Showing papers on "Harmonics published in 2007"

High Performance Current Controller for Selective Harmonic Compensation in Active Power Filters

Abstract: A new current control scheme for selective harmonic compensation is proposed for shunt active power filters The method employs an array of resonant current controllers, one for the fundamental, and one for each harmonic, implemented in fundamental reference frame in order to reduce the overall computational effort The proposed controller design is based on the pole-zero cancellation technique, taking into account the load transfer function at each harmonic frequency Two design methods are provided, which give controller transfer functions with superior frequency response The complete current controller is realized as the superposition of all individual harmonic controllers The frequency response of the entire closed loop control is optimal with respect to filtering objectives, ie, the system provides good overall stability and excellent selectivity for interesting harmonics This conclusion is supported by experimental results on a 76-kVA laboratory filter, indicating a reduction in current THD factor from 34% to 2%, while the highest harmonic compensated is the 37th harmonic current

Non-linear electromagnetic response of graphene

Abstract: It is shown that the massless energy spectrum of electrons and holes in graphene leads to the strongly non-linear electromagnetic response of this system. We predict that the graphene layer, irradiated by electromagnetic waves, emits radiation at higher frequency harmonics and can work as a frequency multiplier. The operating frequency of the graphene frequency multiplier can lie in a broad range from microwaves to the infrared.

A Single-Stage Three-Phase Grid-Connected Photovoltaic System With Modified MPPT Method and Reactive Power Compensation

Abstract: Single-stage grid-connected photovoltaic (PV) systems have advantages such as simple topology, high efficiency, etc. However, since all the control objectives such as the maximum power point tracking (MPPT), synchronization with the utility voltage, and harmonics reduction for output current need to be considered simultaneously, the complexity of the control scheme is much increased. This paper presents the implementation of a single-stage three-phase grid-connected PV system. In addition to realize the aforementioned control objectives, the proposed control can also remarkably improve the stability of the MPPT method with a modified incremental conductance MPPT method. The reactive power compensation for local load is also realized, so as to alleviate grid burden. A DSP is employed to implement the proposed MPPT controller and reactive power compensation unit. Simulation and experimental results show the high stability and high efficiency of this single-stage three-phase grid-connected PV system.

Interactions Between Fuel Cells and Power Converters: Influence of Current Harmonics on a Fuel Cell Stack

Abstract: As fuel cells are likely to be used in many future applications, dedicated power converters must be developed and optimized. A thorough knowledge of the fuel cell operation is thus required for power electronics engineers. This paper proposes a theoretical and experimental study of the behavior of a fuel cell stack subject to current harmonics. The fundamental role of the internal double layer capacitor is demonstrated

Repetitive-Based Controller for a UPS Inverter to Compensate Unbalance and Harmonic Distortion

Abstract: This paper discusses a repetitive-based controller for an uninterruptible power supply (UPS) inverter. It is shown that a bank of resonant filters, used as a refinement term for harmonic compensation in earlier works, is equivalent to a repetitive scheme with a particular structure. The latter is implemented using a simple feedback array with a delay line, thus making the implementation relatively easy. More precisely, the repetitive scheme takes a negative feedback structure plus a feedforward path whenever the odd harmonics are considered for compensation only. The repetitive scheme, equivalent to the bank of resonant filters, acts as a refinement term to reject the harmonic distortion caused by the unbalanced and distorted load current, and thus, allowing the UPS inverter to deliver an almost pure sinusoidal balanced voltage. Experimental results in a 1.5 KVA three-phase inverter are included to show the performance of the proposed controller

Design of a New Cooperative Harmonic Filtering Strategy for Distributed Generation Interface Converters in an Islanding Network

Abstract: Increasing demand for premium electric power, in terms of both quality and reliability, and emerging new energy technologies have led to the development of distributed generation systems Due to rapid penetration of power electronics equipment, many of the loads within distributed generation systems are nonlinear in nature, and the resulting harmonics pollution needs to be addressed This paper proposes a new cooperative harmonic filtering strategy for the interface converters of distributed generation sources A droop control method based on the reactive volt-ampere consumption of harmonics of each interface converter is designed and implemented In this strategy, the overall harmonic filtering workload can be evenly shared without any communications The operation principle is explained in detail Computer simulations and laboratory test results validate the distributed harmonic damping capability of the proposed strategy

Passive Filter Design for Three-Phase Inverter Interfacing in Distributed Generation

Abstract: With the growing use of inverters in distributed generation, the problem of injected harmonics becomes critical. These harmonics require the connection of low pass filters between the inverter and the network. This paper presents a design method for the output LC filter in grid coupled applications in distributed generation systems. The design is according to the harmonics standards that determine the level of current harmonics injected into the grid network. Analytical expressions for the maximum inductor ripple current are derived. The filter capacitor design depends on the allowable level of switching components injected into the grid. Different passive filter damping techniques to suppress resonance affects are investigated and evaluated. Simulated results are included to verify the derived expressions.

A Unified Artificial Neural Network Architecture for Active Power Filters

Abstract: In this paper, an efficient and reliable neural active power filter (APF) to estimate and compensate for harmonic distortions from an AC line is proposed. The proposed filter is completely based on Adaline neural networks which are organized in different independent blocks. We introduce a neural method based on Adalines for the online extraction of the voltage components to recover a balanced and equilibrated voltage system, and three different methods for harmonic filtering. These three methods efficiently separate the fundamental harmonic from the distortion harmonics of the measured currents. According to either the Instantaneous Power Theory or to the Fourier series analysis of the currents, each of these methods are based on a specific decomposition. The original decomposition of the currents or of the powers then allows defining the architecture and the inputs of Adaline neural networks. Different learning schemes are then used to control the inverter to inject elaborated reference currents in the power system. Results obtained by simulation and their real-time validation in experiments are presented to compare the compensation methods. By their learning capabilities, artificial neural networks are able to take into account time-varying parameters, and thus appreciably improve the performance of traditional compensating methods. The effectiveness of the algorithms is demonstrated in their application to harmonics compensation in power systems

Frequency response of cantilever beams immersed in viscous fluids with applications to the atomic force microscope: Arbitrary mode order

Abstract: The frequency response of a cantilever beam is well known to depend strongly on the fluid in which it is immersed. In this article, we present a theoretical model for the frequency response of a rectangular cantilever beam immersed in a viscous fluid that enables the flexural and torsional modes of arbitrary order to be calculated. This extends the previous models of Sader and Green [J. Appl. Phys. 84, 64 (1998); 92, 6262 (2002)], which were formulated primarily for the fundamental mode and the next few harmonics, to the general case of arbitrary mode order by accounting for the three-dimensional nature of the flow field around the cantilever beam. Due to its importance in atomic force microscope applications, results for the thermal noise spectrum are presented and the influence of mode order on the frequency response investigated.

A Flexible Selective Harmonic Mitigation Technique to Meet Grid Codes in Three-Level PWM Converters

Abstract: Due to the development of new grid codes, power converters' output signal harmonic control is currently becoming extremely important in medium- and high-power applications. By taking this new scenario into account, a new method to generate switching three-level pulsewidth-modulation (PWM) patterns to meet specific grid codes is presented. The proposed method, which is named selective harmonic mitigation PWM , generates switching three-level PWM patterns with high quality from the point of view of harmonic content, avoiding the elimination of some specific harmonics and studying all harmonics and the total harmonic distortion as a global problem by using a general-purpose random-search heuristic algorithm. This fact leads to a drastic reduction or even avoidance of the bulky and costly grid connection tuned filters of power systems. Any harmonic shaping can be considered due to the flexibility of the method. Power devices switching constraints are considered to obtain directly applicable results. As a practical example, limits from one actual grid code have been used to get the experimental results by means of a 150-kVA three-level diode-clamped converter test bench. Comparisons between the proposed technique, optimized PWM and Selective Harmonic Elimination methods have been carried out. The results obtained with this new method greatly improve previous ones.

Eliminating ground current in a transformerless photovoltaic application

Abstract: A single phase converter can be used for low-power grid connected applications. In photovoltaic applications it is possible to remove the transformer in the inverter in order to reduce losses, costs and size. Galvanic connection of the grid and the DC sources in transformerless systems can introduce additional leakage currents because of the earth parasitic capacitance. This currents increase conducted and radiated electromagnetic emissions, harmonics injected in the utility grid and system losses. Amplitude and spectrum of ground current depends on the converter topology, on the switching strategy and on the resonant circuit formed by the ground capacitance, the converter, the AC filter and the grid. In this paper, the leakage current in a 1.5 kW photovoltaic installation is measured. The installation includes a string of sixteen panels, a full bridge inverter and a LCL filter. Influence of inverter topology and modulation strategy on the magnitude of the leakage current is presented. Finally, the use of neutral point clamped inverters in transformerless photovoltaic applications is studied.

Electric Power Generation, Transmission, and Distribution

Abstract: ELECTRIC POWER GENERATION: NONCONVENTIONAL METHODS Wind Power Gary L. Johnson Advanced Energy Technologies Saifur Rahman Photovoltaics Roger A. Messenger ELECTRIC POWER GENERATION: CONVENTIONAL METHODS Hydroelectric Power Generation Steven R. Brockschink, James H. Gurney, and Douglas B. Seely Synchronous Machinery Paul I. Nippes Thermal Generating Plants Kenneth H. Sebra Distributed Utilities John R. Kennedy TRANSMISSION SYSTEM Concept of Energy Transmission and Distribution George G. Karady Transmission Line Structures Joe C. Pohlman Insulators and Accessories George G. Karady and R.G. Farmer Transmission Line Construction and Maintenance Wilford Caulkins and Kristine Buchholz Insulated Power Cables used in Underground Applications Michael L. Dyer Transmission Line Parameters Manuel Reta-Hernandez Sag and Tension of Conductor D.A. Douglass and Ridley Thrash Corona and Noise Giao N. Trinh Geomagnetic Disturbances and Impacts upon Power System Operation John G. Kappenman Lightning Protection William A. Chisholm Reactive Power Compensation Rao S. Thallam Environmental Impact of Transmission Lines George G. Karady DISTRIBUTION SYSTEMS Power System Loads Raymond R. Shoults and Larry D. Swift Distribution System Modeling and Analysis William H. Kersting Power System Operation and Control George L. Clark and Simon W. Bowen Hard to Find Information (on Distribution System Characteristics and Protection) Jim Burke Real-Time Control of Distributed Generation Murat Dilek and Robert P. Broadwater ELECTRIC POWER UTILIZATION Metering of Electric Power and Energy John V. Grubbs Basic Electric Power Utilization-Loads, Load Characterization and Load Modeling Andrew Hanson Electric Power Utilization: Motors Charles A. Gross POWER QUALITY Introduction S.M. Halpin Wiring and Grounding for Power Quality Christopher J. Melhorn Harmonics in Power Systems S.M. Halpin Voltage Sags Math H.J. Bollen Voltage Fluctuations and Lamp Flicker in Power Systems S.M. Halpin Power Quality Monitoring Patrick Coleman INDEX

Coherent superposition of laser-driven soft-X-ray harmonics from successive sources

Abstract: High-order harmonic generation from atoms ionized by femtosecond laser pulses has been a promising approach for the development of coherent short-wavelength sources. However, the realization of a powerful harmonic X-ray source has been hampered by a phase velocity mismatch between the driving wave and its harmonics, limiting their coherent build-up to a short propagation length and thereby compromising the efficiency of a single source. Here, we report coherent superposition of laser-driven soft-X-ray (SXR) harmonics, at wavelengths of 2–5 nm, generated in two successive sources by one and the same laser pulse. Observation of constructive and destructive interference suggests the feasibility of quasi-phase-matched SXR harmonic generation by a focused laser beam in a gas medium of modulated density. Our proof-of-concept study opens the prospect of enhancing the photon flux of SXR harmonic sources to levels enabling researchers to tackle a range of applications in physical as well as life sciences.

Neural Network-Based Selective Compensation of Current Quality Problems in Distribution System

Abstract: Active power filters (APFs) have been used to compensate harmonics, reactive current, and negative sequence fundamental frequency current drawn by nonlinear loads. The control of APF is the core issue for their proper operation. The flexibility of selective compensation embedded in the control scheme makes APF versatile for compensation of reactive power, harmonic currents, and unbalance in source currents and their combinations, depending upon the limited rating of voltage source inverter employed as APF. The proposed scheme utilizes neural network-based decomposition of the load current into positive and negative sequence fundamental frequency component, reactive component and harmonic components. The adaline-based current decomposer estimates the reference currents through tracking of unit vectors together with tuning of the weights. The implementation of the control scheme facilitates selective compensation which respects the limited rating of the APF. The simulated results using developed MATLAB model are presented and are validated by experimental results to depict the effectiveness of the proposed control method of APF

Electron Bernstein wave heating and diagnostic

Abstract: This paper gives a review on the experiments with electron Bernstein waves (EBWs) in fusion devices. The different methods of EBW generation are described and compared with experimental results. The influence of density fluctuation and parametric instability on the conversion efficiency is discussed. The related experiments are reported. The EBW propagation is calculated by ray-tracing codes. The results are used to analyse EBW emission, heating and current drive experiments in stellarators and tokamaks. With high power microwave sources EBWs have been excited over a wide range of frequencies for plasma heating and current drive. The experimental results demonstrated that EBW can efficiently heat over-dense plasmas. The local power deposition allows the generation of heat waves for transport studies. Due to their electrostatic character, EBWs can achieve parallel refractive indices (N||) larger than 1, which is favourable for efficient current drive. This could be confirmed by a first current drive experiment. The EBWs also express a strong cyclotron damping, which enabled efficient heating at higher harmonics in several experiments.

Frequencies in the Vibration Induced by the Rotor Stator Interaction in a Centrifugal Pump Turbine

Abstract: The highest vibration levels in large pump turbines are, in general, originated in the rotor stator interaction (RSI). This vibration has specific characteristics that can be clearly observed in the frequency domain: harmonics of the moving blade passing frequency and a particular relationship among their amplitudes. It is valuable for the design and condition monitoring to count on these characteristics. A CFD model is an appropriate tool to determine the force and its characteristics. However, it is time consuming and needs highly qualified human resources while usually these results are needed immediately and in situ. Then, it is useful to determine these characteristics in a simple, quick, and accurate method. At present, the most suitable method indicates a large amount of possible harmonics to appear, without indicating the relative importance of them. This paper carries out a theoretical analysis to predict and explain in a qualitative way these frequencies and amplitudes. The theoretical analysis incorporates the number of blades, the number of guide vanes, the RSI nonuniform fluid force, and the sequence of interaction. This analysis is compared with the method currently in use, and both methods are applied to a practical case. The theoretical analysis gives a resulting force over the pump turbine, which corresponds well to the measured behavior of a pump turbine in terms of its frequencies and the relationship between their amplitudes. A corrective action is proposed as a result of the analysis and after it is carried out in one of the units, the vibration levels are reduced. The vibration induced by the RSI is predicted considering the sequence of interaction and different amplitudes in the interactions between the same moving blade and different stationary blades, giving a different and original interpretation about the source of the vibration characteristics. A successful corrective action is proposed as a consequence of this new interpretation.

An Investigation of the Harmonic Emissions of Wind Turbines

Abstract: Harmonic emissions of variable speed wind turbines (WTs) are a well-known issue, but little information that is based on field measurements on actual machines has been published on the subject. In this paper, the harmonic behavior of modern WTs using extensive measurements performed on commercially available machines is discussed. Specific issues addressed include the shape and the frequency range of the harmonic current spectrum, the variation of the harmonics with the WT operating point, the statistical characteristics of their magnitude and phase angle, the effect of grouping and time-averaging, as well as their symmetrical component characteristics. The objective of the paper is to outline important general characteristics of the harmonic behavior of WTs, rather than to analyze specific machines or compare alternative WT designs

Enhanced phase-matching for generation of soft X-ray harmonics and attosecond pulses in atomic gases

Abstract: Focus Serial: Frontiers of Nonlinear Optics We theoretically investigate the generation of high harmonics and attosecond pulses by mid-infrared (IR) driving fields. Conditions for coherent build-up of high harmonics are revisited. We show that the coherence length dictated by ionization-induced dephasing does not constitute an ultimate limitation to the coherent growth of soft X-ray (> 100 eV) harmonics driven by few-cycle mid-IR driving pulses: perfect phase-matching, similar to non-adiabatic self-phase- matching, can be achieved even without non-linear deformation of the driving pulse. Our trajectory-based analysis of phase-matching reveals several important advantages of using longer laser wavelengths: conversion efficiency can be improved by orders of magnitude, phase-matched build-up of harmonics can be achieved in a jet with a high gas pressure, and isolated attosecond pulses can be extracted from plateau harmonics.

Power Quality Problems Compensation With Universal Power Quality Conditioning System

Abstract: The aim of this paper is to present a universal power quality conditioning system (UPQS) named after unified power quality conditioner, which is extended by adding a shunt active filter at the load side. Its main purpose is to compensate for both supply voltage and load current imperfections, such as: sags, swells, interruptions, imbalance, flicker, harmonics, reactive currents and current unbalance. Such system has better harmonic compensation characteristic, since there are no problems associated with passive filter operation. Converter and control analysis is presented, together with results showing the UPQS modes of operation

Electromechanical energy conversion systems

Abstract: An exemplary power system may include an electric machine with multiple sets of stator windings, each set of windings being coupled through a separate switch matrix to a common voltage bus, and each of which may be spatially arranged in full pitch around the stator such that stator flux harmonics are substantially reduced. The reduced stator flux harmonics may be associated with phase current harmonic content. In an example application, such power systems may operate in a generating mode to transfer mechanical energy to electrical energy on a DC voltage bus. In some illustrative embodiments, the power system may provide both high-power and high-speed (e.g., 1MW at 8000 rpm or above) motoring and/or generating capability suitable, for example, for on-board (e.g., marine, aviation, traction) power systems.

High-order harmonic generation in a laser plasma: a review of recent achievements

Abstract: A review of studies of high-order harmonic generation in plasma plumes is presented. The generation of high-order harmonics (up to the 101st order, λ = 7.9 nm) of Ti:sapphire laser radiation during the propagation of short laser pulses through a low-excited, low-ionized plasma produced on the surfaces of different targets is analysed. The observation of considerable resonance-induced enhancement of a single harmonic (λ = 61.2 nm) at the plateau region with 10−4 conversion efficiency in the case of an In plume can offer some expectations that analogous processes can be realized in other plasma samples in the shorter wavelength range. Recent achievements of single-harmonic enhancement at mid- and end-plateau regions are discussed. Various methods for the optimization of harmonic generation are analysed, such as the application of the second harmonic of driving radiation and the application of prepulses of different durations. The enhancement of harmonic generation efficiency during the propagation of femtosecond pulses through a nanoparticle-containing plasma is discussed.

Leakage current evaluation of a singlephase transformerless PV inverter connected to the grid

Abstract: For low-power grid connected applications a single phase converter can be used. In PV applications it is possible to remove the transformer in the inverter in order to reduce losses, costs and size. Galvanic connection of the grid and the DC sources in transformerless systems can introduce additional leakage currents due to the earth parasitic capacitance. This currents increase conducted and radiated electromagnetic emissions, harmonics injected in the utility grid and losses. Amplitude and spectrum of leakage current depends on the converter topology, on the switching strategy and on the resonant circuit formed by the ground capacitance, the converter, the AC filter and the grid. In this paper, the leakage current in a 1.5 kW PV installation is measured under different conditions and used to build simulation model. The installation includes a string of sixteen PV panel, a full bridge inverter and a LCL filter. This model allows studying the influence of the harmonics injected by the inverter on the leakage current.

Online Wavelet Transform-Based Control Strategy for UPQC Control System

Abstract: In this paper, a new algorithm is proposed to estimate amplitude and phase angle (and frequency) of load currents and source voltages in the presence of harmonics and frequency oscillation. Moreover, a comprehensive control strategy is introduced to extract the compensating signals for the control of series and shunt converters of the unified power-quality conditioner (UPQC). The basic approach used in this algorithm is to extract the fundamental component of load current and source voltage by using wavelet-transform decomposition. Then, the amplitude, phase angle, and frequency of fundamental component are calculated using the well-known LS algorithm, which is relatively simple and fast. The voltage and current references of UPQC active power filters are obtained using the positive sequence of three-phase system voltages and load currents. The proposed control strategy can extract the reference currents and voltages of UPQC fast and accurately in the presence of harmonics and/or frequency oscillation. Simulation results have been presented to show improvement of the UPQC performance in the presence of frequency and amplitude variation condition and various forms of current and voltage distortion

A Harmonically Coupled Admittance Matrix Model for AC/DC Converters

Abstract: This paper proposes a new method to model the harmonic generating characteristics of AC/DC converters. The model transforms the time domain nonlinear characteristic of the converter into a frequency domain linear admittance matrix. The matrix represents the coupling among the converter AC side harmonic voltages and currents accurately and it does not vary with the harmonic conditions of the system. The proposed model opens up a new way to conduct harmonic power flow studies. This paper presents the theoretical foundation and analytical derivation of the admittance model for both single-phase and three-phase bridge rectifiers. Potential applications of the model are discussed.

A DSP Based Optimal Algorithm for Shunt Active Filter Under Nonsinusoidal Supply and Unbalanced Load Conditions

Abstract: The use of power electronic circuits in a wide range of applications has resulted in distorted current waveforms in the power system. This results in nonsinusoidal voltage drops across the transformers and transmission line impedances, resulting in a nonsinusoidal voltage supply at the point of common coupling. Asymmetrical distribution of large, 1-phi loads further complicates the issue by causing imbalance in the line currents of the 3-phi system. Not only should this imbalance be compensated, other parameters, such as the current total harmonic distortion and the power factor should also be maintained as per the norms. However, the requirements of harmonic free current waveforms and good power factor, under nonsinusoidal voltage conditions, are contradictory to each other. Under these conditions, an optimum performance is the best one can achieve. This paper proposes a new scheme for balancing the currents and obtaining the best compromise between the power factor and current distortion under nonsinusoidal voltage conditions. The technique does not use p-q theory and does not require sequence transformation for power calculations, even though the current is unbalanced. All the details of this work are presented

A Comparative Analysis of Real-Time Algorithms for Power Signal Decomposition in Multiple Synchronous Reference Frames

Abstract: The monitoring and rejection of voltage and current harmonics in power electronics applications such as power quality conditioners or distributed generation systems require correct estimation algorithms especially if the harmonic amplitudes are time varying. Power signal decomposition in multiple synchronous rotating reference frames (MSRFs) is considered one of the best solutions. The most commonly employed implementations of this signal transformation are based on phase-locked loops (PLLs), recursive discrete Fourier transforms (RDFT), or discrete Kalman filtering (DKF). In this paper, a rigorous analysis of the performance of these implementations has been carried out. Complete tests have been performed to evaluate the computational burden, the frequency domain response, and the tolerance to low frequency amplitude variations. The results make it possible to select the proper method depending on the requirements of each application.

Dramatic enhancement of high-order harmonic generation.

Abstract: We present a dramatic enhancement [Phys. Rev. Lett. 91, 043002 (2003)] of high-order harmonic generation by simultaneous irradiation of booster harmonics. A key feature of our experiment is the use of mixed gases (Xe and He) with different ionization energies. The harmonics from Xe atoms act as a booster to increase the harmonic yield from He by a factor of $4\ifmmode\times\else\texttimes\fi{}{10}^{3}$. The dominance of the dramatic enhancement effect is supported by simulation with the time-dependent Schr\"odinger equation as well as the observed spatial characteristic of the generated harmonics and dependence on medium conditions.