This paper proposes a single-ended fault detection scheme for long transmission lines using support vector machine(SVM)for multi-terminal direct current systems based on modular multilevel converter(MMC-MTDC).The sche...This paper proposes a single-ended fault detection scheme for long transmission lines using support vector machine(SVM)for multi-terminal direct current systems based on modular multilevel converter(MMC-MTDC).The scheme overcomes existing detection difficulties in the protection of long transmission lines resulting from high grounding resistance and attenuation,and also avoids the sophisticated process of threshold value selection.The high-frequency components in the measured voltage extracted by a wavelet transform and the amplitude of the zero-mode set of the positive-sequence voltage are the inputs to a trained SVM.The output of the SVM determines the fault type.A model of a four-terminal DC power grid with overhead transmission lines is built in PSCAD/EMTDC.Simulation results of EMTDC confirm that the proposed scheme achieves 100%accuracy in detecting short-circuit faults with high resistance on long transmission lines.The proposed scheme eliminates mal-operation of DC circuit breakers when faced with power order changes or AC-side faults.Its robustness and time delay are also assessed and shown to have no perceptible effect on the speed and accuracy of the detection scheme,thus ensuring its reliability and stability.展开更多
提出了两种含VSC-MTDC的交、直流系统潮流控制算法,即节点注入电流法和改进序列潮流算法。节点注入电流法中,以各端公共耦合点PCC(point of common coupling)作为交替求解的接口,VSC-MTDC系统对交流系统的影响通过改变PCC点的注入电流...提出了两种含VSC-MTDC的交、直流系统潮流控制算法,即节点注入电流法和改进序列潮流算法。节点注入电流法中,以各端公共耦合点PCC(point of common coupling)作为交替求解的接口,VSC-MTDC系统对交流系统的影响通过改变PCC点的注入电流来实现。针对节点注入电流法中由于接口变量数目较多而导致收敛性能变差的问题,改进了序列潮流算法,只需设置主站PCC点传输的有功功率作为接口变量进行迭代更新,直至收敛。在新英格兰系统上验证了两种所提算法的有效性和可行性。对比分析两种方法可知,改进序列潮流算法总体性能优于节点注入电流法,为含VSC-MTDC的交、直流系统潮流算法的选择提供参考。展开更多
随着国内柔性直流输电技术的快速发展,基于真双极接线的多端柔性输电技术将被越来越多地应用到实际工程中。不同于单极线路或换流器退出运行时,伪双极系统下会出现的线路过载和切机切负荷现象,对于运行方式更为灵活的真双极系统,由于非...随着国内柔性直流输电技术的快速发展,基于真双极接线的多端柔性输电技术将被越来越多地应用到实际工程中。不同于单极线路或换流器退出运行时,伪双极系统下会出现的线路过载和切机切负荷现象,对于运行方式更为灵活的真双极系统,由于非故障极可转代故障极的部分功率,使得电网可靠性和整体输电能力的提升成为可能。提出了一种适用于真双极多端柔性直流输电(VSC-MTDC)系统的功率转代策略:基于真双极系统正负极电网可独立控制功率的特点,当直流系统在非正常运行状况下出现非对称拓扑时,在确保各元件不越限的前提下使非故障极电网转代故障极电网部分功率,以提高VSC-MTDC系统的总传输容量。同时,基于张北±500 k V柔性直流输电示范工程,搭建了真双极四端柔性直流输电PSCAD/EMTDC仿真系统,对所提策略进行了有效性验证。展开更多
为解决多端直流输电系统(multi-terminal direct current,MTDC)存在能量冲击,影响系统的正常启动的问题,提出一种智能协调启动控制策略。变电站采用模块化多电平换流器(modular multilevel converter,MMC),在MMC的预充电的过程中存在预...为解决多端直流输电系统(multi-terminal direct current,MTDC)存在能量冲击,影响系统的正常启动的问题,提出一种智能协调启动控制策略。变电站采用模块化多电平换流器(modular multilevel converter,MMC),在MMC的预充电的过程中存在预充电“先发优势”的问题,为此搭建了五端MMC-MTDC系统的预充电与协调启动模型,针对几种典型的MMC-MTDC系统故障进行分析,提出智能故障恢复快速启动控制策略,仿真验证结果表明:所提出的控制策略能够有效降低启动时的暂态能量冲击,现场测试结果与预期结果一致。展开更多
In order to overcome the problems of power flow control and fault current limiting in multi-terminal high voltage direct current(MTDC)grids,this paper proposes a modular multi-terminal DC power flow controller(MM-DCPF...In order to overcome the problems of power flow control and fault current limiting in multi-terminal high voltage direct current(MTDC)grids,this paper proposes a modular multi-terminal DC power flow controller(MM-DCPFC)with fault current limiting function.The topology structure,operation principle,and equivalent circuit of MM-DCPFC are introduced,and such a structure has the advantages of modularity and scalability.The power balance mechanism is studied and a hierarchical power balance control strategy is proposed.The results show that MM-DCPFC can achieve internal power exchange,which avoids the use of external power supply.The fault characteristics of MM-DCPFC are analyzed,fault current limiting and self-protection methods are proposed,and the factors affecting the current limiting capability are studied.The simulation models are established in PLECS,and the simulation results verify the effectiveness of MM-DCPFC in power flow control,fault current limiting,and scalability.In addition,a prototype is developed to validate the function and control method of MM-DCPFC.展开更多
Frequency regulation of voltage source converter-based multi-terminal high-voltage direct current(VSC-MTDC)system with offshore wind farms enhances the frequency stability by compensating the power for a disturbed AC ...Frequency regulation of voltage source converter-based multi-terminal high-voltage direct current(VSC-MTDC)system with offshore wind farms enhances the frequency stability by compensating the power for a disturbed AC system.However,it is difficult to reasonably allocate frequency-regulation resources due to a lack of coordination mechanisms between wind farms and the MTDC system.Moreover,it is difficult for the frequency control of the wind farms to manage changes in wind speed;and the risk of wind-turbine stalls is high.Thus,based on the kinetic energy of wind turbines and the power margin of the converters,the frequency-regulation capability of wind turbines is evaluated,and a dynamic frequency-support scheme considering the real-time frequency-support capability of the wind turbines and system frequency evolution is proposed to improve the frequency-support performance.A power adaptation technique at variable wind speeds is developed;the active power in the frequency-support stage and restoration stage is switched according to the wind speed.A hierarchical zoning frequency-regulation scheme is designed to use the frequency-regulation resources of different links in the MTDC system with wind farms.The simulation results show that the novel frequency-regulation strategy maintains frequency stability with wind-speed changes and avoids multiple frequency dips.展开更多
文摘This paper proposes a single-ended fault detection scheme for long transmission lines using support vector machine(SVM)for multi-terminal direct current systems based on modular multilevel converter(MMC-MTDC).The scheme overcomes existing detection difficulties in the protection of long transmission lines resulting from high grounding resistance and attenuation,and also avoids the sophisticated process of threshold value selection.The high-frequency components in the measured voltage extracted by a wavelet transform and the amplitude of the zero-mode set of the positive-sequence voltage are the inputs to a trained SVM.The output of the SVM determines the fault type.A model of a four-terminal DC power grid with overhead transmission lines is built in PSCAD/EMTDC.Simulation results of EMTDC confirm that the proposed scheme achieves 100%accuracy in detecting short-circuit faults with high resistance on long transmission lines.The proposed scheme eliminates mal-operation of DC circuit breakers when faced with power order changes or AC-side faults.Its robustness and time delay are also assessed and shown to have no perceptible effect on the speed and accuracy of the detection scheme,thus ensuring its reliability and stability.
文摘提出了两种含VSC-MTDC的交、直流系统潮流控制算法,即节点注入电流法和改进序列潮流算法。节点注入电流法中,以各端公共耦合点PCC(point of common coupling)作为交替求解的接口,VSC-MTDC系统对交流系统的影响通过改变PCC点的注入电流来实现。针对节点注入电流法中由于接口变量数目较多而导致收敛性能变差的问题,改进了序列潮流算法,只需设置主站PCC点传输的有功功率作为接口变量进行迭代更新,直至收敛。在新英格兰系统上验证了两种所提算法的有效性和可行性。对比分析两种方法可知,改进序列潮流算法总体性能优于节点注入电流法,为含VSC-MTDC的交、直流系统潮流算法的选择提供参考。
文摘随着国内柔性直流输电技术的快速发展,基于真双极接线的多端柔性输电技术将被越来越多地应用到实际工程中。不同于单极线路或换流器退出运行时,伪双极系统下会出现的线路过载和切机切负荷现象,对于运行方式更为灵活的真双极系统,由于非故障极可转代故障极的部分功率,使得电网可靠性和整体输电能力的提升成为可能。提出了一种适用于真双极多端柔性直流输电(VSC-MTDC)系统的功率转代策略:基于真双极系统正负极电网可独立控制功率的特点,当直流系统在非正常运行状况下出现非对称拓扑时,在确保各元件不越限的前提下使非故障极电网转代故障极电网部分功率,以提高VSC-MTDC系统的总传输容量。同时,基于张北±500 k V柔性直流输电示范工程,搭建了真双极四端柔性直流输电PSCAD/EMTDC仿真系统,对所提策略进行了有效性验证。
文摘为解决多端直流输电系统(multi-terminal direct current,MTDC)存在能量冲击,影响系统的正常启动的问题,提出一种智能协调启动控制策略。变电站采用模块化多电平换流器(modular multilevel converter,MMC),在MMC的预充电的过程中存在预充电“先发优势”的问题,为此搭建了五端MMC-MTDC系统的预充电与协调启动模型,针对几种典型的MMC-MTDC系统故障进行分析,提出智能故障恢复快速启动控制策略,仿真验证结果表明:所提出的控制策略能够有效降低启动时的暂态能量冲击,现场测试结果与预期结果一致。
基金supported in part by National Key R&D Program of China(No.2018YFB0904600)National Natural Science Foundation of China(No.51807053)。
文摘In order to overcome the problems of power flow control and fault current limiting in multi-terminal high voltage direct current(MTDC)grids,this paper proposes a modular multi-terminal DC power flow controller(MM-DCPFC)with fault current limiting function.The topology structure,operation principle,and equivalent circuit of MM-DCPFC are introduced,and such a structure has the advantages of modularity and scalability.The power balance mechanism is studied and a hierarchical power balance control strategy is proposed.The results show that MM-DCPFC can achieve internal power exchange,which avoids the use of external power supply.The fault characteristics of MM-DCPFC are analyzed,fault current limiting and self-protection methods are proposed,and the factors affecting the current limiting capability are studied.The simulation models are established in PLECS,and the simulation results verify the effectiveness of MM-DCPFC in power flow control,fault current limiting,and scalability.In addition,a prototype is developed to validate the function and control method of MM-DCPFC.
基金supported by the National Key R&D Program of China(No.2022YFB2402700).
文摘Frequency regulation of voltage source converter-based multi-terminal high-voltage direct current(VSC-MTDC)system with offshore wind farms enhances the frequency stability by compensating the power for a disturbed AC system.However,it is difficult to reasonably allocate frequency-regulation resources due to a lack of coordination mechanisms between wind farms and the MTDC system.Moreover,it is difficult for the frequency control of the wind farms to manage changes in wind speed;and the risk of wind-turbine stalls is high.Thus,based on the kinetic energy of wind turbines and the power margin of the converters,the frequency-regulation capability of wind turbines is evaluated,and a dynamic frequency-support scheme considering the real-time frequency-support capability of the wind turbines and system frequency evolution is proposed to improve the frequency-support performance.A power adaptation technique at variable wind speeds is developed;the active power in the frequency-support stage and restoration stage is switched according to the wind speed.A hierarchical zoning frequency-regulation scheme is designed to use the frequency-regulation resources of different links in the MTDC system with wind farms.The simulation results show that the novel frequency-regulation strategy maintains frequency stability with wind-speed changes and avoids multiple frequency dips.
