Showing papers on "Harmonics published in 1988"

Analysis and realization of a pulsewidth modulator based on voltage space vectors

Abstract: A space vector concept for deriving the switching times for pulsewidth-modulated voltage source inverters is compared with the conventional sinusoidal concept. The switching times are deducted from assumptions for minimum current distortion, the resulting mean voltage values are shown, and the differences between these and the established sinusoidal PWM (pulse-width modulator) are elaborated. Based on an analytical calculation the current distortions and torque ripples are evaluated and compared with the values obtained with the conventional method. The space vector representation results in lower current harmonics and possibly a higher modulation index. A modulator based on an 8086 microprocessor has been implemented, and its performance is reported. >

Multiple-harmonic conversion of 1064 nm radiation in rare gases

Abstract: The authors report the observation of very-high-order odd harmonics of Nd:YAG laser radiation in rare gases at an intensity of about 1013 W cm-2. Harmonic light as high as the 33rd harmonic in the XUV range (32.2 nm) is generated in argon. The key point is that the harmonic intensity falls slowly beyond the fifth harmonic as the order increases. Finally, a UV continuum, beginning at 350 nm and extending down towards the short wavelength region is apparent in xenon.

Programmed PWM techniques to eliminate harmonics - A critical evaluation

Abstract: Programmed pulsewidth modulators (PWMs) eliminating several lower-order harmonics generate high-quality output spectra, which in turn result in minimum current ripple and reduced torque pulsations, thereby satisfying several performance criteria and contributing to overall improved performance. Several programmed PWM switching patterns to eliminate harmonics in the output spectra of single-phase and three-phase inverters are possible. Each of these leads to a specific advantage in single- and three-phase inverters, depending on the application. First, the superiority of programmed PWM techniques over the conventional carrier-modulated PWMs is established. Next, a critical evaluation of the programmed PWM schemes on the basis of applications for single- and three-phase inverters is used to provide a framework and guidelines for the selection of the appropriate technique for each application area. Evaluation criteria include harmonic loss factor and total harmonic distortion factors defined at the input and output of the inverter terminals. Finally, a simple low-cost solution for obtaining the required PWM switching points is proposed. Selected results are verified experimentally. >

A PWM AC to DC converter with fixed switching frequency

Abstract: For a PWM (pulsewidth-modulated) AC-to-DC converter, a predicted current control strategy with fixed switching frequency (PCFF control) is proposed. Its principle and implementation are described, and experimental results are provided which show that the converter under PCFF control has bidirectional power transmission ability with high dynamic performance. The line currents are close to sinusoidal with unit power factor. The PCFF control produces a better switching pattern than hysteresis current control. This results in a reduction of harmonics in the line currents and lower stress on the switching devices. >

Modelling of inductance machines for electric drives

Abstract: The author presents several linear and nonlinear models suitable for transient and steady-state analyses of induction machine drives The models are presented in equivalent circuit form to preserve the identity of nonlinear parameters The circuits, designated as Gamma or inverse Gamma form, are simpler than the conventional T-form circuit Their parameters are readily determined from terminal measurements The major effects of magnetic nonlinearity are included in the models in a way that is more accurate than that usually obtained with the conventional T-form circuits Modeling of time harmonics is also discussed >

Influence of harmonics on power distribution system protection

Abstract: The effects of nonsinusoidal voltages and currents on the performance of static underfrequency and overcurrent relays are experimentally studied. The tests were conducted in such a way that the frequency, amplitude and phase shift of individual harmonics could be adjusted in a controlled manner by using a waveform generator with a phase-locking circuit. The relationship between the harmonic currents and voltages was modeled in terms of the power system impedance within residential distribution systems. It was found that for harmonic voltage and current amplitudes (as they occur in distribution systems), underfrequency relays and the time delay operation of overcurrent relays show a marked deterioration in performance. The instantaneous operating characteristics of overcurrent relays, however, are hardly affected by the presence of harmonic currents. >

A new approach to harmonic compensation in power systems

Abstract: A novel approach to compensate for harmonics in power systems is proposed. The approach differs from conventional passive and active filters in its compensation principle. A practical system configuration to implement the approach is presented. It consists of a small-VA-rating PWM (pulsewidth-modulated) converter and a passive filter. The compensation principle is described, and compensation characteristics are examined theoretically and experimentally. The practicability and validity of the approach are experimentally demonstrated. >

A single-phase controlled-current PWM rectifier

Abstract: The analysis and test results are given for an experimental single-phase controlled-current PWM (pulse-width-modulated) rectifier that operates at unity power factor with near sinusoidal current waveform and that has power reversal capability. The twice-line-frequency AC power is identified as a source of voltage harmonics in the DC link. The harmonics enter into the voltage regulation feedback loop to distort the AC current waveform. These undesirable harmonics can be removed by a low-pass filter. It is noted that the overall design must address the possibility of instability due to the low-pass filter in the feedback path. >

Harmonic current and reactive power compensation with an active filter

Abstract: A static VAr compensator (SVC) using an active filter has been developed that compensates reactive power, harmonic current, negative-phase current, and voltage fluctuations. The system configuration is described, and five types of control scheme for the filter, which are based on practical applications for various loads, and the performance characteristics for each type of control are analyzed. The active filter is shown by simulation to be more effective for suppressing arc-furnace flicker than the TCR (thyristor-controlled-reactor) SVC. >

A new injection method for AC harmonic elimination by active power filter

Abstract: An injection method for an active filter which eliminates the harmonics present in AC lines by injecting PWM harmonic compensating current is proposed. In the proposed method, the active filter produces a pulsewidth modulation (PWM) current that cancels the existing harmonics up to any order completely. To generate such PWM current, both inverter and DC current source is needed. The current source can be replaced by a large inductor without any external power source. This can be achieved by providing the inverter with rectifying capability because the inverter has the same circuit structure as the rectifier. Therefore, the proposed model of PWM injection current includes not only the harmonic components to suppress the existing harmonics up to any order, but also the fundamental one, to raise the inductor current to any desired value. The characteristics of the injection method are investigated through a digital computer simulation. Feasibility is proved by the experimental results. >

Newton-type algorithms for the harmonic phasor analysis of nonlinear power circuits in periodical steady state with special reference to magnetic nonlinearities

Abstract: Computer algorithms of the Newton-Raphson type are derived for the harmonic analysis of systems containing nonlinear dynamic components in periodic steady state. The problem is formulated in the complex conjugate multiharmonic space that inherently represents the harmonic coupling between the different harmonic frequencies. The theory is applied to single-phase systems, including magnetic nonlinearities. >

Voltage harmonics generated by voltage-fed inverters using PWM natural sampling

Abstract: The harmonics generated by natural sampled pulse width modulation are investigated experimentally and by theoretical models for the outputs obtained with triangular and sawtooth carrier signals. These equations are valid for synchronous and asynchronous operation. It is shown that triangular carriers generate less harmonics than sawtooth carriers, and that in synchronous operation with single-phase modulation, the carrier-to-output frequency ratio may be any integer value. >

A novel control of a current source active filter for AC power system harmonic compensation

Abstract: A novel control strategy for an active power filter that consists of a current-source PWM (pulse width modulation) converter and LC filters to eliminate the PWM carrier frequency components is proposed in order to obtain good harmonic compensation characteristics. The PWM converter is controlled with feedback loops of filter input currents and their derivatives to suppress the transient oscillations caused by the LC filters without inserting damping resistors. Optimum values of feedback gains are investigated. The cut-off frequency of the optimum design is about one seventh of the PWM carrier frequency, and the compensation delay time is two times as large as the carrier period. The performance of the active power filter with the proposed control strategy was confirmed by experiments in which the harmonic currents generated by a phase-controlled bridge converter were compensated. Input current after compensation was essentially sinusoidal, and the harmonic content was reduced to less than 4%. >

A novel harmonic power filter

Abstract: A novel harmonic power filter system consisting of passive LC filters and voltage source pulsewidth-modulated (PWM) converters is described. This filter system has the merits of both conventional passive LC filters and conventional active power filters. The operating principle and a control scheme for the system are described. Compensation characteristics are discussed from both a theoretical and experimental point of view. The practicability and validity of compensation for large-capacity thyristor converters is experimentally demonstrated. >

3-phase voltage sourced reversible rectifier

Abstract: A voltage sourced reversible rectifier (VSRR) which achieves bidirectional power flow between a 3-phase AC supply and a DC bus voltage is described. The device features a pulse width modulated voltage control strategy that confines unwanted harmonics to known frequency bands where they can be easily filtered out while operating at near unity power factor, with sinusoidal currents, and maintaining a constant DC bus voltage. The paper considers both the steady state and transient response of such a device, and develops mathematical models to describe its performance under both analogue and digital control. Experimental results obtained with a micro processor controlled prototype VSRR are present ed and compared with theoretical predictions.

Nonlinear photorefractive response at high modulation depths

Abstract: The response of photorefractive materials to sinusoidal intensity patterns of high modulation depths is considered. It is shown that the induced refractive-index response in this regime is highly nonlinear. Concise analytic expressions for the various nonlinear harmonics of the photorefractive response field are developed and compared with exact numerical solutions of the underlying charge-transport equations as well as with the results of previous theoretical models over a broad range of physical parameters. We show that the nonlinear response characteristics are strongly dependent on both the magnitude of applied electric fields and the relative concentrations of charge donor and acceptor sites in the material. For drift dominated recording, in particular, we determine analytically that in the limit of a minimal acceptor/donor ratio, the amplitudes of higher spatial harmonics of the response reach a maximum and eventually decay as functions of increasing applied field, whereas these amplitudes reach a nonzero limit in the case of a near-unity acceptor/donor ratio. Finally, we generalize our results to account for diffusion effects in an appended derivation.

Field-oriented asynchronous pulse-width modulation for high-performance AC machine drives operating at low switching frequency

Abstract: A novel pulse-width modulation (PWM) technique is based on the prediction of the trajectory patterns that the stator current vector describes in a field-oriented coordinate system. The method of field-oriented PWM control minimizes the switching frequency at given torque ripple. The harmonics of the field producing current are left to develop freely, permitting operation at values of switching frequency that are lower than those of the optimal schemes known so far. The optimal switching times are computed off line and are called from a memory during operation. Signal processing is performed in a simple hardware structure. >

Reduction of undesired harmonic components

Abstract: A method and means for reducing the energy of undesired r.f. harmonics generated in the highest power amplifier of a transmitter by providing at least one alternate path for the undesired harmonics incorporating a relatively low powered power amplifier for at least one undesired harmonic component and causing the harmonic in the alternate path to have an amplitude that is substantially equal to the corresponding harmonic output from the main path and also causing the harmonic in the alternate path to be substantially out of phase with the harmonic in the main path and then combining the harmonics of the paths. This procedure is only feasible if the output of the highest power amplifier is processed by the main path circuit connected between the highest power power amplifier and the antenna so as to attenuate the undesired components sufficiently so that the harmonics power levels are a very small percentage of desired output wave.

Plates with regular stiffening in acoustic media: Vibration and radiation

Abstract: Flat plates and cylindrical shells with regular and identical stiffening constitute spatially periodic structures. Specially convenient methods of vibration analysis are available for these, some of which are suitable for the inclusion of the effects of fluid loading from adjacent acoustic media. This paper outlines the nature of free wave motion in periodic structures stiffened either in one direction or in two orthogonal directions. The structural response to distributed harmonic pressure fields is analyzed by using displacement functions consisting of a series of space harmonics. The sound radiated or transmitted by the vibrating structure is also found. The method of space harmonics is next combined with the method of phased array receptance functions to yield equations for the propagation constants of fluid‐loaded periodic plates, stiffened in just one direction. Some calculated propagation constants are presented. Recent developments in the application of the hierarchical finite‐element method to periodic structures are described. Computed results are presented for the propagation constants of an orthogonally stiffened plate, without fluid loading. Consideration is given to those frequency ranges and wave motions that could radiate into an adjacent medium.

Current harmonics and acoustic noise in AC adjustable speed drives

Abstract: AC adjustable-speed drives (ASDs) are finding increased opportunities for applications in which acoustic noise is a major consideration for acceptability. The major source of noise in an ASD is the excitation of the motor by nonsinusoidal currents. For load characteristics that simulate the performance required for fans and compressors, ASDs based on induction motors, permanent-magnet motors, and switched reluctance motors have been examined for correlation between current and acoustic noise. The effects that the specific types of noise produced by drives have on human subjects is examined with regard to temporary and permanent hearing-threshold shifts. >

Application of a three phase nonsinusoidal calibration system for testing energy and demand meters under simulated field conditions

Abstract: The influence of harmonics on commonly used three-phase watthour and kVA demand meters was studied using a computer-controlled three-phase test system capable of generating nonsinusoidal voltages and currents. A description of the three-phase test system and results of preliminary tests of several revenue meters are presented. The performance of the tested meters is compared with the response of time-division and thermal reference meters for different harmonic conditions, primarily field-recorded data. Errors up to 2% were observed during the testing of energy meters and up to 35% for kVA demand meters. >

Interference-term real-time measurement for self-stabilized two-wave mixing in photorefractive crystals.

Abstract: We report the real-time direct interference-term measurement for a two-wave-mixing experiment in photorefractive crystals. Knowledge of the interference term may provide information concerning diffraction efficiency, interference pattern-to-recorded hologram phase shift, and optical activity and anisotropic diffraction properties of these materials. This method comprises phase modulation of one of the interfering beams and synchronous detection of the first and second harmonics in the resulting output irradiance modulation. Simultaneous detection of both harmonics enables the measurement to be made even in strongly perturbed conditions, since one harmonic is used for measuring and the other is used for operating an active stabilization system. Experimental results for Bi12TiO20 are reported.

Three dimensional nonlinear theory of the gyropeniotron amplifier

Abstract: The saturation regime of gyropeniotron interaction in a cylindrical waveguide with slotted wall (magnetron type) is studied by developing a three-dimensional nonlinear theory for the amplifier. The wave amplitude is calculated self-consistently in the presence of the RF current modulation by solving a set of coupled nonlinear differential equations. Extremely high efficiency of about 75% is calculated for interaction between the n-mode and the second cyclotron harmonic of a 70kV, 3·5 A beam with v⊥=/vz2 under ideal conditions of zero velocity spread and on-axis guiding centre location. The effects of spread in beam velocity and guiding centre position are also calculated. An efficiency of about 38% is obtained for a beam with 5% velocity spread and 10% guiding centre spread. The performance characteristics of the amplifier are shown for cyclotron harmonics up to order s = 8. The efficiency drops to 22% at s equals; 8.

Vector control system for induction motors

Abstract: A vector control system for an induction motor driven by a power converter which generates three phase ac currents containing harmonics. In the system, an exciting current command value and a torque current command value in a rotating magnetic field coordinate system are applied, induction motor primary currents in a stator coordinate system is detected at a period which is the same or integral multiple of that of the harmonic components by integration operation, the detected value of the integrated primary currents is subjected to definite integration over the same time interval as the period thereof thereby detecting fundamental wave components of the primary currents without the harmonic components, the fundamental wave components are transformed in the form of rotating magnetic field coordinate system thereby detecting an exciting current component and a torque current component, a deviation between the exciting current command value and the detected actual exciting current component and a deviation between the torque current command value and the detected actual torque current component are detected respectively, and producing and applying to the power converter three phase ac voltage command values in the stator coordinate system tending to reduce the deviations to zero.

Parametric decay in the edge plasma of ASDEX during fast wave heating in the ion cyclotron frequency range

Abstract: Two types of parametric decay instabilities were observed in the scrape-off layer of ASDEX during hydrogen second harmonic heating, in a single as well as in a two ion species plasma. The first type was identified as decay into an ion Bernstein wave and an ion cyclotron quasi-mode, and the second as decay into an ion Bernstein wave and a low frequency electron quasi-mode. The parametric decay processes are due to the high electric fields near the fast wave antennas. Theoretical growth rate calculations predict a threshold for the electric fields near the antennas. In the relevant experiments the electric fields, estimated from a full-wave code, indeed exceeded the thresholds. These parametric decay processes, as well as the harmonics of the generator frequency also seen in the scrape-off layer, provide a mechanism that might contribute to the observed direct energy deposition in the edge plasma and to ICRF induced impurity production.

Single-phase AC-AC converters using power MOSFETs

Abstract: The authors analyze the performance of a single-phase AC-AC converter with particular emphasis on the output harmonic content and input voltage utilization. A complete power circuit that makes use of power MOSFETs is given. An analysis of the output waveform shows that its harmonic content is very low. It is also found that the ratio of the switching frequency to the modulating triangular wave, has negligible effect on the harmonic content. Hence, the ratio can be chosen at nominal value of 4 and it is mainly used to control the amplitude of the fundamental component in the output. The only disadvantage of the single-phase AC-AC converter is that the amplitudes of certain harmonic frequencies become abnormally high. >

Power system harmonic resonance: a documented case

Abstract: Harmonic resonance occurs in a power system when the power system's natural frequency corresponds to the frequency of a source of harmonic current. A case of power system harmonic resonance which resulted in failure of 13.8 kV switchgear and consequential plant losses is documented. Calculations of the power system's natural frequencies are given for various power system configurations of supply reactance, motor reactance, and system-shunt capacitance. Sources of harmonic current are identified, methods of measuring harmonic current and voltage levels throughout the system are described, and results of the power system harmonic current and voltage survey are reported. The corrective action taken to prevent a recurrence of harmonic resonance is also described. >

Novel Langmuir probe technique for the real‐time measurement of the electron temperature

Abstract: A new method for the evaluation of the electron temperature from the voltage–current characteristic of a single Langmuir probe under the application of an ac signal was developed and tested. This method exploits harmonics in the current spectrum which arise from the nonlinearity of the characteristic; from the amplitude of these harmonics the electron temperature can be deduced. Langmuir probe measurements on the TEXTOR edge plasma show a good agreement of the new method with the conventional one. The advantages of this method over other methods are discussed. Proposals for further improvements are given.