The hybrid modular multilevel converter(MMC)based on half-bridge sub-modules(HBSMs)and full-bridge submodules(FBSMs)can operate at low DC voltages and clear DC side fault currents.However,the costs and power losses ar...The hybrid modular multilevel converter(MMC)based on half-bridge sub-modules(HBSMs)and full-bridge submodules(FBSMs)can operate at low DC voltages and clear DC side fault currents.However,the costs and power losses are much higher in hybrid converters.An auxiliary full-bridge converter(AFC)is designed to reconstruct the converter structure in the arm level,and the HBSMs output capacitor voltage through the AFC can attain similar capabilities to hybrid MMCs.The operational principle of the auxiliary full-bridge converter is discussed,and the low voltage operation and non-blocking fault ride through control are verified in a two-terminal DC network simulation.Through economic analysis,the power loss of the AFC is similar to a HBSM MMC but the total investment is lower than a hybrid MMC,making the AFC a promising solution to improve the existing HBSM converter with more controllability.展开更多
Considering the advantages and limitations of traditional identification method,combined with the strategy of active detection,the principle of DC grid pilot protection based on active detection is proposed to improve...Considering the advantages and limitations of traditional identification method,combined with the strategy of active detection,the principle of DC grid pilot protection based on active detection is proposed to improve the sensitivity and reliability of hybrid MMC DC grid protection,and to ensure the accurate identification of fault areas in DC grid.By using the DC fault ride-through control strategy of the hybrid sub-module MMC,the fault current at the converter station DC terminal is limited.Based on the high controllability of hybrid MMC,sinusoidal fault detection signals with the same frequency are injected into the line at each converter station.Based on model recognition,the capacitance model condition is satisfied by the detected signals at both ends during external faults whereas not satisfied during internal faults.The Spearman correlation coefficients is then introduced,and the correlation discriminant of capacitance model is constructed to realize fault area discrimination of DC grid.The simulation results show that the active detection protection scheme proposed in this paper can accurately identify the fault area of DC grid,and is not affected by fault impedance and has low sampling rate requirement.展开更多
A hybrid of line commutated converters(LCCs)and modular multi-level converters(MMCs)can provide the advantages of both the technologies.However,the commutation failure still exists if the LCC operates as an inverter i...A hybrid of line commutated converters(LCCs)and modular multi-level converters(MMCs)can provide the advantages of both the technologies.However,the commutation failure still exists if the LCC operates as an inverter in a hybrid LCC/MMC system.In this paper,the system behavior during a commutation failure is investigated.Both halfbridge and full-bridge MMCs are considered.Control strategies are examined through simulations conducted in PSCAD/EMTDC.Additionally,commutation failure protection strategies for multi-terminal hybrid LCC/MMC systems with AC and DC circuit breakers are studied.This paper can contribute to the protection design of future hybrid LCC/MMC systems against commutation failures.展开更多
针对模块化多电平统一电能质量调节器(modular multilevel unified power quality conditioner, MMC-UPQC)六桥臂结构下的单相桥臂故障问题,提出了一种五桥臂拓扑,这种新型拓扑可实现故障情况下的电能质量补偿。首先,对MMC-UPQC串并联...针对模块化多电平统一电能质量调节器(modular multilevel unified power quality conditioner, MMC-UPQC)六桥臂结构下的单相桥臂故障问题,提出了一种五桥臂拓扑,这种新型拓扑可实现故障情况下的电能质量补偿。首先,对MMC-UPQC串并联侧的数学模型进行分析,提出了一种复合模型预测控制(hybrid model predictive control,H-MPC),所提控制方法结合了有限集模型预测控制(finite-control-set model predictive control, FCS-MPC)以及快速模型预测控制(fast model predictive control, F-MPC)。然后,通过构建两侧独立的价值函数减少了控制方法的计算量,同时也实现了五桥臂解耦控制。最后,相比传统线性(例如PI)和非线性(例如无源控制passivity-based control,PBC)的控制策略,所提复合模型预测控制在电压补偿、负序电压抑制以及谐波电流补偿等方面具有一定优势,并在一定程度上避免了复杂的参数整定及坐标变化环节。仿真实验结果证明了所提控制方法的可行性和优越性。展开更多
基金supported by the National Key Research and Development Program under Grant No.2018YFB0904600the National Natural Science Foundation of China under Grant No.51777072.
文摘The hybrid modular multilevel converter(MMC)based on half-bridge sub-modules(HBSMs)and full-bridge submodules(FBSMs)can operate at low DC voltages and clear DC side fault currents.However,the costs and power losses are much higher in hybrid converters.An auxiliary full-bridge converter(AFC)is designed to reconstruct the converter structure in the arm level,and the HBSMs output capacitor voltage through the AFC can attain similar capabilities to hybrid MMCs.The operational principle of the auxiliary full-bridge converter is discussed,and the low voltage operation and non-blocking fault ride through control are verified in a two-terminal DC network simulation.Through economic analysis,the power loss of the AFC is similar to a HBSM MMC but the total investment is lower than a hybrid MMC,making the AFC a promising solution to improve the existing HBSM converter with more controllability.
基金supported by The National Natural Science Foundation key project(U1766209).
文摘Considering the advantages and limitations of traditional identification method,combined with the strategy of active detection,the principle of DC grid pilot protection based on active detection is proposed to improve the sensitivity and reliability of hybrid MMC DC grid protection,and to ensure the accurate identification of fault areas in DC grid.By using the DC fault ride-through control strategy of the hybrid sub-module MMC,the fault current at the converter station DC terminal is limited.Based on the high controllability of hybrid MMC,sinusoidal fault detection signals with the same frequency are injected into the line at each converter station.Based on model recognition,the capacitance model condition is satisfied by the detected signals at both ends during external faults whereas not satisfied during internal faults.The Spearman correlation coefficients is then introduced,and the correlation discriminant of capacitance model is constructed to realize fault area discrimination of DC grid.The simulation results show that the active detection protection scheme proposed in this paper can accurately identify the fault area of DC grid,and is not affected by fault impedance and has low sampling rate requirement.
文摘基于模块化多电平换流器(modular multilevel converter,MMC)的柔性直流电网是实现大规模可再生能源发电汇集、输送和并网的有效手段。针对传统直流故障穿越(fault ride-through,FRT)方案会引起全网功率传输中断的问题,提出一种新型的直流FRT协调控制策略,其可快速阻断故障电流并维持非故障线路换流站不退出运行。在分析故障电流组成及关键影响因素的基础上,针对故障线路MMC(fault line MMC,FLMMC),提出了自适应于故障线电流变化的负压控制策略,该策略可以提升电流衰减速度,保证故障可靠隔离。针对非故障线路MMC(non-fault line MMC,NFLMMC),考虑到FLMMC的过电流、过电压风险,提出了NFLMMC主动限流(active current-limiting control,ACLC)协同控制方法,设计了限流性能可自适应于直流母线电压变化的参数选取原则,在抑制故障电流的同时兼顾直流电网的快速恢复。最后,基于RTLAB实时数字仿真平台搭建了四端柔性直流电网仿真模型。仿真结果表明,所提协同控制策略能够快速阻断直流故障电流,缩短直流电网功率恢复时间,提高系统安全稳定性。
基金supported by the Science and Technology Project of the State Grid Corporation of China,HVDC Systems/Grids for Transnational Interconnections(Project number:SGTYHT/16-JS-198).
文摘A hybrid of line commutated converters(LCCs)and modular multi-level converters(MMCs)can provide the advantages of both the technologies.However,the commutation failure still exists if the LCC operates as an inverter in a hybrid LCC/MMC system.In this paper,the system behavior during a commutation failure is investigated.Both halfbridge and full-bridge MMCs are considered.Control strategies are examined through simulations conducted in PSCAD/EMTDC.Additionally,commutation failure protection strategies for multi-terminal hybrid LCC/MMC systems with AC and DC circuit breakers are studied.This paper can contribute to the protection design of future hybrid LCC/MMC systems against commutation failures.
文摘针对模块化多电平统一电能质量调节器(modular multilevel unified power quality conditioner, MMC-UPQC)六桥臂结构下的单相桥臂故障问题,提出了一种五桥臂拓扑,这种新型拓扑可实现故障情况下的电能质量补偿。首先,对MMC-UPQC串并联侧的数学模型进行分析,提出了一种复合模型预测控制(hybrid model predictive control,H-MPC),所提控制方法结合了有限集模型预测控制(finite-control-set model predictive control, FCS-MPC)以及快速模型预测控制(fast model predictive control, F-MPC)。然后,通过构建两侧独立的价值函数减少了控制方法的计算量,同时也实现了五桥臂解耦控制。最后,相比传统线性(例如PI)和非线性(例如无源控制passivity-based control,PBC)的控制策略,所提复合模型预测控制在电压补偿、负序电压抑制以及谐波电流补偿等方面具有一定优势,并在一定程度上避免了复杂的参数整定及坐标变化环节。仿真实验结果证明了所提控制方法的可行性和优越性。
