Modular System of Cascaded Converters Based onModel Predictive Control

A modular system of cascaded converters based on model predictive control(MPC)is proposed to meet the application requirements ofmultiple voltage levels and electrical isolation in renewable energy generation systems.The system consists of a Buck/Boost+CLLLC cascaded converter as a submodule,which is combined in series and parallel on the input and output sides to achieve direct-current(DC)voltage transformation,bidirectional energy flow,and electrical isolation.The CLLLC converter operates in DC transformer mode in the submodule,while the Buck/Boost converter participates in voltage regulation.This article establishes a suitable mathematical model for the proposed system topology,and uses MPC to control the system based on this mathematical model.Module parameters are designed and calculated,and simulation is built in MATLAB/Simulink to complete the simulation comparison experiment between MPC and traditional proportional integral(PI)control.Finally,a physical experimental platform is built to complete the physical comparison experiment.The simulation and physical experimental results prove that the control accuracy and response speed ofMPC are better than traditional PI control strategy.

Synchronization analysis on cascaded multilevel converters with distributed control

Cascaded multilevel converters built with integrated modules have many advantages such as increased power density,flexible distributed control,multi-functionality,increased reliability and short design cycles.However,the system performance will be affected due to the synchronization errors among each integrated modules.This paper analyzes the impact of the three kinds of synchronization errors on the whole system performance,as well as detailed synchronization implementation.Some valuable conclusions are derived from the theoretical analysis,simulations and experimental results.

A distributed VSG control method for a battery energy storage system with a cascaded H-bridge in a grid-connected mode

With the high penetration of renewable energy,new challenges,such as power fluctuation suppression and inertial support capability,have arisen in the power sector.Battery energy storage systems play an essential role in renewable energy integration.In this paper,a distributed virtual synchronous generator(VSG)control method for a battery energy storage system(BESS)with a cascaded H-bridge converter in a grid-connected mode is proposed.The VSG is developed without communication dependence,and state-of-charge(SOC)balancing control is achieved using the distributed average algorithm.Owing to the low varying speed of SOC,the bandwidth of the distributed communication networks is extremely slow,which decreases the cost.Therefore,the proposed method can simultaneously provide inertial support and accurate SOC balancing.The stability is also proved using root locus analysis.Finally,simulations under different conditions are carried out to verify the effectiveness of the proposed method.

The General Technique of Selected Harmonics Elimination for Multilevel Converters

Forhigh power applications,multilevel converters have many advantages in comparison with other circuit topologies with output transformers. Cascaded inverters are one type of multilevel converters,they are easy to implement,very suitable for modularized layout and packaging.Their manufacturing cost is low.A multilevel PWM technique,called as General Technique of Selected Harmonics Elimination (GTSHE) ,is proposed in the paper. A general harmonic elimination equation for N cells,M pulses per half cycle,nth harmonic is derived,and verified by simulation results.

Stability-improvement Method of Cascaded DC-DC Converters with Additional Voltage-error Mutual Feedback Control

The interaction between the source and load converters in cascaded DC-DC converters may cause instability.Thus,improving the stability of cascaded DC-DC converters is important.To solve the above-mentioned problem,a flowchart to improve the control method is established by calculating the eigenvalue sensitivity of a time-domain model of cascaded DC-DC converters.Further,an additional voltage-error mutual feedback control method is firstly proposed based on the flowchart provided in this study to improve the stability of cascaded DC-DC converters.Subsequently,the influence of the proposed mutual feedback control on the stability of cascaded DC-DC converters is analyzed.Finally,the effectiveness of the proposed control method is verified by simulation and experiment.

A Qualitative Assessment of a Modified Multilevel Converter Topology M2LeC for Lightweight Low-Cost Electric Propulsion

A Cascade H Bridge (CHB) is evaluated for both electric vehicle motor traction control and off-vehicle charging against the Power ElectronicsUK Automotive Challenge for cost and mass for the year 2035. By combining the power electronics with batteries using low-voltage MOSFET transistors in a series cascade arrangement the cost and mass targets could be met 12 years earlier (in 2023 and 20 times lighter if an application specific integrated circuit (ASIC) is used. A 200 kW peak reference car was used to evaluate cost and mass benefits using four different topologies of power electronics. Vehicle installation is shown to be simplified as only passive cooling is required removing the need for liquid cooling systems and the arrangement is inherently safe;no high voltages are present when the vehicle is stationary. The inherently higher efficiency of CHB increases vehicle range. The converter with integrated batteries can also behave as an integrated on-board battery charger delivering additional off-vehicle benefits by removing the need for costly external chargers.

电压模式控制Buck与源变换器级联的稳定性分析

不同的变换器之间构建一个级联系统,相互作用是其核心问题.在电压模式控制下Buck与其他变换器级联可能产生性能退化和不稳定.本文利用Buck变换器的小信号模型,设计了使变换器具有良好动态性能的PI控制器.提出了阻抗比判据通过图形化判定,以Nyquist稳定判据来评估此级联系统的稳定性,并构建了闭环级联变换器的仿真模型,提供了一个在电压模式控制下Buck变换器在级联应用的解决方案,以保持级联系统的稳定性和良好的动态性能.

级联H桥储能变流器直流侧纹波电流协同抑制方法

H桥模块(单相变流器)作为级联储能变流器的基本单元,其固有功率变换特性会导致直流侧产生大量纹波电流,严重威胁交流侧电能质量和储能电池寿命。文中在全面分析纹波电流产生机理的基础上,提出一种基于串-并联滤波器的级联H桥变流器协同抑制方案,有效提高直流侧电流质量。通过分析无源参数的阻抗特点,给出了双LC滤波器参数配置方案,使所提滤波器既能吸收二倍频纹波电流,又能吸收高频纹波电流。同时,提出一种改进的三次谐波注入法,进一步减小直流侧纹波电流。最后,仿真和实验结果验证了所提方案的有效性。

故障相桥臂转移接地消弧装置直流侧电压控制方法

为解决传统三相直挂式柔性消弧装置非故障相桥臂承担线电压的问题,提出一种故障相桥臂转移接地的消弧方法。该方法通过开关控制故障相桥臂转移接地,母线电压由故障相桥臂与非故障相桥臂共同承担,三相桥臂共同输出电流,以实现消弧。针对级联H桥直流侧需独立供电的问题,提出分布式电压平衡换流调制方法。该调制方法根据H桥输出状态和电流方向对直流侧电容充放电的影响,有选择地改变H桥的输出状态,进而实现直流侧电压的稳定。使用该调制方法的消弧装置无须在直流侧配备独立供电电源,在消弧过程中能保持H桥直流侧电压稳定。最后,采用Matlab/Simulink仿真软件对所提方法进行验证,仿真结果证明了所提方法的有效性。

适用于直流链式储能的电池簇荷电状态自均衡控制策略

电池储能系统凭借其可提高可再生能源消纳能力的优势逐渐成为电网关键环节,其中的生产、使用环节导致的电池不一致性是影响储能系统容量和寿命的重要因素。在直流型高压级联链式储能的拓扑结构基础上采取分级控制策略,基于电压模式和功率模式的一级控制,针对链式电池储能系统电池荷电状态(SOC)不均衡的问题,提出了一种控制参数自适应的SOC自均衡控制策略。该策略综合考虑电池SOC偏差、各模块电容电压水平、直流系统电压控制裕度以及输出调制比上下限,实时计算均衡系数。实验结果验证了所提策略能够在SOC偏差较大以及满功率充放电切换的极端工况下,实现SOC自均衡,均衡效率较高。

基于变截面积流道的液力变矩器叶栅系统设计

流道截流面积分布对液力旋转机械的液力性能具有重要影响.为了减少扩散等液力损失,变矩器流道截面积分布通常采用等截面积分布的形式,但并未充分发挥截面积分布参数在流场调节和性能优化方面的优势.因此,文中研究提出了过流截面积表征模型,并建立了基于可变截面积分布的变矩器循环圆及叶栅设计系统,对变截面积流道的液力变矩器循环圆设计方法进行了研究.通过设计多组不同截面积分布的实例,采用DOE方法进行比较分析,进一步研究了叶轮截面积分布形式对变矩器性能的影响.同时,通过对不同叶轮的组合设计,获得了组合型性能优化的循环圆设计结果.实验结果表明,基于变截面积流道优化设计的变矩器泵轮转矩提升了18.5%,起动变矩比也提高了3.9%.说明基于变截面积分布的液力变矩器循环圆设计方法是一种有效的优化设计方法,能够在保持外形尺寸的同时提升变矩器性能.

大规模风电并网后江苏电网柔直换流站与UPFC的有功功率协调控制方法

随着“双碳”战略的提出与实施,我国将着力建设以新能源为主的新型电力系统。然而,风电等新能源大规模并网将给具有交直流混联特征的新型电力系统带来威胁与挑战。通过以十四五规划末期的江苏电网为例进行分析,发现在大规模风电并网背景下,江苏电网中部分“南北向”500 kV交流输电线路N-1后负载过重的问题亟待解决。为此,基于综合灵敏度计算方法,提出了一种综合利用柔直换流站和统一潮流控制器的有功功率协调控制方法,用于改善系统潮流分布并消除潜在的线路过载。最后,以江苏电网为例进行了仿真分析,对苏州500 kV统一潮流控制器、白鹤滩-江苏直流和某规划中三端柔直换流站进行了有功协调控制。结果表明,所提协调控制方法应用于江苏电网时可有效解决交流输电通道N-1后负载过重问题,且相比于典型最优化方法在计算速度上具有优势。

基于零序无功电流柔性调控的配电网覆冰线路不停电在线融冰方法

提出采用两台级联H桥变流器(cascadedH-bridge converter,CHBC)柔性调控配电网零序无功电流,实现覆冰线路不停电在线融冰。两台CHBC分别串接在覆冰线路首端和末端的接地变压器。通过末端CHBC在绝缘允许范围内调控配电网零序电压,产生的零序对地电容电流作为融冰电流流过三相覆冰导线,实现较单相接地短路电流融冰更佳的效果。通过首端CHBC调控配电网对地电容电流,实现融冰电流大小的任意调节。所提方法无需做单相接地短路,能够精准调控融冰电流,实现三相线路同时融冰。CHBC与配电网仅存在无功交互,节省了直流稳压电源容量,且功率交互之和为零,不影响配电网正常供电电压,全网负荷无需停电。仿真和实验结果验证了所提方法及控制方案的可行性和有效性。

一种新型模块化三相光伏逆变器及其分布式控制策略

级联H桥被认为是最适合光伏发电场景的拓扑之一。针对传统三相级联H桥光伏逆变器电容体积大、寿命短、相间功率失配及控制通信系统复杂的问题,基于磁通抵消原理提出1种新型模块化三相光伏逆变器及其分布式控制策略。首先,介绍了模块化拓扑的基本结构;然后,对磁通抵消功率解耦的基本原理及二倍频电压纹波的影响因素进行了详细分析,提出1种分布式控制策略以实现二倍频电压纹波的抑制并保证三相输出功率平衡;最后,通过仿真与实验验证了分析的正确性与控制策略的可行性。

混合级联换流站直流短路故障解析计算与快速限流方法

在高电压、大容量、远距离输电领域,混合换流器拓扑相比于单一换流器更具技术优势,其中为应对危害性极高的直流短路故障,换流器直流短路故障穿越能力需重点考虑。首先,提出一种基于电网换相换流器(line commutated converter,LCC)和模块化多电平换流器具备直流短路故障穿越能力的混合级联拓扑及其直流短路故障限流控制方法;其次,通过分析直流短路故障期间换流器动态特性,构建分阶段状态方程数学模型,实现对直流短路故障电流进行精确解析计算;再次,选取最严重直流短路故障工况,以最大允许电流变化量作为判据并结合所提出的直流短路故障电流计算方法,给出强制触发角参数动态优化计算机制;最后,以优化设计的强制触发角为基础提出快速限流控制策略,该策略能保证LCC在直流短路故障穿越时不发生换相失败且快速降低故障电流。

基于功率变换器的梯次利用电池系统均衡控制策略

梯次利用是一种处理退役锂电池的经济高效的方法。但是在电池梯次利用过程中,存在不同新旧状态电池混用而导致的不均衡问题,会影响电池系统的安全稳定运行。本工作分析了梯次利用电池直接并联时,由于不均衡导致的电流环流以及系统容量利用率低的问题。为解决这一问题,本工作提出一种基于功率变换器的梯级电池并联系统结构及均衡控制算法。所提方法将每个电池模组和功率变换器相连组成一个功率模组,通过调节电压环路、电流环路、荷电环路的输出,分别控制每个电池模组的充放电电流,最终实现梯次利用电池均衡优化控制策略。本工作给出了系统控制策略的详细分析和实验结果讨论,以评估和验证所提均衡方法的有效性。实验测试表明,在使用相同健康程度电池、不同健康程度电池以及变负载等测试情况下,本工作所提梯次利用电池均衡控制策略都能够稳定、有效控制不同电池电流,保证电池之间的荷电参数均衡,提高了电池系统整体效率。本研究可以促进废旧电池的有效梯次利用,并为电池系统均衡控制提供新思路。通过实现电池的均衡利用,可以延长电池的使用寿命,降低系统故障风险,提高系统的可靠性和稳定性。

虚拟同步机控制策略下的STATCOM相间电容电压平衡方法研究

针对基于直挂式级联H桥拓扑结构的静止同步补偿器在运行时产生的相间电容电压不平衡的问题,本文在虚拟同步机的控制策略基础上提出了一种平衡相间电容电压的方法。虚拟同步机具有响应速度快的优点,但用于静止同步补偿器来作无功补偿时会产生相间电容电压不平衡的问题。因此本文首先引入电容电压平均值来完成虚拟同步机的快速并网,其次将虚拟同步机算法的无功调节分为3部分来实现分相无功补偿,再在电压合成前引入电容电压平均值作反馈,以实现相间电容电压平衡。最后在Matlab/Simulink环境下搭建仿真模型,验证了所提控制策略能有效解决相间电容电压不平衡问题,并且仿真结果表明所提控制策略具有良好的动态响应特性。

Inter-cluster Voltage Balancing Control of Modular Multilevel Cascaded Converter Under Unbalanced Grid Voltage

This paper presents a novel inter-cluster direct current(DC)capacitor voltage balancing control scheme for the single-star configured modular multilevel cascaded converter(MMCC)-based static synchronous compensator(STATCOM)under unbalanced grid voltage.The negative-sequence component of grid voltage at the point of common connection(PCC)causes unbalanced active power flow in the phase limbs of converter.This leads to the imbalance of DC voltages of the sub-module capacitors across the MMCC phases,and consequently,the malfunction of converter.The proposed solution is to inject both negative-sequence current(NSC)and zero-sequence voltage(ZSV)into the phase limbs of MMCC.A quantification factor Qf is used to achieve the sharing of inter-cluster active pow-er between the NSC and ZSV injection methods.Accurate determination of the quantification factor has been presented.In addition to maintaining the DC voltages of sub-module capacitor across the MMCC phases balanced,it also prevents the overcurrent and overvoltage of converter by injecting NSC and ZSV with the right proportion.The control scheme is validated on a 3.54 kV 1.2 MVA power system using MMCC-based STATCOM with 3-level bridge cells as sub-modules.The results show that the proposed scheme provides superior effectiveness in eliminating the voltage imbalance of DC capacitor in the phase limb while maintaining low voltage and current ratings.

Overview on Reliability of Modular Multilevel Cascade Converters

Multi-level converters have been used extensively in modern industry which calls for energy conversion with high-power and high-or medium-voltage.Because of its modularity and scalability,the multi-level converter with modular structure can be extended to different voltage levels and has a variety of forms in practical applications.It has attracted much attention from academia in the past decade,however,as a result of the numerous vulnerable power electronics sub-modules,significant challenges remain with regards to reliability.After summarizing the current research status of modular multilevel cascade converters,the main issues of reliability are reviewed in the paper.Firstly,the failure cases are thoroughly surveyed and classified,and the main failure causes are analyzed.Secondly,the reliability evaluation methods are reviewed and applied to the modular multilevel cascade converters.Thirdly,some promising measures to improve the reliability are presented and discussed,including parameter selection,redundancy design,fault-tolerant control and so on.Then,a complete reliability-oriented design procedure for the modular multilevel cascade converters is proposed.Finally,the challenges and opportunities to improve the reliability are concluded.