Showing papers on "Harmonics published in 2011"
TL;DR: This paper provides a systematic approach to the design of filter-based active damping methods with tuning procedures, performance, robustness, and limitations discussed with theoretical analysis, selected simulation, and experimental results.
Abstract: Pulsewidth modulation (PWM) voltage source converters are becoming a popular interface to the power grid for many applications. Hence, issues related to the reduction of PWM harmonics injection in the power grid are becoming more relevant. The use of high-order filters like LCL filters is a standard solution to provide the proper attenuation of PWM carrier and sideband voltage harmonics. However, those grid filters introduce potentially unstable dynamics that should be properly damped either passively or actively. The second solution suffers from control and system complexity (a high number of sensors and a high-order controller), even if it is more attractive due to the absence of losses in the damping resistors and due to its flexibility. An interesting and straightforward active damping solution consists in plugging in, in cascade to the main controller, a filter that should damp the unstable dynamics. No more sensors are needed, but there are open issues such as preserving the bandwidth, robustness, and limited complexity. This paper provides a systematic approach to the design of filter-based active damping methods. The tuning procedures, performance, robustness, and limitations of the different solutions are discussed with theoretical analysis, selected simulation, and experimental results.
580 citations
TL;DR: In this paper, the minimum ripple energy storage requirement is derived independently of a specific topology, and the feasibility of the active capacitor's reduction schemes is verified based on the minimum energy requirement, which can effectively reduce the energy storage capacitance.
Abstract: It is well known that single-phase pulse width modulation rectifiers have second-order harmonic currents and corresponding ripple voltages on the dc bus. The low-frequency harmonic current is normally filtered using a bulk capacitor in the bus, which results in low power density. However, pursuing high power density in converter design is a very important goal in the aerospace applications. This paper studies methods for reducing the energy storage capacitor for single-phase rectifiers. The minimum ripple energy storage requirement is derived independently of a specific topology. Based on the minimum ripple energy requirement, the feasibility of the active capacitor's reduction schemes is verified. Then, we propose a bidirectional buck-boost converter as the ripple energy storage circuit, which can effectively reduce the energy storage capacitance. The analysis and design are validated by simulation and experimental results.
450 citations
TL;DR: In this article, the authors present a novel control strategy for achieving maximum benefits from these grid-interfacing inverters when installed in 3-phase 4-wire distribution systems, where the inverter is controlled to perform as a multi-function device by incorporating active power filter functionality.
Abstract: Renewable energy resources (RES) are being increasingly connected in distribution systems utilizing power electronic converters. This paper presents a novel control strategy for achieving maximum benefits from these grid-interfacing inverters when installed in 3-phase 4-wire distribution systems. The inverter is controlled to perform as a multi-function device by incorporating active power filter functionality. The inverter can thus be utilized as: 1) power converter to inject power generated from RES to the grid, and 2) shunt APF to compensate current unbalance, load current harmonics, load reactive power demand and load neutral current. All of these functions may be accomplished either individually or simultaneously. With such a control, the combination of grid-interfacing inverter and the 3-phase 4-wire linear/non-linear unbalanced load at point of common coupling appears as balanced linear load to the grid. This new control concept is demonstrated with extensive MATLAB/Simulink simulation studies and validated through digital signal processor-based laboratory experimental results.
428 citations
TL;DR: In this article, the authors applied nearest level control (NLC) to the modular multilevel converter (MMC) and analyzed the voltage harmonics in order to select a proper sampling frequency, which significantly influenced the output voltage levels and voltage total harmonic distortions.
Abstract: This paper applies nearest level control (NLC) to the modular multilevel converter (MMC). Since there are a number of submodules (SMs) in high-voltage applications of MMC and all SM voltages are required to be measured and sorted, if the uniform sampling frequency is not high enough, the SM will not be triggered as quickly as possible. Thus, the converter-output voltage levels and voltage harmonics will be affected. A method to systematically analyze the voltage harmonics is presented, considering the uniform sampling frequency. In order to select a proper sampling frequency, two critical values, which significantly influence the output voltage levels and voltage total harmonic distortions, are calculated. Simulation results based on PSCAD/EMTDC proved the validity of the proposed modulation scheme and the importance of a properly selected sampling frequency.
386 citations
TL;DR: This paper demonstrates a frequency-domain-model-based approach to determine the optimum filter parameters that provide the necessary performance under all operating conditions given the necessary design constraints.
Abstract: This paper describes the design procedure and performance of an LCL grid filter for a medium-voltage neutral-point clamped converter to be adopted for a multimegawatt (multi-MW) wind turbine. The unique filter design challenges in this application are driven by a combination of the medium-voltage converter, a limited allowable switching frequency, component physical size and weight concerns, and the stringent limits for allowable injected current harmonics. Traditional design procedures of grid filters for lower power and higher switching frequency converters are not valid for a multi-MW filter connecting a medium-voltage converter switching at low frequency to the electric grid. This paper demonstrates a frequency-domain-model-based approach to determine the optimum filter parameters that provide the necessary performance under all operating conditions given the necessary design constraints. To achieve this goal, new concepts, such as virtual-harmonic content and virtual filter losses are introduced. Moreover, a new passive-damping technique that provides the necessary damping with low losses and very little degradation of the high-frequency attenuation is proposed.
383 citations
TL;DR: A systematic method is proposed to obtain the highest stability and avoidance of closed-loop anomalous peaks: it is achieved by the minimization of the inverse of the Nyquist trajectory distance to the critical point, i.e., the sensitivity function.
Abstract: The following two types of resonant controllers are mainly employed to obtain high performance in voltage-source converters: 1) proportional + resonant (PR) and 2) vector proportional + integral (VPI). The analysis and design of PR controllers is usually performed by Bode diagrams and phase-margin criterion. However, this approach presents some limitations when resonant frequencies are higher than the crossover frequency defined by the proportional gain. This condition occurs in selective harmonic control and applications with high reference frequency with respect to the switching frequency, e.g., high-power converters with a low switching frequency. In such cases, additional 0-dB crossings (phase margins) appear; therefore, the usual methods for simple systems are no longer valid. In addition, VPI controllers always present multiple 0-dB crossings in their frequency response. In this paper, the proximity to the instability of PR and VPI controllers is evaluated and optimized through Nyquist diagrams. A systematic method is proposed to obtain the highest stability and avoidance of closed-loop anomalous peaks: it is achieved by the minimization of the inverse of the Nyquist trajectory distance to the critical point, i.e., the sensitivity function. Finally, several experimental tests, including an active power filter that operates at a low switching frequency and compensates harmonics up to the Nyquist frequency, validate the theoretical approach.
343 citations
TL;DR: A new compensation-circuit design procedure is proposed that considers high-order current harmonics and results in inverter zero-current switching and is compared with the classical compensation designed for zero phase angle between the inverter voltage and current fundamental components.
Abstract: Inductive power transfer (IPT) is commonly used to transmit power from an extended loop (track) to a number of galvanically isolated movable pick-ups. To maximize the power transfer and minimize converter requirements, various compensation circuits have been proposed for both the track (primary) and the pick-up (secondary). This paper investigates the suitability of the LCC series-parallel compensation for IPT primary design. A new compensation-circuit design procedure is proposed that considers high-order current harmonics and results in inverter zero-current switching. The proposed compensation is compared with the classical compensation designed for zero phase angle between the inverter voltage and current fundamental components. Expressions for the bifurcation boundary, voltampere rating of reactive-compensation elements, and the current at the moment of switching are derived and analyzed. Analytical results are verified both via PSpice simulations and experimentally using a 1-hp MOSFET-based prototype.
273 citations
TL;DR: In this article, a general delayed signal cancellation (DSC) operator was proposed to eliminate any specified harmonic, which can be further cascaded to stepwise reject all undesired harmonics, and then the conditioned voltage signal can be used in PLL loop to achieve fast transient response at high control bandwidth without suffering from the steadystate error caused by harmonics.
Abstract: During the grid synchronization of distributed generation (DG) units, phase-locked loop (PLL) is well accepted as an efficient approach to detect grid phase angle. Conventional PLL schemes used in DG controller have to compromise between steady-state accuracy and transient dynamics when grid voltage is polluted by unbalance and harmonics. To simultaneously realize good steady-state and transient performances, this paper proposes a general delayed signal cancellation (DSC) operator, which can be tailored to eliminate any specified harmonic. The proposed DSC operator can be further cascaded to stepwise reject all undesired harmonics. Then the conditioned voltage signal can be used in PLL loop to achieve fast transient response at high control bandwidth without suffering from the steady-state error caused by harmonics. Based on differently configured DSC operators, two PLL designs are then developed, namely CDSC-PLL1 and CDSC-PLL2. Specifically, CDSC-PLL1 is aimed for grid voltage with unbalance and odd/even harmonics, while CDSC-PLL2 further addresses asymmetrical harmonics, i.e., harmonics arising from asymmetrically distorted three-phase voltages. By introducing a frequency feedback loop, the proposed PLL can operate properly during considerable frequency variations, even when a phase jump or severe harmonics are also present. All proposed PLL designs have very simple structure and can be easily implemented. The superior performance is confirmed by experimental results.
249 citations
TL;DR: In this article, a mathematical model of a doubly fed induction generator (DFIG) in the positive synchronous reference frame under distorted grid voltage conditions is presented, where the fundamental and the fifth and seventh-order components of rotor currents are directly regulated by the PI-R controller without sequential component decompositions.
Abstract: This paper presents a mathematical model of a doubly fed induction generator (DFIG) in the positive synchronous reference frame under distorted grid voltage conditions. The oscillations of the DFIG's electromagnetic torque and the instantaneous stator active and reactive powers are fully described when the grid voltage is harmonically distorted. Four alternative control targets are proposed to improve the system responses during grid harmonic distortions. A new rotor current control scheme implemented in the positive synchronous reference frame is developed. The control scheme consists of a proportional-integral (PI) regulator and a harmonic resonant (R) compensator tuned at six times the grid frequency. Consequently, the fundamental and the fifth- and seventh-order components of rotor currents are directly regulated by the PI-R controller without sequential-component decompositions. The feasibility of the proposed control strategy is validated by simulation studies on a 2.0-MW wind-turbine-driven DFIG system. Compared with the conventional vector control scheme based on standard PI current controllers, the proposed control scheme leads to significant elimination of either DFIG power or torque oscillations under distorted grid voltage conditions.
223 citations
TL;DR: An active power compensator (APC) based on single-phase back-to-back power converter is proposed in this paper to solve problems of power quality of electric railway power supply system.
Abstract: An active power compensator (APC) based on single-phase back-to-back power converter is proposed in this paper to solve problems of power quality of electric railway power supply system. This system adopts a single-phase feeding connection, which is called cophase power supply scheme. In this scheme, APC connects the balance transformer between feeding phase for power supply and another phase for compensation. It has some characteristics, such as active power balancing, reactive power compensating, and harmonics filtering. In order to achieve these characteristics, the control scheme requires seven combination models. In this paper, a multifunctional control algorithm is proposed to realize every conceivable model. A cophase system with APC based on field programmable gate array (FPGA) and YNvd balance transformer is also designed and evaluated. The experimental results obtained from this prototype illustrate that the compensating ability is extremely high in steady-state and dynamic responses, and the power quality of a substation with distorted loads can be improved integrally.
214 citations
TL;DR: A review of techniques and methodologies developed for power quality analysis and power disturbance classification is presented in this article, in order to show their major characteristics, such as harmonics, sags, swells etc.
Abstract: The relevance of power quality (PQ) issues has recently augmented because of the increased use of power electronic equipment, which results in a voltage deviation and current waveforms. The PQ monitoring is covered by two main subjects: the development of PQ indices to quantify the power supply quality and the electrical disturbances detection such as harmonics, sags, swells etc., which allows knowing the conditions of the electric power systems. In this study a review of techniques and methodologies developed for PQ analysis and power disturbance classification is presented in order to show their major characteristics.
TL;DR: The novelty of the proposed system lies in extension of the generic DPC-SVM scheme by additional higher harmonic and voltage dips compensation modules and implementation of the whole algorithm in a single chip floating point microcontroller.
Abstract: Power electronic Grid-Connected Converters (GCCs) are widely applied as grid interface in renewable energy sources. This paper proposes an extended Direct Power Control with Space Vector Modulation (DPC-SVM) scheme with improved operation performance under grid distortions. The real-time operated DPC-SVM scheme has to execute several important tasks as: space vector pulse width modulation, active and reactive power feedback control, grid current harmonics and voltage dips compensation. Thus, development and implementation of the DPC-SVM algorithm using single chip floating-point microcontroller TMS320F28335 is described. It combines large peripheral equipment, typical for microcontrollers, with high computation capacity characteristic for Digital Signal Processors (DSPs). The novelty of the proposed system lies in extension of the generic DPC-SVM scheme by additional higher harmonic and voltage dips compensation modules and implementation of the whole algorithm in a single chip floating point microcontroller. Overview of the laboratory setup, description of basic algorithm subtasks sequence, software optimization as well as execution time of specific program modules on fixed-point and floating-point processors are discussed. Selected oscillograms illustrating operation and robustness of the developed algorithm used in 5 kVA laboratory model of the GCC are presented.
TL;DR: In this article, several methods of power system harmonics estimation are critically reviewed and classified based on the type of analysis tool and applications and the key issues and challenges in harmonics estimations are highlighted.
TL;DR: An all-optical shaper-assisted linear cross-correlation technique was developed and verified the shapes of optical fields and confirmed the critical role of carrier-envelope phase in Fourier synthesis of optical waveforms.
Abstract: Achieving the control of light fields in a manner similar in sophistication to the control of electromagnetic fields in the microwave and radiofrequency regimes has been a major challenge in optical physics research. We manipulated the phase and amplitude of five discrete harmonics spanning the blue to mid-infrared frequencies to produce instantaneous optical fields in the shape of square, sawtooth, and subcycle sine and cosine pulses at a repetition rate of 125 terahertz. Furthermore, we developed an all-optical shaper-assisted linear cross-correlation technique to retrieve these fields and thereby verified their shapes and confirmed the critical role of carrier-envelope phase in Fourier synthesis of optical waveforms.
TL;DR: In this article, the authors further developed harmonics injection and equal area criteria-based four-equation method to realize OPWM for two-level inverters and multilevel inverters with unbalanced dc sources.
Abstract: In medium-/high-power inverters, optimal pulse-width modulation (OPWM) is often used to reduce the switching frequency and at the same time, realize selective harmonic elimination (SHE). For both two-level and multilevel inverters, most selective harmonic elimination (SHE) studies are based on solving multiple variable high-order nonlinear equations. Furthermore, for multilevel inverters, SHE has been often studied based on the assumption of balanced dc levels and single switching per level. In this paper, the authors further developed harmonics injection and equal area criteria-based four-equation method to realize OPWM for two-level inverters and multilevel inverters with unbalanced dc sources. For the cases, where only small number of voltage levels are available, weight oriented junction point distribution is utilized to enhance the performance of the four-equation method. A case study of multilevel inverter at low-modulation index is used as an example. Compared with existing methods, the proposed method does not involve complex equation groups and is much easier to be utilized in the case of large number of switching angles, or multiple switching angles per voltage level in multilevel inverters.
TL;DR: An all-optical signal processing architecture is reported that enables, for the first time, multilevel all-Optical quantization of phase-encoded optical signals.
Abstract: The exponentially increasing capacity demand in information systems will be met by carefully exploiting the complementary strengths of electronics and optics. Optical signal processing provides simple but powerful pipeline functions that offer high speed, low power, low latency and a route to densely parallel execution. A number of functions such as modulation and sampling, complex filtering and Fourier transformation have already been demonstrated. However, the key functionality of all-optical quantization has still not been addressed effectively. Here, we report an all-optical signal processing architecture that enables, for the first time, multilevel all-optical quantization of phase-encoded optical signals. A four-wavemixing process is used to generate a comb of phase harmonics of the input signal, and a two-pump parametric process to coherently combine a selected harmonic with the input signal, realizing phase quantization. We experimentally demonstrate operation up to six levels
TL;DR: In this article, the grid interconnection of a PMSG-based wind turbine with harmonics and reactive power compensation capability at the point of common coupling (PCC) is investigated, where the generator-side converter is used to achieve maximum power point tracking (MPPT).
Abstract: The power electronics plays an important role in the reliable operation of a modern wind energy conversion system (WECS). This study aims at the grid interconnection of a permanent magnet synchronous generator (PMSG)-based wind turbine with harmonics and reactive power compensation capability at the point of common coupling (PCC). The proposed system consists of two back-to-back connected converters with a common dc-link. The generator-side converter is used to achieve maximum power point tracking (MPPT). The grid-side converter is actively controlled to feed generated power as well as to supply the harmonics and reactive power demanded by the non-linear load at PCC, thus enabling the grid to supply only sinusoidal current at unity power factor. A model of directly driven PMSG-based variable speed WECS is developed and simulated in MATLAB/SPS environment. The effectiveness of proposed control approach is validated through extensive simulation and experimental results.
TL;DR: A new dynamic harmonic estimator is presented as an extension of the fast Fourier transform (FFT), which assumes a fluctuating complex envelope at each harmonic, and is able to estimate harmonics that are time varying inside the observation window.
Abstract: A new dynamic harmonic estimator is presented as an extension of the fast Fourier transform (FFT), which assumes a fluctuating complex envelope at each harmonic. This estimator is able to estimate harmonics that are time varying inside the observation window. The extension receives the name “Taylor-Fourier transform (TFT)” since it is based on the McLaurin series expansion of each complex envelope. Better estimates of the dynamic harmonics are obtained due to the fact that the Fourier subspace is contained in the subspace generated by the Taylor-Fourier basis. The coefficients of the TFT have a physical meaning: they represent instantaneous samples of the first derivatives of the complex envelope, with all of them calculated at once through a linear transform. The Taylor-Fourier estimator can be seen as a bank of maximally flat finite-impulse-response filters, with the frequency response of ideal differentiators about each harmonic frequency. In addition to cleaner harmonic phasor estimates under dynamic conditions, among the new estimates are the instantaneous frequency and first derivatives of each harmonic. Two examples are presented to evaluate the performance of the proposed estimator.
TL;DR: In this article, the effect of eddy-current loss reduction by the magnet segmentation in synchronous motors with concentrated windings is investigated, and the effect depends on the types of the rotors, for instance, interior and surface permanent magnet rotors.
Abstract: In this paper, we investigate the effect of eddy-current loss reduction by the magnet segmentation in synchronous motors with concentrated windings in order to understand appropriate segmentation methods. The loss-reduction effects in each harmonic eddy current in the magnets are analyzed by both the theoretical solution and the three-dimensional finite-element analysis with Fourier transformation. The basic experiments using magnet specimens are carried out in order to support the calculated results. It is clarified that the loss-reduction effect varies with the harmonic orders and that the effect depends on the types of the rotors, for instance, interior and surface permanent-magnet rotors.
TL;DR: In this article, a reference signal generation method for the unified power quality conditioner (UPQC) adopted to compensate current and voltage quality problems of sensitive loads is proposed, which consists of a shunt and series converter having a common dc link.
Abstract: This paper proposes a novel reference signal generation method for the unified power quality conditioner (UPQC) adopted to compensate current and voltage-quality problems of sensitive loads. The UPQC consists of a shunt and series converter having a common dc link. The shunt converter eliminates current harmonics originating from the nonlinear load side and the series converter mitigates voltage sag/swell originating from the supply side. The developed controllers for shunt and series converters are based on an enhanced phase-locked loop and nonlinear adaptive filter. The dc link control strategy is based on the fuzzy-logic controller. A fast sag/swell detection method is also presented. The efficacy of the proposed system is tested through simulation studies using the Power System Computer Aided Design/Electromagnetic Transients dc analysis program. The proposed UPQC achieves superior capability of mitigating the effects of voltage sag/swell and suppressing the load current harmonics under distorted supply conditions.
TL;DR: In this paper, a new control strategy of effective fifth and seventh harmonic elimination in the stator output voltage at the point of common coupling for a stand-alone doubly fed induction generator (DFIG) feeding a three-phase diode rectifier was proposed.
Abstract: This paper proposes a new control strategy of effective fifth and seventh harmonic elimination in the stator output voltage at the point of common coupling for a stand-alone doubly fed induction generator (DFIG) feeding a three-phase diode rectifier. This load regularly causes such harmonic distortions, which harmfully affect the performance of other loads connected to the DFIG. In order to allow the DFIG to deliver a pure sinusoidal stator output voltage, these harmonics must be rejected. The proposed elimination method is investigated based on the rotor current controller employing a proportional integral and a resonant controller, which is implemented in the fundamental reference frame. In this frame, both positive seventh and negative fifth voltage harmonic can be eliminated by using only single resonant compensator tuned at six multiples of synchronous frequency in the rotor current controller. The control scheme is developed in the rotor-side converter for the control and operation of the DFIG. Simulations and experimental results with 2.2-kW DFIG feeding a nonlinear load are shown to verify prominent features of the proposed control method.
TL;DR: In this paper, a compensation strategy based on the railway static power conditioners (RPC) is proposed to eliminate the negative-sequence and harmonic currents in the high-speed train traction systems with three-phase V/V transformers.
Abstract: In order to eliminate the negative-sequence and harmonic currents in the high-speed train traction systems with three-phase V/V transformers, a compensation strategy based on the railway static power conditioners (RPC) is proposed in this paper. An RPC contains two converters that are connected back to back by sharing the same dc link. In this paper, the structure and principle to compensate negative-sequence currents for the RPC with a three-phase V/V transformer are explained, and a strategy to provide the compensation references for negative-sequence and harmonic currents is proposed. Also, a method to separate active current, reactive current, and harmonic current references from the total negative-sequence and harmonic current references is given. Moreover, a controller is proposed to maintain the dc-link voltage and to compensate the negative-sequence and harmonic currents. Simulation and experimental results are provided to demonstrate that the proposed strategy is very effective.
TL;DR: In this paper, it is shown that the low-speed limit is mainly a function of the amplitude of the inverter harmonics and that theory and simulation will provide erroneous results when inverter harmonic coefficients are not incorporated.
Abstract: Although it is widely known that back-electromotive-force tracking has a low-speed limit, there is very little published literature evaluating this limit. This paper shows that the low-speed limit is mainly a function of the amplitude of the inverter harmonics and that theory and simulation will provide erroneous results when inverter harmonics are not incorporated. This paper will also provide a method for evaluating this low-speed limit using drive stiffness as the metric. The experimental results show quantitatively how the drive stiffness suffers as speed decreases, which ultimately sets the practical low-speed limit.
TL;DR: In this paper, the authors used artificial neural networks (ANNs) to generate the switching angles in an 11-level full-bridge cascade inverter powered by five varying dc input sources.
Abstract: This work approximates the selective harmonic elimination problem using artificial neural networks (ANNs) to generate the switching angles in an 11-level full-bridge cascade inverter powered by five varying dc input sources. Each of the five full bridges of the cascade inverter was connected to a separate 195-W solar panel. The angles were chosen such that the fundamental was kept constant and the low-order harmonics were minimized or eliminated. A nondeterministic method is used to solve the system for the angles and to obtain the data set for the ANN training. The method also provides a set of acceptable solutions in the space where solutions do not exist by analytical methods. The trained ANN is a suitable tool that brings a small generalization effect on the angles' precision and is able to perform in real time (50-/60-Hz time window).
TL;DR: In this paper, a semiclassical model for plasmon-enhanced high-order harmonic generation (HHG) in the vicinity of metal nanostructures is presented, and the authors show that both the inhomogeneity of the enhanced local fields and electron absorption by the metal surface play an important role in the HHG process and lead to the generation of even harmonics and significantly increased cutoff.
Abstract: We present a semiclassical model for plasmon-enhanced high-order harmonic generation (HHG) in the vicinity of metal nanostructures. We show that, besides the field enhancement, both the inhomogeneity of the enhanced local fields and electron absorption by the metal surface play an important role in the HHG process and lead to the generation of even harmonics and a significantly increased cutoff. For the examples of silver-coated nanocones and bowtie antennas, we predict that the required intensity reduces by up to three orders of magnitude due to plasmonic field enhancement. The study of the enhanced high-order harmonic generation is connected with a finite-element simulation of the electric field enhancement due to the excitation of the plasmonic modes.
TL;DR: The generation of circularly polarized high order harmonics in the extreme ultraviolet range from a linearly polarized infrared laser focused into a neon filled gas cell is reported, being significantly more efficient than currently demonstrated direct generation of elliptically polarized harmonics.
Abstract: We report the generation of circularly polarized high order harmonics in the extreme ultraviolet range (18–27 nm) from a linearly polarized infrared laser (40 fs, 0.25 TW) focused into a neon filled gas cell. To circularly polarize the initially linearly polarized harmonics we have implemented a four-reflector phase-shifter. Fully circularly polarized radiation has been obtained with an efficiency of a few percents, thus being significantly more efficient than currently demonstrated direct generation of elliptically polarized harmonics. This demonstration opens up new experimental capabilities based on high order harmonics, for example, in biology and materials science. The inherent femtosecond time resolution of high order harmonic generating table top laser sources renders these an ideal tool for the investigation of ultrafast magnetization dynamics now that the magnetic circular dichroism at the absorption M-edges of transition metals can be exploited.
TL;DR: In this article, the authors investigated the electrical asymmetry effect in geometrically symmetric capacitively coupled radio frequency discharges operated at multiple consecutive harmonics and found that a significantly stronger dc self-bias can be generated electrically and that the mean ion energies at the electrodes can be controlled separately from the ion flux over a broader range by tuning the phase shifts between individual voltage harmonics.
Abstract: The electrical asymmetry effect (EAE) in geometrically symmetric capacitively coupled radio frequency discharges operated at multiple consecutive harmonics is investigated by a particle-in-cell (PIC) simulation and an analytical model. The model is based on the original EAE model, which is extended by taking into account the floating potentials, the voltage drop across the plasma bulk, and the symmetry parameter resulting from the PIC simulation. Compared with electrically asymmetric dual-frequency discharges we find that (i) a significantly stronger dc self-bias can be generated electrically and that (ii) the mean ion energies at the electrodes can be controlled separately from the ion flux over a broader range by tuning the phase shifts between the individual voltage harmonics. A recipe for the optimization of the applied voltage waveform to generate the strongest possible dc self-bias electrically and to obtain maximum control of the ion energy via the EAE is presented.
15 May 2011
TL;DR: In this paper, a new and high efficiency method is presented to reduce simultaneously the sub-and high MMF harmonics of these winding types, based on doubling the number of stator slots, using two identical winding systems connected in series and shifted to each other.
Abstract: The fractional slots tooth concentrated windings are characterized with high space MMF harmonics which results to undesirable effects on electric machine, such as localised core saturation, eddy current loss in the rotor and noise and vibration. A new and high efficiency method is presented in this paper to reduce simultaneously the sub- and high MMF harmonics of these winding types. The method is based on doubling the number of stator slots, using two identical winding systems connected in series and shifted to each other for a specific angle, using stator core with different tooth width and using different turns per coil for the neighbouring phase coils. With the proposed technique the unwanted sub- and high winding MMF harmonics can be reduced or completely canceled.
TL;DR: In this article, the authors proposed an advanced controller for three-phase phase-locked loops (PLLs), which are employed in grid-connected power converters, which is formed of one or more lead compensators cascaded to the main proportional integral regulator.
Abstract: This paper proposes an advanced controller suitable for three-phase phase-locked loops (PLLs), which are employed in grid-connected power converters. This controller is formed of one or more lead compensators cascaded to the main proportional integral regulator. The proposed lead compensators are second order with pure imaginary roots: they have both a notch peak and a resonant peak (the notch frequency is lower than the resonant frequency). Hence, their phase versus frequency response exhibits phase wraps of ± 180°. Consequently, the parameters of each lead compensator are tuned with two objectives: to eliminate a specific frequency in the synchronous reference frame (SRF) and to enhance stability by a phase lead (phase boost). Through this technique, three-phase PLLs achieve both high bandwidth (fast transient response) and selective cancellation (filtering of unbalance and harmonics ripple in the SRF). The proposed controllers are suitable for simpler three-phase PLL schemes, such as the SRF-PLL. Therefore, big improvement is achieved without adding extra blocks and signals to basic PLL structures. Simulation and real-time implementation (dSpace DS1103) tests, emulating very demanding realistic conditions, have been performed. Key figures from these tests are shown, which prove the high performance and robustness of the proposal.
01 Nov 2011
TL;DR: In this paper, an impedance-based analysis approach is used to characterize the harmonic resonance caused by impedance interactions between the wind inverter and the grid and possible mitigation methods by active damping and grid synchronization design are presented, and their effectiveness demonstrated by simulations and experiments.
Abstract: Harmonic resonance can happen in traditional power systems between power factor correction (PFC) capacitors and transformer leakage inductance. In a distribution network with high penetration of renewable generation sources, inverter PWM harmonic currents can be a source of harmonic excitation for system resonance. This paper discusses harmonic resonance problems caused by impedance interactions between the wind inverter and the grid. An impedance-based analysis approach is used to characterize such resonance. Modeling of inverter output impedance directly in the phase domain to enable such analysis is presented. Possible mitigation methods by active damping and grid synchronization design are presented, and their effectiveness demonstrated by simulations and experiments.