Showing papers on "Harmonics published in 2002"

Grid current regulation of a three-phase voltage source inverter with an LCL input filter

Abstract: Many grid connected power electronic systems, such as STATCOMs, UPFCs and distributed generation system interfaces, use a voltage source inverter (VSI) connected to the supply network through a filter. This filter, typically a series inductance, acts to reduce the switching harmonics entering the distribution network. An alternative filter is a LCL network, which can achieve reduced levels of harmonic distortion at lower switching frequencies and with less inductance, and therefore has potential benefits for higher power applications. However, systems incorporating LCL filters require more complex control strategies and are not commonly presented in literature. This paper proposes a robust strategy for regulating the grid current entering a distribution network from a three-phase VSI system connected via an LCL filter. The strategy integrates an outer loop grid current regulator with inner capacitor current regulation to stabilise the system. A synchronous frame PI current regulation strategy is used for the outer grid current control loop. Linear analysis, simulation and experimental results are used to verify the stability of the control algorithm across a range of operating conditions. Finally, expressions for "harmonic impedance" of the system are derived to study the effects of supply voltage distortion on the harmonic performance of the system.

Role of the intramolecular phase in high-harmonic generation

Abstract: We study numerically the generation of high-order harmonics by two-center molecules for arbitrary angles between the molecular axis and the laser polarization axis. For fixed angle, the harmonic spectrum exhibits a minimum at a frequency which is independent of the laser parameters. The amplitude of each harmonic is strongly angle dependent, and a pronounced minimum is found at the same angle where a sudden jump in the harmonic phase occurs. By calculating the spatial dependence of the harmonic amplitudes and phases, we are able to explain these effects in terms of interfering contributions from various regions within the molecule.

Impact of EV battery chargers on the power quality of distribution systems

Abstract: A summary of the actual state of battery charger harmonics is presented. The effect of harmonic distortion on the distribution system, especially on distribution transformers, is analyzed. A program was developed that allows the consideration of the transformer life consumption as a function of the battery charger characteristics and charging algorithm. The program is considered to be a distribution planning and management tool. The proposed tool can be easily applied to determine the optimum charging time as a function of the existing load, ambient temperature, and time of day. From the study it can be deduced that a direct connect-and-charge scheme can be detrimental to the transformer life, especially under high temperature and large load. Calculations show the existence of a quadratic relationship between the transformer life consumption and the total harmonic distortion (THD) of the battery charger current. Furthermore, the current THD should be limited to 25 30% in order to have a reasonable transformer life expectancy.

Fuzzy logic controlled shunt active power filter for power quality improvement

Abstract: The simulation and experimental study of a fuzzy logic controlled, three-phase shunt active power filter to improve power quality by compensating harmonics and reactive power required by a nonlinear load is presented. The advantage of fuzzy control is that it is based on a linguistic description and does not require a mathematical model of the system. The fuzzy control scheme is realised on an inexpensive dedicated micro-controller (INTEL 8031) based system. The compensation process is based on sensing line currents only, an approach different from conventional methods, which require harmonics or reactive volt-ampere requirement of the load. The performance of the fuzzy logic controller is compared with a conventional PI controller. The dynamic behavior of the fuzzy controller is found to be better than the conventional PI controller. PWM pattern generation is based on carrierless hysteresis based current control to obtain the switching signals. Various simulation and experimental results are presented under steady state and transient conditions.

A frequency-domain detection of stator winding faults in induction machines using an external flux sensor

Abstract: The aim of this paper is to present the advantage of stator fault detection using an external stray flux sensor in a working induction machine. In the past 20 years, stray flux sensors have been used for diagnostic purposes but rarely in the presence of a complex power supply such as a voltage static inverter. In this paper, it is proved that a simple external stray flux sensor is more efficient than the classical stator current signature analysis to detect inter-turn short circuit in three-phase induction machines. The new result is that, even in the presence of power supply harmonics, it is possible to easily detect the stator winding faults in the low-frequency range of the flux spectrum with low-frequency resolution.

Control and performance of a fully-digital-controlled shunt active filter for installation on a power distribution system

Abstract: This paper presents a fully-digital-controlled shunt active filter for harmonic termination of a power distribution system. The main purpose of the active filter based on voltage detection is not to compensate for current harmonics but to damp out harmonic propagation caused by line inductors and shunt capacitors for power factor correction. However, time and phase delays inherent in digital controllers might lead to unsatisfactory harmonic-damping performance although digital controllers are preferable to analog controllers. This paper deals with the design and implementation of a digital controller for a shunt active filter based on voltage detection. Experimental results obtained from a laboratory system developed in this paper verify the viability and effectiveness of the fully-digital-controlled active filter.

Basic acoustic properties of microbubbles.

Abstract: Small (encapsulated) gas bubbles in a contrast medium react to an external oscillating pressure field with volume pulsations. Depending on the magnitude of the ultrasound wave, the vibrations will be related either linearly or nonlinearly to the applied acoustic pressure. For low acoustic pressures, the instantaneous radius oscillates linearly in relation to the amplitude of the applied external pressure field. The oscillation of the bubble is governed by parameters such as resonance frequency, damping coefficients, and shell properties. For higher amplitudes of the external field, the pulsation of the bubbles becomes nonlinear. The spectrum of the scattered ultrasound wave also contains higher harmonics of the emitted frequency in addition to the fundamental frequency. The emitted frequency, bubble size, and nonlinear propagation effects have significant influence on the harmonic generation. For encapsulated bubbles exposed to even higher acoustic amplitudes, their scattering effectiveness increases dramatically and becomes transient. The scattered frequency spectrum broadens, containing higher harmonics. This consequence is due to rupture, disappearance, change of gas content, etc. Using these specific characteristics of the contrast bubbles will open new perspectives in imaging and analysis for medical diagnosis.

Instantaneous power compensation in three-phase systems by using p-q-r theory

Abstract: This paper proposes a novel power compensation algorithm in three-phase four-wire systems by using p-q-r theory. The p-q-r theory is compared with two previous instantaneous power theories, p-q theory and cross vector theory. The p-q-r theory provides two-degrees of freedom to control the system currents by only compensating the instantaneous imaginary power without using any energy storage element. The definition of powers maintains power conservation, and agrees well with the general understanding of power. Simulation results show the superiority of p-q-r theory both in definition and compensation.

Optimum harmonic reduction with a wide range of modulation indexes for multilevel converters

Abstract: This paper proposes a novel modulation technique to be applied to multilevel voltage-source converters suitable for high-voltage power supplies and flexible AC transmission system devices. The proposed technique can generate output stepped waveforms with a wide range of modulation indexes and minimized total voltage harmonic distortion. The main power devices switch only once per cycle, as is suitable for high-power applications. In addition to meeting the minimum turn-on and turn-off time requirements for high-power semiconductor switches, the proposed technique excludes from the synthesized waveform any pulses that are either too narrow or too wide. By using a systematic method, only the polarities and the number of levels need to be determined for different modulation levels. To verify the theory and the simulation results, a cascaded converter-based hardware prototype, including an 8-b microcontroller as well as modularized power stage and gate driver circuits, is implemented. Experimental results indicate that the proposed technique is effective for the reduction of harmonics in multilevel converters, and both the theoretical and simulation results are well validated.

Super harmonic imaging: a new imaging technique for improved contrast detection.

Abstract: For ultrasound contrast agents (UCA), nonlinear imaging now has become fundamental. All of the current contrast-imaging methods are dominantly based on the nonlinear response of UCA bubbles. The discrimination between the perfused tissue and the UCA is the challenge in the field of UCA-imaging. This differentiation is usually associated or expressed by the ratio of the scattered power from the contrast agent to the scattered power from the tissue and is termed "contrast-to-tissue ratio" (CTR). Second harmonic imaging showed a better discrimination between tissue and UCA than fundamental imaging because of a higher CTR. We demonstrate, in this study, that the CTR increases as a function of the order of the harmonic frequency. Currently, due to the limited bandwidth of the transducers, only the second harmonic is selectively imaged, resulting in images with a superior quality to fundamental images, but still degraded and not optimal because of the harmonic generation in the underlying tissue (due to nonlinear propagation) and hence giving a limited CTR. To increase the CTR and to take advantage of the higher harmonics (third, fourth, fifth and the ultraharmonics and termed here super harmonics), we have developed a new phased array transducer. The array transducer contains two different types of elements arranged in an interleaved pattern (odd and even elements). The total number of elements is 96. The elements can operate separately and at a distinct frequency, enabling separate transmission and reception modes. The odd elements (48) operate at typically 2.8 MHz center frequency and 80% bandwidth. The even elements (48) have a center frequency of 900 kHz with a bandwidth of 50%. In vitro measurements using the dual frequency probe show an increase of 40 dB in the CTR for super harmonic components over the conventional second harmonic system. The increase in CTR is in agreement with the calculations using existing models for the response of encapsulated bubbles and known theory of nonlinear propagation. Animal experiments have demonstrated the feasibility of this approach using commercially avail- able UCA and showed a similar increase of the CTR (E-mail: bouakaz@tch.fgg.eur.nl). © 2002 World Federation for Ultrasound in Medicine & Biology.

Extreme-ultraviolet high-order harmonic pulses in the microjoule range

Abstract: We study high-order harmonic generation at a high pumping energy using a long focal length lens. We identify different saturation regimes of the harmonic emission, revealing the interplay between phase matching, absorption, and laser defocusing. In the optimal conditions, high conversion efficiencies are obtained, resulting in an increase of at least one order of magnitude of the harmonic energies compared to previously reported values. In xenon, microjoule energies are reached, opening new perspectives for the applications of this ultrashort coherent radiation. The generation of the high-order harmonics of intense laser pulses in gases @1# has recently opened new perspectives for probing matter in the extreme-ultraviolet ~XUV! pulses on an unprecedented time scale. The ultrashort harmonic pulse duration is used in pump-probe experiments in atomic @2,3# and molecular @ 4‐6 # spectroscopy, as well as in solidstate physics @7,8#. Combined to the high intrinsic beam coherence, it has allowed ultrafast diagnosis of laser-produced plasmas through XUV interferometry @9,10#. However, the harmonic beam energy is still relatively low and many applications would become possible if the number of generated photons were increased: ultrafast XUV holography, diagnosis of dense bright plasmas, or even study of nonlinear processes in the XUV, limited so far to low harmonic orders @11#. Recently a number of studies have demonstrated high conversion efficiencies, using ultrashort laser pulses focused in hollow core fibers @12‐14# or cells @15,16#. However, these efficiencies were obtained at a very low laser energy ~less than 1 mJ in most cases! that imposed a relatively tight focusing geometry in order to reach saturation intensities of the generating rare gases. This resulted in a low harmonic energy, in the nanojoule range. The fact that much larger energies are now available on ultrashort laser systems raises a number of questions: using higher laser energies and looser focusing, is it possible to achieve similar efficiencies, and thus to generate microjoule harmonic pulses? In particular, how will phase matching be affected by these unusual generating conditions? In this Rapid Communication, we report a thorough study of harmonic generation at a high pumping energy using a long focal length lens. Using a pumping energy of 27 mJ and a f 52 m lens, we study the influence of the beam aperture, the medium length, and the atomic density on the harmonic yield produced in a pulsed gas jet. We identify different saturation regimes, thanks to the excellent quantitative agreement obtained with detailed three-dimensional ~3D! simulations. In particular, we clearly observe the interplay between phase matching, absorption, and defocusing, the main limiting factors of the macroscopic emission. Absolute photon number measurements in the optimal conditions give conversion efficiencies as high as those reported using ultrashort (,20 fs) laser pulses, but now with ten times more energy. Using a f 55 m lens, we show that even higher conversion efficiencies can be obtained, resulting in harmonic energies in excess of 1 mJ. The experiments were performed on the LUCA laser facility with an amplified Ti:sapphire system delivering 60 fs pulses at 800 nm, with an energy of up to 100 mJ at 20 Hz. In our experiment, a pumping energy of 27 mJ was focused with either a f 52 m or 5 m lens in a pulsed gas jet. The nozzle was formed by a slit of dimensions 300 mm3 3m m producing a jet at pressure 10‐100 Torr characterized by Mach-Zehnder interferometry. By rotating the jet relative to the laser axis, we can change the length of the generating medium, while keeping the same peak density. Harmonics produced in the jet are analyzed by an XUV spectrometer without entrance slit @17#, and detected with a calibrated XUV photodiode blinded for diffused IR light with two 100 nm Al filters. The absolute spectrometer response~as well as the filter transmission! was measured using the harmonic radiation as a source further monochromatized with another spectrometer, like a synchrotron beam line. Since the total aperture of the laser beam was 40 mm, the

Topology for hybrid multilevel inverter

Abstract: A new topology for a hybrid multilevel inverter is presented, which significantly increases the level number of the output waveform and thereby dramatically reduces the low-order harmonics and total harmonic distortion. To the best of the authors' knowledge, the presented topology has the greatest level number for a given number of stages. Moreover, the stage with higher DC link voltage has lower switching frequency; and thereby reduces the switching losses. Comparison of the results of various multilevel inverters is investigated to reflect the merits of the presented topology. The details of the PWM control using the harmonic elimination technique for the hybrid inverter are presented and confirmed by both simulation and experimental results.

Generation of highly coherent submicrojoule soft x rays by high-order harmonics

Abstract: We demonstrated the energy scaling of high-order harmonics using a self-guided beam under the phase-matched condition. By adjusting the argon gas density and pump laser focusing condition, a total output harmonic energy as high as $0.7 \ensuremath{\mu}\mathrm{J}$ was obtained in the spectral region of 34.8 to 25.8 nm (the corresponding order of the 23rd to 31st harmonic), while the 27th-order harmonic (29.6 nm) energy attained was as high as $0.3 \ensuremath{\mu}\mathrm{J}.$

An investigation on the validity of power-direction method for harmonic source determination

Abstract: The power-direction method has been used widely to identify the locations of harmonic sources in a power system. A number of utility-customer disputes over who is responsible for harmonic distortions have been settled with the help of the method. A closer examination of the method, however, reveals that it is unable to fulfil the task of harmonic source detection. Case studies can easily show that the method yields incorrect results. In this paper, problems associated with the method are investigated using case studies and mathematical analysis. The results show that the power direction method is theoretically incorrect and should not be used to determine harmonic source locations. The main cause of the problem is that the direction of active power flow is a function of the phase-angle difference between the two sources. The direction of reactive power flow, on the other hand, has a better correlation with the source magnitudes.

Generation of 10- microJ coherent extreme-ultraviolet light by use of high-order harmonics.

Abstract: We demonstrate the generation of 10‐µJ coherent extreme-ultraviolet (XUV) light at wavelengths from 73.6 to 42.6 nm, using high-order harmonics. The peak power of this coherent XUV light is estimated to be 0.13 GW at 62.3 nm, and the peak brightness achieved was 3×1028 photons/mm2 mrad2 s. To our knowledge, this XUV energy is the highest value achieved with high-order harmonics.

A new hybrid active power filter (APF) topology

Abstract: In this paper, a new hybrid active power filter topology is presented. A higher-voltage, low-switching frequency insulated gate bipolar transistor (IGBT) inverter and a lower-voltage high-switching frequency metal oxide semiconductor field effect transistor (MOSFET) inverter are used in combination to achieve harmonic current compensation. The function of the IGBT inverter is to support utility fundamental voltage and to compensate for the fundamental reactive power. The MOSFET inverter fulfills the function of harmonic current compensation. To further reduce cost and to simplify control, the IGBT and MOSFET inverters share the same DC-link via a split capacitor bank. With this approach harmonics can be cancelled over a wide frequency range. Compared to the conventional APF topology, the proposed approach employs lower dc-link voltage and generates less noise. Simulation and experimental results show that the proposed active power filter topology is capable of compensating for the load harmonics.

Analysis and Application of Chorochronic Periodicity in Turbomachinery Rotor/Stator Interaction Computations

Abstract: A methodology for predicting the three-dimensional unsteady aerodynamics of the interaction between two turbomachinery blade rows that are in relative angular motion with one another is described. In this case, the kinematics of the blades introduce a chorochronic (space‐time) periodicity. This periodicity is analyzed in detail, and a mathematically straightforward methodology, based on Fourier series in µ (azimuth) and t (time), is presented for treating the interface between the two rows. These results are implemented in a computational method solving the three-dimensional Favre ‐Reynolds-averaged Navier ‐Stokes equations, with a near-wall wall-normalfree Reynolds-stress model. Only one blade passage per blade row is discretized. At the pitchwise boundaries, phase-lagged periodicity is applied using Fourier series in time. Both the pitchwise boundaries time harmonics and the interface chorochronic tµ harmonics are updated using a low-storage moving-averages technique. Computational results are presented and compared with measurements for a 1 1 -stage turbine, where the two stators have the same number of blades enabling the use of chorochronic periodicity. Sample results are also presented for a transonic inlet guide vane/rotor interaction, illustrating the ability of the interface treatment to handle shock waves.

IEEE trial-use standard 1459-2000, definitions for the measurement of electric power quantities under sinusoidal, nonsinusoidal, balanced, or unbalanced conditions

Abstract: The definitions for active, reactive, and apparent powers that are currently used are based on the knowledge developed and agreed upon during the 1940s. Such definitions served the industry well, as long as the current and voltage waveforms remained nearly sinusoidal. Important changes have occurred in the last 50 years. New definitions in the Trial-Use Standard were developed to give guidance with respect to the quantities that should be measured or monitored for revenue purposes, engineering economic decisions, and determination of major harmonic polluters. The panel will discuss these issues.

A comparative study of carrier-frequency modulation techniques for conducted EMI suppression in PWM converters

Abstract: A rigorous mathematical analysis and a comparative study of carrier-frequency modulation (CFM) techniques for the conducted electromagnetic interference (EMI) suppression in pulsewidth-modulated converters is presented. CFM techniques dither the switching period with a small amplitude variation around the nominal value, so that the harmonic power is redistributed over the spectrum of concern. Two types of dithering signals, including the periodic and random signals, are investigated in this paper. The operational characteristics as well as the input and output power spectra of the converters with the two modulating signals are compared. In particular, their characteristics in the low- and high-frequency harmonic power redistribution will be depicted. It is shown that random CFM (RCFM) gives a more effective way to disperse the harmonics around the switching frequency than the periodic CFM (PCFM) with the same frequency deviation. However, RCFM introduces higher low-frequency harmonics than the PCFM at the converter output. Furthermore, effects of the resolution filter bandwidth in the electromagnetic compatibility analyzer on conducted EMI measurement is discussed. The validity of the analyses is confirmed experimentally by using a dc/dc buck converter operating in continuous conduction mode.

A "critical impedance"-based method for identifying harmonic sources

Abstract: This paper proposes a new method to determine whether the utility or the customer side has more contribution to the harmonic currents measured at the point of common coupling. The method is inspired by the observation that the direction of harmonic reactive power, instead of active power, is a more reliable indicator on the location of dominant harmonic sources. The method needs approximate impedance information to operate. Mathematical analysis, simulation studies, and field measurements have shown that this is a useful, reliable, and practical solution for the harmonic source detection problem.

Novel frequency-domain-based technique to detect stator interturn faults in induction machines using stator-induced voltages after switch-off

Abstract: Traditionally, for medium- and high-voltage motors and generators, condition-based monitoring of stator faults is performed by measuring partial discharge activities. For low-voltage machines, negative-sequence impedance or currents are measured for the same. Such diagnostic schemes should be carefully implemented as supply voltage unbalance, manufacturing-related asymmetry, etc., also produce negative-sequence voltages. A few approaches based on motor current signature analysis have already been proposed to detect stator interturn faults. However, little or no physical insight was provided to explain the occurrence of certain harmonics in the line current or the influence of voltage unbalance on these harmonics. Also, in at least one of these papers, a large portion of the stator winding was shorted to emulate the faults. The method proposed in this paper monitors certain rotor-slot-related harmonics at the terminal voltage of the machine, once it is switched off. In the absence of supply voltage, issues such as voltage unbalance, time harmonics do not influence the measurements except as initial conditions, which is a very desirable feature when the machine is fed from an adjustable-speed drive. Satisfactory simulation and experimental results have been obtained with only about 1.5% (5/324) of the total number of turns shorted.

Compensated synchronous PI current controller in overmodulation range and six-step operation of space-vector-modulation-based vector-controlled drives

Abstract: Overmodulation enhances the power utilization of the installed capacity of a voltage-source inverter. A space-vector strategy is used for constant-switching-frequency inverters. In order to achieve the overmodulation, a modified reference signal with nonuniform angular velocity is generated using a preprocessor. Such a reference wave produces low-frequency harmonics in currents. The presence of current harmonics restricts the bandwidth of the synchronous proportional plus integral current controller in the overmodulation range. A compensating current control is presented to allow for high-bandwidth current control in synchronous coordinates during overmodulation and six-step. The proposed scheme allows for an easy upgrade of a conventional vector control scheme to include overmodulation and, thus, reduce the design-to-market time.

A research on photovoltaic energy controlling system with maximum power point tracking

Abstract: This paper introduces a photovoltaic energy system, controlled by a DC-DC converter and a single phase bi-directional PWM converter to realize the inversion. A current controlling (MPPT) method of tracking the maximum power point and forcing the system to operate close to this point is used. An artificial neural network is used in the MPPT system and its robustness and insensitivity to the intermittent weather conditions is enhanced. Uc3854 is used as an inversion current controller, which has a high performance in harmonics and power factor.

Diagnosis of breaks in substation's grounding grid by using the electromagnetic method

Abstract: According to the method of moment, a new method to analyze the grid fed by harmonic currents is presented. The method can be used for the condition that the grid is in at least ten-layer soil with a frequency of the injected currents up to 1 MHz. There can be more than one injected current, and the grounding conductor of the grid can be put in any form. Validation of the method is presented by comparing it with other existing methods. As an application of the presented method, breaks in the substation's grounding grid are diagnosed by the measured voltages on the surface of the ground.

Eliminating harmonics in a multilevel converter using resultant theory

Abstract: A method is given to determine conditions for which the switching angles in a multilevel converter can be chosen to produce the required fundamental voltage while at the same time cancel out higher order harmonics. A complete analysis is given for a 7- level converter where it is shown that for a range of the modulation index m/sub I/, the switching angles can be chosen to produce the desired fundamental V/sub 1/=m/sub I/(s4V/sub dc///spl pi/) while making the 5/sup th/ and 7/sup th/ harmonics identically zero.

AC/DC cascaded power converters having high DC conversion ratio and improved AC line harmonics

Abstract: AC/DC cascaded power converters having high DC conversion ratio and improved AC line harmonics provide low input harmonic currents, high power factor and efficient operation for low voltage DC outputs when coupled directly to a source of unfiltered rectified AC voltage. The power converter incorporates an intermediate storage element that provides most or all of the energy storage capacitance within the power converter and a blocking device that enables continuous energy transfer from AC line to output to achieve unity power factor and regulated output while maintaining low AC input current ripple.

A nonlinear control of the instantaneous power in dq synchronous frame for PWM AC/DC converter under generalized unbalanced operating conditions

Abstract: This paper proposes a new control scheme for regulating the instantaneous power for PWM AC/DC type rectifiers under generalized unbalanced operating conditions. By nullifying the oscillating components of instantaneous power at the poles of the converter instead of the front-end through solving a set of nonlinear control equations in real time, the harmonics in the output DC voltage can be eliminated more effectively under generalized unbalanced operating conditions on the AC input side. The control scheme allows the PWM rectifier to generate a DC output without substantial even-order harmonics and to maintain nearly unity power factor under generalized unbalanced operating conditions, which makes it possible to reduce the size of the DC-link capacitor and AC inductors leading to the possibility of reduced total cost. Simulation results along with experimental results under the two examples of the unbalanced operating conditions confirm the feasibility of the new control method.