基于改进SVPWM的三电平NPC逆变器中点电压平衡的研究

三电平NPC逆变器具有直流电压利用率高、开关耐压低、输出电压谐波低等优点而被应用于高压大功率场合。而中点电压平衡问题一直是三电平NPC逆变器研究的关键。文中在传统的空间矢量调制方法的基础上进行改进,不改变原来矢量的模长,对各扇区的不同小三角形内影响中点电位的中、小矢量进行重新分配和虚拟,理论上能完全消除电压矢量对中点电位的影响,该方法的中点电压平衡能力在全调制度范围内控制灵活且有效。通过仿真和实验验证了所提方法的正确性。     

Evaluation of the low-frequency neutral-point voltage oscillations in the three-level inverter

The nearest vectors to the reference vector are commonly used in space-vector modulation (SVM) strategies. The main advantages of these modulation strategies are the low switching frequencies of the devices, the good output voltage spectra, and the low electromagnetic interference. However, when these techniques are applied to the three-level neutral-point (NP)-clamped inverter, low-frequency oscillations appear in the NP voltage for some operating conditions. As a result, the value of the dc-link capacitors must be increased in order to attenuate such oscillations. In this paper, these amplitudes are quantified for two modulation strategies that use nearest vectors to the reference vector. Owing to the nondimensional variables used in the analysis, the information provided will help for the calculation of the dc-link capacitors in a given specific application. Simulated and experimental examples are presented.     

Soft-switching PWM three-level converters

This paper proposes a family of modulation strategies for PWM three-level (TL) converters. The modulation strategies can be classified into two kinds according to the turn-off sequence of the two switches of the pair of switches. The concept of the leading switches and the lagging switches is introduced to realize soft-switching for PWM TL converters. The realization of soft-switching for both the leading switches and the lagging switches is proposed, based on which, soft-switching PWM TL converters can be classified into two kinds: zero-voltage-switching (ZVS) and zero-voltage and zero-current-switching (ZVZCS), for which the suitable modulation strategies are pointed out respectively from the family of modulation strategies. A novel ZVZCS TL converter is proposed, its operation principle and parameter design are analyzed, and the experimental results are also included     

三电平DC-DC变换器的统一建模研究

三电平变换器的统一建模是对变换器进行控制和优化的基础.但是,对于三电平变换器而言,其开关数目多,工作模态复杂,用传统建模方法对之进行统一建模相当困难.脉冲波形积分法是一种新的统一建模方法,为此,本文首次采用脉冲波形积分法,对三电平变换器的统一建模进行了研究.以非隔离三电平变换器为例,详细阐明了其统一建模原理,并给出了输入输出共地和输入输出不共地的Buck变换器的统一建模方法.仿真分析与实验结果表明,脉冲波形积分法是一种对三电平变换器进行统一建模的有效工具,为三电平变换器的有效控制奠定了基础.     

Novel switching method for single-phase NPC three-level inverter with neutral-point voltage control

This paper proposes a novel switching method with the neutral-point voltage control in a single-phase neutral-point-clamped three-level inverter (SP-NPCI) used in photovoltaic systems. A proposed novel switching method for the SP-NPCI improves the efficiency. The main concept is to fix the switching state of one leg. As a result, the switching loss decreases and the total efficiency is improved. In addition, it enables the maximum power-point-tracking operation to be performed by applying the proposed neutral-point voltage control algorithm. This control is implemented by modifying the reference signal. Simulation and experimental results provide verification of the performance of a novel switching method with the neutral-point voltage control.     

三电平变流器高电压穿越下的中点电压控制研究

针对高电压穿越条件,分析中点钳位型三电平变流器的中点电压数学模型,推导出三相功率与零序电压的传递函数。提出一种新型的注入零序电压控制方案,该方案通过控制中点处的零序总功率为0,来实现中点电压的平衡控制。具体方案是,通过计算中点处的三相功率偏差,经PI调节器输出零序电压调制波,经过3s/2s坐标变换,叠加至三相基波调制波,并采用SVPWM调制算法实现。仿真显示,该方案能够实现中点电压的平衡控制,具有较好的动态响应。     

Novel 20-MW downhill conveyor system using three-level converters

This paper presents a very new drive system used to transport ore from the mine down to the concentrator plant in the copper mine "Los Pelambres". Eight 2500-kW motors are driven by three-level inverters with gate-turn-off thyristors (GTOs). A three-level active front end is used at the input side of each inverter. A GTO chopper is used to provide controlled electrical braking in case of line loss. The paper presents the requirements and alternatives for the drive system and the control strategies for the converters and the belt. A novel application of the selective harmonic elimination method is used to reduce the input current harmonics. Special attention is dedicated to the interaction with the electrical network. The most relevant features of the system are: (1) fuseless operation; (2) adjustable power factor; (3) reduced input current harmonics; (4) smooth transition between motoring and regenerating modes; and (5) 15 MW of regenerated power with more than six months of successful operation.     

Modeling and design of a neutral-point voltage regulator for a three-level diode-clamped inverter using multiple-carrier modulation

The three-level diode-clamped multilevel converter commonly called the neutral-point-clamped converter has become established to be a preferred topology for high-power motor drive applications operating at several kilovolts. Although solutions to the problem of maintaining a stable neutral-point voltage in the converter continue to be the topic of research, a simple solution based on a design-oriented dynamic model of the system is not widely known. This paper presents the design, analysis, and implementation of a simple neutral-point voltage regulator for a three-level diode-clamped multilevel inverter, which uses a multiple-carrier sine-triangle modulator in conjunction with a closed-loop controller for neutral-point regulation. Redundant state choices are controlled via a continuous offset voltage that regulates the dc injection into the midpoint of the dc bus. A small-signal transfer function is developed in closed form, for neutral-point regulation, with the voltage offset as the control variable. Besides maintaining dc-bus voltage balance, the use of the approach leads to a significant reduction in the voltage distortion at the neutral point, allowing a definitive reduction in the required dc bus capacitance. Analytical, computer simulation, and experimental results verifying the approach are presented in this paper.     

The nearest three virtual space vector PWM - a modulation for the comprehensive neutral-point balancing in the three-level NPC inverter

This letter presents a new modulation approach for the complete control of the neutral-point voltage in the three-level three-phase neutral-point-clamped voltage source inverter. The new modulation approach, based on the virtual space vector concept, guarantees the balancing of the neutral-point voltage for any load (linear or nonlinear) over the full range of converter output voltage and for all load power factors, the only requirement being that the addition of the output three-phase currents equals zero. The implementation of the proposed modulation is simple according to the phase duty-ratio expressions presented. These expressions are only dependent on the modulation index and reference vector angle. The performance of this modulation approach and its benefits over other previously proposed solutions are verified experimentally.     

三电平逆变器中点电位平衡的综合控制策略

三电平逆变电路普遍存在中点电位不平衡的问题,中点电位的不平衡主要体现在两个方面:中点电位偏移和中点电位波动。基于传统的SVPWM算法,通过分析造成中点电位不平衡的原因,提出了前馈控制+反馈控制的综合控制策略。该方法通过采集两个电容电压和三相负载电流的实时值,通过切换函数的不同值来选择不同的控制策略,从而实现对中点电位平衡的综合控制。仿真实验证明了此综合控制策略控制精度高,并且不会出现中点电位偏移的问题,很好地控制了中点电位的平衡,弥补了在只有反馈控制策略下电压波动幅值较大的缺点,达到了很好的效果。     

Diode-clamped multilevel converters: a practicable way to balance DC-link voltages

The converter topologies identified as diode-clamped multilevel (DCM) or, equivalently, as multipoint clamped (MPC), are rarely used in industrial applications, owing to some serious drawbacks involving mainly the stacked bank of capacitors that constitutes their multilevel DC link. The balance of the capacitor voltages is not possible in all operating conditions when the MPC converter possesses a passive front end. On the other hand, in AC/DC/AC power conversion, the back-to-back connection of a multilevel rectifier with a multilevel inverter allows the balance of the DC-link capacitor voltages and, at the same time, it offers the power-factor-correction capability at the mains AC input. An effective balancing strategy suitable for MPC conversion systems with any number of DC-link capacitors is presented here. The strategy has been carefully studied to optimize the converter efficiency. The simulation results related to a high-power conversion system (up to 10 MW) characterized by four intermediate DC-link capacitors are shown.     

A family of zero-voltage and zero-current-switching (ZVZCS) three-level DC-DC converters with secondary-assisted regenerative passive snubber

A detailed analysis and optimized-oriented design of a family of three-level soft-switching converters is presented. The zero-voltage-switching (ZVS) of the outer switches and zero-current-switching (ZCS) of the inner switches is realized by employing a pulse-width-modulation (PWM) phase-shift control and a secondary-assisted passive snubber. The switching operation is discussed by comparing the results produced by the use of different passive snubbers (the author's original one and literature-available ones). The voltage on the rectifier diodes is clamped at a reasonable value, specific to each one of the six snubbers taken into consideration. The voltage stress on each transistor is reduced to half of the input voltage. Lower rated transistors and rectifier diodes can be used, thus reducing the conduction losses. Before turning on/off the inner switches, the snubber's capacitor voltage determines the fall of the primary current to zero, thus avoiding wasteful energy circulation and assuring ZCS. The snubber's energy is recuperated to the load. The outcome of these improvements is a high efficiency in energy processing. Soft-switching-oriented constraints on the converter parameters are expressed as implicit equations, whose graphical solution permits the optimized design of the parameters in order to ensure ZVZCS. A comparative analysis of the effective duty cycle and the boost effect of it, due to the use of the secondary snubber, is performed. The influence of the choices of the parameters values on the regulation capability is pointed out. Experimental results prove the expected high performances of the optimized converters.     

NPC型三电平逆变器SVPWM算法及中点电压平衡控制的研究

为解决二极管箝位型(NPc)三电平逆变器中点电压平衡问题,文中详细介绍了三电平逆变器空间矢量调制算法的基本原理,从空间矢量控制的角度分析了中点电压波动的基本规律,提出了一种有效抑制中点电压波动的算法。该算法依照中点电荷守恒原理,通过检测输出三相电流和中点电压波动值,推导出用于精确调节中点电压小矢量冗余开关状态的分配因子表达式。仿真结果验证了该方法的可行性和精确性。     

基于SPWM的Z源T型三电平逆变器中点平衡和升压控制

由于Z源三电平中点钳位(NPC)逆变器存在无源器件多的缺点,采用Z源T型三电平逆变器拓扑,该拓扑具有Z源NPC逆变器相同的升压特性,但是开关器件数目较少,效率高。以电压源型Z源三电平T型逆变器为例,对同相电压偏移(PD)和反相电压偏移(APOD)方法进行分析,研究发现采用PD的方法可以减少开关次数,降低开关损耗。在Matlab/Simulink下,建立Z源T型三电平逆变器PD模型,仿真结果表明,采用PD方法能够获得较好的波形质量,并针对T型三电平逆变器固有的中点不平衡,提出了注入零序分量实现中点平衡控制。通过实验验证了所提方法的有效性。     

三能级原子同时存在辐射和吸收时的量子相干性

量子位和环境的相互作用,将引起量子位能量耗散或相对位相改变,最终导致量子位由相干叠加态退化为混合态或纯态,这种现象被称为量子消相干.针对三能级原子同时存在辐射和吸收时的情况,基于量子力学和Kraus算子理论,利用约化密度矩阵方法,分析了三能级原子密度矩阵元的演化规律,从而研究该系统的量子相干特性.结果表明:初态处于相干叠加态的三能级原子,由于辐射和吸收,可能演化成相干叠加态、未完全混合态、完全混合态、纯态.     

Soft-switching techniques in PWM converters

A number of soft-switching pulse-width-modulated (PWM) converter techniques have been proposed, aimed at combining the desirable features of both the conventional PWM and resonant converters while avoiding their respective limitations. In this paper, three classes of zero-voltage soft-switching (PWM) converters (namely the zero-voltage-switched (ZVS) quasi-square-wave converters, ZVS-PWM converters, and zero-voltage-transition PWM converters) and two classes of zero-current soft-switching PWM converters (namely, the zero-current-switched PWM converters and zero-current-transition PWM converters) are reviewed, and their merits and limitations are assessed. Experimental results of several prototype of converters are presented to illustrate each class of converter     

On the zero-crossing distortion in single-phase PFC converters

Input current distortion in the vicinity of input voltage zero crossings of boost single-phase power factor corrected (PFC) ac-dc converters is studied in this paper. Previously known causes for the zero-crossing distortion are reviewed and are shown to be inadequate in explaining the observed input current distortion, especially under high ac line frequencies. A simple linear model is then presented which reveals two previously unknown causes for zero-crossing distortion, namely, the leading phase of the input current and the lack of critical damping in the current loop. Theoretical and practical limitations in reducing the phase lead and increasing the damping factor are discussed. A simple phase compensation technique to reduce the zero-crossing distortion is also presented. Numerical simulation and experimental results are presented to validate the theory.     

Modeling of multilevel converters

In this paper, the imbricated cells multilevel converters are studied and modeled from a control viewpoint. These converters make use of several switches connected in a series, which allows using switches with reduced voltage ratings; these low voltage switches have lower conduction losses and can switch at higher frequency. In addition to this feature common to all converters using series connected switches, the control signals of multilevel converters can be phase shifted to increase the apparent switching frequency and improve the dynamic performances of the whole converter. It is shown that a multilevel inverter leg, composed of p pairs of switches and p-1 capacitors, forms a multivariable nonlinear system that cannot be properly modeled by standard methods such as state-space averaging. The transient behavior of this system depends on the current harmonics and their phase shift with the different control signals. A specific model is detailed, studied, and used to illustrate the properties of these converters. In particular, the natural balancing of the voltage across the switches is demonstrated and the time constants involved in this process are determined     

Architectural trends in current-steering digital-to-analog converters

Recent interests from the research community in building digitized transmitters have led to numerous architectural and circuit-level innovations and developments towards the design of digital-to-analog converters (DACs) in the GHz space. However, several challenges exist in terms of interface overhead and process capabilities that fundamentally influence the accuracy, linearity and achievable speed. This paper aims to highlight some of the fundamental challenges in DACs and provide the reader with an insight into some of the emerging innovations at the systemic and circuit levels that address these challenges and aid in the transition of high-resolution DACs into the GHz regime.     

Stability in high-output-voltage push-pull current-fed converters

The relationship between the magnetizing inductances of a transformer and of a choke coil in high-voltage power supplies is studied. The analysis defines the conditions for high switching frequency and stable operation of high-voltage power supplies without power loss increases. This approach is confirmed by the implementation of a high-voltage power supply operating at 200 kHz with an efficiency of 86%