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.

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.

Study on the Reversible DC-DC Converter as Interface between Low and Medium Voltage DC Networks

This paper introduces an isolated reversible DC-DC converter with a particular topology, which benefits from both the NPC （neutral point clamped） structure and the series-parallel connection of converters. The key property of the proposed topology is the output voltage elevation above the blocking capabilities of each switch, without taking to a delicate synchronization of series-connected semiconductors. The converter is composed by two identical cells, each containing a full bridge, a medium frequency transformer and an NPC converter, connected in parallel at the input and in series at the output. The operation principle of each cell, into which a trapezoidal modulation was implemented, is similar to a DAB （dual active bridge）. A new model improves the dynamic performance of the controller. Simulation and experimental results verify the proposed topology, its control and start-up strategy.

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.

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.

Isolated Bipolar Modular Multilevel DC-DC Converter with Self-balancing Capability for Interconnection of MVDC and LVDC Grids

Bipolar medium-voltage DC(MVDC)and lowvoltage DC(LVDC)grids have the advantages of flexible integration of distributed renewable-energy generation and reliable power supply.In order to achieve voltage conversion,power transfer,and electrical isolation for bipolar MVDC and LVDC grids,a high-power DC-DC converter is essential.Therefore,this paper proposes an isolated bipolar modular multilevel DCDC converter(BiMMDC)with power self-balancing capability for interconnection of MVDC and LVDC grids.The proposed BiMMDC consists of two series connected MMCs in the MV stage to configure a bipolar MVDC interface,and interleaved converters combined with a dual-transformer are designed in the LV stage to configure the bipolar LVDC interface and to provide a self-balancing capability.Equivalent circuits of two series-connected MMCs and a dual-transformer with interleaved converters are derived.After that,operation principles of the proposed BiMMDC are introduced,considering balanced/unbalanced power transfer of bipolar LVDC grid and monopolar shortcircuit or open-circuit faults at MVDC grid.The control scheme is also presented for the proposed BiMMDC under different operating conditions.Finally,a Matlab simulation and controller hardware-in-the-loop(CHIL)evaluation results are provided to validate the feasibility and effectiveness of the proposed typology and its operating performance.

A Novel Asymmetrical Single-Phase Multilevel Inverter Suitable for Hybrid Renewable Energy Sources

This paper introduces a novel single-phase asymmetrical multilevel inverter suitable for hybrid renewable energy sources. The proposed inverter consists of two isolated DC sources and six power semiconductor controlled switches. The suggested inverter is capable of generating seven-level output when the input DC voltage is taken in the ratio of 1:2. The higher magnitude DC source is fed from Photo Voltaic (PV) panels, whereas the lower magnitude DC source is fed from Wind Turbine (WT) driven Permanent Magnet DC (PMDC) generator. Both the renewable energy sources are connected to the inverter via two DC-DC boost converters connected in cascade (i.e. one for maximum power point tracking and another for DC-link voltage control). The proposed hybrid renewable energy source inverter is connected to single-phase grid via proper control systems. The complete system is simulated using MATLAB/SIMULINK and the results are presented in detail.

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。

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。

Protection of large partitioned MTDC Networks Using DC-DC converters and circuit breakers

This paper proposes a DC fault protection strategy for large multi-terminal HVDC(MTDC)network where MMC based DC-DC converter is configured at strategic locations to allow the large MTDC network to be operated interconnected but partitioned into islanded DC network zones following faults.Each DC network zone is protected using either AC circuit breakers coordinated with DC switches or slow mechanical type DC circuit breakers to minimize the capital cost.In case of a DC fault event,DC-DC converters which have inherent DC fault isolation capability provide‘firewall’between the faulty and healthy zones such that the faulty DC network zone can be quickly isolated from the remaining of the MTDC network to allow the healthy DC network zones to remain operational.The validity of the proposed protection arrangement is confirmed using MATLAB/SIMULINK simulations.

混合级联多端直流暂态过电压机理及抑制措施

混合级联多端直流采用常规直流与柔性直流混合级联的新型拓扑结构,为我国远距离、大容量、多落点的能源传输提供了新模式,但常规直流与柔性直流之间的交互作用也引发了新问题。该文首先揭示实际受端电网交流电压恢复特性导致混合级联多端直流暂态过电压机理以及低端模块化多电平换流器(modular multilevel converter,MMC)闭锁风险;然后分别从送受端两个方面,提出降功率运行、改进送端移相策略和提升MMC有功功率传输极限等暂态过电压抑制措施。最后,基于PSD-PSModel软件,搭建送受端电网机电暂态和混合级联多端直流电磁暂态的混合仿真模型,仿真验证理论分析的准确性和抑制措施的有效性。

基于级联式多电平变换器的静止无功补偿器拓扑研究

电力系统实时无功补偿对提升电能质量至关重要,传统的无功功率补偿常采用大容量的电抗器、电容器,体积大、重量大、效率低。静止无功补偿器采用全控开关器件构成,具有可控,动态响应速度快,损耗低,效率高的优点。以单相H桥为级联单位串联而成的级联式多电平变换器可通过对各个H桥进行移相控制,输出多电平波形,提高输出电压正弦度,降低谐波含量,减小滤波器的体积,且具有扩展性强,适应性强,实现容易等优点。介绍了级联式多电平变换器的拓扑与工作原理及适用于该变换器的基于三角载波移相技术的PWM调制策略,通过降低开关管的开关频率,提高系统的效率。最后搭建了仿真模型验证了该拓扑与调制策略的有效性。

Research on cascaded multilevel inverter and its application in STATCOM

Because of the broad application of multilevel converters in the high-power area,a cascaded multilevel voltage-source inverter with phase-shifted SPWM(PS-SPWM)switching scheme is proposed as a static syn-chronous compensator(STATCOM).This can eliminate the bulky and weighty transformers and reduce power loss.In addition,the equivalent carrier frequency can be doubled and the output harmonics will be reduced compared with the STATCOM being put into operation.The operating principle and control methods are analyzed in detail and the feasibility is validated by simulation with MATLAB.

基于相移级联多电平逆变器的STATCOM研究

针对目前静态同步补偿器(STATCOM)存在的问题,提出了将级联多电平逆变器作为STATCOM的主拓扑,运用相移SPWM(PSSPWM)调制的解决方案,详细阐述了其工作原理和控制方法。MATLAB的仿真结果表明,它既能省去庞大笨重的变压器,减小功耗;还可以成倍提高等效载波频率,降低输出谐波含量,使级联逆变器拓扑成为STATCOM的首选。

级联型多电平变换器一般构成方式及原则研究

级联型多电平变换器类似于积木化的结构,本文按模块单元的结构形式,将级联型多电平变换器分为相同单元和不同单元两类,以输出最大电平数为目标,系统地分析模块单元个数与最大输出电平数、输入直流电压与最大输出电平数、以及输出电平数与谐波畸变率之间的关系,从中归纳得到构造级联型多电平变换器的指导原则。仿真和实验证明了文中理论研究的正确性。

一种新型的级联型逆变器PWM信号随机分配方法研究

级联型逆变器具有开关冗余和隔离直流电源冗余。该文提出采用随机分配的方法来均衡开关和电源的利用率。当控制器给出输出电平后,电源和开关器件的工作状态是由两个随机函数决定的。随机函数具有均匀分布特性,可以由线性同余法来实现。当输出波形正负对称时,逆变器的每个单元处于工作状态的概率是相同的。随着逆变器工作时间的延长,各单元的利用率自然趋于相同。仿真和实验结果证明了该方法的可行性。

主从式级联多电平变换器及其控制方法的研究

针对混合级联多电平变换器的输出电压等级和中间单元功能不明确的缺点,提出了一种主从式级联多电平功率变换器的拓扑组合结构。主变换器用来实现基波功率输出,从变换电路用来改善输出波形质量。该主从式级联多电平变换器具有输出波形质量高、开关损耗小、系统整体效率高等显著优点,特别适用于高压或超高压功率变换系统,是一种非常实用的电路拓扑组合方式。文中给出了适合于该功率变换器拓扑结构的两种控制策略:三角载波调制控制策略和特定谐波消除(SHEPWM)控制策略,并给出了50Hz下三角载波控制策略的仿真结果。仿真结果证明了所提的主从式级联多电平功率变换及其控制策略的有效性和实用性。

基于级联型错时采样空间矢量调制多电平变流器的APF研究

该文提出并分析了错时采样空间矢量调制(STS-SVM)技术的工作原理及其在级联型多电平变流器上的实现方法。实现的多电平变流器结合了STS-SVM 和级联型多电平变流器两者的优点,使用元件数目少、直流侧均压容易实现、易于模块化设计和调试,实验结果验证了理论分析的正确性。最后通过仿真研究了级联型STS-SVM 多电平变流器在有源电力滤波器中的应用特点。

级联型高压变频器控制算法的研究及实现

介绍了级联型高压变频器的拓扑结构和多载波控制算法,从理论上分析了载波移相式算法的优点。针对在实际设计中使用多载波算法时存在的困难,给出了一种基于数字信号处理器(DSP)和现场可编程门阵列(FPGA)芯片实现错位移相式算法的方法。从实验结果中可以看出,级联型错位移相式变频器的输出相电压和线电压的阶梯数目较多,波形非常接近正弦波,且电压的变化率小,进而证明了文章所提到的实现方法的可行性。

级联多电平逆变器在STATCOM的应用研究

随着多电平变换器在高压大功率方面的广泛应用,针对目前静态同步补偿器(STATCOM)存在的问题,提出了将级联多电平逆变器作为STATCOM的主拓扑,运用相移SPWM(PSSPWM)进行调制的解决方案,该方案既能省去庞大笨重的变压器,减小功耗;还可以成倍提高等效载波频率,降低输出谐波含量.文章详细阐述了其工作原理和控制方法,并通过MATLAB仿真实验验证了方案的可行性.