Showing papers on "Pulse-width modulation published in 1999"

Stationary frame current regulation of PWM inverters with zero steady state error

Abstract: Current regulators for AC inverters are commonly categorised as hysteresis, linear PI or deadbeat predictive, with a further subclassification into stationary ABC frame and synchronous DQ frame implementations. Synchronous frame controllers are generally accepted to have a better performance than stationary frame controllers do, as they operate on DC quantities and hence can eliminate steady state errors. This paper establishes a theoretical connection between these two classes of regulators and proposes a new type of stationary frame controller, which achieves the same steady state performance as a synchronous frame controller. The new controller is applicable to both single phase and three phase inverters.

Dual current control scheme for PWM converter under unbalanced input voltage conditions

Abstract: Voltage unbalance in a three-phase system causes performance deterioration of PWM power converters by producing 120 Hz voltage ripples in the DC link and by increasing the reactive power. To eliminate the DC link voltage ripple and the DC component of the reactive power, both positive- and negative-sequence currents should be controlled simultaneously, according to the paper by Rioual et al (1996). The authors used two synchronous reference frames: a positive-sequence current regulated by a proportional integral (PI) controller in a positive synchronous reference frame (SRF); and a negative-sequence current regulated by a PI controller in a negative SRF. In the positive SRF, which rotates counterclockwise, the positive sequence appears as DC, while the negative sequence appears as 120 Hz. In contrast, in the negative SRF, which rotates clockwise, the negative sequence appears as DC, while the positive sequence appears as 120 Hz. By deleting 120 Hz components using a notch filter in each SRF, one can measure positive- and negative-sequence currents separately, and use them for constructing two feedback controllers. Since the negative-sequence current is also controlled in its own SRF by a DC command, this approach yields better performance without increasing the control gain. Note that, since the controller is implemented by a software routine in the digital signal professor chip, using two SRFs does not require additional hardware. The authors demonstrated the effectiveness of the proposed control scheme by using computer simulation and experiments.

A fast space vector modulation algorithm for multilevel three-phase converters

Abstract: This paper introduces a general space vector modulation algorithm for n-level three-phase power converters. This algorithm is computationally extremely efficient and is independent of the number of levels of the power converter. At the same time, it provides excellent insight into the operation of multilevel power converters.

Simple analytical and graphical methods for carrier-based PWM-VSI drives

Abstract: This paper provides analytical and graphical methods for the study, performance evaluation and design of the modern carrier-based pulse width modulators (PWMs), which are widely employed in PWM voltage-source inverter (VSI) drives. Simple techniques for generating the modulation waves of the high-performance PWM methods are described. The two most important modulator characteristics-the current ripple and the switching losses-are analytically modeled. The graphical illustration of these often complex multivariable functions accelerate the learning process and help one understand the microscopic (per-carrier cycle) and macroscopic (per fundamental cycle) behavior of all the modern PWM methods. The analytical formulae and graphics are valuable educational tools. They also aid the design and implementation of the high-performance PWM methods.

On-line dead-time compensation technique for open-loop PWM-VSI drives

Abstract: A new on-line dead-time compensation technique for low-cost open-loop pulsewidth modulation voltage-source inverter (PWM-VSI) drives is presented. Because of the growing numbers of open-loop drives operating in the low-speed region, the synthesis of accurate output voltages has become an important issue where low-cost implementation plays an important role. The so-called average dead-time compensation techniques rely on two basic parameters to compensate for this effect: the magnitude of the volt seconds lost during each PWM cycle and the direction of the current. In a low-cost implementation, it is impractical to attempt an on-line measurement of the volt-seconds error introduced in each cycle-instead an off-line measurement is favored. On the other hand, the detection of the current direction must be done on line. This becomes increasingly difficult at lower frequencies and around the zero crossings, leading to erroneous compensation and voltage distortion. This paper presents a simple and cost-effective solution to this problem by using an instantaneous back calculation of the phase angle of the current. Given the closed-loop characteristic of the back calculation, the zero crossing of the current is accurately obtained, thus allowing for a better dead-time compensation. Experimental results validating the proposed method are presented.

Opportunities for harmonic cancellation with carrier-based PWM for a two-level and multilevel cascaded inverters

Abstract: Pulsewidth modulation (PWM) strategies have been the subject of many years of research effort, and the merits of the various alternatives investigated have been argued extensively. In general, it is now accepted that natural or asymmetrical regular sampled PWM with a third harmonic injected or a space-vector centered reference waveform gives the best harmonic performance, with discontinuous modulation having some advantages for higher modulation ratios. This paper uses a general analytical solution for carrier-based PWM to mathematically identify the harmonic cancellation that occurs in various PWM implementations and converter topologies. This solution provides a formal justification for the superiority of natural and asymmetrical regular sampling techniques in eliminating half of their sideband harmonics simply by virtue of their phase leg switching. Then, the use of fundamental reference phase shifting between phase legs to create single- and three-phase 1-1 output voltages is reviewed, and the harmonic cancellation that occurs is identified. One significant result from this analysis is the realization that an odd/triplen carrier/fundamental ratio has no intrinsic harmonic benefit, contrary to the generally accepted wisdom. Finally, opportunities for harmonic elimination in multilevel cascaded inverter systems by carrier phase shifting are investigated, and the optimum phase shift is identified.

Adaptive SVM to compensate DC-link voltage ripple for four-switch three-phase voltage-source inverters

Abstract: An adaptive space vector modulation (SVM) approach to compensate the DC-link voltage ripple in a B4 inverter is proposed and examined in detail. The theory, design, and performance of this pulsewidth modulation (PWM) method are presented, and the method effectiveness is demonstrated by extensive simulations and experiments. High-quality output currents are guaranteed by this approach even with substantial DC-voltage variations that might be caused by an unbalanced AC supply system, the diode rectification of the line voltages, and circulation of one output phase current through the split capacitor bank. The application of this approach to induction machine drives is also discussed. It is concluded that the DC-voltage ripple effect on the B4 inverter output can be minimized by an adaptive SVM algorithm with the advantage of improving the response of the DC-link filter and the output quality of the inverter becoming high.

A comprehensive study of neutral-point voltage balancing problem in three-level neutral-point-clamped voltage source PWM inverters

Abstract: This paper explores the fundamental limitations of neutral-point voltage balancing problem for different loading conditions of three level voltage source inverters. A new model in DQ coordinate frame utilizing current switching functions is developed, as a means to investigate theoretical limitations and lend more intuitive insight into the problem. The low frequency ripple of the neutral point caused by certain loading conditions is observed and quantified.

Interleaved PWM with discontinuous space-vector modulation

Abstract: This paper describes the effect of interleaved discontinuous space-vector modulation (SVM) in paralleled three-phase systems using three-phase pulsewidth modulation (PWM) rectifiers as an example. At the discontinuous point of the SVM, the phase shift between the switching signals of the paralleled modules generates a zero-sequence excitation to the system. Because the conventional control in a balanced three-phase system with only dq channels cannot reject this disturbance, a beat-frequency circulating current will develop on the zero axis. Based on this observation, a SVM without using zero vectors is used to eliminate the cause of pure zero-sequence current for parallel operation. Using this SVM, the circulating current is observable in dq channels. It can be suppressed dynamically by strong current loops of power-factor-correction (PFC) circuits. The concept is verified experimentally on a breadboard system.

PWM-VSI inverter assisted stand-alone dual stator winding induction generator

Abstract: This paper presents a novel usage of a dual stator winding, three-phase induction machine as a stand-alone generator with both controlled output load voltage magnitude and frequency. This generator, with both three-phase power and control windings housed in the stator structure, has the load connected to the power winding and a three-phase PWM voltage source inverter sourcing the control winding. The input to the PWM inverter is either a battery source or a charged DC capacitor. The operational characteristics of these generator schemes with either of the two inverter sources is investigated and shown to have desirable performance. How the load voltage magnitude depends on the various control and design parameters such as rotor speed, compensating capacitance and load impedance are determined using a detailed mathematical model of the system which is confirmed to accurately predict dynamic and steady-state performance characteristics.

Hybrid topology for multilevel power conversion

Abstract: The present invention provides a multilevel electric power converter including a plurality of DC voltage sources providing different DC source voltage levels. The DC source voltage levels are preferably multiples of each other and may vary in a binary fashion or in a geometric progression with a factor of three to provide a large number of output voltage levels for a given number of inverter levels. The multilevel inverter is preferably implemented as a series connected set of H-bridge inverters, with each H-bridge inverter having an independent DC voltage source providing the desired DC source voltage level. A hybrid modulation strategy may be employed whereby the lowest voltage level inverter is modulated at a high frequency, e.g., by pulse width modulation, and higher voltage level inverters in the multilevel inverter are modulated to provide a low frequency stepped waveform. The combined high frequency pulse width modulated and low frequency stepped waveform has good spectral quality. A high voltage high quality waveform may be generated in this manner by taking advantage of the high voltage blocking capability of switching devices, such as GTO thyristors, in the high voltage inverters in the multilevel inverter, and the high frequency switching characteristics of switching devices, such as IGBTs, in the lowest voltage level inverter in the multilevel inverter. A multilevel inverter in accordance with the present invention may be employed in a single-phase or multi-phase applications.

Motor shaft voltages and bearing currents and their reduction in multi-level medium voltage PWM voltage source inverter drive applications

Abstract: This paper presents test results on problems and solutions of motor shaft voltages and bearing currents in medium-voltage pulsewidth modulated (PWM) drive systems. Tests show that multilevel medium-voltage PWM voltage-source inverter drives can cause motor bearing currents, similar to a low-voltage PWM drives, even with one motor bearing insulated. Common-mode voltages generated as a result of PWM switching are observed on motor windings and capacitively coupled to the motor shaft, leading to bearing currents. Potential solutions, including altering common-mode circuitry, changing the grounding scheme, providing common-mode filtering, and grounding the motor shaft, are investigated. Test results on their effectiveness are presented.

Multilevel PWM methods at low modulation indices

Abstract: When utilized at low amplitude modulation indices, existing multilevel carrier-based PWM strategies have no special provisions for this operating region, and several levels of the inverter go unused. This paper proposes some novel multilevel PWM strategies to take advantage of the multiple levels in both a diode-clamped inverter and a cascaded H-bridge inverter by utilizing all of the levels in the inverter even at low modulation indices. Simulation results show what effects the different strategies have on the active device utilization. A prototype 6-level diode-clamped inverter and an 11-level cascaded 11-bridge inverter have been built and controlled with the novel PWM strategies proposed in this paper.

A new phase detecting method for power conversion systems considering distorted conditions in power system

Abstract: In this paper, a new phase detecting method is proposed for PWM rectifiers and active filters considering distorted utility conditions. The accurate phase angle information of utility voltage is essential to the active power conversion systems tied to the utility line. The angle information can be easily corrupted by the distorted utility voltage such as voltage unbalance and harmonics in the waveform. Thus, large harmonics may be caused in the current or voltage reference signal whose phase angle is based on the detected voltage phase angle. First, focusing on the distorted utility condition, effects of the several phase angle detecting methods on the operation of PWM rectifiers and active filters are investigated. Then a new voltage phase angle detector is proposed using voltage sequence detector and PI controller. The proposed method does not cause harmonics in the reference signal by adequately eliminating unbalance components and high-order harmonic terms in the utility voltage. The basic principle of the proposed method is described in detail. Experimental results prove the feasibility of the proposed strategy compared with the conventional method.

A new soft-switched quasi-single-stage (QSS) bi-directional inverter/charger

Abstract: A new soft-switched quasi single-stage (QSS) bidirectional inverter/charger topology is proposed in this paper. It realizes seamless four-quadrant operation in inverter mode, and rectifier operation with unity power factor in charger mode. Single stage power conversion, standard half-bridge connection of devices, soft-switching for all the power devices, low conduction loss, simple center-aligned PWM control, and high efficiency are among the salient features. The principles of circuit operation, PWM control and synthesis, and topological extension are discussed in this paper. The experimental results on a 3 kVA prototype (12 V DC to and from 110 V AC) are also presented.

Adaptive repetitive control of PWM inverters for very low THD AC-voltage regulation with unknown loads

Abstract: An adaptive repetitive control scheme is proposed and applied to the control of a pulsewidth-modulated (PWM) inverter used in a high-performance AC power supply. The proposed control scheme can adaptively eliminate periodic distortions caused by unknown periodic load disturbances in an AC power supply. The proposed adaptive repetitive controller consists of a voltage regulator using state feedback control, a repetitive controller with tuning parameters and an adaptive controller with a recursive least-squares estimator (LSE). This adaptive repetitive controller designed for AC voltage regulation has been realized using a single-chip digital signal processor (DSP) TMS320C14 from Texas Instruments. Experimental verification has been carried out on a 2 kVA PWM inverter. Simulation and experimental results show that the DSP-based adaptive repetitive controller can achieve both good dynamic response and low total harmonic distortion (THD) under large-load disturbances and uncertainties.

DC ripple current reduction on a single-phase PWM voltage source rectifier

Abstract: A novel topology of single-phase pulsewidth modulation (PWM) voltage-source rectifier capable of achieving not only a sinusoidal input current, but also a zero-ripple output current, is presented. The rectifier consists of a conventional single-phase PWM voltage-source rectifier, a pair of additional switches and an inductor. Hence, the proposed rectifier requires neither a large DC capacitor nor a passive L-C resonant circuit. The input current control is achieved by the conventional PWM current control technique. However, DC ripple current reduction control is difficult because one of the switching legs in the DC ripple current reduction circuit is shared with the PWM rectifier circuit. Two control methods, referred to here as the DC C inductor method and the AC inductor method, are proposed for DC ripple reduction, and the characteristics of these control methods are discussed. These control methods are implemented using a microprocessor, and the effectiveness of the circuit is confirmed experimentally. This rectifier has useful applications in uninterruptible power systems and DC power supplies, especially for cases in which the batteries are connected in parallel to the DC line.

Power line conditioner using cascade multilevel inverters for voltage regulation, reactive power correction, and harmonic filtering

Abstract: A power line conditioner using cascade multilevel inverter used for voltage regulation, reactive power (var) compensation and harmonic filtering, including the control schemes for operating the cascade inverter for voltage regulation and harmonic filtering in distribution systems. The cascade M-level inverter consists of (M-1)/2 H-bridges in which each bridge has its own separate DC source. This new inverter (1) can generate almost sinusoidal waveform voltage with only one time switching per line cycle, (2) can eliminate transformers of multipulse inverters used in the conventional static VAR compensators, and (3) makes possible direct connection to the 13.8 kV power distribution system in parallel and series without any transformer. In other words, the power line conditioner is much more efficient and more suitable to VAR compensation and harmonic filtering of distribution systems than traditional multipulse and pulse width modulation (PWM) inverters. It has been shown that the new inverter is specially suited for simultaneous VAR compensation and harmonic filtering.

Opportunities for Harmonic Cancellation with Carrier Based PWM for Two-Level and Multi-Level Cascaded Inverters

Abstract: PWM strategies have been the subject of many years of research effort, and the merits of the various alternatives investigated have been argued extensively. In general, it is now accepted that natural or asymmetrical regular sampled PWM with a third harmonic injected or a space vector centered reference waveform gives the best harmonic performance, with discontinuous modulation having some advantages for higher modulation ratios. This paper uses a general analytical solution for carrier based PWM to mathematically identify the harmonic cancellation that occurs in various PWM implementations and converter topologies. This solution provides a formal justification for the superiority of natural and asymmetrical regular sampling techniques in eliminating half of their sideband harmonics simply by virtue of their phase leg switching. Then the use of fundamental reference phase shifting between phase legs to create single and three phase I-I output voltages is reviewed, and the harmonic cancellation that occurs is identified. One significant result from this analysis is the realisation that an odd/triplen carrier/fundamental ratio has no intrinsic harmonic benefit, contrary to the generally accepted wisdom. Finally, opportunities for harmonic elimination in multi-level cascaded inverter systems by carrier phase shifting are investigated, and the optimum phase shift is identified.

Self-excited induction generator controlled by a VS-PWM bidirectional converter for rural applications

Abstract: This paper concerns an application of a three-phase cage induction machine (IM) as a self-excited generator connected to the AC side of a voltage-source PWM bidirectional inverter. The generator is supposed to be driven by a low-head unregulated shaft hydraulic turbine. The proposed system is intended to be applied in rural plants as a low-cost source of high-quality AC sinusoidal regulated voltage with constant frequency. Simulation results are obtained based on the /spl alpha//spl beta//spl gamma/ stationary reference frame model of the IM. The experimental results demonstrated that the system presents satisfactory behavior when feeding AC loads and during the startup of induction motors.

A line-voltage-sensorless synchronous rectifier

Abstract: A line-voltage-sensorless control for three-phase pulsewidth-modulated (PWM) synchronous rectifiers is presented. A line synchronization and unity power factor control are described. Indirect synchronization without sensing the line voltage allows a standard vector-controlled inverter to be used as a synchronous rectifier without requiring any additional hardware. Furthermore, the line synchronization can be properly operated under line voltage distortion or notching and line frequency variation. All control functions are implemented with a single-chip microcontroller. It is shown via experimental results that the proposed controller gives good performance for the synchronous rectifier.

A neural network based space vector PWM controller for voltage-fed inverter induction motor drive

Abstract: A neural network based implementation of space vector modulation of a voltage-fed inverter has been proposed in this paper that fully covers the undermodulation and overmodulation regions linearly extending operation smoothly up to square wave. The neural network has the advantage of very fast implementation of SVM algorithm that can increase the converter switching frequency, particularly when a dedicated ASIC chip is used in the modulator. Two ANN-based SVM techniques have been validated: an indirect method with the help of a timer that generates the PWM waveforms from the command voltage vector at the input, and a direct method that synthesizes waveforms directly without any timer. The indirect method has been fully implemented and extensively evaluated in a volts/Hz controlled 5 hp, 60 Hz, 230 V induction motor drive. The performances of the drive with ANN-based SVM are excellent. The scheme can be easily extended to vector-controlled drive. The direct method, although has a simpler topology, needs very large training data and training time.

On line thermal model and thermal management strategy of a three phase voltage source inverter

Abstract: A third order dynamical thermal model suitable for modeling diodes and IGBTs is proposed. The method for deriving model parameters from manufacturers data sheets is derived and verified. Models of the power circuit devices, IGBTs and diodes, together with a heat sink model are combined into a dynamical thermal model of 3-phase inverter bridge. The model, together with thermal management strategy is implemented in a microprocessor. The instantaneous values of junction temperatures of IGBTs and free wheeling diodes (FWDs) are estimated on-line with a one step ahead prediction algorithm. If the temperature on any of the power devices reaches a critical value, the PWM frequency is reduced or the PWM method changed in order to reduce losses. If switching losses are still too high and minimum allowable switching frequency is reached; the thermal model starts to reduce inverter current in order to keep the junction temperature of the hottest device of the power circuit below a critical level. The dynamical model with thermal management strategy becomes most useful at low frequency, i.e. stall speed like operation of the drive with high instantaneous loading on particular devices in the power circuit.

Sliding-mode control of boost-type unity-power-factor PWM rectifiers

Abstract: A robust sliding-mode controller, suitable for the output voltage control of voltage-sourced unity-power-factor three-phase pulsewidth modulation (PWM) rectifiers, presenting no steady-state errors, is described. This "just-in-time" switching controller controls the output voltage and the line input currents, while providing bidirectional power flow, near-unity-power-factor operation, low harmonic content, fast dynamic response of the output voltage, and minimum switching frequency due to a new /spl alpha/-/spl beta/ space-vector current regulator. The voltage controller performance is compared with the behavior of the conventional proportional integral output voltage control, aided by PWM current-mode modulators, and with the nonrobust fast and slow manifold sliding-mode approach. The comparison shows that the proposed controller confers faster dynamics and does not present steady-state errors. Test results confirm that the performance of the controller is independent of system parameters and load and exceeds the performance of existing hysteretic current-mode control systems.

VIENNA rectifier II-a novel single-stage high-frequency isolated three-phase PWM rectifier system

Abstract: Based on an analysis of basic realization possibilities, the structure of the power circuit of a new single-stage three-phase boost-type pulsewidth modulated (PWM) rectifier system (VIENNA Rectifier II) is developed. This system has continuous sinusoidal time behavior of the input currents and high-frequency isolation of the output voltage, which is controlled in a highly dynamic manner. As compared to a conventional two-stage realization, this system has substantially lower complexity and allows the realization of several isolated output circuits with minimum effort. The basic function of the new PWM rectifier system is described based on the conduction states occurring within a pulse period. Furthermore, a straightforward space- vector-oriented method for the system control is proposed which guarantees a symmetric magnetization of the transformer. Also, it makes possible a sinusoidal control of the mains phase currents in phase with the associated phase voltages. By digital simulation, the theoretical considerations are verified and the stresses on the power semiconductors of the new converter system are determined. Finally, results of an experimental analysis of a 2.5-kW laboratory prototype of the system are given, and the direct startup and the short-circuit protection of the converter are discussed. Also, the advantages and disadvantages of the new converter system are compiled in the form of an overview.

Improved digital current control methods in switched reluctance motor drives

Abstract: This paper proposes a method to avoid current feedback filters in fast digital-based current loops in switched reluctance drives. Symmetrical pulsewidth modulation (PWM) and synchronized sampling of the phase current allow a noise-free current sampling with no antialiasing filter. This paper also proposes more efficient methods to chop the two transistors in the asymmetric inverter used with switched reluctance drives. A fast field-programmable gate array (FPGA)-based test system is used for validation of the new methods. Test results show a significant improvement in dynamic and steady-state current loop control compared with traditional methods. The new chopping method is found to reduce the switching losses and increase the drive efficiency.

Minimum-loss strategy for three-phase PWM rectifier

Abstract: In this paper, the conduction and switching losses of a voltage-fed three-phase pulsewidth modulation (PWM) rectifier are analyzed for various PWM schemes. On the basis of this result, a novel PWM strategy which minimizes the loss of a three-phase PWM rectifier is developed. This minimization result is derived from the following two factors: (1) less switching frequency ratio; and (2) the absence of switching in the vicinity of peak input current. As a result, it is anticipated that the switching loss of the rectifier is reduced by 46%, compared with continuous space-vector PWM rectifiers, and 20% compared with conventional discontinuous space-vector PWM rectifiers. Moreover, the proposed PWM scheme can produce the highest available output voltage because it is based on the concept of the voltage space vector. The effectiveness of the proposed PWM strategy is verified by experiments.

Compressor pulse width modulation

Abstract: A scroll compressor includes a capacity modulation system. The capacity modulation system has a piston that is connected to the non-orbiting scroll that disengages the non-orbiting scroll from the orbiting scroll when a pressure chamber is placed in communication with the suction chamber of the compressor. The non-orbiting scroll member moves into engagement with the orbiting scroll when the chamber is placed in communication with the discharge chamber. The engagement between the two scrolls is broken when the pressure chamber is placed in communication with fluid from the suction chamber. A solenoid valve controls the communication between the pressure chamber and the suction chamber. By operating the valve in a pulsed width modulated mode, the capacity of the compressor can be infinitely varied between zero and one hundred percent.

Reference frames fit for controlling PWM rectifiers

Abstract: The authors argue that one should keep things simple when controlling bidirectional pulsewidth modulation rectifiers by considering the utility grid as a virtual electric machine. The advantage is that the air-gap flux of this big machine can be directly measured in a straightforward way. Therefore, as shown in this paper, principles of field orientation can be applied to control the power flow, yielding high-dynamic performance.

Hopf bifurcation and chaos from torus breakdown in a PWM voltage-controlled DC-DC boost converter

Abstract: The behavior of a closed loop dc-dc boost converter is investigated when the pulse width modulation (PWM) period is varied. The dynamics are analyzed both by using analytical solutions of the state equations and by the stroboscopic map. This analysis shows that Hopf bifurcation occurs at a certain value of the parameters. Phase-locking periodic windows, the period adding sequence, and the transition from quasi-periodicity to period doubling via torus breakdown are also obtained. An experimental prototype was built to check the numerical results. Parasitic elements, such as the equivalent series resistance of the inductor and the conducting voltage of the diode, are included in the model to obtain better concordance with experiments.