Closed-Loop Control of a Three-Phase Neutral-Point-Clamped Inverter Using an Optimized Virtual-Vector-Based Pulsewidth Modulation

This paper presents a closed-loop control scheme for the three-level three-phase neutral-point-clamped dc-ac converter using the optimized nearest three virtual-space-vector pulsewidth modulation, which is a modulation that produces low output-voltage distortion with a significant reduction of the dc-link capacitance. A new specific loop modifying the modulating waveforms is proposed to rapidly control possible perturbations in the neutral-point voltage balance. An online estimation of the load displacement angle and load linear/nonlinear nature is introduced at no extra cost. The remaining part of the control is analogous to the control for a two-level converter with an appropriate interfacing to the selected modulation. The closed-loop control is designed for the case of a renewable-energy source connected to the ac mains, and its performance is analyzed through simulation and experiments.     

An SVM Algorithm to Balance the Capacitor Voltages of the Three-Level NPC Active Power Filter

A requirement of the three-level neutral-point-clamped voltage source inverter, with or without a separately supporting dc link, is to maintain the balance of the two dc-link capacitor voltages. In this paper, balancing of the capacitor voltages for these two dc-link voltage source possibilities is analyzed and an algorithm is proposed to independently balance the capacitor voltages. The algorithm considers the average energy effect of each vector on capacitor voltage and the necessary optimal vector state sequence in each modulation cycle. The algorithm minimizes the voltage rating overheads of the hardware, reduces the voltage ripple, and attenuates the negative effects associated with dc-link voltage oscillations.      

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.     

Three-Level Inverter-Based Shunt Active Power Filter in Three-Phase Three-Wire and Four-Wire Systems

This paper presents a direct current-space-vector control of an active power filter (APF) based on a three-level neutral-point-clamped (NPC) voltage-source inverter. The proposed method indirectly generates the compensation current reference by using an equivalent conductance of the fundamental component using APF's dc-link voltage control. The proposed control can selectively choose harmonic current components by real-time fast Fourier transform to generate the compensation current. The compensation current is represented in a rotating coordinate system with chosen switching states from a switching table implemented in a field-programmable gate array. In addition, a three-phase four-wire APF based on a three-level neutral-point-clamped inverter is also presented. The proposed APF eliminates harmonics in all three phases as well as the neutral current. A three-phase three-wire NPC inverter system can be used as a three-phase four-wire system since the split dc capacitors provide a neutral connection. To regulate and balance the split dc-capacitor voltages, a new control method using a sign cubical hysteresis controller is proposed. The characteristics of the APF system with an LCL-ripple filter are investigated and compared with traditional current control strategies to evaluate the inherent advantages. The simulation and experimental results validated the feasibility of the proposed APF.      

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.     

$H^infty$Control of the Neutral Point in Four-Wire Three-Phase DC–AC Converters

Inverters used as the interface for a distributed generator in a three-phase four-wire system sometimes operate with a large neutral current because of unbalanced loads and single-phase (possibly nonlinear) loads. Voltage balance within the dc-link of the inverter is important for proper operation of the inverter, and the neutral current is a significant disturbance to this. It is preferable to use fast acting control rather than dissipative balancing or large-valued capacitors. This paper develops a linear model of an actively balanced split dc-link and applies H^infin control design to provide high-bandwidth robust control. The approach is developed for a conventional two-level inverter, but it remains valid (without change) for a three-level neutral-point clamped inverter. The controller achieves very small deviations of the neutral point (better than 0.5 in 800 V) from the midpoint of the dc source despite the large neutral current (32 A_RMS). The controller design is described and verified first in a PSCAD simulation and second in experimental testing of a 30-kW 415-V (line) inverter     

Optimal pulse-width modulation for three-level inverters

The three-level neutral-point-clamped voltage source inverter is widely used in high power, medium voltage applications. This paper studies continuous and discontinuous pulse-width modulation for this inverter. Detailed analysis of discontinuous modulation shows that the average switching frequency is not directly proportional to the carrier or sampling frequency, since additional switching transitions occur between different regions of discontinuity. At typical switching frequencies for high power applications (up to 2 kHz) these additional transitions contribute significantly to the inverter's total losses, so that a proper comparison of the harmonic performance can only be carried out under constant loss conditions with varying carrier frequency. This comparison is performed for a typical industrial medium voltage inverter. The paper then considers the major issues of neutral-point voltage balancing and loss distribution within the inverter, for the identified optimal modulation schemes.     

基于中点电荷预估控制的三电平PWM整流器研究

研究了二极管箝位型三电平PWM整流器的基本原理,针对直流侧存在中点电位不平衡问题,提出了一种基于中点电荷预估控制的中点电位控制算法。建立了电网电压定向下三电平PWM整流器在d-q坐标系的数学模型,推导了平衡因子的精确计算方程,在简化三电平SVPWM算法的基础上,搭建了基于MATLAB的仿真模型,并进行了仿真研究,仿真结果验证了该算法的正确性和有效性。     

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.     

Three-phase power quality compensator under the unbalanced sources and nonlinear loads

A three-phase voltage-source inverter for a power quality compensator under the unbalanced mains and nonlinear loads is proposed to provide balanced three-phase source current and improve input power factor. The proposed converter is based on the conventional three-phase voltage-source inverter with three additional ac power switches to achieve three-level pulsewidth modulation. The voltage stress of three ac power switches is clamped to half the dc-link voltage. The balanced reference mains currents are estimated using the dc-bus voltage and load currents. A proportional-integral voltage controller is used in the outer loop to compensate the switching losses of the voltage-source inverter. To perform the integrated power quality compensation, a hysteresis current control scheme is adopted to track the balanced line current command in phase with mains voltage. Three voltage levels are generated on the ac terminal of the proposed inverter. Computer simulation and experimental results are provided to verify the effectiveness of the proposed control scheme.     

Novel Direct Current-Space-Vector Controlfor Shunt Active Power Filters Basedon the Three-Level Inverter

The growing number of electric drives with non-sinusoidal line currents has given increased interest in active power filters (APF), to avoid grid problems caused by harmonic distortions. In this paper, a novel direct current-space-vector control scheme (DCSVC) is presented for a three-level, neutral-point-clamped voltage source inverter, which is employed as an active power filter. The proposed method generates the compensation current reference indirectly generating an equivalent ohmic conductance for the fundamental component by means of the APF's dc-link voltage control. Based on the fast Fourier transform the compensation of the reactive fundamental current and selectable harmonics can be cancelled, confining the operation to only harmonic compensation and thus saving the APF's apparent power. The novel DCSVC, operating in synchronously rotating coordinates is implemented in a field programmable gate array, realizing the switching states from switching tables. The proposed control reduces the average switching frequency and thus, the switching power loss significantly, compared with a previous DCSVC, operating in stationary coordinates. Simulation and experimental results validate the feasibility and highly dynamic performance of the proposed control, both for harmonic and total non-active current compensation.      

A New Multilevel Conversion Structure for Grid-Connected PV Applications

A novel scheme for three-phase grid-connected photovoltaic (PV) generation systems is presented in this paper. The scheme is based on two insulated strings of PV panels, each one feeding the dc bus of a standard two-level three-phase voltage-source inverter (VSI). The inverters are connected to the grid by a three-phase transformer having open-end windings on the inverter side. The resulting conversion structure performs as a multilevel power active filter (equivalent to a three-level inverter), doubling the power capability of a single VSI with given voltage and current ratings. The multilevel voltage waveforms are generated by an improved space-vector-modulation algorithm, suitable for the implementation in industrial digital signal processors. An original control method has been introduced to regulate the dc-link voltages of each VSI, according to the voltage reference given by a single maximum power point tracking controller. The proposed regulation system has been verified by numerical simulations and experimental tests with reference to different operating conditions.     

无电压传感器的三电平PWM整流器定频直接功率控制

给出了三电平PWM整流器的数学模型,在传统的直接功率控制和电压定向控制的基础上,结合虚拟磁链控制的优点,提出了一种新的三电平PWM整流器定频直接功率控制方法。该控制方法省略了电网电压传感器,实现了有功功率和无功功率的动态解耦,调制环节采用空间电压矢量调制,开关频率固定。仿真结果表明,该控制方法实现了单位功率因数运行,网侧电流谐波小,具有良好的动静态性能,保证了中点电位的平衡。     

Control of Three-Level PWM Converter Applied to Variable-Speed-Type Turbines

This paper proposes advanced control of a three-phase three-level neutral-point-clamped pulsewidth-modulated (PWM) converter connecting a permanent-magnet synchronous generator to a grid. The control scheme is mainly based on active and reactive power loops and contains the following additional blocks: virtual flux and filter-capacitor voltage estimators for sensorless operation, active damping (AD) of possible resonances in the LCL filter that connects the converter to the grid, and a PWM modulator with dc-link voltage balancing and minimization of switching losses. It is shown that the proposed control method exhibits several features such as sensorless operation, robust algorithm, minimization of switching losses, and simple tuning procedure of AD. The simulation and experimental results have proven an excellent performance and verified the validity of the proposed system.     

Three Phase Three-Level PWM Switched Voltage Source Inverter With Zero Neutral Point Potential

A new three phase three-level pulsewidth modulation (PWM) switched voltage source inverter with zero neutral point potential is proposed. It consists of three single-phase inverter modules and each module is composed of a switched voltage source and inverter switches. The major advantage is that the peak value of the phase output voltage is twice as high as that of the conventional neutral-point-clamped PWM inverter. Thus, the proposed inverter is suitable for applications with low voltage sources such as batteries, fuel cells, or solar cells. Furthermore, three-level waveforms of the proposed inverter can be achieved without the switch voltage unbalance problem. Since the average neutral point potential of the proposed inverter is zero, a common ground between the input stage and the output stage is possible. Therefore, it can be applied to a transformerless power conditioning system. The proposed inverter is verified by a PSpice simulation and experimental results based on a laboratory prototype.     

A Novel Four-Level Voltage Source Inverter—Influence of Switching Strategies on the Distribution of Power Losses

In this paper, a novel four-level inverter will be presented and analyzed. The proposed inverter topology, which is composed of a conventional two-level and a three-level neutral-point clamped (NPC) inverter, is suitable for high-voltage and high-power applications. The proposed inverter, when it is compared with the conventional four-level NPC pulsewidth modulation inverter, exhibits the following advantages: a) ability of changing the power losses distribution profile among the devices by selecting a suitable switching strategy; b) reduction of total inverter power semiconductor device losses; c) ability of bidirectional operation for all power semiconductor switches; and d) easy implementation using existing power semiconductor modules. The effect of conduction and switching losses profiles of the proposed inverter for different switching strategies is examined under different loads, power factors, and modulation indices. The dc-link capacitors voltages are effectively balanced via a proposed self-voltage balancing topology, without the need of isolated dc voltage sources or additional voltage stabilizing circuits. Finally, the theoretical results are confirmed by simulation and experimental results     

Capacitor Voltage Balance for the Neutral-Point- Clamped Converter using the Virtual Space Vector Concept With Optimized Spectral Performance

This paper presents a new pulsewidth modulation (PWM) for the complete control of the neutral-point voltage in the three-level three-phase neutral-point-clamped (NPC) direct current-alternating current (dc-ac) converter. The balancing of the neutral-point voltage is achieved over the full range of converter output voltages and for all load power factors with the minimum output voltage distortion at around the switching frequency. The simple phase duty-ratio expressions in d-q-0 coordinates that define this modulation are presented. The performance of this modulation approach and its benefits over other previously proposed solutions are verified through simulation and experiments.     

5kW光伏并网发电系统及其仿真研究

随着新能源发电的迅速发展,越来越多的可再生能源被转化为电能并通过并网逆变器输送到电网。利用MATLAB仿真工具箱建立了由光伏阵列输出、Boost升压电路、逆变器、控制器、电网等组成的5 kW光伏并网发电系统的仿真模型,研究了光伏并网系统的特性。采用变结构模糊PID控制器实现5 kW光伏发电系统的MPPT;采样电网电压作为逆变器电流的参考信号,利用滞环比较法控制逆变器,实现系统输出电流与电网电压同频同相,功率因素近似为1。仿真结果表明,系统较好地实现了光伏发电系统的MPPT及安全并网,对实际光伏并网系统的设计有参考意义。     

有源前端型三电平逆变器中点电位平衡的一种控制策略

由三电平有源前端(AFE)和三电平逆变器构成的中高压变频器在能量再生,功率因数,输入和输出谐波方面具有良好的性能.在这种变频器中,为了实现直流侧中点电位平衡控制(NPB),以往的方法都要对逆变控制采用的SVPWM算法进行调整,这不可避免地会增加逆变输出谐波.本文提出一种无需调整SVPWM算法而只在变频器前端解决NPB问题的策略,避免了逆变输出谐波的增加,使负载电机旋转磁场具有较为理想的圆度.文中介绍了这种NPB的基本原理,对在前端控制与在逆变侧控制的输入和输出性能进行了比较分析.仿真实验研究结果证明了该控制策略的正确性和有效性.     

逆变器载波PWM方法的研究

多电平逆变器具有输出容量大、输出电压高、电流谐波含量小等优点,广泛应用于高速动车组、中高压变频调速、电力系统有源滤波等方面。文章对三电平二极管中点箝位式逆变器的中点电位平衡问题进行了分析,讨论了几种常用的载波PWM方法,并在此基础上提出了一种新的将相位偏移、注入零序分量相结合的PWM法。仿真结果证明新方法有良好的中点电位控制能力及输出电压谐波特性。