Unidimensional Modulation Technique for Cascaded Multilevel Converters

This paper presents a simple and low-computational-cost modulation technique for multilevel cascaded H-bridge converters. The technique is based on geometrical considerations considering a unidimensional control region to determine the switching sequence and the corresponding switching times. In addition, a simple strategy to control the dc-voltage ratio between the H-bridges of the multilevel cascaded converter is presented. Examples for the two-cell topology are shown, but the proposed technique can be applied to develop modulation methods for a higher number of H-bridges. Experimental results are presented to validate the proposed technique.      

DC-Voltage-Ratio Control Strategy for Multilevel Cascaded Converters Fed With a Single DC Source

Recently, a multilevel cascaded converter fed with a single dc source has been presented. An analysis of the steady-state working limits of this type of converter is presented in this paper. Limits of the maximum output voltage and the minimum and maximum loading conditions for stable operation of the converter are addressed. In this paper, a way to achieve any dc voltage ratio (inside the stable operation area of the converter) between the H-bridges of the single-dc-source cascaded H-bridge converter is presented. The proposed dc-voltage-ratio control is based on a time-domain modulation strategy that avoids the use of inappropriate states to achieve the dc-voltage-ratio control. The proposed technique is a feedforward-modulation technique which takes into account the actual dc voltage of each H-bridge of the converter, leading to output waveforms with low distortion. In this way, the dc voltage of the floating H-bridge can be controlled while the output voltage has low distortion independently of the desired dc voltage ratio. Experimental results from a two-cell cascaded converter are presented in order to validate the proposed dc-voltage-ratio control strategy and the introduced concepts.      

Selective Harmonic Elimination PWM Control for Cascaded Multilevel Voltage Source Converters: A Generalized Formula

This paper proposes a generalized formulation for selective harmonic elimination pulse-width modulation (SHE-PWM) control suitable for high-voltage high-power cascaded multilevel voltage source converters (VSC) with both equal and nonequal dc sources used in constant frequency utility applications. This formulation offers more degrees of freedom for specifying the cost function without any physical changes to the converter circuit, as compared to conventional stepped waveform technique, and hence the performance of the converter is greatly enhanced. The paper utilizes the merits of the hybrid real coded genetic algorithm (HRCGA) in finding the optimal solution to the nonlinear equation system with fast and guaranteed convergence. It is confirmed that multiple independent sets of solutions exist. Different operating points for both five- and seven-level converters including single- and three-phase patterns are documented. Selected experimental results are reported to verify and validate the theoretical and simulation findings.      

A Comparison of Diode-Clamped and Cascaded Multilevel Converters for a STATCOM With Energy Storage

The progression of distributed generation within a bulk power system will lead to the need for greater control of transmission-line power flows. Static synchronous compensators (STATCOMs) provide a power-electronics-based means of embedded control of transmission-line voltage and power flows. The integration of energy storage with a STATCOM can extend traditional STATCOM capabilities to four-quadrant power flow control and transient stability improvement. This paper discusses energy storage systems (ESSs) integrated with conventional and multilevel bidirectional power converters for a hybrid STATCOM/ESS. Conventional, diode-clamped, and cascaded multilevel converter-based STATCOM/ESSs are developed, and their performances for a variety of power system applications are compared using battery energy storage. The advantages and disadvantages of each topology are presented. Both simulation and experimental results are provided to validate the conclusions     

基于重复补偿观测器的四象限变流器无差拍控制

分析了采用无差拍电流控制的四象限变流器的电流稳定性受控制延时以及电网电流采样通道低通滤波器影响的机理。提出了基于重复控制思想状态观测器的电流预测方法,减小了电网电流预测的误差,具有算法简单,预测精度高的特点。实验结果验证了本文提出的电流预测方法的正确性和有效性。     

多电平PWM控制技术发展

多电平PWM控制技术一直是多电平变换器研究的核心内容之一。基于传统两电平PWM技术研究的经验，经过近十几年的发展，多电平PWM控制技术已形成了儿类不同的实现方法，同时新的控制方法还在涌现。与两电平相比，多电平PWM方法需要面对一些新出现的问题，并拓展PWM控制的内涵，进而形成新的PWM控制思路。按照目前的发展情况，多电平PWM控制方法一般有多电平载波PWM方法、多电平空间矢量PWM方法，以及其他优化的PWM方法。本文对已有多电平PWM控制技术进行了归纳和分析，最后指出多电平空间矢量法和载波调制法在一定条件下具有内在的一敛性。     

Feed-Forward Space Vector Modulation for Single-Phase Multilevel Cascaded Converters With Any DC Voltage Ratio

Modulation techniques for multilevel converters can create distorted output voltages and currents if the dc-link voltages are unbalanced. This situation can be avoided if the instantaneous dc voltage error is not taken into account in the modulation process. This paper proposes a feed-forward space vector modulation method for a single-phase multilevel cascade converter. Using this modulation technique, the modulated output voltage of the power converter always generates the reference determined by the controller, even in worst case voltage unbalance conditions. In addition, the possibility of optimizing the dc voltage ratio between the H-bridges of the power converter is introduced. Experimental results from a 5-kVA prototype are presented in order to validate the proposed modulation technique.      

H桥级联多电平逆变器相移SVPWM技术的研究

载波相移SPWM调制法是级联型逆变器的主流控制方法,其等效载波频率高、谐波特性好、功率单元之间输出功率平衡。而空间矢量法谐波特性好、电压利用率高、控制方法简单便于数字实现。文中针对H桥级联型多电平逆变器的结构特点,综合采用载波移相SPWM法和空间矢量法,并用一种新的空间矢量算法取代传统的方法。该算法中无需三角函数和无理数计算,计算过程非常简单,节约了计算时间,结果更为准确。最后通过系统仿真验证该算法的正确性和有效性。     

级联型多电平逆变器中多载波PWM控制方法的改进

级联型多电平逆变器由于其模块化结构、便于生产高度、更换容易等特点，受到电力电子工程技术人员的广泛关注。本文通过对于级联型多电平逆变器几种典型的控制方法的分析比较，提出了一种改进多载波PWM控制方法。该方法通过改变载波的幅值，使级联逆变器的各个逆变单元开关规律相同，并使所有的开关器件的损耗一致，有利于逆变单元的统一设计和可靠运行。     

Modified Phase-Shifted PWM Control for Flying Capacitor Multilevel Converters

The issue of voltage imbalance remains a challenge for the flying capacitor multilevel converter. The phase-shifted pulsewidth modulation (PS-PWM) method has a certain degree of self-balancing properties. However, the method alone is not sufficient to maintain balanced capacitor voltages in practical applications. The paper proposes a closed-loop modified PS-PWM control method by incorporating a novel balancing algorithm. The algorithm takes advantage of switching redundancies to adjust the switching times of selected switching states and thus maintaining the capacitor voltages balanced without adversely affecting the system's performance. Key techniques of the proposed control method, including selection of switching states, calculation of adjusting times for the selected states, and determination of new switching instants of the modified PS-PWM are described and analyzed. Simulation and experimental results are presented to confirm the feasibility of the proposed method     

基于电网电压补偿的多电平变频器控制方法研究

提出了一种基于电网电压补偿的多电平变频器控制方法，从主回路拓扑结构出发，以低压变频器规则采样法为基本理论，详细介绍了适合于多电平变频器控制的理论分析和算法推导过程。并给出了适合数字信号处理的采样原理和采样时序。直接根据电网电压的变化，实时调节输出电压，能获得与闭环系统相媲美的理想机械特性和运行效果。该方法不仅具有计算步骤简单、输出SPWM对称性好、精度高和控制方便的特点，而且适用于任何电平数目及拓扑结构。试验结果表明，和其他多电平控制方法相比，使用效果好，有很高的利用价值。     

Voltage Balancing Control of Diode-Clamped Multilevel Converters With Passive Front-Ends

In the previous literature, it has been reported that it is not possible to guarantee the balance of the dc-link capacitor voltages of multilevel three-phase diode-clamped dc–ac converters with passive front-ends for high modulation indices, especially for more than three levels. This paper proposes a novel closed-loop control approach capable of guaranteeing such balance for all operating conditions of the converters without the need for additional hardware. Three different phase duty-ratio perturbation schemes are proposed. They are compared through simulation for the case of a four-level three-phase diode-clamped dc–ac converter operated with a virtual-vector-based modulation. The most simple and effective perturbation scheme, only requiring the sensing of all dc-link capacitor voltages, is tested experimentally in the same four-level converter. The results demonstrate the feasibility of guaranteeing the dc-link capacitor voltage balance for all converter operating conditions.      

一种新型的级联型多电平逆变器研究

本文提出一种新的级联型多电平逆变器拓扑结构，通过负载将两个电压型三相逆变器级联在一起，该逆变器能作为一个三电平或四电平逆变器运行。本文对这种逆变器的基本结构和控制原理进行了分析。     

皮秒脉冲在级联光纤光栅中的传输特性研究

基于波分复用(WDM)系统中级联光纤光栅色散特性,研究了皮秒脉冲宽度以及光纤光栅长度和耦合系数等参数对脉冲压缩的影响;以高斯脉冲为例,运用等效色散的方法数值模拟了皮秒脉冲经过常规光纤和级联光纤光栅的演化过程.结果表明,选择合适的参数,级联光纤光栅可以对在普通光纤中色散展宽的脉冲进行色散补偿.还发现,进一步增加光栅长度,可使脉冲得到有效压缩.     

H桥级联型多电平逆变器调制策略对比分析

针对H桥级联型多电平逆变器,对比分析了传统的多电平PWM调制策略,指出其存在的输出电压谐波性能问题。提出一种改进的调制策略,不仅抑制了直流母线电压波动引起的低次谐波含量,而且有效地降低了高、次谐波含量。最后通过对比实验,验证了所提出方法的正确性与有效性。     

基于伪PLL的DCDC转换器自适应斜坡补偿电路

针对时钟外同步DCDC转换器不同,应用频率所需电感值不同对斜坡补偿斜率的影响,提出了一种基于伪PLL的自适应斜坡补偿电路。该电路能够根据DCDC转换器不同的外同步频率自动调节斜坡补偿斜率的大小,在保证转换器稳定工作的同时,不会影响转换器的带载能力和环路的反应速度。通过基于0.35 μm 的标准CMOS工艺,对不同的外同步频率下伪PLL电路产生自适应斜坡补偿斜率的过程进行了仿真验证,在250 kHz~1.5 MHz的频率范围内,伪PLL均能产生自适应的斜坡补偿斜率,保证DCDC工作环路的稳定性。     

Control of a Single-Phase Cascaded H-Bridge Multilevel Inverter for Grid-Connected Photovoltaic Systems

This paper presents a single-phase cascaded H-bridge converter for a grid-connected photovoltaic (PV) application. The multilevel topology consists of several H-bridge cells connected in series, each one connected to a string of PV modules. The adopted control scheme permits the independent control of each dc-link voltage, enabling, in this way, the tracking of the maximum power point for each string of PV panels. Additionally, low-ripple sinusoidal-current waveforms are generated with almost unity power factor. The topology offers other advantages such as the operation at lower switching frequency or lower current ripple compared to standard two-level topologies. Simulation and experimental results are presented for different operating conditions.     

Synthesis of Multilevel Converters Based on Single- and/or Three-Phase Converter Building Blocks

This paper synthesized a family of multilevel converters that is constructed by using multiple well-developed single- and/or three-phase converter building blocks. The resulting advantages include a modular structure that leads to convenient construction and maintenance as well as easy extension to higher voltage levels. One successful example from this family is the cascaded H-bridge converter, which is now widely used for volt-amperes reactive (VAR) compensation and motor drive applications due to its many advantages. Another emerging member is the Hexagram converter, composed of six interconnected three-phase converter modules, which has many eminent features such as lowered component count and lowered dc energy storage requirement compared to the cascaded H-bridge converter, and thus, is a promising candidate for high-power applications.     

混合逆变器改进调制算法的研究

目前,在混合级联型逆变器的研究领域中,改变串联单元的母线电压比是一种比较典型的方法。但是,该类拓扑结构存在着不同电压比例母线之间的“能量回流”问题,在不同的调制比下运行,可能会出现导致母线电压不平衡的情况。针对该问题,本文在分析“能量回流”问题产生机理的基础上,提出了一种纯软件算法的解决方案,该方案针对不同的调制比,实时的改变主逆变器的导通比较电压,且同时动态的调整从逆变器的零序分量,从而实现了全调制比范围内克服“能量回流”的问题,并且改善了输出电压谐波性能等目标。最后,本文对所提出的控制算法进行了仿真和实验的验证。