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.      

Fundamental-Frequency-Modulated Six-Level Diode-Clamped Multilevel Inverter for Three-Phase Stand-Alone Photovoltaic System

This paper presents a fundamental-frequency-modulated diode-clamped multilevel inverter (DCMLI) scheme for a three-phase stand-alone photovoltaic (PV) system. The system consists of five series-connected PV modules, a six-level DCMLI generating fundamental-modulation staircase three-phase output voltages, and a three-phase induction motor as the load. In order to validate the proposed concept, simulation studies and experimental measurements using a small-scale laboratory prototype are also presented. The results show the feasibility of the fundamental frequency switching application in three-phase stand-alone PV power systems.     

Voltage Balancing Control for a Three-Level Diode-Clamped Converter in a Medium-Voltage Transformerless Hybrid Active Filter

This paper discusses a transformerless hybrid active filter integrated into the 6.6-kV, 1-MW adjustable-speed motor drive having a three-phase diode rectifier at the front end. The hybrid filter consists of an active filter using a three-level diode-clamped pulsewidth modulator converter rated at 60 kVA, and a 250-kVA passive filter tuned to the seventh harmonic frequency. They are directly connected in series without a transformer. This circuit configuration enables one to use 1.2-kV insulated gate bipolar transistors because the dc voltage of the three-level converter is 1.32 kV (20% of 6.6 kV). Voltage balancing control characterized by superimposing a sixth harmonic zero-sequence voltage on the active filter voltage reference in each phase is introduced to the three-level converter with triangle carrier modulation. Experimental waveforms obtained from a 400-V, 15-kW downscaled system verify the viability and effectiveness of the proposed hybrid filter, keeping the two dc capacitor voltages well-balanced.     

电网不平衡条件下模块化多电平变换器控制策略

当电网处于电压不平衡状态时,MMC的内部会产生环流.环流中存在二倍频的正序、负序和零序分量,增加了系统的不稳定性.针对电网不平衡情况下MMC二倍频环流抑制的问题,文中提出一种基于线性自抗扰控制器的环流抑制策略.根据瞬时功率理论推导出环流参数的数量关系,在此基础上设计了基于线性自抗扰控制策略的环流控制器来抑制环流.最后利...     

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。     

A Simple Voltage Balancing Scheme for m-Level Diode-Clamped Multilevel Converters Based on a Generalized Current Flow Model

This paper presents a simple voltage balancing scheme for m-level diode-clamped multilevel converters (DCMC). This paper first introduces a novel and simple current flow model for general $m$ -level DCMCs. The superiority of the proposed model is its independence from the modulation scheme and its simplicity. It also provides a new perspective for voltage sharing accessibility among the dc link capacitors. The proposed current flow model, a cost function, and a space vector modulation (SVM) switching strategy are then used to balance the capacitors' voltages of the DCMCs in a very simple and optimized manner. Such a voltage balancing scheme was not developed for converters of five levels or greater due to the complexity of the converter and its modeling approaches. To validate the feasibility of the proposed voltage balancing scheme, this paper presents analytical and simulation results obtained from a five-level DCMC. In particular, this paper introduces a stability region within which the voltage balancing strategy converges. The impact of the cost function on the stability margins and converter performance is examined and discussed by means of comparison with different cost functions, and it is shown that the proposed cost function will improve the stability margins. The performance of the strategy for unbalanced and polluted loads shows that, unexpectedly, in some specific cases output current harmonics will improve the stability margin.      

用于光伏发电的串联全桥DC-DC变换器

光伏发电系统中DC-DC变换器的作用是使太阳能光伏电池电压达到光伏系统逆变环节的直流母线电压的要求。其中,全桥结构具有功率开关管电压应力小、变压器利用率高、容易实现软开关的特点,现将标准的全桥DC-DC变换器重新设计成串联形式,用计算机仿真来评估其工作状况,结果表明,开关的工作峰值电流被显著地减小了,减少了损耗并提高了...     

Analytical Modelling of Voltage Balance Dynamics for a Flying Capacitor Multilevel Converter

This paper presents a strategy for the analytic determination of the natural voltage balancing dynamics of flying capacitor converters. The approach substitutes double Fourier series representations of the pulsewidth modulation (PWM) switching signals into a nonlinear dynamic circuit model of the converter. The result reduces to a linearized state-space model that can be readily solved, with the Fourier solution coefficients defining the state-space matrix terms. The solution can be readily developed for converters of any level, and allows rapid analytical investigation of the dynamic (and static) balancing behavior over a wide range of conditions. Furthermore, the approach allows powerful strategies such as root locus to be used to investigate the converter's performance as a function of changes in parameters such as modulation index and load. The analysis approach has been fully verified by comparing it against experimental results on a low voltage prototype converter.     

General Modulation Strategy for Seven-Phase Inverters With Independent Control of Multiple Voltage Space Vectors

This paper focuses on the analysis of a seven-phase voltage-source inverter for high-performance motor drives. The problem of the modulation strategy of the inverter is solved by combining the multiple space vector representation with traditional carrier-based pulsewidth modulation. This approach leads to the definition of a general modulation strategy that can be usefully utilized in multimotor drives, as well as in multiphase motor drives for improving the torque density. The inverter output voltage capability is investigated, showing that the proposed modulation strategy is able to fully exploit the dc input voltage either in sinusoidal or nonsinusoidal operating conditions. The results obtained in the analytical investigation are confirmed by experimental tests.     

Single-Stage AC/DC Converter With Input-Current Dead-Zone Control for Wide Input Voltage Ranges

A single-phase single-stage ac/dc converter with input-current dead-zone control is proposed. It is based on flyback topology operating in discontinuous conduction mode (DCM). The current charging into the link capacitor is controlled according to line changes by adjusting the input-current blocking angle to alleviate an excessive increase of the link voltage. The reduced voltage stress can maintain an almost-constant voltage irrespective of load conditions by operating in dc/dc stage in DCM. Experimental results of a 60-W (5-V 12-A output) prototype converter show that the link voltage is limited within 384 V and that the measured power factor is more than 0.91 under universal voltage inputs and entire load conditions. In addition, the maximum efficiency is measured to be about 81% at the rated condition     

光伏并网逆变器电压型控制技术研究

本文主要对电压源电压型并网控制技术进行了研究.根据逆变器并联运行中的无功调幅.有功调频的控制原理,将此方法应用到光伏并网逆变器中实现了有功和无功的独立控制.使得逆变器在输出功率上控制更加灵活详细讨论了电压型并网控制算法,并给出了关键模块I_(d)、I_(q)。的计算方法和正弦波合成器的实现过程最后.在Matlab/Simulink中建立了仿真模型,并进行了实验验证.仿真和实验结果表明电压型并网控制方案具有可行性。     

风力发电机网侧变换器的磁极约束控制

对风力发电机来说,机械部分和电网部分的优良动力性能是至关重要的。在电网侧,需要满足故障穿越和低短路功率弱电网连结的要求。本文采用联排电力电子器件,给出了一个磁约束控制(pole-restraining control,PRC)的网侧变换器。和现有的控制方法比较,磁约束控制可消除电力电子变换器内部系统的不稳定性,并能极大的增强动态性能。改进的性能通过一个风力发电机的网侧变换器做了验证。并对磁约束控制的原理作了说明,仿真结果表明了瞬态性能有所提高。     

降压型直流转换器三角波电压前馈控制技术

研究了一种降压型直流转换器的三角波电压前馈控制技术.根据其开环模型,推导出连续导通模式下的开环输入/输出电压小信号关系,并给出前馈系数的取值范围.在输入电压Vi=7～15 V、输出电压Vo=5 V的条件下,对前馈控制电路进行测试.结果表明,前馈确实提高了电路输入瞬态响应与负载调节能力,其电压调节能力与电源抑制能力取决于对前馈通路交流传递函数的设置.     

A Cascade Multilevel Converter Topology With Reduced Number of Switches

This paper introduces a new multilevel converter topology that has many steps with fewer power electronic switches. The proposed circuit consists of series-connected submultilevel converters blocks. The optimal structures of this topology are investigated for various objectives, such as minimum number of switches and capacitors, and minimum standing voltage on switches for producing maximum output voltage steps. A new algorithm for determination of dc voltage sources’ magnitudes has also been presented. The proposed topology results in reduction of the number of switches, losses, installation area, and converter cost. The operation and performance of the proposed multilevel converter has been verified by the simulation and experimental results of a single-phase 53-level multilevel converter.      

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.     

Digital Current Control of a Voltage Source Converter With Active Damping of LCL Resonance

Inductance–capacitor–inductance (LCL)-filters installed at converter outputs offer higher harmonic attenuation than L-filters, but careful design is required to damp LCL resonance, which can cause poorly damped oscillations and even instability. A new topology is presented for a discrete-time current controller which damps this resonance, combining deadbeat current control with optimal state-feedback pole assignment. By separating the state feedback gains into deadbeat and damping feedback loops, transient overcurrent protection is realizable while preserving the desired pole locations. Moreover, the controller is shown to be robust to parameter uncertainty in the grid inductance. Experimental tests verify that fast well-damped transient response and overcurrent protection is possible at low switching frequencies relative to the resonant frequency.     

A Novel Topology for Photovoltaic DC/DC Full-Bridge Converter With Flat Efficiency Under Wide PV Module Voltage and Load Range

In this paper, a novel topology for a photovoltaic (PV) dc/dc converter that can dramatically reduce the power rating and increase the efficiency of a PV system by analyzing PV module characteristics is proposed. Based on the analysis, in the proposed topology, only 30.7% power of the total PV system is needed for a dc/dc converter. Furthermore, the dc/dc converter efficiency curve is flat under wide PV module voltage and all load ranges. In particular, the converter efficiency at the lower duty range is dramatically improved. The total PV system is implemented for a 250-kW PV power conditioning system (PCS). This system has only three dc/dc converters with a 25-kW power rating. It is only one-third of the total PV PCS power. The 25-kW prototype PV dc/dc converter is introduced to experimentally verify the proposed topology. In addition, an experimental result shows that the proposed topology exhibits a good performance.     

基于电压直接控制的光伏系统最大功率点跟踪策略

在分析光伏系统的数学模型和输出特性的基础上,对传统的扰动测量法进行了改进,提出一种直接基于太阳能输出电压的最大输出功率点跟踪策略。利用Simulink对该方法进行仿真,并且应用到实际光伏系统中。测量结果表明了方法的优越性,能够适应天气变化较快的场合,输出功率能够快速跟踪太阳能最大功率点。     

双星型模块组合型多电平变换器的PWM控制策略

双星型模块组合型多电平变换器(MMC)具有良好的可扩展性、模块化、故障无损特性,是一种能很好满足未来高压或中压功率变换需求的拓扑。然而,很少文章对其PWM调制策略及参考电压指令生成方法进行详细的分析和讨论。基于MMC的电路结构特点和基本工作原理,本文提出一种优化的载波移相SPWM控制策略和电压控制策略,无需额外的硬件电路,即可实现对MMC功率单元直流电容电压良好的均压控制效果。通过一台5电平三相MMC的仿真结果,详细分析了其动态特性、静态特性,验证了本文控制策略的有效性以及MMC的可行性。