| In recent years,the engineering application of Flexible HVDC technology is of great significance to the establishment of economic,environmental protection,advanced intelligent power transmission and distribution system and the improvement of power grid security and stability.Modular Multilevel Converter(MMC),with the obvious advantages of topology and output characteristics,has been attracting more and more attentions from academia and engineering circles,and has been widely used in the field of field of Flexible HVDC.This paper focuses on the analysis of the AC faults and DC faults of the MMC Based High Voltage Direct Current(MMC-HVDC)and the corresponding protection and control strategies.Through combined the methods of mechanism modeling analysi and real-time simulation verification on the following aspects:(1)Detailed analysis of the topological and basic operational principle of the MMC is analyzed.The mathematic model of MMC main circuit based on switching function is established.Then,the carrier phase shifting modulation,module voltage control and circulating current suppressing strategy are presented for the flexible HVDC.The physical model and mathematical model of the MMC-HVDC system are established.The control method and control objectives of the MMC-HVDC system are elaborated on the basis of the three-layer control.The concrete design scheme of the controller is given to establish the theoretical foundation for the following fault analysis.(2)The mechanism of three kinds of faults on the DC side of MMC-HVDC are studied deeply,including monopolar grounding fault,pole-to-pole fault,line breaking fault.A circuit model analysis method which can ignore the instantaneous change of sub-module switching during transient processing is adopted.The failure mechanism of the MMC-HVDC system is analyzed by studying the equivalent circuit model of three typical faults on the DC side.The results of mechanism modeling and analysis on transient characteristic of DC fault are validated by using a 2-terminal active MMC-HVDC model on the RT-LAB platfor,which provides a theoretical basis for the rational design of DC fault protection and control strategy.(3)The corresponding control strategy for the DC side fault are given.One is the over-current suppression method based on virtual impedance,which can effectively reduce overcurrent level without affecting steady state operation of system.The other is the typical MMC sub-module topologies with DC fault self-cleaning capability.And the key performance of the sub-module is modeled and simulated from the view of isolation speed.(4)The transient characteristics of three typical faults on AC side of MMC-HVDC system are analyzed briefly.By analyzing the three failure mechanisms on the AC side of MMC-HVDC system,the mathematical expression of the fault voltage and current is deduced,and the failure mechanism of the AC side of the system is analyzed.The correctness and effectiveness of the mechanism modeling methods and results are validated by using a 2-terminal active MMC-HVDC model on the RT-LAB platform.(5)According to the low voltage traversing technique under the AC side fault,the control strategy based on a positive and negative sequence separation technique under the voltage fault of the power grid is studied.Then,the general reference current calculation formula under different control objectives is deduced and the design of the control system is carried out.The impact of the unbalanced control scheme under different control objectives on the MMC is analyzed and compared.And the correctness of the theoretical analysis is verified by the simulation results. |
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