Showing papers on "Harmonics published in 2004"

A new control structure for grid-connected LCL PV inverters with zero steady-state error and selective harmonic compensation

Abstract: The PI current control of a single-phase inverter has well known drawbacks: steady-state magnitude and phase error and limited disturbance rejection capability. When the current controlled inverter is connected to the grid, the phase error results in a power factor decrement and the limited disturbance rejection capability leads to the need of grid feed-forward compensation. However the imperfect compensation action of the feed-forward control results in high harmonic distortion of the current and consequently noncompliance with international standards. In this paper a new control strategy aimed to mitigate these problems is proposed. Stationary-frame generalized integrators are used to control the fundamental current and to compensate the grid harmonics providing disturbance rejection capability without the need of feed-forward grid compensation. Moreover the use of a grid LCL-filter is investigated with the proposed controller. The current control strategy has been experimentally tested with success on a 3 kW PV inverter.

Multi frequency solution of chatter stability for low immersion milling

Abstract: Finish milling is usually required in the peripheral milling of thin aircraft webs with long end mills, where the structures are flexible and radial depths of cut are small. The spindle speed and depth of cut must be selected optimally to avoid both forced and chatter vibrations, which in turn enables production of the parts within specified tolerances. Recent articles show that stability pockets differ at certain speeds when the radial immersion in milling is low and the machining process is highly intermittent. This paper presents a stability theory which predicts chatter stability lobes that are not covered by classical chatter theories in which the coupling between the spindle speed and process stability are neglected. The dynamics of low radial immersion milling are formulated as an eigenvalue problem, where harmonics of the tooth spacing angle and spread of the transfer function with the harmonics of the tooth passing frequencies are considered. It is shown that the stability lobes are accurately predicted with the presented method. This paper details the physics involved when the tooth passing frequencies alter the effective transfer function of the structure in the stability solution. The products of the harmonics of the directional coefficients and transfer functions of the structure are evaluated at the natural mode under the influence of tooth passing frequency harmonics in order to obtain the exact locations of chatter stability lobes.

Improved current control strategy for power conditioners using sinusoidal signal integrators in synchronous reference frame

Abstract: In this paper, a current control scheme, based on proportional-integral regulators using sinusoidal signal integrators (SSIs), is proposed for shunt type power conditioners. The aim is to simplify the implementation of SSI-based current harmonic compensation for industrial implementations where strict limitations on the harmonic distortion of the mains' currents are required. To compensate current harmonics, the SSIs are implemented to operate both on positive and negative sequence signals. One regulator, for the fundamental current component, is implemented in the stationary reference frame. The other regulators, for the current harmonics, are all implemented in a synchronous reference frame rotating at the fundamental frequency. This allows the simultaneous compensation of two current harmonics with just one regulator, yielding a significant reduction of the computational effort compared with other current control methods employing sinusoidal signal integrators implemented in stationary reference frame. A simple and robust voltage filter is also proposed by the authors to obtain a smooth and accurate position estimation of the voltage vector at the point of common coupling (PCC) under distorted mains' voltages. The whole control algorithm has been implemented on a 16-b, fixed-point digital signal processor (DSP) platform controlling a 20-kVA power conditioner prototype. The experimental results presented in this paper for inductive and capacitive loads show the validity of the proposed solutions.

Repetitive-based control for selective harmonic compensation in active power filters

Abstract: This paper proposes a repetitive-based controller for active power filters, which compensates selected current harmonics produced by distorting loads. The approach is based on the measurement of line currents and performs the compensation of selected harmonics using a closed-loop repetitive-based control scheme based on a finite-impulse response digital filter. Compared to conventional solutions based on stationary-frame current control, this approach allows full compensation of selected frequencies, even if the active filter has limited bandwidth. Compared to synchronous-frame harmonic regulations on line currents, the complexity of the proposed algorithm is independent of the number of compensated harmonics. Moreover, it is more appropriate for digital signal processor implementation and less sensitive to rounding and quantization errors when finite word length or fixed-point implementation is considered. Experimental results on a 5-kVA prototype confirm the theoretical expectations.

A complete solution to the harmonic elimination problem

Abstract: The problem of eliminating harmonics in a switching converter is considered. That is, given a desired fundamental output voltage, the problem is to find the switching times (angles) that produce the fundamental while not generating specifically chosen harmonics. In contrast to the well known work of Patel and Hoft and others, here all possible solutions to the problem are found. This is done by first converting the transcendental equations that specify the harmonic elimination problem into an equivalent set of polynomial equations. Then, using the mathematical theory of resultants, all solutions to this equivalent problem can be found. In particular, it is shown that there are new solutions that have not been previously reported in the literature. The complete solutions for both unipolar and bipolar switching patterns to eliminate the fifth and seventh harmonics are given. Finally, the unipolar case is again considered where the fifth, seventh, 11th, and 13th harmonics are eliminated along with corroborative experimental results.

Improved analytical modelling of rotor eddy current loss in brushless machines equipped with surface-mounted permanent magnets

Abstract: An improved analytical model for predicting the rotor eddy current loss in brushless machines equipped with surface-mounted permanent magnets is presented. It is formulated in polar co-ordinates and based on the calculation of the two-dimensional electromagnetic field in the airgap/magnet regions, with due account of the eddy current reaction field. It enables the eddy current loss in the permanent magnets and the retaining sleeve, if fitted, to be calculated, and caters for motors having either overlapping or non-overlapping stator windings, as well as any specified load condition. The analysis accounts for both time and space mmf harmonics, but neglects the influence of stator slotting. The model is applied to a brushless DC traction machine in which the rotor loss is due predominantly to time harmonics in the armature reaction field which result from commutation events. The predicted rotor loss is compared with the loss deduced from thermometric measurements and from an analytical magnetostatic model which neglects the eddy current reaction field. Good agreement between predictions and measurements is achieved over the complete operating speed range.

An adaptive notch filter for frequency estimation of a periodic signal

Abstract: Online frequency estimation of a pure sinusoidal signal is a classical problem that has many practical applications. Recently an ANF with global convergence property has been developed for this purpose. There exist some practical applications in which signals are not pure sinusoidal and contain harmonics. Therefore, online frequency estimation of periodic but not necessarily sinusoidal signals espoused by such applications becomes quite important. This note presents an alternative stability analysis for a modified ANF that permits the presence of harmonics in the incoming signal. Also, this stability analysis is simpler and alleviates the problem complexity even in the case of pure sinusoidal signal. Simulation results confirm theoretical issues.

Optimizing uniformly excited linear arrays through time modulation

Abstract: The letter shows that, by the proper use of time modulation in equispaced linear arrays with uniform excitation distribution, it is possible to maintain the sidelobe zone of the radiated power below a certain - previously stipulated - level, whereas the undesired harmonics are minimized. In addition to that, the further extension of the technique to non-equispaced arrays permits a broadband response to be obtained, by simply searching the positions of the elements that reach the desired power pattern behavior within the required bandwidth.

Z-source inverter for motor drives

Abstract: This paper presents a Z-Source inverter system and control for general-purpose motor drives. The Z-source inverter system employs a unique LC network in the DC link and a small capacitor on the AC side of the diode front end. By controlling the shoot-through duty cycle, the Z-source can produce any desired output AC voltage, even greater than the line voltage. As a result, the new Z-source inverter system provides ride-through capability under voltage sags, reduces line harmonics, improves power factor and reliability, and extends output voltage range. Analysis, simulation, and experimental results will be presented to demonstrate these new features.

Decision tree-based methodology for high impedance fault detection

Abstract: This paper presents a high impedance fault (HIF) detection method based on decision trees (DTs). The features of HIF, which are the inputs of DTs, are those well-known ones, including current [in root mean square (rms)], magnitudes of the second, third, and fifth harmonics, and the phase of the third harmonics. The only measurements needed in the proposed method are the current signals sampled at 1920 Hz. It will reduce the cost of hardware compared with methods that use high sampling rates. A new HIF model is also used. The data of current signals are from the simulation of Electromagnetic Transients Program (EMTP). The DT algorithm trained can successfully distinguish the HIFs from most normal operations on simulation data, including switching loads, switching shunt capacitors, and load transformer inrush currents. Testing on experimental data is recommended for future work.

Uninterruptible power supplies and active filters

Abstract: UNINTERRUPTIBLE POWER SUPPLIES Classification Batteries for UPS Applications Flywheels for UPS Applications Comparative Analysis of Flywheels and Electrochemical Batteries Applications of UPS Systems Parallel Operation Performance Evaluation of UPS Systems Power Factor Correction in UPS Systems Control of UPS Systems Converters for UPS Systems Battery Charger/Discharger References ACTIVE FILTERS Harmonic Definition Harmonic Sources in Electrical Systems Effects of Harmonics Harmonic Mitigation Methods Classification of Active Filters Active Filters for DC/DC Converters Modeling and Analysis Control Strategies Stability Assessment Conclusion References UNIFIED POWER QUALITY CONDITIONERS Series-Parallel Configuration Current Control Voltage Control Power Flow and Characteristic Power References REDUCED-PARTS UNINTERRUPTIBLE POWER SUPPLIES Concept of Reduced-Parts Converters Applied to Single-Phase On-Line UPS Systems New On-Line UPS Systems Based on Half-Bridge Converters New On-Line UPS Systems Based on a Novel AC/DC Rectifier New Three-Phase On-Line UPS System with Reduced Number of Switches New Single-Phase to Three-Phase Hybrid Line-Interactive/On-Line UPS System References REDUCED-PARTS ACTIVE FILTERS Reduced-Parts Single-Phase and Three-Phase Active Filters Reduced-Parts Single-Phase Unified Power Quality Conditioners Reduced-Parts Single-Phase Series-Parallel Configurations Reduced-Parts Three-Phase Series-Parallel Configurations References MODELING, ANALYSIS, AND DIGITAL CONTROL System Modeling Using Generalized State Space Averaging Method Digital Control References

Texture decomposition by harmonics extraction from higher order statistics

Abstract: In this paper, a method of harmonics extraction from Higher Order Statistics (HOS) is developed for texture decomposition. We show that the diagonal slice of the fourth-order cumulants is proportional to the autocorrelation of a related noiseless sinusoidal signal with identical frequencies. We propose to use this fourth-order cumulants slice to estimate a power spectrum from which the harmonic frequencies can be easily extracted. Hence, a texture can be decomposed into deterministic components and indeterministic components as in a unified texture model through a Wold-like decomposition procedure. The simulation and experimental results demonstrated that this method is effective for texture decomposition and it performs better than traditional lower order statistics based decomposition methods.

On the modeling and design of dual-stator windings to minimize circulating harmonic currents for VSI fed AC machines

Abstract: The major drawback of usual dual-stator ac machines, when supplied by a voltage-source inverter (VSI), is the occurrence of extra harmonic currents. These extra currents circulate only in the stator windings and cause additional losses. This paper deals with the modeling and design of dual-stator winding ac machines for safe operation with a VSI. A new reference frame model for a dual-stator induction machine (DSIM), including mutual leakage coupling, is proposed. This model allows us to highlight the previously mentioned circulating harmonic currents. The leakage inductance associated with these harmonics is shown to have quite a small value, highly depending on the coil pitch. It is also shown that full pitch is required, and that special slot shape designs should be investigated, to limit the magnitude of circulating currents. Experimental results on a prototype of a DSIM are presented and they show a very good correlation with theoretical curves.

Modeling of power electronic loads in AC distribution systems using the generalized State-space averaging method

Abstract: Most of the loads in ac distribution systems have positive incremental impedance characteristic. However, power electronic loads, when tightly regulated, sink constant power from the system. Therefore, they have negative incremental impedance characteristic. This can cause negative impedance instability. Power electronic loads usually have a controlled or uncontrolled rectifier at the front end. In this paper, these loads are modeled using the generalized state-space averaging method. An assessment of their effects in ac distribution systems is also presented. Experimental results are presented to verify the proposed analysis.

Modeling characteristics of harmonic currents generated by high-speed railway traction drive converters

Abstract: This paper presents a study for modeling harmonic currents injected by three-level pulse-width-modulated (PWM) converters of the high-speed railway traction drive in steady-state motoring mode. An analytical solution for converter harmonics based on the double Fourier series theory is described. The time-domain simulation results obtained by the use of PSpice are then compared with those obtained by the proposed model. It is shown that the harmonic currents determined according to the proposed model agree well with those results obtained by using the time-domain simulation tool.

Resonant harmonic response in tapping-mode atomic force microscopy

Abstract: Higher harmonics in tapping-mode atomic force microscopy offers the potential for imaging and sensing material properties at the nanoscale. The signal level at a given harmonic of the fundamental mode can be enhanced if the cantilever is designed in such a way that the frequency of one of the higher harmonics of the fundamental mode (designated as the resonant harmonic) matches the resonant frequency of a higher-order flexural mode. Here we present an analytical approach that relates the amplitude and phase of the cantilever vibration at the frequency of the resonant harmonic to the elastic modulus of the sample. The resonant harmonic response is optimized for different samples with a proper design of the cantilever. It is found that resonant harmonics are sensitive to the stiffness of the material under investigation.

A novel control algorithm for the DG interface to mitigate power quality problems

Abstract: Distributed Generation (DG) exists in distribution systems and is installed by either the utility or the customers. This paper proposes a novel utilization of the existing DG nonlinear interface not only to control the active power flow, but also to mitigate unbalance and harmonics, and to manage the reactive power of the system. The proposed Flexible Distributed Generation (FDG) is similar in functionality to FACTS, but works at the distribution level. Moreover, a novel ADAptive LINEar neuron (ADALINE) structure is presented. The new structure is applied to multi output (MO) systems for parameter tracking/estimation, and is called MO-ADALINE. It is dedicated to symmetrical components estimation. The control loop combines a Fuzzy Logic Controller (FLC) for voltage regulation, and a processing unit-based ADALINE to deal with unbalance, harmonics and reactive power compensation. One advantage of the proposed control system is its insensitivity to parameter variation, a necessity for distribution system applications. Simulations of the suggested FDG based control algorithm are conducted to evaluate the performance of the novel system.

Power smoothing in wind generation systems using a sensorless vector controlled induction Machine driving a flywheel

Abstract: This paper presents a novel control strategy for power smoothing in generation systems in which power flow variations can occur. These variations are the norm in wind energy generation. The system is based on a sensorless vector controlled induction machine driving a flywheel. The induction machine is controlled to operate in a wide speed range by using flux weakening above rated speed. A speed observer is used to obtain the rotational speed in the whole speed range. In order to tune the speed observer and compensate for the parameter variation and uncertainties, a separate estimation of the speed is obtained from the rotor slot harmonics using an algorithm for spectral analysis. This algorithm can track fast dynamic changes in the rotational speed, with high accuracy. The control strategies have been experimentally verified on a 3.5-kW experimental setup with an induction machine and flywheel. The experimental results show the high level of performance obtained with the proposed sensorless vector control system.

Low-divergence coherent soft x-ray source at 13 nm by high-order harmonics

Abstract: We have generated a few tens of nanojoules of coherent soft x-ray pulses at a wavelength around 13 nm with high-order harmonics. Output energy was estimated to be 25 nJ/pulse at the 59th harmonic (13.5 nm), and the beam divergence was measured to be 0.35 mrad full width at half maximum. Since a Mo/Si mirror covers two harmonics in the high reflectivity region, we can use ∼50 nJ harmonic energy per pulse in the 13 nm wavelength. This high-coherent low-emittance x-ray source is useful for the characterization and inspection of an optical system for extreme-ultraviolet lithography.

A detailed model of induction machines with saturation extendable for fault analysis

Abstract: A review of the literature suggests that modeling of induction machines with saturation has received considerable attention Most of these models are very application specific None of them, however, seems to be completely adequate for the purpose of fault diagnosis This is because for a noninvasive detection technique such as Motor Current Signature Analysis (MCSA), one has to detect different frequency components in the line current of the machine Incorporation of saturation in the induction machine model will help in identifying and analyzing harmonics in the line current spectra that may arise due to the interaction of saturation with a specific fault The present paper attempts to create such a model The model uses modulation of air-gap permeance in order to emulate saturation Modified winding function approach (MWFA) has been used to model the induction machine with saturation To demonstrate the effectiveness of the model, certain high-frequency harmonics that can be predicted theoretically are shown to be present in the line current of the simulated machine Experimental results also confirm the presence of these harmonics The author plans to extend this model in the future for fault analysis

Fuzzy approach for optimal placement and sizing of capacitor banks in the presence of harmonics

Abstract: This paper presents a fuzzy-based approach for optimal placement and sizing of fixed capacitor banks in radial distribution networks in the presence of voltage and current harmonics. The objective function includes the cost of power losses, energy losses, and capacitor banks. Constraints include voltage limits, number/size and locations of installed capacitors, and the power quality limits of IEEE-519 standard. Candidate buses for capacitor placement are selected using the sensitivities of constraints and the objective function with respect to reactive power injection at each bus. Using fuzzy set theory, a suitable combination of objective function and constraints is generated as a criterion to select the most suitable bus for capacitor placement. The /spl alpha/-cut process is applied at each iteration to guarantee simultaneous improvements of objective function and satisfying given constraints. Simulation results for the 18 bus IEEE distorted network show the advantages of the proposed method as compared to the maximum sensitivities selection algorithm.

Detection of mechanical imbalances of induction machines without spectral analysis of time-domain signals

Abstract: Mechanical rotor imbalances and rotor eccentricities are reflected in electric, electromagnetic, and mechanical quantities. Therefore, many surveillance schemes determine the Fourier spectrum of a single line current in order to monitor the motor condition. Mechanical imbalances give rise to two first-order current harmonics. Due to the interaction of the currents and voltages, both these current harmonics are also reflected by a single harmonic component in the frequency spectrum of the electric power. This single component is easier to assess than both the current harmonics. The technique proposed in this contribution evaluates this imbalance-specific modulation of the electric power. The proposed approach does not determine the Fourier spectrum of a time-domain signal, though. First, the imbalance specific oscillation of the electric power is extracted by a bandpass filter. Then, the averaged pattern of this component is determined by means of an angular data clustering technique. In that way, the oscillation of the electric power in the time domain becomes mapped into a discrete waveform in an angular domain. The amplitude of the fundamental harmonic of these discrete data serves as the imbalance indicator of the proposed scheme. This technique, therefore, overcomes small load and slip fluctuations. Measured results of a mechanically unbalanced machine and a case of combined static and dynamic eccentricity are presented.

Characterization of noise and vibration sources in interior permanent-magnet brushless DC motors

Abstract: This work characterizes electromagnetic excitation forces in interior permanent-magnet (IPM) brushless direct current (BLDC) motors and investigates their effects on noise and vibration. First, the electromagnetic excitations are classified into three sources: 1) so-called cogging torque, for which we propose an efficient technique of computation that takes into account saturation effects as a function of rotor position; 2) ripples of mutual and reluctance torque, for which we develop an equation to characterize the combination of space harmonics of inductances and flux linkages related to permanent magnets and time harmonics of current; and 3) fluctuation of attractive forces in the radial direction between the stator and rotor, for which we analyze contributions of electric currents as well as permanent magnets by the finite-element method. Then, the paper reports on an experimental investigation of influences of structural dynamic characteristics such as natural frequencies and mode shapes, as well as electromagnetic excitation forces, on noise and vibration in an IPM motor used in washing machines.

Distributed Active Filter Systems (DAFSs): A New Approach to Power System Harmonics

Abstract: This paper proposes a distributed active filter system (DAFS) for alleviating the harmonic distortion of power systems. The proposed DAFS consists of multiple active filter units installed on the same location or different locations within the power system. The active filter units of the proposed DAFS can cooperate, without any communication among them, to reduce the voltage harmonic distortion of the power lines. Each individual active filter unit functions like a harmonic conductance to reduce voltage harmonics. A droop relationship between the harmonic conductance and the volt-ampere of the active filter unit is programmed into the controller of each unit so multiple active filter units can share the workload of harmonic filtering. The slope of the droop is determined by the volt-ampere rating of the active filter unit in order to distribute the harmonic filtering workload in proportion to the rated capacity of each unit. The principle of operation is explained in this paper and test results based on computer simulation and laboratory test bench are provided to validate the functionalities of the proposed DAFS.

A simple new control technique for unified power quality conditioner (UPQC)

Abstract: It is expected that, in future, utility service providers will enforce more strict power factor and harmonic standards. Therefore, efforts are being made so that power electronic system or any other load drawing reactive volt-amperes is made to appear as linear load drawing current in-phase with the utility voltage. One of the solutions towards this end is to employ active power filters (APF). This paper presents a control technique for unified power quality conditioner (UPQC), which is a combination of series APF and shunt APF. A control strategy based on unit vector template generation is discussed in this paper with the focus on the mitigation of voltage harmonics present in the utility voltage. Simulation results based on Simpowersystem (SPS) of Matlab/Simulink are presented to verify the effectiveness of the proposed control technique.

High-order harmonic generation up to 250 eV from highly ionized argon.

Abstract: We demonstrate the generation of very high-order harmonics, up to 250 eV, using argon gas. This extends by 100 eV the highest harmonics previously observed using Ar and exceeds the energies observed using any other medium besides helium. This advance is made possible by using a waveguide geometry to limit plasma-induced laser beam defocusing, making it possible to generate high harmonics from Ar ions. This work shows that high harmonic emission from ions can extend laser-based coherent up-conversion into the soft x-ray region of the spectrum.

A signal Processing system for extraction of harmonics and reactive current of single-phase systems

Abstract: A signal processing system for extraction of harmonic and reactive current components is introduced and its performance is evaluated. The extraction system is adopted as part of the control system of a single-phase active power filter (APF) to provide the required signals for harmonic filtering and reactive power compensation. Performance of the overall system is evaluated based on digital time-domain simulation studies. The APF control system including the signal processing algorithms are implemented in Matlab/Simulink Fixed-Point Blockset to accommodate bit-length limitation which is a crucial factor in digital implementation. The power system including the APF, load and the supply system are simulated with the PSCAD/EMTDC software to which the Matlab-based control model is interfaced. The simulation results indicate that the signal processing unit can provide the required signals for APF to perform filtering/compensation within the transient period of 2 to 3 cycles.

An adaptive controller in stationary reference frame for D-statcom in unbalanced operation

Abstract: This paper presents a passivity-based controller for a Distribution Static synchronous Compensator (D-Statcom) aimed at compensating reactive power and unbalanced harmonics in the general case of distorted and unbalanced source voltages and load currents. The proposed approach is based on the measurements of line currents, and ensures precise compensation for selected harmonics. Moreover, in order to compensate for the unavoidable uncertainty in certain system parameters, adaptation is added to the passivity-based controller. One of the major advantages of the proposed solution compared to conventional alternatives is that we are able to perform precise tracking (including higher order harmonics) even in the presence of a relatively low switching frequency, i.e., in the presence of an inverter with limited bandwidth. Simulation and experimental results are provided to illustrate the merits of our solution.

On applying a minimization technique to the harmonic elimination PWM control: the bipolar waveform

Abstract: The well-known harmonic elimination pulse-width modulation (HE-PWM) method for inverter control is revisited. The HE-PWM waveform presents many challenges. It has multiple solutions that not only need to be found as easily and as fast as possible, but must also be evaluated in order to identify the best technique when overall harmonic performance is concerned. Algorithms presented so far rely on starting values that are close to the exact solutions to ensure convergence. A new method based on resultant theory promises limited success since it can only work when a small number of harmonics is to be eliminated. In this paper, it is shown that a minimization technique in combination with a random search results in a relatively simple approach that finds all possible sets of solutions. It is confirmed that numerous independent sets of solutions exist and the ones that offer better harmonic performance are identified. Three cases are reported in detail, including when two, four and six nontriplen odd harmonics are to be eliminated.

A control structure for fast harmonics compensation in active filters

Abstract: Shunt active filters are a means to improve power quality in distribution networks. Typically, they are connected in parallel to disturbing loads in order to reduce the injection of non sinusoidal load currents into the utility grid. The high power active filter investigated in this paper is based on a pulse-width modulation (PWM) controlled voltage source inverter. Its inner current control is realized with a dead-beat controller that allows fast tracking of stochastically fluctuating load currents. For the mitigation of stationary load current harmonics, an outer control loop is required that compensates for the persistant phase error caused by the delay of the inner loop. The outer loop developed in this paper is based on integrating oscillators tuned to the major load current harmonics. Mathematically, they are equivalent to I-controllers in rotating reference frames. Some of these oscillators are located within a closed control loop. For frequencies where the feedback would excite grid resonances they are placed in a prefilter with phase shifting elements. Since all oscillators share a common feedback full selectivity of the harmonic analysis is achieved. For every harmonic the degree of compensation can be adjusted individually. In addition to the oscillators, a direct path is provided that feeds forward the load current to the inner control loop. Thus, load current transients can be tracked with the full speed of the dead-beat controller. The direct path does not affect the harmonic analysis performed by the oscillators.