Research Of MMC-HVDC System Topology With DC Fault Blocking Capability

In recent years,Modular Multilevel Converter(MMC)has become a research hotspot in the field of High Voltage Direct Current(HVDC),a traditional half bridge submodule(Half Bridge).Sub-Module,HBSM)has the advantages of topology,simple control scheme,etc.,and it can well solve the problem of IGBT series dynamic voltage equalization and trigger consistency in traditional two-level converters,which greatly reduces the manufacturing difficulty of inverters..The MMC neutron module is highly modular,and it is easier to design a redundant configuration,which greatly improves the reliability of the inverter operation.When the number of bridge sub-modules is larger,the AC output voltage waveform of the inverter is better.When the level number reaches a certain level,there is no need to install a special filter and save space.The optimized modulation method can greatly reduce the switching frequency of the device and greatly reduce the operating loss.However,HBSM-MMC has large defects in the field of high-voltage large-capacity overhead line HVDC transmission.When a short-circuit fault occurs on the DC side,the huge short-circuit current cannot be effectively removed,seriously threatening the operational safety of the inverter.In view of the deficiencies in the HBSM-MMC topology,scholars at home and abroad have proposed many solutions.First,the DC circuit breaker is the simplest and effective solution to the DC-side fault problem.However,the DC current has no natural zero-crossing and the arc extinguishment is extremely difficult,which results in slow research and difficult technical research.At present,DC circuit breakers are still rarely popularized in high-pressure and large-capacity applications.Secondly,research on the topology of the converters makes the converters have the ability of auto-blocking DC-side faults like the traditional DC transmission technology.It is also an effective means to block DC-side faults,and uses the converter's own control to achieve direct current.The strategy of self-clearing of side faults has the advantages of no mechanical action and quick system recovery,and is particularly suitable for large-capacity long-distance DC overhead line transmission systems.However,the self-clearing fault topology proposed by scholars at home and abroad has many problems such as complex topology,difficulty in module encapsulation,high construction cost,or difficulty in designing control systems,high complexity,or large loss in topology operation.For this reason,three MMC topologies are proposed in this paper in order to solve some problems existing in the self-clearing MMC topology of existing faults.For the existing fault self-clearing topology,there are a large number of IGBT modules,complex topology,difficulty in module packaging,and high converter cost.Starting with the sub-modules,a new type of sub-module—a half-bridge sub-module is proposed.(Similarity Half Bridge Sub-Module,SHBSM)topology.SHBSM topology has the advantages of simple structure and small module packaging difficulty.SHBSM does not increase the number of IGBTs compared to HBSM.It only requires a small amount of diodes,and the entire submodule has little increase in cost,which is negligible,and the operating principle is consistent with that of HBSM.Control system can transplant mature HBSM control scheme.SHBSM-MMC has the capability of auto-clearing DC faults.When the inverter IGBT triggering system meets a certain degree of latch-up,SHBSM-MMC has a good effect of clearing DC faults.The SHBSM operating power loss proposed in this paper is relatively large-equivalent to the FBSM power loss.In order to further reduce the operating power loss,and at the same time further reduce the unit level requires the number of switching devices,the class of half-bridge sub-module and half-bridge sub-modules are passed Auxiliary diodes are mixed.In this paper,based on the proposed SHBSM topology,a new class of half-bridge-half-bridge hybrid sub-module topology is proposed.This topology mixes SHBSM and HBSM through auxiliary diodes.When dealing with DC faults,SHB-HBHSM-MMC does not need to close all IGBT pulse signals at the same time in the converter,which eliminates the spike voltage and current phenomena in the SHBSM-MMC and greatly reduces the complexity and accuracy of the control system.At the same time SHB-HBHSM further reduces the number of power devices,only need to increase the number of diodes on the basis of traditional HBSM-MMC,without increasing the number of IGBT,further reducing the cost of MMC,and further reducing the SHBSM-MMC operating power loss.Compared with HBSM-MMC,the SHB-HBHSM-MMC still has a large operating loss.At the same time,in order to solve the over-current and over-voltage problems in the Reverse Blocking Half Bridge Sub-Module(RBSM),the hybrid The SHBSM lower tube is improved,and the half-bridge module is introduced to replace the original SHBSM.At the same time,in order to solve the overvoltage problem of the SHBSM-MMC,a comprehensive improvement is made to the RBSM and the SHBSM,and a reverse resistance-half bridge hybrid is proposed.Reverse Blocking-Half Bridge Hybrid Topology(RB-HBHT)MMC.RB-HBHT-MMC has a very low power loss during normal operation,which is equivalent to the traditional HBSM-MMC operating loss.It also satisfies the overvoltage problem of RBSM-MMC and does not need to be strictly met when RB-HBHT-MMC blocks DC faults.Inverter IGBT lockout is consistent,greatly reducing the inverter control system complexity.At the same time,it has advantage such as the control system being portable and the mature HBSM-MMC control scheme.A simulation example was built on the PSCAD simulation platform to verify the effectiveness of the proposed three MMC topology to eliminate DC faults.The relevant parameters were analyzed and compared.Meanwhile,the three MMC topologies proposed in this paper were not strictly met in the IGBT trigger system.When it is consistent,the effects of DC fault removal and the electrical stress of each power electronic device in the inverter are obtained.Finally,the three MMC topologies proposed in this paper are comprehensively compared horizontally.Under the same level of output,they are compared and analyzed from the aspects of converter IGBT inputs,operating losses and control system complexity.The amount of converter IGBT input looks.HBSM-MMC has the lowest power consumption and the best economical efficiency.However,there are DC faults that cannot be removed and isolated by self-service.In the topology with DC fault self-cleaning capability,the IGBT inputs for FBSM-MMC are the most,and SHBSM-MMC and SHB-HBHSM-MMC are the least.Control system complexity,The three topologies presented in this paper all have the features of a portable traditional HBSM-MMC control system scheme,and the corresponding control system complexity is not greatly increased compared to HBSM-MMC.The complexity of the overall control system does not increase much,and it still maintains a good control system's simplicity.Compared with the FBSM-MMC,the complexity of the CDSM-MMC control system is greatly reduced.The operating loss of RB-HBHT-MMC is the lowest in terms of operating loss,which is comparable to that of HBSM-MMC.The highest is SHBSM-MMC,which is about twice the operating loss of HBSM-MMC.Finally,combining the fault removal effect and the electrical stress of the switching devices in the process of elimination,the respective advantages of the three MMC topologies proposed in this paper are determined,and relevant recommendations are made for the three most suitable transmission scenarios for MMC topology.