Analysis of the compound split transmission based on the four-port power split device

In order to identify all the appropriate system schemes for the compound split systems formed primarily with a four-port mechanical power split device,power transmission characteristics of the compound split systems was analyzed.Considering the structural symmetry and according to the different connection arrangement of the four ports,compound split system was classified into four types.Using black-box modeling method,the generalized models of the speed ratio,the torque ratio and the power split ratio were established.Moreover,a semi-invert diagram was used to distinguish the different schemes in each type.The characteristics of the speed ratio,the torque ratio and the power split ratio in each domain were also analyzed and compared.Through the semi-invert diagram,a selection method based on the rated-power speed ranges in different schemes was presented and all suitable compound split systems were identified,which can be used as references for the scheme selection of this kind of continuously variable power split transmission.

考虑长距离输电缆特性的逆变器并网系统谐波不稳定分析及抑制

长距离输电缆(LTC)与并网逆变器交互作用易引发系统谐波不稳定的风险,且忽略线路频变特性可能会导致稳定性误判。文中计及输电线路的分布参数特性和频变特性,研究如何评估这类逆变器并网系统的稳定性。首先,基于矢量匹配法对含LTC的交流系统进行等值,同时考虑逆变器的内外环控制、锁相环、控制延时等环节,建立dq旋转坐标系下逆变器并网系统高频状态空间模型;其次,采用参与因子分析方法辨识出导致谐波不稳定的关键影响环节,并基于根轨迹方法分析了LTC频变特性、延迟时间、控制器参数、电缆长度和电网阻抗对系统稳定性的影响;再次,提出了基于参数灵敏度分析和矩阵相似变换的系统稳定性改善方法,并对改进后系统的谐波稳定性进行分析;最后,基于RT-LAB构建硬件在环实验平台进行了验证,实验结果验证了理论分析的正确性。

常规直流逆变站交流送出线路距离保护适应性分析与对策

常规直流逆变站交流送出线路故障通常会引发换相失败,存在故障电压和电流波形畸变等问题,可能导致工频相量距离保护性能劣化。首先,分析了工程中广泛应用的正序电压极化距离保护在常规直流逆变站交流送出线路中的适应性,分析结果表明,当逆变站运行于单回交流出线工况时,传统正序电压极化距离保护不再适用;然后,基于交流线路的RL模型,提出了基于波形相关系数的时域距离元件,可以不受逆变站非线性输出特性的影响,并针对时域距离元件存在的出口近区故障方向判别问题,利用电流畸变程度判断故障方向;最后,通过仿真验证了时域距离保护方案的有效性。

AC fault ride through control strategy on inverter side of hybrid HVDC transmission systems

Hybrid high-voltage direct current(HVDC)transmission systems employ a new type of HVDC transmission topology that combines the advantages of the linecommutated converter system and the voltage-source converter system.They can improve the efficiency and reliability of long-distance power transmission.However,realizing alternating-current(AC)grid-fault ride through on the inverter side of a hybrid HVDC transmission system is a challenge considering that a voltage-source converter based HVDC(VSC-HVDC)is used on the inverter side.In this study,a control strategy for an overvoltage fixed trigger angle based on the power-balance method is developed by fully utilizing the operation characteristics of a hybrid HVDC transmission system.The strategy reduces the inverter-side overvoltage of the HVDC system under a fault in the inverter-side AC system.Simulations based on Gezhou Dam are conducted to validate the effectiveness of the proposed strategy.

Stability Analysis of Multi-paralleled Grid-connected Inverters Including Distribution Parameter Characteristics of Transmission Lines

The sparse distribution characteristics of renewable energy resources can lead to there being tens of kilometers of transmission lines between a grid-connected inverter and the actual grid.Accurate analysis of the stability of such gridconnected inverter systems currently involves using a complex hyperbolic function to shaped model of the transmission line circuit.This has proved to be problematic,so,drawing upon the distribution parameter characteristics of transmission lines,this paper looks at how to use impedance-based stability criteria to assess the stability of multi-paralleled grid-connected inverters.First,the topology of multi-paralleled inverters connected to the grid via transmission lines is established,using each transmission line terminal as a grid connection point.Each grid-connected system is taken to be equivalent to a small-signal circuit model of the“current source-grid”.Euler’s formula and the Nyquist stability criterion are combined to assess the stability of the associated grid-connected current transfer functions and evaluate the stability of the grid-connected current.Finally,a simulation analysis circuit is constructed to verify whether power line intervention will cause stability problems in the grid-connected system.Overall,it is found that long-distance transmission lines are more likely to cause unstable output of the grid-connected current.It is also found that the number of grid-connected inverters,the short-circuit ratio(SCR),the distorted grid and the inverter parameters can all have a significant impact on the stability of the grid-connected current.

A fast boundary protection for an AC transmission line connected to an LCCHVDC inverter station

For AC transmission lines connected to an LCC-HVDC inverter station,commutation failure can lead to the wrong operation of traditional protection.To solve the problem,this paper proposes a fast protection scheme using transient information from one end of the AC line.The boundary frequency characteristics of the AC line connected to LCC-HVDC inverter are analyzed first.This reveals the existence of significant attenuation on both high frequency signals and some specific frequency signals.Based on the boundary characteristics,a novel boundary protection principle utilizing a backward traveling wave is then proposed.A PSCAD/EMTDC simulation model of a±800 kV LCC-HVDC and 500 kV AC transmission system is established,and different fault cases are simulated.The simulation results prove that the novel protection principle is immune to commutation failure,fault resistance and fault type.

Transmission line protection challenges influenced by inverter-based resources:a review

High penetration of renewable energy sources(RES)leads to new challenges for protection devices.Protection schemes are typically designed according to the dynamic behavior of rotating machines as generation sources,while the RES dynamic response,mainly governed by inverters,is not considered.Consequently,some relevant algorithms of transmission line protection are experiencing challenges because of the fact that magnitude and phase angle com-parison,amount of negative-sequence,and short-circuit current level are affected by the RES.Therefore,an in-depth study of this issue is necessary,one which considers the main causes and new methodological criteria solutions.This work presents an extensive literature review of the evaluation of electrical protection performance and the effects of RES connected to a power grid through inverters.Bibliographic data on many representative publications related to this topic are obtained to show the current research lines and their proposed solutions.In addition,this work identi-fies the main protection functions affected and describes the new protection schemes that consider RES.Finally,an analysis and discussion of the selected bibliography are presented.

一种高效光伏逆变器的PWM策略

光伏(PV)逆变器正朝着更高效、更高功率密度的方向发展。此处针对目前PV逆变器输出效率较低、功率密度较小的问题,提出了一种提高PV逆变器效率的脉宽调制(PWM)策略。该调制策略工作原理如下:采用调制波分层、分区域的正弦脉宽调制(SPWM)方式,获取输出五电平电压的开通关断信号,使其两个开关桥臂分别工作在高频和低频的状态,来减少低频开关桥臂的损耗,进而实现对传输效率的提高。此处选用T-NPC作为PV逆变器的逆变拓扑,对所提调制策略进行实验验证与对比分析,实验结果表明,所提PWM策略可以有效提高T-NPC型PV逆变器的传输效率,进而获得更高的功率密度。

级联H桥变频调速系统电机端过电压抑制方案研究

用于中高压电机驱动的级联H桥变频器具有输出波形谐波含量低、器件电压应力小以及效率高等优势。然而与传统的两电平变频器一样,当其经长电缆驱动电机时存在电机端过电压的问题。为此,本文对级联H桥变频器的结构进行了改进,将一个辅助双向开关放入每一相的某一子单元中,使该子单元能够产生±Vdc/2电平,即在总输出电压的每个上升和下降沿中插入一个短暂的固定时间的中间电平。此举可将上升/下降沿分为两段,并且两者之间存在两倍于电压波在电缆上传播时间的延迟。由于延迟的存在,第二段正好可以抵消第一段的反射波,所以电机端由反射现象导致的过电压会被明显抑制。最后,仿真及实验均验证了所提出方案的有效性。

弱电网下海上低频风电系统网侧控制和稳定性分析研究

低频交流输电技术广泛应用于海洋风电的输送,但其送出的电流质量和系统稳定性会受到网侧变流器和弱电网线路阻抗的影响。文章针对海上低频风电系统的网侧控制,研究了αβ坐标系下基于虚拟同步机(Virtual Synchronous Generator,VSG)控制方法,并引入虚拟阻抗环节实现VSG的功率解耦,使得网侧变流器能在电网频率突变时自发调频,提高了其并网电能质量和弱电网下的稳定性。考虑到VSG并网系统与弱电网之间的交互作用,利用谐波线性化方法建立了αβ坐标系下VSG并网系统的序阻抗模型,结合阻抗分析法和Nquist稳定判据分析了并网系统在弱电网下的稳定性。最后,通过仿真验证了分析的正确性。

基于E类逆变器的WPT系统线圈抗偏移研究

现有无线电能传输(WPT)系统中普遍存在元器件参数选型难,且常见线圈类型抗偏移能力普遍较弱。本文基于推挽式E类逆变器构建SS型拓扑结构磁耦合谐振式WPT(MCR-WPT)系统,计算系统中元器件参数,并推导其最大效率负载条件,同时提出同半径圆角矩形线圈设计,对同面积内不同条件线圈传输效率及抗偏移能力进行比较。仿真结果表明:耦合线圈结构与参数对传输性能有明显影响,在同等最佳负载条件下,所提线圈较其他常见类型线圈传输性能有所提升,最后通过实验验证仿真结果。

新能源场站全工况下多逆变器并联驱动MGP的UPF控制

针对逆变器并联驱动同步电机对(MGP)的id=0矢量控制,在MGP出力较大时,难以保证新能源通过MGP并网的有功传输效率的问题,结合同步电机在单位功率因数(UPF)状态下的特点,提出多逆变器并联驱动MGP的UPF控制策略。通过计算逆变器输出全为有功状态下的同步电动机定子d、q轴电流,来保证新能源通过MGP并网的有功传输效率;再利用磁链ψ与同步电动机定子d轴电流具备的负反馈特性来简化计算,并通过电流分配系数ki,使其适用于多台逆变器容量相等或不等、出力相同或不同的全工况应用场景,吻合工程实际。理论研究和仿真分析均表明,id=0矢量控制模式难以保证新能源通过MGP并网的有功传输效率,而多逆变器并联驱动MGP的仿真结果表明,所提控制策略可以保证新能源场站全工况下MGP的有功传输效率。

基于无功电流波形相关性的逆变型电源电站送出线故障方向判别原理

受逆变型电源弱馈、电源阻抗变化等故障特性的影响,使用正序电压或记忆电压作为极化电压的方向元件可能会出现误动作的情况。为了解决该问题,提出了一种基于无功电流波形相关性的故障方向判别原理。利用故障期间逆变型电源与同步发电机正序电流的差异,通过计算无功电流测量波形与参考波形的相关系数判断故障方向。当逆变型电源侧发生正向故障时,无功电流测量波形与参考波形基本一致;发生反向故障时,无功电流测量波形与参考波形存在显著差异。仿真结果表明:提出的保护判据在不同的故障情况下都能够可靠、灵敏地判断故障方向。

倒虹吸水力特性研究进展

倒虹吸是一种工程量小、施工方便、造价成本低的交叉输水建筑物,在我国输水工程中有广泛的应用。通过查阅国内外倒虹吸水力特性研究资料,介绍了倒虹吸的基本构造和布置形式,同时分析了倒虹吸水力特性的研究意义,并根据倒虹吸水力特性的不同研究方法对国内外学者的研究成果展开了分析。大量研究表明:倒虹吸的基本水流流态和水头损失的计算方法在大型倒虹吸研究中存在部分局限性,复杂条件下倒虹吸的水力特性仍需要深入研究;现有研究中大流量倒虹吸水力特性成果十分有限,未来该方面的研究将会是重点;目前多研究方法互相验证是研究倒虹吸复杂条件下水力特性的主要手段之一;已建成倒虹吸的维护与修缮技术也需要关注。在满足工程实况需求与学科发展需要的前提下,可以认为未来倒虹吸的研究将主要集中于复杂条件下大流量倒虹吸研究和倒虹吸与计算机多学科交叉研究,以期为后续倒虹吸水力特性的研究提供一定的借鉴。

基于移相调制的无线供电与信息协同传输技术

该文提出一种基于移相调制的无线供电与信息协同传输技术。通过控制全桥逆变器的频率和移相角,将工作状态分为两种:当系统仅需要无线供电时,移相全桥逆变器的工作频率等于系统一次和二次线圈谐振频率fp,实现无线电能的高效率传输;当系统需要同时进行无线供电与信号传输时,移相全桥逆变器控制频率采用fp/3作为低频通信频率,通过调制移相角实现信号传输,利用其3次谐波频率fp进行高效率无线电能传输。理论上,通信时所选择的工作相位点仅影响基波频率点幅值分量以实现解码,而对实现无线供电谐波频率点幅值没有影响。该技术采用二次侧双谐振回路构造能量接收电路和信号解调电路,可在同一套硬件装置上实现信道复用,实现低频通信信号的传输和无线电能的高效率传输,在海洋等对通信频率敏感的介质条件下具有明显优势。

基于MMC-HVDC的井下直流输电换流站设计

为解决煤炭井下设备电压等级偏低和闪变两大故障,考虑将具有模块化多电平换流站的柔性直流输电(MMC-HVDC)系统直接应用于煤矿井下设备输电。介绍了换流站的基本物理构成,引入了PWM调制和均压方式的控制器,控制部分分为内环控制和外环控制,内环采用电流控制,外环采用电压控制,对控制参数进行了整定。通过Simulink开展稳态试验,证明该系统在井下运行时有良好的稳定性,可实现井下用电设备有功功率和无功功率的单独调节;若系统受到外部扰动时,可快速稳定直流电压;可将逆变站输出的电压幅值和相位保持在额定值。试验结果验证了井下柔性直流输电系统的实用性。

换流阀在高压直流输电系统中的应用及技术展望

本文简述了直流输电系统中换流阀的组成结构、工作原理及设备类型,论述了高压直流输电换流技术的理论基础,分析了6脉动换流器的工作过程,换流器吸收的无功功率及容量的计算方式,指出了该项技术的应用前景与发展方向。

一种新型液压同步倒装提升方式桁架式输电线路跨越装置的研究与应用

结合输电线路跨越施工用的新型液压同步倒装提升方式桁架式跨越装置,重点从装置的主要结构及参数、主体结构设计、顶升套架结构、安装使用和拆卸、工程实际应用等方面进行了介绍,分析了其特点,为类似跨越提供了方法借鉴与参考。

中压柔直换流阀系统控制策略研究

随着配电系统的不断发展,对中压配电网柔直输电系统的应用需求日益广泛。主要阐述三电平换流阀在中压柔直系统中的应用,并对中压柔直系统关键技术问题进行了仿真分析和试验验证。实践表明,提到的中压柔直系统的实现方案具有经济性、可行性。自适应控制的应用明显提高了柔直系统的自由度和可靠性。

直流输电系统换相失败的研究(一)——换相失败的影响因素分析

作为直流输电系统最常见的故障之一,换相失败与很多因素有关,主要有换流母线电压、换流变压器变比、直流电流、换相电抗、越前触发角、不对称故障时换相线电压的过零点相位移、换流阀的触发脉冲控制方式和交流系统的频谱特性等,以CIGREHVDC标准模型为研究对象比较详细地分析了以上因素对逆变器换相失败的影响,得出了逆变器发生换相失败的一般规律。