Current optimization-based control of dual three-phase PMSM for low-frequency temperature swing reduction

In this paper,a control scheme based on current optimization is proposed for dual three-phase permanent-magnet synchronous motor(DTP-PMSM)drive to reduce the low-frequency temperature swing.The reduction of temperature swing can be equivalent to reducing maximum instantaneous phase copper loss in this paper.First,a two-level optimization aiming at minimizing maximum instantaneous phase copper loss at each electrical angle is proposed.Then,the optimization is transformed to a singlelevel optimization by introducing the auxiliary variable for easy solving.Considering that singleobjective optimization trades a great total copper loss for a small reduction of maximum phase copper loss,the optimization considering both instantaneous total copper loss and maximum phase copper loss is proposed,which has the same performance of temperature swing reduction but with lower total loss.In this way,the proposed control scheme can reduce maximum junction temperature by 11%.Both simulation and experimental results are presented to prove the effectiveness and superiority of the proposed control scheme for low-frequency temperature swing reduction.

Electrical characteristics of new three-phase traction power supply system for rail transit

A novel three-phase traction power supply system is proposed to eliminate the adverse effects caused by electric phase separation in catenary and accomplish a unifying manner of traction power supply for rail transit.With the application of two-stage three-phase continuous power supply structure,the electrical characteristics exhibit new features differing from the existing traction system.In this work,the principle for voltage levels determining two-stage network is dissected in accordance with the requirements of traction network and electric locomotive.The equivalent model of three-phase traction system is built for deducing the formula of current distribution and voltage losses.Based on the chain network model of the traction network,a simulation model is established to analyze the electrical characteristics such as traction current distribution,voltage losses,system equivalent impedance,voltage distribution,voltage unbalance and regenerative energy utilization.In a few words,quite a lot traction current of about 99%is undertaken by long-section cable network.The proportion of system voltage losses is small attributed to the two-stage three-phase power supply structure,and the voltage unbal-ance caused by impedance asymmetry of traction network is less than 1‰.In addition,the utilization rate of regenerative energy for locomotive achieves a significant promotion of over 97%.

Zero-sequence Current Suppressing Strategy for Dual Three-phase Permanent Magnet Synchronous Machines Connected with Single Neutral Point

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.

Three-Phase Heterojunction NiMo-Based Nano-Needle for Water Splitting at Industrial Alkaline Condition

Constructing heterojunction is an effective strategy to develop high-performance non-preciousmetal-based catalysts for electrochemical water splitting(WS).Herein,we design and prepare an N-doped-carbon-encapsulated Ni/MoO_(2) nano-needle with three-phase heterojunction(Ni/MoO_(2)@CN)for accelerating the WS under industrial alkaline condition.Density functional theory calculations reveal that the electrons are redistributed at the three-phase heterojunction interface,which optimizes the adsorption energy of H-and O-containing intermediates to obtain the best ΔG_(H*) for hydrogen evolution reaction(HER)and decrease the ΔG value of ratedetermining step for oxygen evolution reaction(OER),thus enhancing the HER/OER catalytic activity.Electrochemical results confirm that Ni/MoO_(2)@CN exhibits good activity for HER(ƞ_(-10)=33 mV,ƞ_(-1000)=267 mV)and OER(ƞ_(10)=250 mV,ƞ_(1000)=420 mV).It shows a low potential of 1.86 V at 1000 mA cm^(−2) for WS in 6.0 M KOH solution at 60℃ and can steadily operate for 330 h.This good HER/OER performance can be attributed to the three-phase heterojunction with high intrinsic activity and the self-supporting nano-needle with more active sites,faster mass diffusion,and bubbles release.This work provides a unique idea for designing high efficiency catalytic materials for WS.

Vector Control of Three-Phase Solar Farm Converters Based on Fictive-Axis Emulation

In this paper,a new method for adjusting the current of three-phase voltage source DC-AC converter in orthogonal(DQ)reference frame is presented.In the DQ reference system,AC variable appears in the constant form of DC,making the controller design the same as the DC-DC converter[1].It provides controllable gain benefits at the steady-state operating point,and finally realizes zero steady-state error[2].In addition,the creative analytical model is dedicated to building up a series of virtual quantities orthogonal to the actual single-phase system.In general,orthogonal imaginary numbers get the reference signal by delaying the real quantity by a quarter period.However,the introduction of such time delay makes the dynamic response of the system worse.In this paper,orthogonal quantities are generated from a virtual axis system parallel to the real axis,which can effectively improve the dynamic performance of traditional methods without increasing the complexity of controller structure.Through PSCAD simulation,the ideal experimental results are obtained.

IGBT并联应用均流控制技术综述

绝缘栅双极型晶体管IGBT(insulated-gate bipolar transistor)在现代电力电子技术中应用广泛,在某些单个器件性能达不到设计要求的工作场合,IGBT的并联使用成为一种经济可行的方法。多模块IGBT并联应用可以简化电路结构,增大变流器输出功率,提高装置功率密度。IGBT并联应用过程中,器件本体的动、静态特性及结温的差异,驱动电路结构及功率回路不对称性,伴随IGBT长期使用出现的老化或失效等问题,都会引起并联IGBT支路电流的不均衡,影响系统的可靠性和稳定性。对国内外IGBT并联应用所关注的研究热点进行了调研分析,总结了IGBT并联动、静态电流不均衡产生的原理及影响,分析了电流均衡控制原理的差异。从功率回路均流控制和驱动回路均流控制两个方面,对IGBT并联应用均流控制的工作特性进行了分析总结和技术对比,并对IGBT并联均流技术的发展方向进行了展望。

基于虚拟阻抗的三相Buck整流器输入不平衡控制策略

三相输入电压不平衡时,传统双环控制三相Buck整流器的输出电压存在二倍频脉动,输入电流发生明显畸变,严重影响整流器的性能和可靠性。为抑制输出电压脉动和输入电流畸变,该文通过建立三相Buck整流器在两相静止坐标系下的等效电路模型及功率传输数学模型,分析输出电压二倍频脉动及输入电流畸变产生的机理,在此基础上提出一种基于虚拟阻抗的输入不平衡控制策略,最后通过仿真和1 kW的实验样机验证了理论分析的正确性和所提控制策略的有效性。

匝间短路故障对永磁同步电机定子侧电流影响分析

定子绕组的匝间短路故障是永磁同步电机最为常见的故障之一,故障发生时被短路部分绕组中将会产生较大的电流,这将对电机的安全运行造成巨大威胁。为实现匝间短路的故障诊断,将分析匝间短路故障对电机定子侧电流的影响。通过有限元仿真和试验测试获取不同匝间短路严重程度下的定子侧电流,从电流不平衡度、负序电流和小波能量谱三个角度分析匝间短路故障对电机定子侧电流的影响。分析结果表明,当短路匝数增加、故障点接触电阻减小时,电流不平衡度、负序电流和小波能量谱中的D1频段能量变化率都将增大。因此,这三个指标的增加可以作为匝间短路故障的判断依据。

基于电流不平衡度的GIS倒母线刀闸位置判别方法

针对当前220 kV GIS站倒母线过程中刀闸传动连杆断裂或传动机构脱落无法在线监测的问题,从倒母线过程中某间隔刀闸分、合前后母联三相电流变化的特点出发,通过分析母联三相电流的不平衡度和变化量的不平衡度,找出了刀闸分、合前后母联电流的变化特征,并根据此特征实现了三相刀闸位置的正确判断,同时设计了刀闸位置不对应在线报警逻辑判别表,实现了刀闸位置异常时能够发出报警信号提醒操作人员,为同类刀闸或开关位置不对应的问题提供了很好的解决方案。

不平衡电网电压下虚拟同步发电机模型预测控制

虚拟同步发电机(Virtual Synchronous Generator,VSG)控制算法,可为电网提供惯性并参与电压及频率调节,但在电网电压出现不平衡时会导致并网电流不平衡及有功、无功功率波动的问题,严重影响了并网电能质量。针对此问题,提出了一种不平衡电网下VSG模型预测控制策略,通过基于快速电压矢量选择的模型预测控制策略来控制所重构的正序及负序电流分量,达到有功功率恒定、无功功率恒定及电流平衡的控制目标。此外还引入负序电流调节系数,进而实现这三个目标的协调控制。

Research on Open-circuit Fault Tolerant Control of Six-phase Permanent Magnet Synchronous Machine Based on Fifth Harmonic Current Injection

This paper proposes a novel control approach for fault-tolerant control of dual three-phase permanent magnet synchronous motor(PMSM) under one-phase open-circuit fault.A modified six-phase static coordinate transformation matrix and an extended rotating coordinate transformation matrix are investigated considering the influence of the fifth harmonic space on fault-tolerant control. These mathematical models are further analyzed in the fundamental space and the fifth harmonic space after the fault and to eliminate the coupling between the d-q axis voltage equation in the fundamental wave space and the d-q axis voltage equation in the fifth harmonic space, a secondary rotation coordinate transformation matrix is proposed. To achieve the purpose of reducing torque ripple, the fault-tolerant control method proposed in this paper not only takes the minimum copper loss as the constraint condition, but also injects the fifth harmonic current. The experimental result of current and torque is used to verify the accuracy of fault-tolerant control.

单相光伏级联多电平逆变器漏电流抑制与功率均衡控制综述

单相级联多电平逆变器(CMI)因其可实现组件级最大功率点跟踪(MPPT)、组件可关断、多电平输出电压谐波含量低、集优化器与并网逆变器为一体的结构等优势,近年来在光伏并网系统的研究领域受到关注。以单相非隔离型独立直流母线结构的光伏级联多电平逆变器(PV-CMI)为研究对象,针对其存在的两个问题进行综述:一是因缺少隔离环节可能导致的漏电流问题:二是单相PV-CMI各单元间相互独立的直流光伏输入可能导致的功率不均衡问题。这两个问题对单相PVCMI的安全稳定运行造成了威胁,从而极大地限制了单相PV-CMI的推广和应用。文中分析了单相PV-CMI的基本原理和拓扑特点,分别就其漏电流与光伏功率不均衡两大关键问题,综述现有的研究方案并进行了总结。最后,展望了PV-CMI未来的发展趋势及其存在的技术挑战,为后续研究提供了参考。

计及三相不平衡的有源配电网重构

针对单相光伏的大量接入引起配电网运行的三相不平衡、线路有功损耗增大问题,提出以有功损耗和三相不平衡度为目标函数,构建基于三相电流注入法的配电网动态重构模型,采用大M法对节点电压方程进行松弛,避免断开线路的约束条件对模型的影响,并采用多边形内近似法将非线性约束线性化。通过修改的IEEE 33节点配电系统的分析计算,验证了算法的有效性,提高了算法的求解效率和求解精度。

计及线路阻抗与负荷影响的不平衡配电网电压消弧方法

在单相接地故障和接地故障消弧的分析中,通常假设配电网是三相平衡的。已有的消弧方法在对地参数和负载不平衡的配电网单相接地故障时消弧效果不佳。针对上述问题,提出一种计及线路阻抗和配电网参数不平衡的电压消弧方法。分析考虑配电网参数不平衡和线路阻抗的配电网等效模型,推导出精确电压消弧指令值,通过故障前后各注入一次电流计算电压指令值,有源消弧装置调控零序电压至该指令值从而抑制故障点电压、电流为0。为了避免任意注入电流引起故障电流增大,将传统电流消弧作为电压消弧的过渡能有效补偿故障电流。通过和已有考虑线路阻抗电压消弧方法、忽略线路阻抗电压消弧方法对比,仿真结果表明,在不同的不平衡度、故障位置和接地电阻下,所提方法消弧性能最佳。

基于电压、电流不平衡度差值的干式变压器匝间短路故障识别方法

干式变压器绕组发生轻微匝间短路时,相电压、相电流等电气量变化甚微,不能作为表征匝间短路故障的敏感特征量,导致相应保护措施缺失,运行过程中设备烧毁事故时有发生。通过建立干式变压器“场-路”耦合仿真模型,利用实际试验和工程计算获取的状态参数,验证模型的准确性。通过建立和分析其绕组匝间短路故障数学模型,提取相电压、相电流不平衡度标幺值的差值,作为判断匝间短路故障的特征量。通过仿真分析不同工况下其绕组发生匝间短路故障时不同电气量的变化情况,论证了所提新故障特征量不仅能提前感知绕组匝间短路故障,而且能够克服固有的三相不对称及不平衡运行带来的影响,有效性、灵敏性兼顾,为实时监测干式变压器绕组匝间绝缘状态提供一种新方法。

一种提高级联H桥逆变器功率不平衡运行能力的控制策略

在光伏逆变等以传递有功功率为主的应用场合,由于部分光伏模块光照被遮挡、老化等原因造成模块间功率不平衡较严重时,传统调制波注入策略控制的级联H桥并网逆变器会出现并网电流畸变的问题。传统调制波注入策略中不平衡模块的调制信号微调量为部分调制波,而该文提出由方波(与电流同相位)和共同调制波的部分差值量作为微调量,从而实现功率不平衡下的控制。该文所提方法除了具有谐波补偿法的优点(最高调制比达1.27),同时还具有以下两个特点:(1)能够进一步提高功率较高模块的工作能力(所提方法不仅使高功率模块的调制波基波峰值可达到1.27,而且使此模块的基波电压和电流同相位,从而传输最大有功功率);(2)能够克服谐波补偿法中的低功率模块因叠加过多反向谐波而部分区域过调制导致的并网电流畸变问题。仿真和实验验证了所提方法的有效性。

财政政策、经常账户失衡及其治理——来自局部投影工具变量法的经验证据

识别财政政策对经常账户失衡的影响对于理论检验和政策制定具有重要意义。以往的经验研究普遍存在内生性问题或狭小样本量问题,难以提供可靠的证据支持。基于1990—2018年144个国家的面板数据,采用局部投影工具变量法,以军费支出作为政府支出工具变量,识别财政政策对经常账户的影响。研究发现,积极的财政政策会导致经常账户逆差,这一效应较明显且具有持续性。在经过军费支出排他性、外生性以及其他一系列讨论之后,该结论依然成立。异质性分析表明,这一影响在较为固定的汇率制度和较高的经济开放水平下更大。研究为更好地利用财政政策进行国际政策协调治理全球失衡和平衡“稳增长”与“防风险”的关系提供了坚实的证据支持。

Analysis on Circulating Current and Split Capacitor Voltage Balance for Modular Multilevel Converter Based Three-phase Four-wire Split Capacitor DSTATCOM

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.

同塔四回输电线路三相不平衡电流仿真分析

同塔四回线路各导线电磁耦合紧密,三相电流不平衡现象更为严重。为解决同塔四回线路三相不平衡电流过大的问题,首先,以实际输电线路为例,基于电磁暂态与电力电子仿真软件(electromagnetic transient&power electronics,EMTPE)建立了电磁暂态模型,并通过与实测数据对比验证模型的有效性;随后,利用所建模型分析线路三相电流不平衡的主要影响因素,同时根据分析结论指出导线相序排列方式及空间位置分布不恰当是引起宁夏某同塔四回输电线路三相电流不平衡的主要原因,并提出更换一回线相序排列为逆相序的解决方案;最后,结合工程规划情况,给出了该同塔四回线路各导线的最优相序组合方式,为后续工程建设提供指导建议。