Showing papers on "Harmonic published in 2018"
TL;DR: A general theory of space-time modulated digital coding metasurfaces is proposed to obtain simultaneous manipulations of EM waves in both space and frequency domains, i.e., to control the propagation direction and harmonic power distribution simultaneously.
Abstract: The recently proposed digital coding metasurfaces make it possible to control electromagnetic (EM) waves in real time, and allow the implementation of many different functionalities in a programmable way. However, current configurations are only space-encoded, and do not exploit the temporal dimension. Here, we propose a general theory of space-time modulated digital coding metasurfaces to obtain simultaneous manipulations of EM waves in both space and frequency domains, i.e., to control the propagation direction and harmonic power distribution simultaneously. As proof-of-principle application examples, we consider harmonic beam steering, beam shaping, and scattering-signature control. For validation, we realize a prototype controlled by a field-programmable gate array, which implements the harmonic beam steering via an optimized space-time coding sequence. Numerical and experimental results, in good agreement, demonstrate good performance of the proposed approach, with potential applications to diverse fields such as wireless communications, cognitive radars, adaptive beamforming, holographic imaging.
608 citations
TL;DR: Efficient terahertz harmonic generation—challenging but important for ultrahigh-speed optoelectronic technologies—is demonstrated in graphene through a nonlinear process that could potentially be generalized to other materials.
Abstract: Multiple optical harmonic generation—the multiplication of photon energy as a result of nonlinear interaction between light and matter—is a key technology in modern electronics and optoelectronics, because it allows the conversion of optical or electronic signals into signals with much higher frequency, and the generation of frequency combs. Owing to the unique electronic band structure of graphene, which features massless Dirac fermions1–3, it has been repeatedly predicted that optical harmonic generation in graphene should be particularly efficient at the technologically important terahertz frequencies4–6. However, these predictions have yet to be confirmed experimentally under technologically relevant operation conditions. Here we report the generation of terahertz harmonics up to the seventh order in single-layer graphene at room temperature and under ambient conditions, driven by terahertz fields of only tens of kilovolts per centimetre, and with field conversion efficiencies in excess of 10−3, 10−4 and 10−5 for the third, fifth and seventh terahertz harmonics, respectively. These conversion efficiencies are remarkably high, given that the electromagnetic interaction occurs in a single atomic layer. The key to such extremely efficient generation of terahertz high harmonics in graphene is the collective thermal response of its background Dirac electrons to the driving terahertz fields. The terahertz harmonics, generated via hot Dirac fermion dynamics, were observed directly in the time domain as electromagnetic field oscillations at these newly synthesized higher frequencies. The effective nonlinear optical coefficients of graphene for the third, fifth and seventh harmonics exceed the respective nonlinear coefficients of typical solids by 7–18 orders of magnitude7–9. Our results provide a direct pathway to highly efficient terahertz frequency synthesis using the present generation of graphene electronics, which operate at much lower fundamental frequencies of only a few hundreds of gigahertz. Efficient terahertz harmonic generation—challenging but important for ultrahigh-speed optoelectronic technologies—is demonstrated in graphene through a nonlinear process that could potentially be generalized to other materials.
366 citations
TL;DR: In this paper, the authors proposed a new module for asymmetrical multilevel inverters with a low number of components, which is a square combination of two back-to-back T-type inverters and some other switches.
Abstract: This paper introduces a new module for asymmetrical multilevel inverters with a low number of components. The module is a square combination of two back-to-back T-type inverters and some other switches. A square T-type module produces 17 levels by 12 switches and 4 unequal dc sources (two 3 V DC and two 1 V DC). Also, it can be extended as a cascade connection in two strategies to achieve more levels. The module and its cascade connection are suitable for the applications in several dc sources systems such as photovoltaic farms, which lead to a modular topology with more voltage levels at higher voltages. Inherent creation of the negative voltage levels without any additional circuit (such as H-bridge circuit) is one of the main features of the proposed module. The low total harmonic distortion of the output voltage/current and low number of semiconductors are among the other advantages of the proposed module. A nearest level control method as a switching technique is used to produce high quality output voltage with lower harmonic contents. Simulations have been performed in MATLAB/Simulink and a prototype is implemented in the Power Electronics Laboratory; both the simulation and experimental results show a good performance.
246 citations
TL;DR: This work designed and experimentally characterized a reflective time-domain digital coding metasurface, with independent control of the harmonic amplitude and phase, and paves the way for efficient harmonic control for applications in communications, radar, and related areas.
Abstract: Harmonic manipulations are important for applications such as wireless communications, radar detection and biological monitoring. A general approach to tailor the harmonics involves the use of additional amplifiers and phase shifters for the precise control of harmonic amplitudes and phases after the mixing process; however, this approach leads to issues of high cost and system integration. Metasurfaces composed of a periodic array of subwavelength resonators provide additional degrees of freedom to realize customized responses to incident light and highlight the possibility for nonlinear control by taking advantage of time-domain properties. Here, we designed and experimentally characterized a reflective time-domain digital coding metasurface, with independent control of the harmonic amplitude and phase. As the reflection coefficient is dynamically modulated in a predefined way, a large conversion rate is observed from the carrier signal to the harmonic components, with magnitudes and phases that can be accurately and separately engineered. In addition, by encoding the reflection phases of the meta-atoms, beam scanning for multiple harmonics can be implemented via different digital coding sequences, thus removing the need for intricate phase-shift networks. This work paves the way for efficient harmonic control for applications in communications, radar, and related areas.
193 citations
TL;DR: The combination of strong-field laser physics with topological condensed matter opens up new possibilities to electronically control strong- field-based light or particle sources or-conversely-to steer by all optical means topological electronics.
Abstract: High-harmonic generation in the two topological phases of a finite, one-dimensional, periodic structure is investigated using a self-consistent time-dependent density functional theory approach. For harmonic photon energies smaller than the band gap, the harmonic yield is found to differ by up to 14 orders of magnitude for the two topological phases. This giant topological effect is explained by the degree of destructive interference in the harmonic emission of all valence-band (and edge-state) electrons, which strongly depends on whether or not topological edge states are present. The combination of strong-field laser physics with topological condensed matter opens up new possibilities to electronically control strong-field-based light or particle sources or-conversely-to steer by all optical means topological electronics.
160 citations
TL;DR: With the proposed methods, the microgrid system reliability and flexibility can be enhanced and the knowledge of the line impedance is not required and the quality of the voltage at PCC can be greatly improved.
Abstract: This paper investigates the issue of accurate reactive, harmonic, and imbalance power sharing in a microgrid. Harmonic and imbalance droop controllers are developed to proportionally share the harmonic power and the imbalance power among distributed generation (DG) units and improve the voltage quality at the point of common coupling (PCC). Further, a distributed consensus protocol is developed to adaptively regulate the virtual impedance at fundamental frequency and selected harmonic frequencies. Additionally, a dynamic consensus based method is adopted to restore the voltage to their average voltage. With the proposed methods, the microgrid system reliability and flexibility can be enhanced and the knowledge of the line impedance is not required. And the reactive, harmonic, and imbalance power can be proportionally shared among the DG units. Moreover, the quality of the voltage at PCC can be greatly improved. Simulation and experimental results are presented to demonstrate the proposed method.
157 citations
TL;DR: This systematic review can facilitate better understanding of harmonics associated with renewable energy based DG units and provide guidelines on advanced control schemes to realize ancillary harmonic compensation service through DG interfacing inverters.
Abstract: With more renewable energy based distributed generation (DG) units connected to utility power grids, deterioration of power quality at the point of common coupling (PCC) becomes a major concern. There are two types of harmonics associated with DG units, and together they may cause excessive harmonic distortion at the PCC. The first type of harmonics is generated by power electronic devices in DG units such as photovoltaic systems, which contains high-frequency harmonic components at multiples of the carrier frequency of the DG interfacing inverter. Such harmonics are first reviewed in this paper, and the potential operational effect at the system level due to LCL or LC filters installed at the inverter output to mitigate such harmonics are discussed. The second type of harmonics is generated by other nonlinear local, PCC, and utility loads in the system, which are common type of harmonics at multiples of the power grid frequency, 50/60 Hz. Harmonic mitigation for such harmonics achieved through advanced control of the DG interfacing inverter operated as a power quality conditioner are reviewed and summarized. This systematic review can facilitate better understanding of harmonics associated with renewable energy based DG units and provide guidelines on advanced control schemes to realize ancillary harmonic compensation service through DG interfacing inverters.
156 citations
TL;DR: Using a simple one-dimensional two-band model, it is demonstrated that the observed odd harmonics is directly related to the orientation dependence of the magnitude of the transition dipole, while even harmonic is directlyrelated to the phase of the Transition dipole.
Abstract: Since the first observation of odd and even high-order harmonics generated from ZnO crystals in 2011, the dependence of the harmonic yields on the orientation of the laser polarization with respect to the crystal axis has never been properly interpreted. This failure has been traced to the lack of a correct account of the phase of the transition dipole moment between the valence band and the conduction band. Using a simple one-dimensional two-band model, here we demonstrate that the observed odd harmonics is directly related to the orientation dependence of the magnitude of the transition dipole, while even harmonics is directly related to the phase of the transition dipole. Our result points out the essential role of the complex transition dipole moment in understanding harmonic generation from solids that has long been overlooked so far.
155 citations
TL;DR: In this article, a multiscale simulation of solid-state high-order-harmonic generation was performed for dielectrics and it was shown that mesoscopic effects of the extended system, in particular the realistic sampling of the entire Brillouin zone, the pulse propagation in the dense medium, and the inhomogeneous illumination of the crystal, have a strong effect on the harmonic spectra.
Abstract: High-order-harmonic generation by a highly nonlinear interaction of infrared laser fields with matter allows for the generation of attosecond pulses in the XUV spectral regime This process, well established for atoms, has been recently extended to the condensed phase Remarkably well-pronounced harmonics up to order $\ensuremath{\sim}30$ have been observed for dielectrics We establish a route toward an ab initio multiscale simulation of solid-state high-order-harmonic generation We find that mesoscopic effects of the extended system, in particular the realistic sampling of the entire Brillouin zone, the pulse propagation in the dense medium, and the inhomogeneous illumination of the crystal, have a strong effect on the harmonic spectra Our results provide an explanation for the formation of clean harmonics and have implications for a wide range of nonlinear optical processes in dense media
149 citations
TL;DR: An optimal design method for controller parameters is proposed in this paper in order to guarantee the small-signal stability of the interconnected system from a system point of view and time-domain simulations validate the effectiveness of the theoretical analysis and the proposed ideal design method.
Abstract: A subsynchronous oscillation (SSO) phenomenon has been observed in a modular multilevel converter-based high-voltage dc (MMC-HVDC) transmission system for wind farm integration in the real world, which is independent of the type of wind turbine generator. This kind of oscillation appears different from those in doubly fed induction generator-based wind farm with series-compensation line or wind farm integration through two-level voltage-source converter-HVDC transmission system, because the internal dynamics of the MMC may have significant impact on the oscillation. By far, however, very few papers have reported it. In this paper, the generation mechanism of the SSO phenomenon in an MMC-HVDC transmission system for wind farm integration is revealed from an impedance point of view. The harmonic state-space modeling method is applied to model the multifrequency behavior of the MMC, based on which, the ac-side small-signal impedance of the MMC is analytically derived according to harmonic linearization theory. As a general rule, the controller parameters of the wind power inverter and the HVDC converter are designed separately, to meet the performance requirements of the single converter under ideal conditions, but this practice does not guarantee the stability of the interconnected system. Therefore, an optimal design method for controller parameters is proposed in this paper in order to guarantee the small-signal stability of the interconnected system from a system point of view. Finally, time-domain simulations validate the effectiveness of the theoretical analysis and the proposed optimal design method.
130 citations
TL;DR: A new decentralized hierarchical control scheme is presented to improve power sharing of multidistributed energy resources microgrids including nonlinear and sensitive loads and exploits the nonlinear mapping ability of radial basis function neural networks to solve harmonic power flow and obtain voltage harmonics and active and reactive powers.
Abstract: A new decentralized hierarchical control scheme is presented to improve power sharing of multidistributed energy resources microgrids including nonlinear and sensitive loads. In this systems, electronically coupled distributed energy resources are responsible to perform the compensation to reduce the voltage harmonics at the point-of-common coupling. The proposed control scheme adds a new virtual impedance scheme, power calculation unit, and also a complementary loop to improve small- and large-signal stability margins and includes detailed modeling for all hierarchical control levels (either for grid-connected or islanded modes). Compared to conventional virtual impedance methods that add only line current feedforward terms to the voltage reference, here, the line current and voltage at the point-of-common coupling regulate the virtual impedance at fundamental and harmonic frequencies, respectively. So, mismatches in the feeder and line impedances are compensated. Moreover, a power calculation method based on harmonic power flow is presented, which exploits the nonlinear mapping ability of radial basis function neural networks to solve harmonic power flow and obtain voltage harmonics and active and reactive powers. To show the effectiveness of the proposed control scheme, offline time-domain simulation studies have been done on a test microgrid by MATLAB/SIMULINK software and verified experimentally using OPAL-RT real-time digital simulator.
TL;DR: An overview of the harmonic and resonance problems in the RES, including harmonic problem composition, harmonic modeling, available influential factors assessment, harmonic resonance, and associated suppression methods, is presented.
Abstract: Harmonic distortion and harmonic resonance problems have been widely concerned and reported in railway electrification systems (RESs) due to the harmonic injection from the nonlinear electric train, especially high-speed/high-capacity/large-power trains. This paper presents an overview of the harmonic and resonance problems in the RES, including harmonic problem composition, harmonic modeling, available influential factors assessment, harmonic resonance, and associated suppression methods. The harmonic problem mainly consists of the background harmonics brought from the utility system, resonance-region harmonics interacted by capacitive and inductive network elements, and characteristic harmonics generated from the switching process of the onboard power conversion system. The mathematical modeling and analysis methods are introduced, including frequency scanning analysis, S-domain analysis, resonance mode analysis, and modal sensitivity analysis. Available influential factors/parameters have been fully investigated against above-mentioned harmonic problems. Finally, different harmonic suppression methods have been compared and summarized in this paper.
TL;DR: A super-twisting sliding mode direct power control (SSM-DPC) strategy for a brushless doubly fed induction generator (BDFIG) that is robust to uncertainties toward parameter variations and achieves constant converter switching frequency by using space vector modulation.
Abstract: This paper proposes and implements a super-twisting sliding mode direct power control (SSM-DPC) strategy for a brushless doubly fed induction generator (BDFIG). DPC has fast and robust response under transient conditions; however, it suffers from active and reactive power ripples and current distortions, which degrades the quality of the output power. In contrast, vector control has good steady-state current harmonic spectra; however, it is not robust to machine parameters variations, and thus needs a phase-locked loop for synchronous coordinate transformations. The SSM-DPC strategy controls active and reactive power directly without the need of the phase-locked loop. Moreover, its transient performance is similar to DPC and its steady-state performance is the same as vector control. The proposed controller is robust to uncertainties toward parameter variations and achieves constant converter switching frequency by using space vector modulation. Simulation and experimental results of a 2 MW and 3 kW laboratory-scale BDFIG are provided and compared with those of integral sliding mode and DPC to validate the effectiveness, correctness, and the robustness of the proposed strategy.
TL;DR: In this paper, the second harmonic generation due to non-linear interaction of Lamb waves with a fatigue crack was investigated in 3D finite element simulations and experimental studies were carried out to provide physical insight into the mechanism of second harmonic generator.
TL;DR: A novel HIFD algorithm based on the identification of nonlinear voltage–current characteristic profiles (VCCPs) is proposed, with the focus on the quenching and restrike dynamic process of an arc flash.
Abstract: High-impedance fault detection (HIFD) is crucial in an effectively grounded distribution system because of the potential threat of fire and electric shock. HIFD has been extensively researched for more than 30 years, and a harmonic component in zero-sequence current is typically used in detection algorithms since waveform distortion is often produced by arc flash. For a stable HIF, it would be invalid or has lower sensitivity because of the limited harmonic content in zero-sequence current waveform. To overcome this problem, in this paper, a novel detection algorithm is proposed for detecting waveform distortion in the time domain. First, for the analysis of arc flash, the solid dielectric electrical breakdown theory is verified to be more suitable than theories based on the heat accumulation theory, and a nonlinear arc model that can be used for analyzing arc flash is then proposed. Furthermore, a novel HIFD algorithm based on the identification of nonlinear voltage–current characteristic profiles (VCCPs) is proposed, with the focus on the quenching and restrike dynamic process of an arc flash. The high performance of the algorithm is verified by performing various simulations and examining field data. An HIFD prototype using the proposed algorithm was developed, and it showed excellent performance in real-time digital simulator tests. The VCCP based disturbance detection approach could also be popularized in the applications of relay protection, harmonic source location and early fault alarming in smart grid.
TL;DR: In this paper, the authors used high-temperature gas mixtures as the generation medium to increase the gas translational velocity, thereby reducing the steady-state plasma in the laser focus.
Abstract: Laser-driven high-order harmonic generation1,2 provides spatially3 and temporally4 coherent tabletop sources of broadband extreme-ultraviolet (XUV) light. These sources typically operate at low repetition rates, frep ≲ 100 kHz, where phase-matched HHG is readily achieved5,6. However, many applications demand the improved counting statistics or frequency-comb precision afforded by high repetition rates, frep > 10 MHz. Unfortunately, at such high frep, phase matching is prevented by steady-state plasma accumulated in the generation volume7–11, strongly limiting the XUV average power. Here, we use high-temperature gas mixtures as the generation medium to increase the gas translational velocity, thereby reducing the steady-state plasma in the laser focus. This allows phase-matched XUV emission inside a femtosecond enhancement cavity at frep = 77 MHz, enabling a record generated power of ~ 2 mW in a single harmonic order. This power scaling opens up many demanding applications, including XUV frequency-comb spectroscopy12,13 of few-electron atoms and ions for precision tests of fundamental physical laws and constants14–20. Using high-temperature gas mixtures as the generation medium to increase the translational velocity of Xe atoms through the focus of a femtosecond enhancement cavity, phase-matched extreme-ultraviolet emission at a repetition rate of 77 MHz and with an average power of ~ 2 mW in a single harmonic order is achieved.
TL;DR: A new decentralized robust power/current/voltage/frequency control strategy for islanded and grid-connected modes to enhance power-sharing and small- and large-signal stability of a wind/photovoltaic/fuel cell microgrid and improve its performance for nonlinear and unbalanced loads is presented.
Abstract: This paper presents a new decentralized robust power/current/voltage/frequency control strategy for islanded and grid-connected modes to enhance power-sharing and small- and large-signal stability of a wind/photovoltaic/fuel cell microgrid and improve its performance for nonlinear and unbalanced loads. First, the distributed energy resources are modeled for unbalanced voltage condition. Then, in order to improve power sharing, regulate voltage and active/reactive powers injected by these resources, and moreover, harmonic and negative-sequence current control in the presence of nonlinear and unbalanced loads, three separate controllers for positive-sequence voltage and power control and negative sequence current control are designed based on the sliding mode control, Lyapunov function theory, and fractional-order sliding mode control, respectively. The theoretical concept of the proposed control strategy, including the mathematical modeling of microgrid components, basic theorems, controller design procedure, and robustness/closed-loop stability analysis, is outlined. Also, this direct power/current/voltage/frequency control scheme is governed by a hierarchical control scheme that exploits a voltage compensation scheme and harmonic virtual impedance loop. To show the effectiveness of the proposed control scheme, offline time-domain simulation studies are performed on a wind/photovoltaic/fuel cell microgrid with nonlinear and unbalanced loads in MATLAB/Simulink environment and the results are verified by OPAL-RT real-time digital simulator.
TL;DR: A new control method to suppress current harmonics for permanent magnet synchronous linear motor (PMSLM) that is applied in the miniature microsecond laser cutting system and can reduce the overshoot and thrust ripple as well is presented.
Abstract: This paper presents a new control method to suppress current harmonics for permanent magnet synchronous linear motor (PMSLM) that is applied in the miniature microsecond laser cutting system. In the control method, the resonant–two-degree-of-freedom (R–2DOF) proportional–integral–derivative (PID) controller is proposed by combining a resonant controller and a two-degree-of-freedom (2DOF) PID controller. The current harmonic components are first analyzed. The resonant controller is subsequently added to the current loop in parallel to the traditional PI controller to suppress the current harmonic components. However, with the current harmonics suppression, the resonant controller can result in the overshoot in the current loop response. The 2DOF PID controller is adopted to reduce the overshoot. Thus, an R–2DOF PID controller is developed by combining the resonant controller and 2DOF PID controller. Meanwhile, the stability of the proposed controller is analyzed. Compared with the traditional PID controller and the Kalman filter, the proposed controller not only can suppress the current harmonics but can reduce the overshoot and thrust ripple as well. Finally, the simulation and experimental comparison results confirm the validity of the proposed control algorithm.
TL;DR: Experimental results show great coincidence with theoretical analysis, demonstrating the superiority of the newly proposed parameter tuning method.
Abstract: Wireless power transfer (WPT) has attracted a large amount of attention owing to its inherent advantages such as convenience, safety, low maintenance, being weather proof, etc. A parameter tuning method is crucial for a WPT system due to its function of reducing reactive power and improving system efficiency. Three deficiencies of the conventional double-sided inductor–capacitor–inductor (DS- LCL ) compensated system, i.e., low design freedom, weak high-order harmonic suppression capability, and discontinuous input current of a full-wave diode rectifier (FDR), are first analyzed by means of theoretical derivation, numerical calculation, and Pspice simulation. In order to overcome these disadvantages, a novel parameter tuning method is proposed. The characteristic of constant current output of the proposed DS- LCL system is analyzed, followed by a detailed derivation about secondary compensation inductance. Theoretical analysis indicates that the proposed system has four attractive characteristics: higher design freedom, reduced coil current, enhanced high-order harmonic suppression capability, and continuous input current of the FDR. The overall efficiencies of two comparative WPT prototypes, tuned by conventional and proposed methods, are 87.3% and 90.2%, respectively. Experimental results show great coincidence with theoretical analysis, demonstrating the superiority of the newly proposed parameter tuning method.
TL;DR: In this paper, the authors proposed a FCS-MPCC scheme with duty-ratio optimization for five-phase inverters, where a group of the reconstructed virtual voltage vectors (V3) with low-order harmonic elimination is defined and employed as the control sets in FCS predictive current control (FCS-PC) for multiphase inverters.
Abstract: As is well known, the additional low-order harmonic currents and the huge computation amount of the cost function value with all available control sets are the two special issues of finite-control-set model predictive current control (FCS-MPCC) for multiphase inverters Thus, this paper proposes a FCS-MPCC scheme with duty-ratio optimization for five-phase inverters First, a group of the reconstructed virtual voltage vectors (V3) with low-order harmonic elimination is defined and employed as the control sets in FCS-MPCC scheme Thus, it leads to no requirement for harmonic items and weighting factors existing in the cost function Then, a combination of zero vectors and the selected V3 during each control horizon is adopted to rearrange the pulse sequence, which can realize constant switching frequency In addition, the durations of the selected V3 and zero vectors during each control horizon are also estimated to minimize the steady-state current ripples in different subspaces based on the cost function optimization Finally, a comparison of the proposed V3-FCS-MPCC scheme and the other FCS-MPCCs are presented in simulation and experimental test Simulation and experimental results verify that the proposed V3-FCS-MPCC scheme can remain the simplified structure, fast dynamic performance, constant switching frequency, and achieve minimum current ripples in different subspaces
TL;DR: The proposed fault-tolerant MPDPC with power compensation method ensures the continuous and reliable operation of the bidirectional ac/dc converter with high power quality and validate the proposed control strategy under symmetrical unbalanced grid voltages with switch device faults.
Abstract: The bidirectional ac/dc converter is widely used to realize the power conversion between ac and dc microgrid, but the faults of switch devices and unbalanced grid voltages may lead to the decline of power quality and affect normal operation of the converter. The four-switch three-phase (FSTP) fault-tolerant structure is reconstructed from a six-switch three-phase structure with switch device fault. In order to reduce harmonic currents and output power fluctuations under unbalanced grid voltages, finite states model predictive direct power control (MPDPC) with power compensation method is proposed for FSTP structure and predictive power model of the bidirectional FSTP ac/dc converter is established. The power compensation values are expressed by grid voltages and their quadrature signals that lagging 90 electrical degrees in the αβ stationary coordinate system. Compared with the conventional method, phase-locked loop, pulse width modulation, and complex positive-/negative-sequence extraction of grid voltage are not required. Ripples of active power or reactive power under unbalanced grid voltages are eliminated. The proposed fault-tolerant MPDPC with power compensation method ensures the continuous and reliable operation of the bidirectional ac/dc converter with high power quality. Simulation and experimental results are presented to validate the proposed control strategy under symmetrical unbalanced grid voltages with switch device faults.
TL;DR: In this paper, the harmonic instability and resonance frequencies in large permanent magnet synchronous generator (PMSG) based WPPs with full-scale converters are considered, where linearized models of inner control loops of the power converters were considered.
Abstract: Compared to the conventional power systems, large wind power plants (WPPs) present a more challenging system, where the interactions between the passive elements and the wideband control systems of power converters may result in harmonic instability and new resonance frequencies. Most of researches about harmonic stability focus on small-scale systems, and it has not paid much attention yet to identify the mentioned resonance frequencies. This paper models and analyzes the harmonic instability and resonance frequencies in large permanent magnet synchronous generator (PMSG) based WPPs with full-scale converters, where linearized models of inner control loops of the power converters are considered. A large PMSG-based WPP introduces as a multi-input mulit-output (MIMO) control system, therefore, the stability of the whole power system is analyzed based on the real parts of the poles of the introduced MIMO system and the resonance frequencies are identified based on the element amplitudes of the MIMO matrix. An active damping controller is used to set the poles of the WPP in a desired location in order to mitigate the harmonic instability problems. Multiple case studies are provided to depict that wind turbine connections or disconnections in a WPP, as well as grid impedance variations can affect the harmonic stability and resonance frequencies. The effectiveness of the presented theoretical analysis is validated by time-domain simulations of a 400-MW WPP in PSCAD/EMTDC software environment.
TL;DR: A simplified model predictive torque control (MPTC) for an asymmetrical dual three-phase permanent magnet synchronous motor (PMSM), which can significantly improve the machine performances and the computational burden can be considerably alleviated.
Abstract: The conventional model predictive control for dual three-phase motors only evaluates the largest voltage vectors to alleviate the computation burden, but at the costs of large current harmonics and torque ripple. This paper presents a simplified model predictive torque control (MPTC) for an asymmetrical dual three-phase permanent magnet synchronous motor (PMSM), which can significantly improve the machine performances. First, the 36 active voltage vectors in the outer three layers are considered with the proposed control method. In order to suppress the stator harmonic currents, the voltage vectors are preselected based on a switching table according to the flux position and torque deviation in the $\alpha \hbox{-} \beta, \text{x--}y$ subspace. Simultaneously, the number of prediction voltage vectors can also be reduced, thus effectively decreasing the computation time. In particular, with the proposed MPTC method, the harmonic currents can be effectively suppressed. Furthermore, the computational burden can be considerably alleviated. In the meantime, the merits of the conventional MPTC such as fast dynamic response and intuitive implementation are preserved. Finally, both simulation and experimental results are performed to verify the validation of the proposed MPTC methodology when comparing with the conventional MPTC for the dual three-phase PMSMs.
TL;DR: An adaptive tuning method of the virtual resistor is proposed, which can automatically regulate thevirtual resistor to the critical value to stabilize the system, and thus reduce the power loss.
Abstract: When a grid-connected inverter is connected to a weak grid, the system may be unstable. An active damper can be connected to the point of common coupling (PCC), which simulates a virtual resistor to dampen the resonance and thus stabilize the system. In this paper, an adaptive tuning method of the virtual resistor is proposed, which can automatically regulate the virtual resistor to the critical value to stabilize the system, and thus reduce the power loss. Furthermore, the active damper is designed not to respond to the dominant low-frequency harmonic components in the PCC voltage introduced by the grid background harmonics, so that its power loss can be further reduced. In order to make the active damper more accurately simulate the virtual resistor in a wide frequency range, a harmonic-current-reference compensation method is proposed. The prototypes of a 6-kW grid-connected inverter and a 1-kVA active damper are built and tested to verify the effectiveness of the proposed control scheme of the active damper.
TL;DR: Simulation and experimental results at different distribution frequencies, power levels, and output harmonic content are provided to demonstrate the feasibility of the proposed multilevel inverter topology.
Abstract: This paper proposes a switched-capacitor multilevel inverter for high-frequency ac power distribution systems. The proposed topology produces a staircase waveform with higher number of output levels employing fewer components compared to several existing switched-capacitor multilevel inverters in the literature. This topology is beneficial where asymmetric dc voltage sources are available, e.g., in case of renewable energy farms based ac microgrids and modern electric vehicles. Utilizing the available dc sources as inputs for a single inverter solves the major problem of connecting several inverters in parallel. Additionally, the need to stack voltage sources, like batteries or supercapacitors, in series which demand charge equalization algorithms, are eliminated as the voltage sources employed share a common ground. The inverter inherently solves the problem of capacitor voltage balancing as each capacitor is charged to the value equal to one of the input voltage every cycle. State analysis, losses, and the selection of capacitance are examined. Simulation and experimental results at different distribution frequencies, power levels, and output harmonic content are provided to demonstrate the feasibility of the proposed multilevel inverter topology.
TL;DR: Imaging with nonlinear metalenses that combine the function of an ultrathin planar lens with simultaneous frequency conversion is demonstrated, which offers the capability of nonlinear information processing with novel optoelectronic devices.
Abstract: The abrupt phase change of light at metasurfaces provides high flexibility in wave manipulation without the need for accumulation of propagating phase through dispersive materials. In the linear optical regime, one important application field of metasurfaces is imaging by planar metalenses, which enables device miniaturization and aberration correction compared to conventional optical microlens systems. With the incorporation of nonlinear responses into passive metasurfaces, optical functionalities of metalenses are anticipated to be further enriched, leading to completely new application areas. Here, imaging with nonlinear metalenses that combine the function of an ultrathin planar lens with simultaneous frequency conversion is demonstrated. With such nonlinear metalenses, imaging of objects with near infrared light while the image appears in the second harmonic signal of visible frequency range is experimentally demonstrated. Furthermore, the functionality of these nonlinear metalenses can be modified by switching the handedness of the circularly polarized fundamental wave, leading to either real or virtual nonlinear image formation. Nonlinear metalenses not only enable infrared light imaging through a visible detector but also have the ability to modulate nonlinear optical responses through an ultrathin metasurface device while the fundamental wave remains unaffected, which offers the capability of nonlinear information processing with novel optoelectronic devices.
TL;DR: The paper presents a detailed analytical and experimental analysis of the behavior of a closed-loop controlled permanent magnet machine drive under interturn fault conditions, showing that significant second harmonic components in the dq voltages, currents, instantaneous active power (IAP), and instantaneous reactive power (IRP) are generated during turn fault conditions.
Abstract: Fast and reliable detection of stator faults is of key importance for fail-safe and fault-tolerant machine drives in order to immediately trigger appropriate fault mitigation actions. The paper presents a detailed analytical and experimental analysis of the behavior of a closed-loop controlled permanent magnet machine drive under interturn fault conditions. It is shown that significant second harmonic components in the dq voltages, currents, instantaneous active power (IAP), and instantaneous reactive power (IRP) are generated during turn fault conditions. The analyses further show that the increase of the second harmonic in IAP and IRP during fault conditions is comparatively higher than that of voltage and current, making them ideal candidates as turn fault indicators. A turn fault detection technique based on second harmonic in IAP and IRP is implemented and demonstrated for a triple-redundant, fault-tolerant permanent magnet assisted synchronous reluctance machine drive. The effectiveness of the proposed detection technique over the whole operation region is assessed, demonstrating fast and reliable detection over most of the operating region under both motoring and generating mode.
TL;DR: It is demonstrated from ab initio simulations that it is possible to generate high-order harmonics from free-standing monolayer materials, with an energy cutoff similar to that of atomic and molecular gases, and the electron-electron interaction plays an important role in harmonic generation from monollayer materials because of strong local-field effects.
Abstract: The generation of high-order harmonics from atomic and molecular gases enables the production of high-energy photons and ultrashort isolated pulses. Obtaining efficiently similar photon energy from solid-state systems could lead, for instance, to more compact extreme ultraviolet and soft x-ray sources. We demonstrate from ab initio simulations that it is possible to generate high-order harmonics from free-standing monolayer materials, with an energy cutoff similar to that of atomic and molecular gases. In the limit in which electrons are driven by the pump laser perpendicularly to the monolayer, they behave qualitatively the same as the electrons responsible for high-harmonic generation (HHG) in atoms, where their trajectories are described by the widely used semiclassical model, and exhibit real-space trajectories similar to those of the atomic case. Despite the similarities, the first and last steps of the well-established three-step model for atomic HHG are remarkably different in the two-dimensional materials from gases. Moreover, we show that the electron-electron interaction plays an important role in harmonic generation from monolayer materials because of strong local-field effects, which modify how the material is ionized. The recombination of the accelerated electron wave packet is also found to be modified because of the infinite extension of the material in the monolayer plane, thus leading to a more favorable wavelength scaling of the harmonic yield than in atomic HHG. Our results establish a novel and efficient way of generating high-order harmonics based on a solid-state device, with an energy cutoff and a more favorable wavelength scaling of the harmonic yield similar to those of atomic and molecular gases. Two-dimensional materials offer a unique platform where both bulk and atomic HHG can be investigated, depending on the angle of incidence. Devices based on two-dimensional materials can extend the limit of existing sources.
TL;DR: The Clarke transformation constructs a second signal that has exactly 90° phase angle difference from the original single-phase input signal and helps eliminate the estimation errors for a wide frequency range and can be used for other DFT-based synchrophasor algorithms in order to achieve higher synchROphasor measurement accuracy under dynamic conditions.
Abstract: Despite its wide applications in power grid monitoring, the classic discrete Fourier transform (DFT)-based synchrophasor estimation algorithms suffer from significant errors when the power system operates under off-nominal frequency conditions. This phenomenon is caused by spectral leakage of DFT and becomes even more severe for single-phase synchrophasor estimation. To address this issue, a theory to eliminate the spectral leakage-caused errors is proposed and a Clarke transformation-based DFT synchrophasor estimation algorithm is proposed to implement the theory in this paper. The Clarke transformation constructs a second signal that has exactly 90° phase angle difference from the original single-phase input signal and helps eliminate the estimation errors for a wide frequency range. The proposed algorithm is tested under the conditions required in the phasor measurement unit standard C37.118.1-2011 and C37.118.1a-2014, as well as the harmonic and noise conditions not required in the standard to verify its performance. More importantly, the idea of using Clarke transformation can be used for other DFT-based synchrophasor algorithms in order to achieve higher synchrophasor measurement accuracy under dynamic conditions. An example is presented at last to demonstrate the expandability of the proposed idea.
TL;DR: In this paper, the authors discuss the design rationale and guidelines to build magnet-less circulators based on spatiotemporal modulation of resonant junctions consisting of first-order bandstop filters connected in a delta topology.
Abstract: In this paper, we discuss the design rationale and guidelines to build magnet-less circulators based on spatiotemporal modulation of resonant junctions consisting of first-order bandstop filters connected in a delta topology. Without modulation, the junction does not allow transmission between its ports; however, when the natural oscillation frequencies of the constituent $LC$ filters are modulated in time with a suitable phase pattern, a synthetic angular-momentum bias can be effectively imparted to the junction and a transmission window opens at one of the output ports, thus realizing a circulator. We develop a rigorous small-signal linear model and find analytical expressions for the harmonic $S$ -parameters of the proposed circuit, which significantly facilitate the design process. We validate the theory with simulations and further discuss the large-signal response, including power handling, nonlinearity, and noise performance. Finally, we present measured results with unprecedented performance in all metrics for a printed circuit board prototype using off-the-shelf discrete components.