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Showing papers on "Harmonic published in 2014"


Journal ArticleDOI
TL;DR: In this article, the authors derived an analytical formula for the harmonic Hall voltages to evaluate the effective field for both out-of-plane and in-plane magnetized systems, and the results illustrate the versatility of harmonic Hall voltage measurement for studying current induced torques in magnetic heterostructures.
Abstract: Solid understanding of current induced torques is a key to the development of current and voltage controlled magnetization dynamics in ultrathin magnetic heterostructures. To evaluate the size and direction of such torques, or effective fields, a number of methods have been employed. Here, we examine the adiabatic (low-frequency) harmonic Hall voltage measurement that has been used to study the effective field. We derive an analytical formula for the harmonic Hall voltages to evaluate the effective field for both out of plane and in-plane magnetized systems. The formula agrees with numerical calculations based on a macrospin model. Two different in-plane magnetized films, Pt|CoFeB|MgO and CuIr|CoFeB|MgO are studied using the formula developed. The effective field obtained for the latter system shows relatively good agreement with that estimated using spin torque switching phase diagram measurements reported previously. Our results illustrate the versatile applicability of harmonic Hall voltage measurement for studying current induced torques in magnetic heterostructures.

425 citations


Journal ArticleDOI
Junjun Deng, Siqi Li1, Sideng Hu1, Chunting Chris Mi1, Ruiqing Ma 
TL;DR: An inductor-inductor-capacitor (LLC) resonant dc-dc converter design procedure for an onboard lithium-ion battery charger of a plug-in hybrid electric vehicle (PHEV) is presented.
Abstract: In this paper, an inductor–inductor–capacitor (LLC) resonant dc–dc converter design procedure for an onboard lithium-ion battery charger of a plug-in hybrid electric vehicle (PHEV) is presented. Unlike traditional resistive load applications, the characteristic of a battery load is nonlinear and highly related to the charging profiles. Based on the features of an LLC converter and the characteristics of the charging profiles, the design considerations are studied thoroughly. The worst-case conditions for primary-side zero-voltage switching (ZVS) operation are analytically identified based on fundamental harmonic approximation when a constant maximum power (CMP) charging profile is implemented. Then, the worst-case operating point is used as the design targeted point to ensure soft-switching operation globally. To avoid the inaccuracy of fundamental harmonic approximation approach in the below-resonance region, the design constraints are derived based on a specific operation mode analysis. Finally, a step-by-step design methodology is proposed and validated through experiments on a prototype converting 400 V from the input to an output voltage range of 250–450 V at 3.3 kW with a peak efficiency of 98.2%.

356 citations


Journal ArticleDOI
TL;DR: In this paper, a stochastic implementation of the self-consistent harmonic approximation valid to treat anharmonicity at any temperature in the nonperturbative regime is presented.
Abstract: Harmonic calculations based on density-functional theory are generally the method of choice for the description of phonon spectra of metals and insulators. The inclusion of anharmonic effects is, however, delicate as it relies on perturbation theory requiring a considerable amount of computer time, fast increasing with the cell size. Furthermore, perturbation theory breaks down when the harmonic solution is dynamically unstable or the anharmonic correction of the phonon energies is larger than the harmonic frequencies themselves. We present here a stochastic implementation of the self-consistent harmonic approximation valid to treat anharmonicity at any temperature in the nonperturbative regime. The method is based on the minimization of the free energy with respect to a trial density matrix described by an arbitrary harmonic Hamiltonian. The minimization is performed with respect to all the free parameters in the trial harmonic Hamiltonian, namely, equilibrium positions, phonon frequencies, and polarization vectors. The gradient of the free energy is calculated following a stochastic procedure. The method can be used to calculate thermodynamic properties, dynamical properties, and even anharmonic corrections to the Eliashberg function of the electron-phonon coupling. The scaling with the system size is greatly improved with respect to perturbation theory. The validity of the method is demonstrated in the strongly anharmonic palladium and platinum hydrides. In both cases, we predict a strong anharmonic correction to the harmonic phonon spectra, far beyond the perturbative limit. In palladium hydrides, we calculate thermodynamic properties beyond the quasiharmonic approximation, while in PtH, we demonstrate that the high superconducting critical temperatures at 100 GPa predicted in previous calculations based on the harmonic approximation are strongly suppressed when anharmonic effects are included.

269 citations


Journal ArticleDOI
TL;DR: In this article, an improved circulating current control method by applying a digital plug-in repetitive controller is discussed for harmonic elimination of a carrier-phase-shift pulse-width-modulation (CPS-PWM)-based modular multilevel converter (MMC).
Abstract: An improved circulating current control method by applying a digital plug-in repetitive controller is discussed for harmonic elimination of a carrier-phase-shift pulse-width-modulation (CPS-PWM)-based modular multilevel converter (MMC) in this paper. The performance of the controller is analyzed in detail based on an improved circulating current control model with the consideration of the submodule voltage disturbance. It is shown that the proposed control method has the merits of simplicity, versatility, and better performance of circulating harmonic current elimination than the traditional proportional integral controller. The stability analysis of the proposed method are also discussed in the paper as well as the design principles. Finally, the experimental results including the steady-state performance and the transient response are given, which validates the feasibility and excellent performance of the proposed control scheme.

267 citations


Book
01 Jan 2014
TL;DR: In this paper, the authors present an AC-DC conversion and frequency domain analysis for transmission systems, including the following: AC--DC conversion, frequency domain, and harmonic domain.
Abstract: Fourier Analysis. Transmission Systems. Direct Harmonic Solutions. AC--DC Conversion----Frequency Domain. Harmonic Instabilities. Machine Non--linearities----Harmonic Domain. AC--DC Conversion----Harmonic Domain. Iterative Harmonic Analysis. Converter Harmonic Impedances. Appendices. Index.

256 citations


Journal ArticleDOI
TL;DR: In this paper, an enhanced current control approach, which seamlessly integrates system harmonic mitigation capabilities with the primary DG power generation function is proposed, which effectively eliminates the impacts of steady-state fundamental current tracking errors in the DG units.
Abstract: The increasing application of nonlinear loads may cause distribution system power quality issues. In order to utilize distributed generation (DG) unit interfacing converters to actively compensate harmonics, this paper proposes an enhanced current control approach, which seamlessly integrates system harmonic mitigation capabilities with the primary DG power generation function. As the proposed current controller has two well-decoupled control branches to independently control fundamental and harmonic DG currents, local nonlinear load harmonic current detection and distribution system harmonic voltage detection are not necessary for the proposed harmonic compensation method. Moreover, a closed-loop power control scheme is employed to directly derive the fundamental current reference without using any phase-locked loops (PLL). The proposed power control scheme effectively eliminates the impacts of steady-state fundamental current tracking errors in the DG units. Thus, an accurate power control is realized even when the harmonic compensation functions are activated. In addition, this paper also briefly discusses the performance of the proposed method when DG unit is connected to a grid with frequency deviation. Simulated and experimental results from a single-phase DG unit validate the correctness of the proposed methods.

220 citations


Journal ArticleDOI
TL;DR: A new structure based on the use of the SOGI filter as prefilter for the previous structures is proposed to achieve a faster time response and higher harmonic rejection of a grid voltage sequence detection scheme based on a second-order generalized integrator.
Abstract: This paper deals with the improvement of the transient response and harmonic, subharmonic, and dc-offset voltage rejection capability of a grid voltage sequence detection scheme based on a second-order generalized integrator (SOGI). To perform that, the SOGI structure is first analyzed in deep, emphasizing both its tradeoff limits between settling time and harmonic attenuation and the sensitivity to grid subharmonics and dc-offset voltage. Then, a study of the effect of grid voltage harmonics and subharmonics in SOGI and in the SOGI-FLL and MSOGI-FLL structures is introduced. Hence, to overcome these problems, a new structure based on the use of the SOGI filter as prefilter for the previous structures is proposed to achieve a faster time response and higher harmonic rejection. This structure is used in a sequence detection scheme for the detection of the grid voltage components in the αβ-frame and it is applied in a three-phase PV system. Experimental and comparative results are shown to validate this proposal.

216 citations


Journal ArticleDOI
TL;DR: In this article, the authors proposed a load compensator based on the decomposition of output current, in addition to the outer droop-based power controller and the inner voltage and current controllers, which can counteract the harmonic voltage drops across the grid-side inductance of the DG inverter and dampen out harmonic resonance propagation in the microgrid.
Abstract: Harmonic current filtering and resonance damping have become important concerns in the operation and control of the islanded microgrids To address these challenges, this paper proposes a control method for the inverter-interfaced distributed generation (DG) units to autonomously share the harmonic currents and resonance damping burdens The approach employs a load compensator based on the decomposition of output current, in addition to the outer droop-based power controller and the inner voltage and current controllers The load compensator consists of a virtual-fundamental-impedance loop for the enhanced reactive power sharing and a variable-harmonic-impedance loop which allows to counteract the harmonic voltage drops across the grid-side inductance of the DG inverter and also to dampen out harmonic resonance propagation in the microgrid Finally, the laboratory tests on a three-phase islanded microgrid setup are carried out to validate the performance of the proposed control scheme

189 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the potential of using a piezoelectric energy harvester to concurrently harness energy from base excitations and vortex-induced vibrations, and the results showed that there is a significant improvement in the level of the harvested power which can attain 150% compared to using two separate harvesters.
Abstract: We investigate the potential of using a piezoelectric energy harvester to concurrently harness energy from base excitations and vortex-induced vibrations. The harvester consists of a multilayered piezoelectric cantilever beam with a circular cylinder tip mass attached to its free end which is placed in a uniform air flow and subjected to direct harmonic excitations. We model the fluctuating lift coefficient by a van der Pol wake oscillator. The Euler–Lagrange principle and the Galerkin procedure are used to derive a nonlinear distributed-parameter model for a harvester under a combination of vibratory base excitations and vortex-induced vibrations. Linear and nonlinear analyses are performed to investigate the effects of the electrical load resistance, wind speed, and base acceleration on the coupled frequency, electromechanical damping, and performance of the harvester. It is demonstrated that, when the wind speed is in the pre- or post-synchronization regions, its associated electromechanical damping is increased and hence a reduction in the harvested power is obtained. When the wind speed is in the lock-in or synchronization region, the results show that there is a significant improvement in the level of the harvested power which can attain 150 % compared to using two separate harvesters. The results also show that an increase of the base acceleration results in a reduction in the vortex-induced vibrations effects, an increase of the difference between the resonant excitation frequency and the pull-out frequency, and a significant effects associated with the quenching phenomenon.

182 citations


Patent
03 Jul 2014
TL;DR: In this article, an apparatus of a power amplifier is described, which includes an input boosting circuit configured to match a second harmonic input signal using a harmonic control circuit of an input stage to maximize an efficiency and an output power.
Abstract: An apparatus of a power amplifier is provided. The apparatus includes an input boosting circuit configured to match a second harmonic input signal using a harmonic control circuit of an input stage to maximize an efficiency and an output power, a die cell configured to receive and amplify an output signal of the input boosting circuit, and an output boosting circuit configured to receive an output signal of the die cell and to match a second harmonic output signal of the output signal of the die cell using a harmonic control circuit of an output stage to maximize the efficiency and the output power.

182 citations


Journal ArticleDOI
Lifen Chen1, Jun Zang1, Andrew Hillis1, Gerald Morgan, Andrew Plummer1 
TL;DR: In this article, OpenFOAM is applied to non-linear wave interactions with offshore structures for ranges of wave conditions, and the numerical results for wave interaction with a vertical surface piercing cylinder have been compared with physical experiments performed at Danish Hydraulic Institute (DHI).

Journal ArticleDOI
TL;DR: It is shown that harmonics can be enhanced by one to two orders of magnitude without an increase in the total laser power if the laser's waveform is optimized by synthesizing two- or three-colour fields.
Abstract: High-order harmonics extending to the X-ray region generated in a gas medium by intense lasers offer the potential for providing tabletop broadband light sources but so far are limited by their low conversion efficiency. Here we show that harmonics can be enhanced by one to two orders of magnitude without an increase in the total laser power if the laser's waveform is optimized by synthesizing two- or three-colour fields. The harmonics thus generated are also favourably phase-matched so that radiation is efficiently built up in the gas medium. Our results, combined with the emerging intense high-repetition MHz lasers, promise to increase harmonic yields by several orders to make harmonics feasible in the near future as general bright tabletop light sources, including intense attosecond pulses.

Journal ArticleDOI
TL;DR: In this article, a proportional-resonant, multiloop controller is proposed to reduce the number of resonators in a synchronous reference frame, compared with the solution using proportional-integral controllers in the harmonic-reference frame.
Abstract: This paper deals with reactive power compensation and harmonics elimination in medium-voltage industrial networks using a hybrid active power filter. It proposes a hybrid filter as a combination of a three-phase, two-level, voltage-source converter connected in parallel with the inductor of a shunt, single-tuned, passive filter. This topological structure greatly decreases the voltage and current stress over the elements of the active filter. Since the topology is composed of a single-tuned branch, the control algorithm also has to ensure sufficient filtering at other harmonic frequencies. We propose using a proportional-resonant, multiloop controller. Since the controller is implemented in a synchronous-reference frame, it allows us to use half the number of resonators, compared with the solution using proportional-integral controllers in the harmonic-reference frame. Theoretical analyses and simulation results obtained from an actual industrial network model in PSCAD verify the viability and effectiveness of the proposed hybrid filter. In addition, the simulation results are validated by a comparison with the results obtained from a real-time digital simulator.

Journal ArticleDOI
TL;DR: In this article, the distribution system harmonic compensation methods using the distributed generation units with a focus on the DG local controllers are reviewed, and a hybrid control method (HCM) is also discussed, which features better compensation performance and more flexible compensation strategies.
Abstract: Today, many renewable energy-based distributed generation (DG) units are connected to the grid with interfacing converters. Therefore, the harmonic compensation functions can be realized through flexible control of these converters. This article reviews the distribution system harmonic compensation methods using the DG units with a focus on the DG local controllers. How to realize the harmonics compensation functions in the traditional current controlled and voltage-controlled DG systems is presented. A hybrid-control method (HCM) is also discussed, which features better compensation performance and more flexible compensation strategies. Other issues, including harmonics current sharing among the DG units, are also discussed in this article. With the harmonics compensation functions, future DG systems could actively participate in the distribution system power quality control activities.

Journal ArticleDOI
TL;DR: Thanks to the easy software-based reconfigurability, the multiple harmonic beamforming, and the security capability, 4-D arrays can be considered as an enabling technology for future cognitive radio systems.
Abstract: Time-modulation (TM) in four-dimensional (4-D) arrays is implemented by using a set of radio-frequency switches in the beam forming network to modulate, by means of periodic pulse sequences, the static excitations and thus control the antenna radiation features. The on-off reconfiguration of the switches, that can be easily implemented via software, unavoidably generates harmonic radiations that can be suitably exploited for multiple channel communication purposes. As a matter of fact, harmonic beams can be synthesized having different spatial distribution and shapes in order to receive signals arriving on the antenna from different directions. Similarly, the capability to generate a field having different frequency and spatial distribution implies that the signal transmitted by time-modulated 4-D arrays is direction-dependent. Accordingly, such a feature is also exploited to implement a secure communication scheme directly at the physical layer. Thanks to the easy software-based reconfigurability, the multiple harmonic beamforming, and the security capability, 4-D arrays can be considered as an enabling technology for future cognitive radio systems. In this paper, these potentialities of time-modulated 4-D arrays are presented and their effectiveness is supported by a set of representative numerical simulation results.

Journal ArticleDOI
TL;DR: This paper proposes an analytical procedure for computation of all pairs of valid switching angles used in pattern generation in five-level H-bridge cascaded inverters and returns the exact boundaries of all valid modulation index intervals.
Abstract: This paper proposes an analytical procedure for computation of all pairs of valid switching angles used in pattern generation in five-level H-bridge cascaded inverters. The proposed procedure eliminates harmonic components from inverter output voltage and, for each harmonic, returns the exact boundaries of all valid modulation index intervals. Due to its simple mathematical formulation, it can be easily implemented in real time using a digital signal processor or a field-programmable gate array. In this paper, after a detailed description of the method, simulation and experimental results demonstrate the high quality of achievable results.

Journal ArticleDOI
TL;DR: In this article, a flicker-free electrolytic capacitor-less single-phase ac/dc light emitting diode (LED) driver is investigated, which is composed of a power-factor-correction (PFC) converter and a bidirectional converter.
Abstract: In order to achieve high-efficiency, high-power-factor, high-reliability, and low-cost, a flicker-free electrolytic capacitor-less single-phase ac/dc light emitting diode (LED) driver is investigated in this paper. This driver is composed of a power-factor-correction (PFC) converter and a bidirectional converter. The bidirectional converter is used to absorb the second harmonic component in the output current of the PFC converter, thus producing a pure dc output to drive the LEDs. The spectrum of the output capacitor voltage of the bidirectional converter is analyzed, indicating that the output capacitor voltage contains harmonic components at multiples of twice the line frequency apart from the dc component and second harmonic component. A feed-forward control scheme with a series of calculation operation is proposed to obtain the desired modulation signal, which contains the corresponding harmonic components, to ensure the bidirectional converter fully absorb the second harmonic current in the output of the PFC converter. Finally, a 33.6 W prototype is fabricated and tested in the lab, and the experiment results show that the proposed control scheme greatly reduces the ripple of the LED driving current.

Journal Article
TL;DR: In this paper, a circuit analysis of frequency selective surfaces is reviewed with the aim to underline range of validity of different models and their advantages in terms of simplicity and physical insight.
Abstract: Circuit analysis of frequency selective surfaces is reviewed with the aim to underline range of validity of different models and their advantages in terms of simplicity and physical insight. The circuit approach is based on an equivalent representation of the FSSs with series or shunt connections of inductances and capacitances. Dense non-resonant periodic surfaces (i.e.: grid or patch arrays) can be analyzed analytically by computing the values of inductors or capacitors via the homogenization theory. As the lattice period increases with respect to the operating wavelength or the element shape becomes resonant, a fully analytical circuital approach fails, in particular, in the presence of thin substrates. However, simple circuit approaches can still be employed by deriving lumped parameters values via a quick pre-processing and then generalizing them. The results are accurate up to the resonant frequency region of the element. By including an additional lumped element it is possible, taking into account the effect of the first high order Floquet harmonic. The multi-mode formulation is also able to catch the highly nonlinear response of FSS screens in the grating lobe region provided that the current profile of the element does not change significantly.

Journal ArticleDOI
TL;DR: This paper deals with the synthesis problem of multiple resonant controllers for uninterruptible-power-supply systems in a robust control framework by means of a convex optimization problem subject to a set of linear-matrix-inequality constraints.
Abstract: This paper deals with the synthesis problem of multiple resonant controllers for uninterruptible-power-supply systems in a robust control framework. The controller parameters aiming the tracking of sinusoidal reference signals (with zero steady-state error) and the rejection of harmonic components of the output voltage (when supplying energy to time-varying nonlinear loads) are determined by means of a convex optimization problem subject to a set of linear-matrix-inequality constraints. Results from a 5.0-kVA experimental setup are considered to illustrate and validate the proposed approach.

Journal ArticleDOI
24 Jun 2014-Energies
TL;DR: In this paper, a detailed analysis of the frequency response influence between the Proportional Resonant (PR) current controller and the LCL filter is provided, and a systematic method for designing controller parameters and the capacitor current feedback coefficient factor of LCL filters active damping is presented.
Abstract: The Proportional Resonant (PR) current controller provides gains at a certain frequency (resonant frequency) and eliminates steady state errors. Therefore, the PR controller can be successfully applied to single grid-connected PV inverter current control. On the contrary, a PI controller has steady-state errors and limited disturbance rejection capability. Compared with the L- and LC filters, the LCL filter has excellent harmonic suppression capability, but the inherent resonant peak of the LCL filter may introduce instability in the whole system. Therefore, damping must be introduced to improve the control of the system. Considering the controller and the LCL filter active damping as a whole system makes the controller design method more complex. In fact, their frequency responses may affect each other. The traditional trial-and-error procedure is too time-consuming and the design process is inefficient. This paper provides a detailed analysis of the frequency response influence between the PR controller and the LCL filter regarded as a whole system. In addition, the paper presents a systematic method for designing controller parameters and the capacitor current feedback coefficient factor of LCL filter active-damping. The new method relies on meeting the stable margins of the system. Moreover, the paper also clarifies the impact of the grid on the inverter output current. Numerical simulation and a 3 kW laboratory setup assessed the feasibility and effectiveness of the proposed method.

Journal ArticleDOI
TL;DR: Several commonly used methods for power system stationary and time-varying harmonic and interharmonic estimation are reviewed and compared according to the viewpoint of the frequency identification to provide suitable guidelines for the future studies.
Abstract: Frequency is one of the most important factors for power system harmonic and interharmonic estimation. Accurate spectral analysis relies much on the correct identification of frequencies of the measured signals. In this paper, several commonly used methods for power system stationary and time-varying harmonic and interharmonic estimation are reviewed and compared according to the viewpoint of the frequency identification. It is expected to provide suitable guidelines for the future studies.

Journal ArticleDOI
TL;DR: In this paper, a robust passive damping method for LLCL-filter-based grid-tied inverters is proposed, which effectively can suppress the possible resonances even if the grid inductance varies in a wide range.
Abstract: In order to minimize the effect of the grid harmonic voltages, harmonic compensation is usually adopted for a grid-tied inverter. However, a large variation of the grid inductance challenges the system stability in case a high-order passive filter is used to connect an inverter to the grid. Although in theory, an adaptive controller can solve this problem, but in such a case the grid inductance may need to be detected online, which will complicate the control system. This paper investigates the relationship between the maximum gain of the controller that still keeps the system stable and the Q-factor for a grid-tied inverter with an RL series or an RC parallel damped high-order power filter. Then, a robust passive damping method for LLCL-filter-based grid-tied inverters is proposed, which effectively can suppress the possible resonances even if the grid inductance varies in a wide range. Simulation and experimental results are in good agreement with the theoretical analysis.

Journal ArticleDOI
TL;DR: In this paper, the authors developed a density-matrix formalism in the length gauge to calculate the nonlinear response of intrinsic monolayer graphene at terahertz frequencies and found that the interplay of the interband and intraband dynamics leads to strong harmonic generation at moderate field amplitudes.
Abstract: We develop a density-matrix formalism in the length gauge to calculate the nonlinear response of intrinsic monolayer graphene at terahertz frequencies. Employing a tight-binding model, we find that the interplay of the interband and intraband dynamics leads to strong harmonic generation at moderate field amplitudes. In particular, we find that at low temperature, the reflected field of undoped suspended graphene exhibits a third harmonic amplitude that is 32% of the fundamental for an incident field of 100 V/cm. Moreover, we find that up to the seventh harmonic and beyond are generated.

Journal ArticleDOI
TL;DR: In this paper, the authors show that 400 nm driven high harmonic extreme-ultraviolet trARPES is superior to using 800 nm laser drivers since it eliminates the need for any spectral selection, thereby increasing photon flux and energy resolution to <150 meV.

Journal ArticleDOI
TL;DR: In this article, an alternative control method is proposed to improve the harmonic suppression efficiency of the active power filter in a distorted and an unbalanced power system to compensate for the perturbations caused by the unbalanced nonlinear loads.
Abstract: In this paper, an alternative control method is proposed to improve the harmonic suppression efficiency of the active power filter in a distorted and an unbalanced power system to compensate for the perturbations caused by the unbalanced non-linear loads. The proposed method uses a self-tuning filter (STF) to process the grid voltage in order to provide a uniform reference voltage to obtain the correct angular position of the phase locked loop. Moreover, the required compensation currents are obtained by implementing another STF in the transformed set of currents in order to separate the fundamental and the harmonic currents. This allows the calculation of a precise reference current for the unbalanced, the non-linear and the variable load conditions. The proposed control method gives an adequate compensating current reference even for a non-ideal voltage and unbalanced current conditions. The real-time control of the filter under the distorted and the unbalanced power system is developed in an RT-LAB real-time platform. The results obtained in the software-in-the-loop configuration are presented to verify the effectiveness of the proposed control technique.

Journal ArticleDOI
01 Nov 2014
TL;DR: In this article, the harmonic interactions of current controllers in multiparalleled grid-connected inverters are investigated and the potential harmonic instability phenomenon, which features oscillations above the fundamental frequency, is evaluated by the impedance-based stability criterion.
Abstract: This paper investigates the harmonic interactions of current controllers in multiparalleled grid-connected inverters. Potential harmonic instability phenomenon, which features oscillations above the fundamental frequency, is evaluated by the impedance-based stability criterion. The possible reasons of the destabilized inverter with different types of grid impedances are explained by the passivity-based analysis and impedance-based stability criterion. Thereafter, case studies are provided, which shows how the nonpassive grid connected inverters interact with the varying grid impedance due to load changes or by the new connections of other grid-connected inverters. Time domain simulations in the PSCAD/EMTDC environment and experimental results show the corresponding harmonic interaction problems may exist in nowadays power system and the impedance-based stability analysis can be an effective tool to assess those problems.

Journal ArticleDOI
TL;DR: This paper presents a multifunctional control strategy for the autonomous operation of a multi-bus medium voltage (MV) microgrid under nonlinear and unbalanced load conditions to effectively compensate the harmonic currents of nonlinear loads using electronically-coupled distributed generation (DG) units.
Abstract: This paper presents a multifunctional control strategy for the autonomous operation of a multi-bus medium voltage (MV) microgrid under nonlinear and unbalanced load conditions. The main objective is to effectively compensate the harmonic currents of nonlinear loads using electronically-coupled distributed generation (DG) units. The proposed control strategy consists of a multi proportional resonant controller (MPRC) with adjustable resonance frequency and a harmonic impedance controller (HIC). The MPRC and HIC are, respectively, proposed to regulate the microgrid voltage and to share the harmonic currents of the nonlinear loads among the DG units. A conventional droop scheme is also employed to share the average powers of the loads between the DG units. Moreover, a virtual negative-sequence impedance control loop is augmented to the proposed control system to compensate the negative-sequence currents of the unbalanced loads. The proposed strategy minimizes the flow of harmonic and negative-sequence currents in the MV lines of the microgrid by local compensation of harmonic and negative-sequence currents of the feeders. The performance of the proposed control scheme is verified by using digital time-domain simulation studies in the PSCAD/EMTDC software environment.

Journal ArticleDOI
TL;DR: This paper suggests a new approach for the optimal sizing of hybrid active power filter parameters, which is presented for three-phase industrial power systems, and uses Fortran Feasible Sequential Quadratic Programming to determine the filter optimal sizing.
Abstract: The development of new passive, active, and hybrid filtering techniques is important; and the issues of higher quality, reduced complexity, higher efficiency, and lower cost are important conditions that need to be addressed with regard to the expected stringent trends in power quality obligations. This paper suggests a new approach for the optimal sizing of hybrid active power filter (HAPF) parameters, which is presented for three-phase industrial power systems. Hybrid filter topology can be used to compensate harmonic currents, as well as for power factor corrections, without concern for importing and exporting harmonics, or simply the series and parallel resonance that may occur. The new trend in harmonic power filter design is not to obtain the best solution from a single objective optimization but to obtain a good compromise solution accomplished under other conflicting objectives. Fortran Feasible Sequential Quadratic Programming is used to determine the proposed filter optimal sizing to minimize the total voltage harmonic distortion as the main objective function, where maintaining the load power factor at an acceptable limit is desired. If the total harmonic voltage distortion achieves the specified goal, then the objective is redirected into minimizing the resultant voltage and current total harmonic distortions. The optimal design of the HAPF is analyzed by means of three case studies.

Journal ArticleDOI
TL;DR: In this paper, a novel converter topology of high-voltage direct current (HVDC) transmission for integrating the offshore wind farm (WF) with the grid is proposed, which consists of a series connection of the 12-pulse diode rectifiers and a voltage-source converter (VSC).
Abstract: In this letter, a novel converter topology of high-voltage direct current (HVDC) transmission for integrating the offshore wind farm (WF) with the grid is proposed, which consists of a series connection of the 12-pulse diode rectifiers and a voltage-source converter (VSC). For this topology, the PCC (point of common coupling) voltage of the offshore WF side is regulated at a constant magnitude and constant frequency by the WFVSC. Also, the current at the PCC becomes almost sinusoidal since the WFVSC functions as an active power filter for the 11th- and 13th-order harmonic current components. By virtue of the diode rectifier, the cost and power loss of HVDC converters is reduced compared with the case of the conventional fully rated VSC, whereas the performance is kept almost the same. The PSCAD/EMTDC simulation results for a 500-MW HVDC transmission system have been shown to verify the validity of the proposed scheme.

Journal ArticleDOI
TL;DR: A new method, based on natural frequency changes, able to detect damages in beam-like structures and to assess their location and severity, considering the particular manner in which the natural frequencies of the weak-axis bending vibration modes change due to the occurrence of discontinuities is presented.