak ac networks,Voltage Source Converter Based high voltage di-rect current(VSC-HVDC)is well adapted to the above occations.Modular Multilevel Converter is one of the excellent topologies of VSC-HVDC.Besides the advantages mentioned above,MMC has other advantages such as less switching lossed,high output waveform quality,easy for redundancy etc,is the new trend of VSC-HVDC.Thus to carry out studies concerning MMC is of certain significance both in theory and in practice.The control strategies under submodule fault and data interaction in control system are investigated in this research,by combining theorical analysis and simulation verification.The main content of this research is as follows:Sub-module fault is one of the common fault types of MMC,MMC would turn into arm asymmetrical operation after the faulty SMs have been bypassed.The char-acteristics of MMC under arm asymmetrical operation are analyzed detailedly in this paper,and it is pinpointed that the fundamental component of phase current would no longer distribute equally between upper and lower converter arms due to the equiva-lent capacitor inconsistency between them;in addition,fundamental component and the 3rd harmonic component will appear in circulating current,and the 2nd harmonic components which already exist will increase slightly.In order to ensure the stable operation of the system after the fault occurs,a complete set of fault tolerance strategy is proposed,including a faulty SM localization method based on capacitor comparison and a method to suppress the fundamental component of circulating current based on PR controller.Finally,the study system is conducted in PSCAD/EMTDC software environment,and the simulation results demonstrate the validity of the theoretical analysis and the effectiveness of the proposed fault tolerance strategy.In the high-voltage and high-power applications of modular multilevel converters,large amount of SM capacitor voltages need to be sampled and maintained in an ac-ceptable range,but the increase of the number of voltage sensors compromises both economy and reliability of the overall system.To solve this problem,a new method to measure SM capacitor voltages based on voltage estimation and group voltage meas-urement is proposed in this paper,the method proposed divides SMs in the same con-verter arm into groups,each group is equipped with only one voltage sensor,thus the number of voltage sensors is reduced.On the basis of the new measuring method,a SM open-circuit diagnosis strategy is introduced,which can detect and locate faulty SM rapidly by comparing voltage measurement and estimation;and voltage sensor redundant technique is given to guarantee the system normal operation in case voltage sensor fault occurred.A 21-level MMC-HVDC simulation system is conducted in PSCAD/EMTDC software environment,and the simulation results demonstrate that the new method has approximate measuring accuracy with the traditional one;mean-while,SM open-circuit fault and voltage sensor fault can be diagnosed within a rela-tively short time.