The electric networks for the distribution to low voltage costumers can be configured in different layouts. Two main approaches are used: the European system composed by three-phase distribution transformers or the No...The electric networks for the distribution to low voltage costumers can be configured in different layouts. Two main approaches are used: the European system composed by three-phase distribution transformers or the North American system composed by single-phase distribution transformers and three-phase transformer banks of single-phase transformers. With respect to harmonic analysis, much more attention has been focused on the three-phase balanced systems arrangements than on the unbalanced four-wire delta system extensively used to supply low voltage loads of 120/240 V. Different authors have shown the three-phase power systems modeling on a phase-coordinates frame. However, the presence of significant asymmetries in the network forces the need of adding a new phase-coordinates model to represent the three-phase transformers banks of two or three single-phase transformers in its various connections. Several papers treat the use of harmonic analysis programs based on a phase-coordinates frame to study the Wye or Delta connected three-phase systems. However, the commonly used four-wire delta connected systems are not fully treated in literature. This paper presents a phase-coordinates model for the representation of the commonly used three-phase transformer banks of three or two single-phase transformers, and single-phase distribution transformers for the harmonic analysis of the four-wire delta connected systems. The harmonic analysis method based on the presented model is used to examine the characteristics of this kind of distribution system with respect to the penetration of harmonics currents from loads to the primary system.展开更多
The coordinated control of parallel three-phase fourwire converters in autonomous AC microgrids is investigated in this paper.First,based on droop control,virtual impedance is inserted in positive-,negative-and zero-s...The coordinated control of parallel three-phase fourwire converters in autonomous AC microgrids is investigated in this paper.First,based on droop control,virtual impedance is inserted in positive-,negative-and zero-sequences to enhance system damping and imbalance power sharing.Then,to facilitate virtual impedance design,small signal models of the three-sequence equivalent circuits are established respectively.Corresponding indexes are proposed to comprehensively evaluate the impact of sequence virtual impedance on current sharing accuracy,voltage quality at the point of common coupling(PCC)and system stability.In addition,constraint of DClink voltage is also considered to avoid over modulation when subjected to unbalanced loads.Furthermore,to address the PCC voltage degradation resulting from virtual impedance,a voltage imbalance compensation method,based on low-bandwidth communication,is proposed.Finally,simulation and experimental results are provided to verify the correctness of the theory model,indicating that the proposed method can achieve PCC voltage restoration while guaranteeing the current sharing accuracy with desirable dynamics.展开更多
We propose a modular multilevel converter(MMC)based three-phase four-wire(3P4W)split capacitor distribution static synchronous compensator(DSTATCOM),aiming at compensating unbalanced and reactive load currents.Due to ...We propose a modular multilevel converter(MMC)based three-phase four-wire(3P4W)split capacitor distribution static synchronous compensator(DSTATCOM),aiming at compensating unbalanced and reactive load currents.Due to the zero-sequence current compensation,the circulating current char-acteristics of 3P4W MMC-DSTATCOM are different from conventional MMCs.Moreover,the distinct working principle of IMMC would affect the features of split capacitor voltage.The decoupled positive-,negative-and zero-sequence second-order and DC components of the circulating current are deduced explicitly.Two proportional-integral controllers with dual dq transformation are employed to suppress the positive-and negative-sequence components of second-order circulating current,while quasi proportional-resonance controller is designed to eliminate the zero-sequence component.Besides,the phenomenon of the unbalanced split capacitor voltages is revealed,and fast-tracking balancing method by controlling zero-sequence current flowing through the split capacitors is provided.Digital simulation results verify the accuracy of the analysis and the feasibility of the suppression methods.展开更多
Low voltage three-phase four-wire AC distribution grids may experience high neutral current,mainly caused by asymmetrical distribution of single-phase loads in three phases.High neutral current will not only increase ...Low voltage three-phase four-wire AC distribution grids may experience high neutral current,mainly caused by asymmetrical distribution of single-phase loads in three phases.High neutral current will not only increase line losses but also result in neutral potential variations.For the LV AC distribution grid established by a grid-forming inverter(e.g.,uninterruptible power supply and solid-state-transformer),it also suffers from the same neutral current issues.Therefore,this paper comparatively studies several neutral current control approaches and their impacts on grid voltage balance,which is required by grid code.Then,this paper proposes an optimal neutral current control approach,which can obtain maximum neutral current suppression with less impact on grid voltage balance.The correctness of the theoretical analysis is validated through both simulation and experimental results.展开更多
目的:观察四黄散联合间断垂直钢丝缝合加张力带对髌骨下极骨折患者膝关节活动度及骨折愈合时间的影响。方法:选取2021年9月至2022年9月丰城市中医院收治的髌骨下极骨折患者40例,按照随机数字表法分为对照组和试验组,每组20例。两组患者...目的:观察四黄散联合间断垂直钢丝缝合加张力带对髌骨下极骨折患者膝关节活动度及骨折愈合时间的影响。方法:选取2021年9月至2022年9月丰城市中医院收治的髌骨下极骨折患者40例,按照随机数字表法分为对照组和试验组,每组20例。两组患者均给予切开复位内固定,对照组给予间断垂直钢丝缝合联合张力带治疗,试验组给予四黄散联合间断垂直钢丝缝合加张力带治疗。观察两组患者术后6周、3个月、6个月、1年及末次随访时膝关节疼痛评分、屈伸活动范围(range of motion, ROM)、Bostman评分、骨折愈合时间、各项临床指标及并发症发生情况。结果:试验组术后6周、3个月、6个月、1年膝关节疼痛评分均低于对照组,差异有统计学意义(P<0.05)。试验组术后6周、3个月、6个月、1年ROM、Bostman评分高于对照组,差异有统计学意义(P<0.05)。试验组术后1年骨折愈合时间、各项临床指标均优于对照组,并发症发生率低于对照组,差异有统计学意义(P<0.05)。结论:四黄散联合间断垂直钢丝缝合加张力带能改善髌骨下极骨折患者膝关节功能,降低术后并发症发生率,促进骨折早日愈合。展开更多
文摘The electric networks for the distribution to low voltage costumers can be configured in different layouts. Two main approaches are used: the European system composed by three-phase distribution transformers or the North American system composed by single-phase distribution transformers and three-phase transformer banks of single-phase transformers. With respect to harmonic analysis, much more attention has been focused on the three-phase balanced systems arrangements than on the unbalanced four-wire delta system extensively used to supply low voltage loads of 120/240 V. Different authors have shown the three-phase power systems modeling on a phase-coordinates frame. However, the presence of significant asymmetries in the network forces the need of adding a new phase-coordinates model to represent the three-phase transformers banks of two or three single-phase transformers in its various connections. Several papers treat the use of harmonic analysis programs based on a phase-coordinates frame to study the Wye or Delta connected three-phase systems. However, the commonly used four-wire delta connected systems are not fully treated in literature. This paper presents a phase-coordinates model for the representation of the commonly used three-phase transformer banks of three or two single-phase transformers, and single-phase distribution transformers for the harmonic analysis of the four-wire delta connected systems. The harmonic analysis method based on the presented model is used to examine the characteristics of this kind of distribution system with respect to the penetration of harmonics currents from loads to the primary system.
文摘The coordinated control of parallel three-phase fourwire converters in autonomous AC microgrids is investigated in this paper.First,based on droop control,virtual impedance is inserted in positive-,negative-and zero-sequences to enhance system damping and imbalance power sharing.Then,to facilitate virtual impedance design,small signal models of the three-sequence equivalent circuits are established respectively.Corresponding indexes are proposed to comprehensively evaluate the impact of sequence virtual impedance on current sharing accuracy,voltage quality at the point of common coupling(PCC)and system stability.In addition,constraint of DClink voltage is also considered to avoid over modulation when subjected to unbalanced loads.Furthermore,to address the PCC voltage degradation resulting from virtual impedance,a voltage imbalance compensation method,based on low-bandwidth communication,is proposed.Finally,simulation and experimental results are provided to verify the correctness of the theory model,indicating that the proposed method can achieve PCC voltage restoration while guaranteeing the current sharing accuracy with desirable dynamics.
基金This work was supported in part by the National Natural Science Foundation of China(No.51807073).
文摘We propose a modular multilevel converter(MMC)based three-phase four-wire(3P4W)split capacitor distribution static synchronous compensator(DSTATCOM),aiming at compensating unbalanced and reactive load currents.Due to the zero-sequence current compensation,the circulating current char-acteristics of 3P4W MMC-DSTATCOM are different from conventional MMCs.Moreover,the distinct working principle of IMMC would affect the features of split capacitor voltage.The decoupled positive-,negative-and zero-sequence second-order and DC components of the circulating current are deduced explicitly.Two proportional-integral controllers with dual dq transformation are employed to suppress the positive-and negative-sequence components of second-order circulating current,while quasi proportional-resonance controller is designed to eliminate the zero-sequence component.Besides,the phenomenon of the unbalanced split capacitor voltages is revealed,and fast-tracking balancing method by controlling zero-sequence current flowing through the split capacitors is provided.Digital simulation results verify the accuracy of the analysis and the feasibility of the suppression methods.
基金the National Natural Science Foundation of China(NSFC)under Grant 51767017National Science and Technology Major Project of Gansu Province under Grant 19ZD2GA003.
文摘Low voltage three-phase four-wire AC distribution grids may experience high neutral current,mainly caused by asymmetrical distribution of single-phase loads in three phases.High neutral current will not only increase line losses but also result in neutral potential variations.For the LV AC distribution grid established by a grid-forming inverter(e.g.,uninterruptible power supply and solid-state-transformer),it also suffers from the same neutral current issues.Therefore,this paper comparatively studies several neutral current control approaches and their impacts on grid voltage balance,which is required by grid code.Then,this paper proposes an optimal neutral current control approach,which can obtain maximum neutral current suppression with less impact on grid voltage balance.The correctness of the theoretical analysis is validated through both simulation and experimental results.
文摘目的:观察四黄散联合间断垂直钢丝缝合加张力带对髌骨下极骨折患者膝关节活动度及骨折愈合时间的影响。方法:选取2021年9月至2022年9月丰城市中医院收治的髌骨下极骨折患者40例,按照随机数字表法分为对照组和试验组,每组20例。两组患者均给予切开复位内固定,对照组给予间断垂直钢丝缝合联合张力带治疗,试验组给予四黄散联合间断垂直钢丝缝合加张力带治疗。观察两组患者术后6周、3个月、6个月、1年及末次随访时膝关节疼痛评分、屈伸活动范围(range of motion, ROM)、Bostman评分、骨折愈合时间、各项临床指标及并发症发生情况。结果:试验组术后6周、3个月、6个月、1年膝关节疼痛评分均低于对照组,差异有统计学意义(P<0.05)。试验组术后6周、3个月、6个月、1年ROM、Bostman评分高于对照组,差异有统计学意义(P<0.05)。试验组术后1年骨折愈合时间、各项临床指标均优于对照组,并发症发生率低于对照组,差异有统计学意义(P<0.05)。结论:四黄散联合间断垂直钢丝缝合加张力带能改善髌骨下极骨折患者膝关节功能,降低术后并发症发生率,促进骨折早日愈合。
