Showing papers on "Harmonics published in 1989"

Radio-frequency generator with automatic power control for radio-frequency surgery

Abstract: The invention describes a radio-frequency generator for radio-frequency surgery, with a power amplifier operating in switched mode, a modulation device for controlling the output, and a harmonics measuring device for measuring the harmonic frequencies of the generator frequency which are generated by the arc produced between the surgical probe and the tissue to be cut or to be coagulated. At the instant of start-up, the power amplifier is driven by an auxiliary oscillator. After the start-up, the drive of the power amplifier is switched over to selfexcitation, voltage feedback being selected when the load impedance exceeds a particular value, and current feedback when the load impedance falls below a certain value. During the feedback mode, the auxiliary generator is locked to the output frequency and output phase of the radio-frequency generator by means of a phase-locked loop. The harmonics measuring device contains a phase-controlled rectifier, the control signal of which is derived from the auxiliary generator. The advantage of such a generator lies in its low power dissipation, its high operational reliability and its low filtering complexity in the output circuit.

Update of harmonic standard IEEE-519: IEEE Recommended Practices and Requirements for Harmonic Control in Electric Power Systems

Abstract: The authors present the changes in IEEE STD 519-1981, titled IEEE Guide for Harmonic Control and Reactive Compensation of Static Power Converters, that are to be made in 1988 as the result of a two-year review. The voltage distortion criteria in the 1981 edition could not be used to distribute among users, the ability of the utility system to absorb harmonic currents. This problem is addressed in the revision. There are now two criteria that are used to evaluate harmonic distortion. The first is a limitation in the harmonic current that a user can transmit into the utility system. The second criterion is the quality of the voltage that the utility must furnish the user. Examples of the application of the revised standard, which has been upgraded to a Recommended Practice, are given. >

Instability, subharmonics and chaos in power electronic systems

Abstract: The concept of chaos is applied to a variety of nonlinear power electronic circuits. With the onset of instability, the phenomena of subharmonics, quasi-periodicity, and chaos are predicted and observed. The following examples are dealt with: diodes with charge storage (with application to resonant converters); a ferroresonant circuit; a controlled thyristor rectifier circuit; and a Buck converter controlled by pulse-width modulation (PWM). The examples were chosen to model aspects of real power electronic systems. Analytical and experimental results demonstrate the complex nonlinear phenomena such as subharmonics, quasi-periodicity and chaos can arise in even the simplest systems. >

Acoustic noise reduction in sinusoidal PWM drives using a randomly modulated carrier

Abstract: Acoustic noise in an inverter-driven AC electric machine can be reduced by avoiding the concentration of harmonic energy in distinct tones. One method to spread out the harmonic spectrum without the use of programmed PWM (pulse-width modulation) is to cause the switching pattern to be random. It is proposed that the switching pattern can be randomized by modulating the triangle carrier in sinusoidal PWM (pulse-width modulation) with bandlimited white noise. All the advantages of sinusoidal PWM are preserved with this technique. These include real-time control, linear operation, good transient response, and a constant average switching frequency. By controlling the bandwidth and RMS value of the pink noise modulation, it is shown that the instantaneous variation in switching frequency as well as the bandwidth of the energy spectrum in the machine can be specified within predetermined limits. Experimental results show the absence of acoustic noise concentrated at specific tones which is present with conventional sinusoidal modulation. >

A survey of neutral currents in three-phase computer power systems

Abstract: Neutral current in three-phase power systems is often thought to be only the result of the imbalance of the phase currents. With computer systems, very high neutral currents have been observed even when the phase currents are balanced. Measurements from a sample of computer power systems in the United States are presented to determine the extent of the neutral current problem. The author explores the cause of high neutral currents in three-phase computer power systems, potential problems, and recommended remedies. It is found that, while very high neutral currents are possible due to the additive nature of triple harmonic currents, a low percentage of data-processing sites in the US are actually experiencing neutral currents in excess of the rated phase current. However, recent trends in computer systems make high harmonic neutral currents more likely. Power system problems associated with high harmonic neutral currents include overloaded transformers, voltage distortion, and common-mode noise. It is suggested that whenever three-phase, wye power systems are used to supply power to computer systems or other similar electronic loads, the power system design should allow for the possibility of high harmonic neutral current to avoid potential problems. >

Dynamics of a collisional, capacitive RF sheath

Abstract: A self-consistent solution for the dynamics of a high-voltage, capacitive RF sheath driven by a sinusoidal current source is obtained, under the assumptions of time-independent, collisional ion motion and inertialess electrons. Values of the ion current density, the sheath capacitance per unit area for the fundamental voltage harmonic, and the conductance per unit area for stochastic heating by the oscillating sheath are calculated. The ratio of the DC to the peak value of the oscillating voltage is found to be 0.40, while the second and third voltage harmonics are, respectively, 19.3% and 5.3% of the fundamental. >

Implementation of a controlled rectifier using AC-AC matrix converter theory

Abstract: It is well known that a PWM-controlled rectifier can offer advantages of reduced low-order harmonics and unity input power factor when compared to a conventional thyristor converter. However, theoretically optimum PWM strategies are often difficult to implement physically or are not easily extended to regenerative operation. The authors propose an alternative PWM strategy based on AC-AC matrix converter theory, which generates only high-order switching harmonics, presents a unity power factor load to the supply, implicitly extends to regeneration (and operation with a center tapped DC output), and is feasible to physically implement for real-time output voltage control. Both the theory and physical simulation results are presented. >

Critical-state model for harmonic generation in high-temperature superconductors.

Abstract: High-temperature superconductors exhibit harmonic generation when immersed in an ac magnetic field. To explain this phenomenon, we propose a macroscopic critical-state model as an alternative to the loop model used by Jeffries et al. While the original Bean model of the critical state only predicts odd harmonics, our extended model also predicts even harmonics by taking into account the dependence of the critical current upon magnetic field. The results of our measurements of harmonic signals as a function of ac magnetic field, dc magnetic field, temperature, and harmonic number are consistent with the proposed model. In particular, we find that, as the magnetic field is increased, the critical current crosses over from the Bean regime, where ${J}_{c}$ is independent of field, to the Anderson-Kim regime, where ${J}_{c}$ is approximately inversely proportional to the field.

Identification of harmonic sources by a state estimation technique

Abstract: A reverse power flow procedure is described to identify the sources of harmonic signals in electric power systems. Methods have been reported elsewhere to analyze the propagation of these harmonics signals throughout the network. Here the reverse problem of how to identify the source (and potentially the type) of harmonic signals is considered. Line and bus data at several points in the network are used with a least-squares estimator to calculate the injection spectrum at buses suspected of being harmonic sources. When energy at harmonic frequencies is found to be injected into the network at a bus, that bus is identified as a harmonic source. >

Harmonics-causes, effects, measurements and analysis-update

Abstract: Rectification generates harmonic voltages and currents that can cause problems, e.g. insulation failures due to overheating and overvoltages, malfunction of solid-state equipment, communication interference, etc. The common addition of power factor correction capacitors to such a system generally increases the likelihood of such problems occurring. Online data acquisition and analysis of the harmonic content of system voltages and currents may be necessary to identify the cause properly and recommend a solution. In response to these problems, a field-proven portable harmonic-measurement/analysis system is described, as well as the advantages and versatility inherent in its use. This portable, computer-based system provides on-site data collection, precise harmonic identification via a fast Fourier transform, data analysis, and report preparation via a custom spreadsheet. Specific examples involving the application of this measurement/analysis system in the cement, steel, and carbon industries are presented. >

PWM-CSI inverter for induction motor drives

Abstract: The authors discuss a number of issues involved in designing a current source inverter system for a large induction motor drive. Using two modulation techniques-selected harmonic elimination in the upper frequency range and trapezoidal modulation in the lower frequency range-control of voltage, current, and torque harmonics is achieved while limiting the GTO switching frequency to 180 Hz. Each modulation range is divided into a number of subranges to exploit the available switching capacity and to avoid harmonic resonances involving the capacitor and the motor inductance. In addition to the development of basic principles, the authors include simulation waveforms and test results from a laboratory experimental system. >

Low-order harmonic interactions on AC/DC systems

Abstract: A technique is described to evaluate harmonic interaction. It is based on linearizing the harmonic interaction between AC and DC sides of the converter, including the effect of control action. The technique provides an efficient tool for the evaluation of possible adverse amplification of low-order harmonics as well as the design of mitigating measures. It is especially valuable for complex HVDC systems where performance must be evaluated for many contingencies. >

Measurement of magnitude and phase of harmonics generated in nonlinear microwave two-ports

Abstract: A method for simultaneously measuring the magnitude and phase of the harmonics generated by a microwave two-port is reported. The two-port under test is driven with a sinusoidal microwave signal strong enough to force it into nonlinear operation. Its output harmonics are measured in the frequency domain with a setup that includes a vector network analyzer. For phase calibration at the harmonic frequencies, a millimeter-wave Schottky diode is used as a reference device. The system allows the measurement of harmonics with a phase accuracy of about +or-10 degrees at 20 GHz (referred to f/sub 1/=5 GHz). It can be built for any frequency ( >

A method for calculating harmonic currents of a three-phase bridge uncontrolled rectifier with DC filter

Abstract: A practical method is proposed for calculating the harmonic currents of a three-phase bridge uncontrolled rectifier with a DC filter, taking into account the AC source reactance. The method is based on the frequency-domain method and the rectifier switching functions. Analytical equations for the harmonic currents on both the DC and AC sides are derived. The validity of the method is demonstrated by comparison with the results of time simulation. The approach can be extended to the harmonic analysis of a thyristor rectifier as well as a rectifier with unbalanced line conditions. >

Reactive and unbalanced currents compensation in three-phase asymmetrical circuits under nonsinusoidal conditions

Abstract: The present work is a continuation of the author's previous work (see IEEE Trans. Instrum. Meas., vol.37, p.30-34, Mar. 1988), where a new power theory of three-phase circuits under nonsinusoidal conditions was suggested. The main objective of the present work is to determine whether the theory suggested has practical applications that justify the need of measuring quantities introduced in it, particularly, whether it improves a theoretical basis for improving power properties of such circuits. It is shown that the three-phase linear asymmetrical load supplied from a symmetrical source of nonsinusoidal voltage composed of a finite number of harmonics can be converted with a reactance circuit into a symmetrical three-phase circuit of almost unity power factor. It is shown how such a reactance circuit can be designed and which quantities should be measured for this purpose. >

Nonlinear effect of an oscillating electric field on membrane proteins

Abstract: The nonlinear response of a two‐state chemical transition to an oscillating electric field is examined. A reaction for which this analysis is particularly relevant is a conformational transition of a membrane protein exposed to an ac electric field. Even a modest externally applied field leads to a very large local field within the membrane. This gives rise to nonlinear behavior. The applied ac field causes harmonics in the polarization and can cause a dc shift in the state occupancy, both of which can be observed and used to determine kinetic parameters. Fourier coefficients are calculated for the enzyme state probability in the ac field, exactly for infinite frequency, and in powers of the field for finite frequency. Kramers–Kronig relations are proved and response functions are given for the leading terms of the harmonics. The results are extended to the spherical symmetry relevant to suspensions of spherical cells, vesicles, or colloidal particles. If the protein catalyzes a reaction, free energy is t...

Direct frequency convertor with sinusoidal line currents for speed-variable AC motors

Abstract: A novel concept for a static three-phase to three-phase power converter for an AC drive with a unity power factor and reduced harmonics on the line side is presented. The power circuit comprises two back-to-back connected six-pulse bridges having no energy storage elements in the DC link. This permits pulse-width modulation (PWM) control in both bridges while requiring active turn-off semiconductor switches in only one bridge. The line-side harmonics are suppressed by a three-phase second-order filter. The method of predictive optimization is used for the control of the power converter. The complex control structure of the system is based on an online prediction of space vector trajectories. The steady-state operation of the system is exemplified by simulation results. >

Rectifier design for minimum line current harmonics and maximum power factor

Abstract: Rectifier line current harmonics interfere with proper power system operation, reduce rectifier power factor, and limit the power available from a given service. The rectifier's output filter inductance determines the rectifier line current waveform, the line current harmonics, and the power factor. Classical rectifier analysis usually assumes a near-infinite output filter inductance, which introduces significant error in the estimation of line current harmonics and power factor. A quantitative analysis of single and three-phase rectifier line current harmonics and power factor as a function of the output filter inductance is presented. For the single phase rectifier, one value of finite output filter inductance produces maximum power factor and a different value of finite output filter inductance produces minimum line current harmonics. For the three phase rectifier, a near-infinite output filter inductance produces minimum line current harmonics and maximum power factor, and the smallest inductance that approximates a near-infinite inductance is determined. >

The propagation of harmonic currents generated by inverter-fed locomotives in the distributed overhead supply system

Abstract: Pulse-width-modulated converter-fed locomotives generate current harmonics that give rise to traveling waves in the overhead supply system. The waves are partially reflected at the feeding substances, causing parallel and series resonances at various discrete frequencies. An investigation based on the wave propagation approach demonstrates the influence of the track topography and the varying position of locomotives within the track. The distribution of harmonic currents in the overhead supply system is evaluated and discussed. These currents are shown to be much higher in certain locations of the railway track than the harmonic current injected by a locomotive. They may also appear at distant tract locations. The natural resonances in the overhead supply system determine the intensity of electromagnetic interference with the track-side communication lines. >

Excessive neutral currents in three-phase fluorescent lighting circuits

Abstract: Causes for excessive neutral currents amounting to 75% and 157% of phase current values in two installations supplying predominantly fluorescent lighting circuits have been identified. These high neutral currents are accounted for by three main components, namely fundamental phase current unbalance, third-harmonic currents caused by saturation of ballasts, and more third-harmonic current caused by third-harmonic voltage waveform distortion in the supply. Solutions are presented to overcome the problem, including a scheme that cancels third-harmonic currents along a common neutral conductor by means of a six-phase system supplied by two transformers connected in antiphase. >

Nonlinear magnetic flux response in high temperature superconductors

Abstract: We have spectrum analysed the voltage induced in a pick-up coil wound around a cylindrical bulk sample of YBa 2 Cu 3 O 7−δ when driven by a weak low frequency AC magnetic field, superimposed on a weak DC magnetic field, at 77 K. The harmonics generated by the highly nonlinear response are explained quantitatively in great detail by employing a critical state model with a field independent pinning force density, where intergranular vortices sweep in and out of the Josephson weak link regions during each AC cycle. The symmetry relation for the local flux density, which relates the first and second half of an AC cycle, is broken in the presence of a DC magnetic field, causing even harmonics to appear. This suggests high-sensitivity applications in magnetometry.

Interactions of currents and weakly nonlinear water waves in shallow water

Abstract: Two-dimensional Boussinesq-type depth-averaged equations are derived for describing the interactions of weakly nonlinear shallow-water waves with slowly varying topography and currents. The current velocity varies appreciably within a characteristic wavelength. The effects of vorticity in the current field are considered. The wave field is decomposed into Fourier time harmonics. A set of evolution equations for the wave amplitude functions of different harmonics is derived by adopting the parabolic approximation. Numerical solutions are obtained for shallow-water waves propagating over rip currents on a plane beach and an isolated vortex ring. Numerical results show that the wave diffraction and nonlinearity are important in the examples considered.

Method of determining drill bit wear

Abstract: A method is provided for determining the state of wear of a multicone drill bit. Vibrations generated by the working drill bit are detected and converted into a time oscillatory signal from which a frequency spectrum is derived. The periodicity of the frequency spectrum is extracted. The rate of rotation of at least one cone is determined from the periodicity and the state of wear of the drill bit is derived from the rate of cone rotation. The oscillatory signal represents the variation in amplitude of the vertical or torsional force applied to the drill bit. To extract periodicity, a set of harmonics in the frequency spectrum is given prominence by computing the cepstrum of the frequency spectrum or by obtaining an harmonic-enhanced spectrum. The fundamental frequency in the set of harmonics is determined and the rate of cone rotation is derived from the fundamental frequency.

Core loss in buried magnet permanent magnet synchronous motors

Abstract: The steady-state core-loss characteristics of buried-magnet synchronous motors operating from a sinusoidal constant frequency voltage supply are investigated. Measured and calculated core loss, with constant shaft load, is shown to increase with decreasing terminal voltage due to an increase in armature reaction-induced stator flux-density time harmonics. Finite-element modeling is used to show that the additional loss due to the time-harmonic fields can increase core loss by a factor of six over the loss associated with only the fundamental component field at low motor flux levels. A simple air-gap model of motor flux components shows that this increased loss is due to localized rotor saturation. Thus, stator-core harmonic fields should be expected for all buried-magnet rotor synchronous motors (with or without a cage) operating at low flux levels. This factor becomes increasingly important when the motors are operated in the high-speed low-flux mode in conjunction with a variable-speed drive. >

Method of source coding and harmonic cancellation for vibrational geophysical survey sources

Abstract: A method is provided for significantly reducing the distortion and crossfeed from any selected order harmonic for any number of vibratory seismic sources operated concurrently, at the same time providing for separation of the signals from the different sources and for improving the signal-to-noise ratio. After determining the highest order harmonic likely to cause distortion, a number of sweeps of each source in each position is selected. This number depends upon the number of sources and the highest order harmonic to be suppressed. Initial phase angles for each sweep of each source are then selected to permit signal separation while suppressing harmonics up to and including that highest order harmonic.

Summation of probabilistic harmonic vectors (power systems)

Abstract: A method of determining the magnitude of the sum of random harmonic vectors of arbitrary probability characteristics is presented. Utilization of summation technique, in conjunction with harmonic load flow, to evaluate net harmonic magnitudes due to distributed sources in both deterministic and stochastic networks is discussed. It is demonstrated and stochastic networks is discussed. It is demonstrated that the widely used form of probability density function of the magnitude of the sum of random vectors arises from simplification of the general expressions developed here. To assess its validity, a comparative study between the method developed and Monte Carlo simulation is carried out, showing good agreement. However, the analytical method is a lot faster and provides closed-form expressions for the probability density characteristics of the sum of random vectors. >

The determination of crystal size and disorder from X-ray diffraction photographs of polymer fibres. 1. The accuracy of determination of Fourier coefficients of the intensity profile of a reflection

Abstract: Methods which determine the number and disorder of lattice planes in a crystal from the Fourier cosine coefficients of the intensity profile of an X-ray reflection use only the low harmonics and require that the coefficients be normalized so that the zero harmonic is unity. Experimentally, the profiles can only be recorded over a smaller range of scattering angle than required by the theory, and it is necessary to subtract background, which is likely to be estimated with considerable error, before determining the coefficients. It is shown that with polymer fibres this causes serious errors in the normalization, and in the values of those low harmonics used in the size and disorder determination, and prevents reliable values being obtained. Methods which avoid normalization and use only high harmonics are needed. It is shown that disorder may be obtained in such a way, but not size, for which low-order normalized coefficients are essential. A method of extrapolation is described and tested which enables the accurate high harmonics to be used to improve the estimates of the low ones. Whilst this will yield more reliable values of crystal size than are obtainable from existing methods, the accuracy depends entirely on the validity of the extrapolation, which cannot be tested in many cases of interest.

Optimal power control strategies for neutral point clamped (NPC) inverter topology

Abstract: A programmed pulsewidth modulation (PWM) technique for selectively eliminating several lower-order harmonics at the output of a neutral point clamped (NPC) inverter topology is investigated. The switching function approach is utilized to derive relevant analytical expressions for input/output variables. A thorough evaluation of the NPC inverter topology based on the switching function approach is described. Optimal power control strategies for an NPC inverter employing programmed PWM patterns are proposed. For a constant-frequency variable-voltage NPC inverter power supply, the proposed strategy is to maintain a minimum specified total harmonic distortion employing a low-output impedance filter. In the case of an NPC inverter powering an AC motor drive, the proposed strategy is to maintain a minimum specified harmonic current factor. The proposed power control strategies are achieved without substantial increase in inverter switching frequency and are therefore suitable for high-power applications employing gate-turn-off-thyristor (GTO) type devices. >