Extension of operation of space vector PWM strategies with low switching frequencies using different overmodulation algorithms

This paper investigates the operation of four space vector-based synchronized pulse-width modulation (PWM) strategies in the overmodulation zone using three different overmodulation algorithms. It is shown that the symmetries in the PWM waveforms generated can be preserved in the overmodulation zone also. With any given overmodulation algorithm, the voltage control characteristics (i.e., fundamental voltage versus control variable) are found to vary with PWM strategy, pulse number and type of clamping. The inverse of the appropriate voltage control curve is used during premodulation to maintain the modulator gain constant. The differences in the nature of the voltage control characteristics with the different overmodulation algorithms are brought out. These characteristics are compared and contrasted against those at high switching frequencies. The harmonic distortion in the different cases is evaluated and compared. It is shown that the bus clamping strategies perform better than the conventional strategy with any given overmodulation algorithm employed. These strategies, which exploit the flexibilities in the space vector approach, are useful in high power drives on account of their superior waveform quality at low switching frequencies and high DC bus utilization.     

Incorporating the overmodulation range in space vector patterngenerators using a classification algorithm

Operating voltage-source pulsewidth-modulated (PWM) inverters in the overmodulation region extends their voltage and power ranges. Proper operation in this region is of particular importance in AC motor drives. This paper presents a classification algorithm for the implementation of the space vector modulation (SVM) in the pulse-dropping region. It is demonstrated that the proposed algorithm allows smooth transitions through the entire operating range up to square wave operation. It does not require approximations and guarantees exact positioning of the switching instants. In addition, it is shown that the harmonic distortion can be kept lower than with conventional techniques in most of the overmodulation region. The technique requires less computational time, when compared with the conventional SVM methods. The results of the theoretical and mathematical analysis are verified on a 2-kVA prototype unit using a digital signal processor (DSP)-based controller     

Torque ripple analysis and dynamic performance of a space vector modulation based control method for AC-drives

A constant switching frequency torque control method is presented in this paper, that uses flux error vector based space vector modulation (SVM) to achieve steady state and dynamic control of torque. The effect of SVM switching on torque ripple has been analyzed using "flux ripple vectors". This approach is used to develop an insight of torque ripple and to estimate it for any operating angular velocity. We propose a method of compensation to maintain steady state control of torque in the overmodulation region of operation. During torque dynamic, the optimum maximum switching state vector is selected. This gives a response similar to that obtained using direct torque control (DTC) and direct self control (DSC) methods. The proposed method is verified experimentally.     

Carrier-Based Modulation Technique for Matrix Converter

This paper presents a carrier-based modulation method for a matrix converter. By using the offset voltage and changing the slope of carrier, it is possible to synthesize the sinusoidal input currents with the unity power factor and desired output voltages. The proposed method is equivalent to the so called space vector pulsewidth modulation method. The proposed method uses a new point of view to understand the matrix converter modulation method such as the voltage source inverter (VSI) modulation method. Using the proposed method, this paper presents the two-phase/three-phase modulation method and dynamic/steady-state overmodulation method for the matrix converter. These methods are well developed in the study of a VSI. By the proposed steady-state overmodulation method, it is possible to synthesize the fundamental component of output voltage to be equal to that of input voltage at the cost of some distortion of input current. The feasibility of the proposed modulation method has been verified by a computer simulation and experimental results. These results show that the proposed carrier-based modulation method can be implemented easily without any tables. It can be used for the application where a higher voltage transfer ratio is essential     

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.     

A General PWM Strategy for Four-Switch Three-Phase Inverters

A general pulsewidth modulation (PWM) method for control of four-switch three-phase inverters is presented. The proposed vector PWM offers a simple method to select three or four vectors that effectively synthesize the desired output voltage, even in presence of voltage oscillations across the two dc-link capacitors. The method utilizes the so called space vector modulation, and includes its scalar version. Different vector combinations are compared. The effect of Wye and delta motor winding connections over the pulse width modulator is also considered. The common mode voltage generated by the four-switch three-phase converter is evaluated and compared to that provided by the standard six-switch three-phase inverter. Simulation and experimental results are presented to demonstrate the feasibility of the proposed approach     

Space vector pulse width modulation of three-level inverter extending operation into overmodulation region

Multilevel voltage-fed inverters with space vector pulse width modulation have established their importance in high power high performance industrial drive applications. The paper proposes an overmodulation strategy of space vector PWM of a three-level inverter with linear transfer characteristic that easily extends from the undermodulation strategy previously developed by the authors for neural network implementation. The overmodulation strategy is very complex because of large number of inverter switching states, and hybrid in nature, that incorporates both undermodulation and overmodulation algorithms. The paper describes systematically the algorithm development, system analysis, DSP based implementation, and extensive evaluation study to validate the modulator performance. The modulator takes the command voltage and angle information at the input and generates symmetrical PWM waves for the three phases of an IGBT inverter that operates at 1.0 kHz switching frequency. The switching states are distributed such that the neutral point voltage always remains balanced. An open loop volts/Hz controlled induction motor drive has been evaluated extensively by smoothly varying the voltage and frequency in the whole speed range that covers both undermodulation and overmodulation (nearest to square-wave) regions, and performance was found to be excellent. The PWM algorithm can be easily extended to vector-controlled drive. The algorithm development is again fully compatible for implementation by a neural network.     

Performance evaluation of a direct torque controlled drive in thecontinuous PWM-square wave transition region

This paper investigates the operation of a direct torque controlled drive when operating under transient conditions and when operating in overmodulation conditions or in the “transition region” to six-step operation. The direct torque control is a dead-beat control of the torque and flux magnitude. In the steady-state, the stator voltage vector which drives the torque and flux to the reference value is calculated during each fixed switching period. Under transient or overmodulation conditions, an alternative switching algorithm must be used since dead-beat control is no longer possible. Two alternatives are presented for operation in overmodulation. The first involves a determination of the switching state a priori, and calculating the duty cycle for each phase based on the torque and flux error. A much simpler scheme is presented which utilizes the voltage reference vector from the direct torque control algorithm. This scheme, although not resulting in dead-beat control, is shown to provide very satisfactory performance in overmodulation. The direct torque control method shows great promise for light traction applications where a large quasi-constant power region is required. The scheme operates very satisfactorily in overmodulation, compared with existing current regulated PWM-based schemes, due to the fact that the voltage space vectors are directly controlled. A complete experimental evaluation of the proposed scheme operating in the transition region is also given     

An integrated 42-V drive design for automobile loads with a low-distortion overmodulation strategy

This paper (also presented at [27]) presents an integrated design approach to small drives for emerging 42-V automotive systems. The emphasis is on motors for ancillary loads. The final result offers a simple and low cost solution with high efficiency and high power density. Motor and inverter designs are combined to gain advantages in power factor and operating range. A near-minimum-distortion method is introduced for the overmodulation regime to improve power factor without sacrificing harmonic performance. The design is addressed in several steps: integration of the induction motor into a 42-V environment, selection of the inverter modulation scheme, and final system integration. The proposed system consists of an induction motor wound for the proper voltage range and driven by an inverter. The inverter is used partly in the overmodulation regime when bus voltage is low. A signal is formed based on triangle injection to minimize distortion. Some alternative modulation approaches are also analyzed and compared with the proposed solution. Experimental results confirm that a low-distortion overmodulation approach can be implemented successfully.     

Extended Voltage Swell Ride-Through Capability for PWM Voltage-Source Rectifiers

Voltage swells are one of the most harmful disturbances present in industrial power systems, being capable of severely damaging, breaking, or tripping converters. In the case of pulsewidth-modulation voltage-source rectifiers (PWM-VSRs), swells first saturate their control system and then force them into six-pulse operation if no precautions are taken. This paper presents an extended ride-through strategy enabling these converters to deal and cope with swells of up to 1.8 p.u. (IEEE Std. 1159 swell definition). The proposed strategy first fully exploits the dc-link voltage capacity by dynamically entering the overmodulation region, and secondly by drawing inductive current in case the former action does not suffice. Dynamic overmodulation makes possible the straight ride-through of 15% and 42% three-phase and single-phase swells, whereas the modulation index supervisor/control loop drawing inductive current enables the ride-through for greater magnitude disturbances, all the while avoiding saturation of the converter control system. This is realized using decision-making space vector modulation, and a control system built over a nonlinear control law directly obtained from the converter complex state variable model. In this way, linear and decoupled$d$–$q$axes dynamics are attained, ensuring a constant dynamic response throughout the whole operating range. Finally, experimental results from a TMS320C32 digital-signal-processor-based 5-kVA laboratory prototype subjected to typical industry single- and three-phase swells ranging from 5% to 50% are presented. These confirm the predicted performance and feasibility of the proposed voltage swell ride-through strategy for PWM-VSRs.     

Direct method of overmodulation with integrated closed loop stator flux vector control

Overmodulation achieves full utilization of installed dc link voltage, by modifying the magnitude and the phase of reference voltage vector. Due to the nonlinear behavior in this region, closed loop vector control becomes difficult. A simple method of overmodulation with closed loop control of stator flux vector is proposed in this paper. Instead of modifying the voltage reference by nonlinear equations or look up table, we propose a direct method to generate the switching times. In so doing, the average angular velocity of the stator flux vector is controlled linearly throughout the modulation range. The proposed method shows lower harmonic distortion compared with existing methods. It greatly reduces the complexity and cost of achieving overmodulation over existing schemes.     

基于电压空间矢量的三相PWM整流器控制研究

首先建立了三相电压型PWM整流器的数学模型，并对其矢量控制策略进行了分析，在此基础上提出了一种简单的电压空间矢量算法，该算法根据参考电压矢量在坐标系上的分量直接计算电压空间矢量在各个扇区内的作用时间，实验结果验证了所提出控制算法的可行性。     

Control of PWM voltage inverters in the pulse dropping region

The overmodulation of pulse width modulated (PWM) inverters causes a nonlinearity in the feedforward channel. The type of modulator, sine wave, space vector or third harmonic, establishes the characteristics of the transition region's nonlinearity. The characteristics for a number of modulation strategies are introduced. Test results from commercially available volts per hertz (V/F) drives reveal their inability to provide rated voltage even at rated input conditions. The adverse effects of the overmodulation region on current regulated AC inverters are demonstrated by experimental results. A compensated modulation technique (CMT), adaptable to continuous and discontinuous modulators, provides the exact inverse of the nonlinearity; thus it produces a smooth transition to six-step operation without inducing a voltage transient. Experimental results presented in the paper demonstrate the CMT's smooth transition to six-step and the improved performance of the CMT-PWM. Finally, a comparison of the CMT with the other known overmodulation strategy shows the CMT provides a simple technique with essentially identical harmonic characteristics     

Three-Dimensional Feedforward Space Vector Modulation Applied to Multilevel Diode-Clamped Converters

Simplified space vector modulation (SVM) techniques for multilevel converters are being developed to improve factors such as the computational cost, number of commutations, and voltage distortion. The feedforward SVM presented in this paper takes into account the actual DC capacitor voltage unbalance of the multilevel power converter. The resulting technique is a low-cost generalized feedforward 3-D SVM method and is particularized for three-phase multilevel diode-clamped converters. This new modulation technique can be applied to topologies where the gamma component may not be zero. The computational cost of the proposed method is similar to those of comparable methods, and it is independent of the number of levels of the power converter. Experimental results using a three-level diode-clamped converter are presented to validate the proposed modulation technique.     

三相PWM整流器电压空间矢量控制的实现

提出了一种便于数字实现的三相PWM整流器电压空间矢量的控制算法，该算法采用输入电压空间矢量定向，根据参考电压矢量直接计算空间电压矢量的位置和作用时间；利用DSP（数字处理器）实现了三相PWM整流器全数字化控制，并得出了最终实验结果。     

三相电压型PWM整流器空间矢量控制研究及仿真分析

以电压空间矢量控制的基本原理和概念为基础,结合Matlab／Simulink软件包构建了三相PWM整流器空间矢量控制系统的仿真模型,并详细给出各模型的具体参数。仿真结果显示,该方法简单,控制精度高,用于三相PWM整流器中具有良好的动、静态性能。     

基于分类算法的空间电压矢量PWM过调制策略研究

在电压源型PWM逆变器中使用过调制技术可以提高电压利用率。本文采用了一种基于分类算法的SVPWM过调制技术。该方法仅需要简单的线性计算,就可实现线性SVPWM和六阶梯波之间的线性平滑过渡,并且可有效补偿六边形外电压矢量损失,避免了传统SVPWM算法中大量的三角函数运算和求根运算,节省处理器运算时间。MATLAB仿真结果...     

Digital Simulation of PWM Induction Motor Drives for Transient and Steady-State Performance

In this paper, the influence of nonsinusoidal voltage excitation applied to a squirrel cage induction motor is investigated using direct three-phase machine simulation method. Computer packages are developed to generate various types of inverter modulation techniques, namely the sine, step, and current impressed schemes. These packages together with a three-dimensional plotting subroutine are used to investigate the characteristics of an inverter-fed induction motor system. The effects of switching frequency ratio, overmodulation, and start-on machine waveform characteristics are investigated. Harmonic distortion comparisons between sine and step modulation schemes are presented. A vector controller-driven current-impressed scheme are simulated. The effect of varying hysteresis band in a current-impressed scheme is discussed.     

Evaluation of modulation schemes for three-phase to three-phase matrix converters

This paper presents a method for evaluating different modulation schemes employed with three-phase to three-phase matrix converters. The evaluation method addresses three important modulator characteristics: the output waveform quality, the input waveform quality and the switching losses associated with the modulation schemes. The method is used to evaluate four different modulation strategies, all based on the direct space-vector modulation approach. Further, regarding the switching losses, the paper proposes a new space-vector approach by which the switching losses can be reduced by 15%-35%, depending on the output load angle. This new modulation approach is applicable whenever the output voltage reference is below half the input voltage and the output voltage quality is then superior to that of the conventional space vector modulation scheme. The functionality of the new modulation scheme is validated by both simulations and experimental results and compared to waveforms obtained by using exiting space vector modulation schemes. The output voltage of the proposed scheme turns out to be comparable to the best of the conventional schemes while the input current is more distorted.     

基波电压线性输出的两相SVPWM算法

本文提出了一种基于基波电压幅值线性输出控制的两相SVPWM过调制算法,该算法不需要存储数据.能够使逆变器平滑的从线性调制工作状态过渡到方波工作状态,非常适合数字化应用。文中分析了应用该算法以后,逆变器输出相电压的谐波畸变（THD）情况,以及主要谐波成分百分含量的变化情况,并和传统两相SVPWM控制算法进行了技术指标对比。分析及仿真证明,该算法具有输出基波电压增益线形和实现简单的优点,非常适合电机实际应用。