Definite-time zero-sequence over-current protection is presently used in systems whose neutral point is grounded by a low resistance(low-resistance grounding systems).These systems frequently malfunction owing to thei...Definite-time zero-sequence over-current protection is presently used in systems whose neutral point is grounded by a low resistance(low-resistance grounding systems).These systems frequently malfunction owing to their high settings of the action value when a high-impedance grounding fault occurs.In this study,the relationship between the zero-sequence currents of each feeder and the neutral branch was analyzed.Then,a grounding protection method was proposed on the basis of the zero-sequence current ratio coefficient.It is defined as the ratio of the zero-sequence current of the feeder to that of the neutral branch.Nonetheless,both zero-sequence voltage and zero-sequence current are affected by the transition resistance,The influence of transition resistance can be eliminated by calculating this coefficient.Therefore,a method based on the zero-sequence current ratio coefficient was proposed considering the significant difference between the faulty feeder and healthy feeder.Furthermore,unbalanced current can be prevented by setting the starting current.PSCAD simulation results reveal that the proposed method shows high reliability and sensitivity when a high-resistance grounding fault occurs.展开更多
To reduce current harmonics caused by switching frequency,T-type grid-connected inverter topology with LCL filter is adopted.In view of the disadvantages of the slow response speed of the traditional current control a...To reduce current harmonics caused by switching frequency,T-type grid-connected inverter topology with LCL filter is adopted.In view of the disadvantages of the slow response speed of the traditional current control and the failure to eliminate the influence of the LCL filter on the grid-connected current by using current PI control alone,a current double closed loop PI current tracking control is proposed.Through the theoretical analysis of the grid-connected inverter control principle,the grid-connected inverter control model is designed,and the transfer functionmodel of each control link is deduced,and the current loop PI regulator is designed at last.The simulation results show that the control strategy is feasible.展开更多
Dual three-phase permanent-magnet synchronous machines(DTP-PMSM)connected with a single neutral point provide a loop for zero-sequence current(ZSC).This paper proposes a novel space vector pulse width modulation(SVPWM...Dual three-phase permanent-magnet synchronous machines(DTP-PMSM)connected with a single neutral point provide a loop for zero-sequence current(ZSC).This paper proposes a novel space vector pulse width modulation(SVPWM)strategy to suppress the ZSC.Five vectors are selected as basic voltage vectors in one switching period.The fundamental and harmonic planes and the zero-sequence plane are taken into consideration to synthesis the reference voltage vector.To suppress the ZSC,a non-zero zero-sequence voltage(ZSV)is generated to compensate the third harmonic back-EMF.Rather than triangular carrier modulation,the sawtooth carrier modulation strategy is used to generate asymmetric PWM signals.The modulation range is investigated to explore the variation of modulation range caused by considering the zero-sequence plane.With the proposed method,the ZSC can be considerably reduced.The simulated and experimental results are presented to validate the effectiveness of the proposed modulation strategy.展开更多
Compared with the traditional three-phase star connection winding,the open-end winding permanent magnet synchronous motor(OW-PMSM)system with a common direct current(DC)bus has a zero-sequence circuit,which makes the ...Compared with the traditional three-phase star connection winding,the open-end winding permanent magnet synchronous motor(OW-PMSM)system with a common direct current(DC)bus has a zero-sequence circuit,which makes the common-mode voltage and the back electromotive force(EMF)harmonic generated by the inverters produce the zero-sequence current in the zero-sequence circuit,and the zero-sequence current has great influence on the operation efficiency and stability of the motor control system.A zero-sequence current suppression strategy is presented based on model predictive current control for OW-PMSM.Through the mathematical model of OW-PMSM to establish the predictive model and the zero-sequence circuit model,the common-mode voltage under different voltage vector combinations is fully considered during vector selection and action time calculation.Then zero-sequence loop constraints are established,so as to suppress the zero-sequence current.In the end,the control strategy proposed in this paper is verified by simulation experiments.展开更多
We propose a bulk negative refractive index (NRI) metamaterial composed of periodic array of tightly coupled metallic cross-pairs printed oR the six sides of a cube for applications of superlenses. The structural ch...We propose a bulk negative refractive index (NRI) metamaterial composed of periodic array of tightly coupled metallic cross-pairs printed oR the six sides of a cube for applications of superlenses. The structural characteristics of the three-dimensional (3D) metamaterial consist in the high symmetry and the superposition of metallic cross-pairs, which can increase the magnetic inductive coupling between adjacent cross-pairs and realize a broadband and isotropic NRI. The proposed 3D structure is simulated using the CS~ Microwave Studio 2006 to verify the design validity. The simulation results show that the proposed structure can not only realize simultaneously an electric and magnetic response to an incident electromagnetic (EM) wave, but also exhibit a broadband NRI whose relative bandwidth can reach up to 56.7%. In addition, the NRI band is insensitive to tile polarization and the incident angle of the incident EM wave. Therefore, the proposed metamaterial is a good candidate material as three-dimensional broadband isotropic NRI metamaterial.展开更多
With the advent of Computer Algebra System (CAS) such as Mathematica [1], challenging symbolic longhand calcula-tions can effectively be performed free of error and at ease. Mathematica’s integrated features allow th...With the advent of Computer Algebra System (CAS) such as Mathematica [1], challenging symbolic longhand calcula-tions can effectively be performed free of error and at ease. Mathematica’s integrated features allow the investigator to combine the needed symbolic, numeric and graphic modules all in one interactive environment. This assists the author to focus on interpreting the output rather than exerting the efforts of relating the scattered separate modules. In this note the author, utilizing these three features, explores the magneto-static field and its associated vector potential of a steady looping current. In particular by deploying the numeric features of Mathematica the exact value of the vector potential of the looping current conducive to its 3D graph is presented.展开更多
A thin metallic wire loop of arbitrary curvature is rotated with respect to an arbitrary axis of its plane. The device is excited by an electric dipole of infinite length and constant current. The resistance of the lo...A thin metallic wire loop of arbitrary curvature is rotated with respect to an arbitrary axis of its plane. The device is excited by an electric dipole of infinite length and constant current. The resistance of the loop is computed rigorously as function of the position of the source. In this way, the induced voltage along the wire, under any kind of axial excitation, is given in the form of a superposition integral. The measured response is represented for various shapes of the coil, with respect to the time, the rotation angle and the position of the source. These diagrams lead to several technically applicable conclusions which are presented, discussed and justified.展开更多
同步参考坐标系锁相环是高压直流(high voltage direct current,HVDC)同步触发控制系统中广泛应用的一种窄带宽锁相环,在交流系统故障引起相位跳变情况下,其动态响应缓慢。为增大锁相环的带宽,一种滑动平均滤波器(moving average filter...同步参考坐标系锁相环是高压直流(high voltage direct current,HVDC)同步触发控制系统中广泛应用的一种窄带宽锁相环,在交流系统故障引起相位跳变情况下,其动态响应缓慢。为增大锁相环的带宽,一种滑动平均滤波器(moving average filter,MAF)被前置于锁相环路,然而MAF本身存在响应延迟,制约了锁相环的同步速度。为了缓解响应延迟问题,文中提出一种考虑MAF延时和前馈补偿的HVDC快速锁相环。首先,利用MAF线性暂态特征预测相位变化,并分别针对故障接入和切除引起的相位跳变问题提出不同的补偿策略;接着,利用不变性原理对锁相环路进行前馈补偿,在负反馈控制和前馈补偿共同构成的复合校正控制系统的作用下,锁相环能够在较小PI参数下实现快速响应;最后,将所提快速锁相环在CIGRE HVDC标准模型和三峡—上海直流工程模型中进行仿真验证。结果表明,该快速锁相环能够有效缓解滤波器响应延迟的制约,缩短失锁时间,进而提高高压直流逆变侧抵御换相失败的能力。展开更多
针对极端灾害天气下配电网的故障恢复问题,计及极端灾害的时空演变特性,提出了一种利用移动储能(mobile energy storage system,MESS)的灵活性主动参与抑制合环冲击电流,辅助配电网故障恢复的策略。首先,建立了暴风雨等不同气象灾害条...针对极端灾害天气下配电网的故障恢复问题,计及极端灾害的时空演变特性,提出了一种利用移动储能(mobile energy storage system,MESS)的灵活性主动参与抑制合环冲击电流,辅助配电网故障恢复的策略。首先,建立了暴风雨等不同气象灾害条件下配电线路的故障率统计模型。其次,将复杂交通网与配电网进行耦合,建立了以最小化MESS调度时空成本的上层预布局模型。然后,分析了MESS出力对冲击电流的影响,以抑制合环冲击电流、最大化负荷恢复率和最小化MESS调度成本为目标,构建了MESS参与网络重构的下层调度模型。最后,在改进的IEEE 123节点系统中进行测试,验证了MESS对配网故障恢复安全性的提升效果。结果表明,采用MESS主动参与调控能够有效应对移动的极端气象灾害引发的配电网动态故障恢复问题,并抑制合解环过程中电流的波动,提升网络重构方案的可行性和增强配电网的韧性。展开更多
针对逆变侧交流故障下高压直流(line commutated converter-high voltage direct current,LCC-HVDC)输电系统易引发连续换相失败问题,该文分阶段分析了连续换相失败的产生机理。明确了当产生触发角偏差后,直流电流调节效果与实际触发角...针对逆变侧交流故障下高压直流(line commutated converter-high voltage direct current,LCC-HVDC)输电系统易引发连续换相失败问题,该文分阶段分析了连续换相失败的产生机理。明确了当产生触发角偏差后,直流电流调节效果与实际触发角不匹配是连续换相失败的重要诱因。同时不对称故障后,负序分量通过极控、阀控两级控制系统对实际触发角造成的影响,增加连续换相失败的风险。因此提出了一种基于实际触发角响应的连续换相失败抑制方法,通过实际触发角修正整流侧电流指令值。与此同时,该方法通过陷波器降低负序电流对极控层触发角指令值的影响,通过相序解耦控制器和换相电压相位补偿降低负序电压对阀控层触发角实际值的影响。该方法能实现动态、自适应的电流指令值调整,提升恢复过程中直流电流与实际触发角的匹配度,提升直流系统抵御连续换相失败能力。在CIGRE标准测试模型和实际工程模型中,验证了理论分析的正确性和优化方法的有效性。展开更多
该文研究了电网换相换流器高压直流输电(line commutated converter high voltage direct current,LCC-HVDC)系统逆变侧采用不同锁相环时,锁相环控制回路比例–积分(proportional integral,PI)参数变化对直流控制回路稳定性的影响。首先...该文研究了电网换相换流器高压直流输电(line commutated converter high voltage direct current,LCC-HVDC)系统逆变侧采用不同锁相环时,锁相环控制回路比例–积分(proportional integral,PI)参数变化对直流控制回路稳定性的影响。首先,分别建立了逆变侧锁相环采用滑动平均滤波(mo ving average filter,MAF)和级联延时消去滤波(cascaded delayed signal cancellation,CDSC)的LCC-HVDC小干扰动态模型,并通过电磁暂态仿真验证了该模型的正确性。其次,基于拉普拉斯变换获得系统定电压控制回路的传递函数,利用奈奎斯特稳定判据以及稳定裕度指标分析不同锁相环对定电压控制回路稳定性的影响,并进行了机理分析,同时在PSCAD/EMTDC的电磁暂态模型上进行了验证。最后,进一步在LCC-HVDC工程模型上对该文所得结论的普适性进行了仿真验证。展开更多
基金supported in part by National Key Research and Development Program of China(2016YFB0900603)Technology Projects of State Grid Corporation of China(52094017000W).
文摘Definite-time zero-sequence over-current protection is presently used in systems whose neutral point is grounded by a low resistance(low-resistance grounding systems).These systems frequently malfunction owing to their high settings of the action value when a high-impedance grounding fault occurs.In this study,the relationship between the zero-sequence currents of each feeder and the neutral branch was analyzed.Then,a grounding protection method was proposed on the basis of the zero-sequence current ratio coefficient.It is defined as the ratio of the zero-sequence current of the feeder to that of the neutral branch.Nonetheless,both zero-sequence voltage and zero-sequence current are affected by the transition resistance,The influence of transition resistance can be eliminated by calculating this coefficient.Therefore,a method based on the zero-sequence current ratio coefficient was proposed considering the significant difference between the faulty feeder and healthy feeder.Furthermore,unbalanced current can be prevented by setting the starting current.PSCAD simulation results reveal that the proposed method shows high reliability and sensitivity when a high-resistance grounding fault occurs.
基金Supported by Science and Technology Projects of State Grid Corporation ofChina(J2022019).
文摘To reduce current harmonics caused by switching frequency,T-type grid-connected inverter topology with LCL filter is adopted.In view of the disadvantages of the slow response speed of the traditional current control and the failure to eliminate the influence of the LCL filter on the grid-connected current by using current PI control alone,a current double closed loop PI current tracking control is proposed.Through the theoretical analysis of the grid-connected inverter control principle,the grid-connected inverter control model is designed,and the transfer functionmodel of each control link is deduced,and the current loop PI regulator is designed at last.The simulation results show that the control strategy is feasible.
基金supported in part by the National Natural Science Foundation of China under Grant 51977099。
文摘Dual three-phase permanent-magnet synchronous machines(DTP-PMSM)connected with a single neutral point provide a loop for zero-sequence current(ZSC).This paper proposes a novel space vector pulse width modulation(SVPWM)strategy to suppress the ZSC.Five vectors are selected as basic voltage vectors in one switching period.The fundamental and harmonic planes and the zero-sequence plane are taken into consideration to synthesis the reference voltage vector.To suppress the ZSC,a non-zero zero-sequence voltage(ZSV)is generated to compensate the third harmonic back-EMF.Rather than triangular carrier modulation,the sawtooth carrier modulation strategy is used to generate asymmetric PWM signals.The modulation range is investigated to explore the variation of modulation range caused by considering the zero-sequence plane.With the proposed method,the ZSC can be considerably reduced.The simulated and experimental results are presented to validate the effectiveness of the proposed modulation strategy.
基金Fundamental Research Funds for the Central Universities,China(No.2232019D3-53)Initial Research Funds for Young Teachers of Donghua University,China(104070053029)Shanghai Rising-Star Program,China(No.19QA1400400)。
文摘Compared with the traditional three-phase star connection winding,the open-end winding permanent magnet synchronous motor(OW-PMSM)system with a common direct current(DC)bus has a zero-sequence circuit,which makes the common-mode voltage and the back electromotive force(EMF)harmonic generated by the inverters produce the zero-sequence current in the zero-sequence circuit,and the zero-sequence current has great influence on the operation efficiency and stability of the motor control system.A zero-sequence current suppression strategy is presented based on model predictive current control for OW-PMSM.Through the mathematical model of OW-PMSM to establish the predictive model and the zero-sequence circuit model,the common-mode voltage under different voltage vector combinations is fully considered during vector selection and action time calculation.Then zero-sequence loop constraints are established,so as to suppress the zero-sequence current.In the end,the control strategy proposed in this paper is verified by simulation experiments.
基金supported by the National Natural Science Foundation of China (Grant No. 51005001)the National Science Foundation for Post-doctoral Scientists in China (Grant No. 20090450226)+1 种基金the Research Foundation of Education Bureau of Heilongjiang Province, China (Grant No. 11551098)the Youth Foundation of Harbin University of Science and Technology, China (Grant No. 2009YF024)
文摘We propose a bulk negative refractive index (NRI) metamaterial composed of periodic array of tightly coupled metallic cross-pairs printed oR the six sides of a cube for applications of superlenses. The structural characteristics of the three-dimensional (3D) metamaterial consist in the high symmetry and the superposition of metallic cross-pairs, which can increase the magnetic inductive coupling between adjacent cross-pairs and realize a broadband and isotropic NRI. The proposed 3D structure is simulated using the CS~ Microwave Studio 2006 to verify the design validity. The simulation results show that the proposed structure can not only realize simultaneously an electric and magnetic response to an incident electromagnetic (EM) wave, but also exhibit a broadband NRI whose relative bandwidth can reach up to 56.7%. In addition, the NRI band is insensitive to tile polarization and the incident angle of the incident EM wave. Therefore, the proposed metamaterial is a good candidate material as three-dimensional broadband isotropic NRI metamaterial.
文摘With the advent of Computer Algebra System (CAS) such as Mathematica [1], challenging symbolic longhand calcula-tions can effectively be performed free of error and at ease. Mathematica’s integrated features allow the investigator to combine the needed symbolic, numeric and graphic modules all in one interactive environment. This assists the author to focus on interpreting the output rather than exerting the efforts of relating the scattered separate modules. In this note the author, utilizing these three features, explores the magneto-static field and its associated vector potential of a steady looping current. In particular by deploying the numeric features of Mathematica the exact value of the vector potential of the looping current conducive to its 3D graph is presented.
文摘A thin metallic wire loop of arbitrary curvature is rotated with respect to an arbitrary axis of its plane. The device is excited by an electric dipole of infinite length and constant current. The resistance of the loop is computed rigorously as function of the position of the source. In this way, the induced voltage along the wire, under any kind of axial excitation, is given in the form of a superposition integral. The measured response is represented for various shapes of the coil, with respect to the time, the rotation angle and the position of the source. These diagrams lead to several technically applicable conclusions which are presented, discussed and justified.
文摘同步参考坐标系锁相环是高压直流(high voltage direct current,HVDC)同步触发控制系统中广泛应用的一种窄带宽锁相环,在交流系统故障引起相位跳变情况下,其动态响应缓慢。为增大锁相环的带宽,一种滑动平均滤波器(moving average filter,MAF)被前置于锁相环路,然而MAF本身存在响应延迟,制约了锁相环的同步速度。为了缓解响应延迟问题,文中提出一种考虑MAF延时和前馈补偿的HVDC快速锁相环。首先,利用MAF线性暂态特征预测相位变化,并分别针对故障接入和切除引起的相位跳变问题提出不同的补偿策略;接着,利用不变性原理对锁相环路进行前馈补偿,在负反馈控制和前馈补偿共同构成的复合校正控制系统的作用下,锁相环能够在较小PI参数下实现快速响应;最后,将所提快速锁相环在CIGRE HVDC标准模型和三峡—上海直流工程模型中进行仿真验证。结果表明,该快速锁相环能够有效缓解滤波器响应延迟的制约,缩短失锁时间,进而提高高压直流逆变侧抵御换相失败的能力。
文摘针对极端灾害天气下配电网的故障恢复问题,计及极端灾害的时空演变特性,提出了一种利用移动储能(mobile energy storage system,MESS)的灵活性主动参与抑制合环冲击电流,辅助配电网故障恢复的策略。首先,建立了暴风雨等不同气象灾害条件下配电线路的故障率统计模型。其次,将复杂交通网与配电网进行耦合,建立了以最小化MESS调度时空成本的上层预布局模型。然后,分析了MESS出力对冲击电流的影响,以抑制合环冲击电流、最大化负荷恢复率和最小化MESS调度成本为目标,构建了MESS参与网络重构的下层调度模型。最后,在改进的IEEE 123节点系统中进行测试,验证了MESS对配网故障恢复安全性的提升效果。结果表明,采用MESS主动参与调控能够有效应对移动的极端气象灾害引发的配电网动态故障恢复问题,并抑制合解环过程中电流的波动,提升网络重构方案的可行性和增强配电网的韧性。
文摘针对逆变侧交流故障下高压直流(line commutated converter-high voltage direct current,LCC-HVDC)输电系统易引发连续换相失败问题,该文分阶段分析了连续换相失败的产生机理。明确了当产生触发角偏差后,直流电流调节效果与实际触发角不匹配是连续换相失败的重要诱因。同时不对称故障后,负序分量通过极控、阀控两级控制系统对实际触发角造成的影响,增加连续换相失败的风险。因此提出了一种基于实际触发角响应的连续换相失败抑制方法,通过实际触发角修正整流侧电流指令值。与此同时,该方法通过陷波器降低负序电流对极控层触发角指令值的影响,通过相序解耦控制器和换相电压相位补偿降低负序电压对阀控层触发角实际值的影响。该方法能实现动态、自适应的电流指令值调整,提升恢复过程中直流电流与实际触发角的匹配度,提升直流系统抵御连续换相失败能力。在CIGRE标准测试模型和实际工程模型中,验证了理论分析的正确性和优化方法的有效性。
文摘该文研究了电网换相换流器高压直流输电(line commutated converter high voltage direct current,LCC-HVDC)系统逆变侧采用不同锁相环时,锁相环控制回路比例–积分(proportional integral,PI)参数变化对直流控制回路稳定性的影响。首先,分别建立了逆变侧锁相环采用滑动平均滤波(mo ving average filter,MAF)和级联延时消去滤波(cascaded delayed signal cancellation,CDSC)的LCC-HVDC小干扰动态模型,并通过电磁暂态仿真验证了该模型的正确性。其次,基于拉普拉斯变换获得系统定电压控制回路的传递函数,利用奈奎斯特稳定判据以及稳定裕度指标分析不同锁相环对定电压控制回路稳定性的影响,并进行了机理分析,同时在PSCAD/EMTDC的电磁暂态模型上进行了验证。最后,进一步在LCC-HVDC工程模型上对该文所得结论的普适性进行了仿真验证。