Showing papers on "Electrical impedance published in 2017"
TL;DR: A comprehensive overview on the contributions and their classification on the inverter- and grid-side damping measures are presented and some promising damping methods for industrial applications will be discussed.
Abstract: Grid-tied voltage source inverters using LCL filter have been widely adopted in distributed power generation systems (DPGSs). As high-order LCL filters contain multiple resonant frequencies, switching harmonics generated by the inverter and current harmonics generated by the active/passive loads would cause the system resonance, and thus the output current distortion and oscillation. Such phenomenon is particularly critical when the power grid is weak with the unknown grid impedance. In order to stabilize the operation of the DPGS and improve the waveform of the injected currents, many innovative damping methods have been proposed. A comprehensive overview on those contributions and their classification on the inverter- and grid-side damping measures are presented. Based on the concept of the impedance-based stability analysis, all damping methods can ensure the system stability by modifying the effective output impedance of the inverter or the effective grid impedance. Classical damping methods for industrial applications will be analyzed and compared. Finally, the future trends of the impedance-based stability analysis, as well as some promising damping methods, will be discussed.
279 citations
TL;DR: In this article, an updated review of EIS main implementations and applications is presented, as well as a broad range of applications as a quick and easily automated technique to characterize solid, liquid, semiliquid, organic as well and inorganic materials.
Abstract: . Electrical impedance spectroscopy (EIS), in which a sinusoidal test voltage or current is applied to the sample under test to measure its impedance over a suitable frequency range, is a powerful technique to investigate the electrical properties of a large variety of materials. In practice, the measured impedance spectra, usually fitted with an equivalent electrical model, represent an electrical fingerprint of the sample providing an insight into its properties and behavior. EIS is used in a broad range of applications as a quick and easily automated technique to characterize solid, liquid, semiliquid, organic as well as inorganic materials. This paper presents an updated review of EIS main implementations and applications.
234 citations
TL;DR: In this paper, a frequency-selective surface (FSS) with high in-band transmission at high frequency and wideband absorption at low frequency is presented. But the PLC structure is not considered.
Abstract: This communication presents a novel frequency-selective surface (FSS) with high in-band transmission at high frequency and wideband absorption at low frequency. It consists of a resistive sheet and a metallic bandpass FSS separated by a foam spacer. The resistive element is realized by inserting a strip-type parallel $LC$ (PLC) structure into the center of a lumped-resistor-loaded metallic dipole. The PLC resonates at the passband of the bandpass FSS and exhibits an infinite impedance, which splits the resistive dipole into two short sections per the surface current; this setup allows for high in-band transmission at high frequency. Below the resonance frequency, the PLC becomes finite inductive and the entire FSS performs as an absorber with the metallic FSS as a ground plane. The surface current distribution on the resistive element can be controlled at various frequencies via the PLC structure. The wideband absorption and high in-band transmission of the proposed design are verified by both numerical simulation and experimental measurements. The potential extension to polarization-insensitive designs is also discussed.
224 citations
TL;DR: It is shown that both stability analysis methods can effectively determine the stability of the system and that the eigenvalue-based method cannot unambiguously predict sustained harmonic oscillations in voltage source converter (VSC) based high voltage dc (HVdc) systems caused by pulse-width modulation (PWM) switching.
Abstract: This paper investigates the small-signal stability of power electronics-based power systems in frequency domain. A comparison between the impedance-based and the eigenvalue-based stability analysis methods is presented. A relation between the characteristics equation of the eigenvalues and poles and zeros of the minor-loop gain from the impedance-based analysis have been derived analytically. It is shown that both stability analysis methods can effectively determine the stability of the system. In the case of the impedance-based method, a low phase-margin in the Nyquist plot of the minor-loop gain indicates that the system can exhibit harmonic oscillations. A weakness of the impedance method is the limited observability of certain states given its dependence on the definition of local source-load subsystems, which makes it necessary to investigate the stability at different subsystems. To address this limitation, the paper discusses critical locations where the application of the method can reveal the impact of a passive component or a controller gain on the stability. On the other hand, the eigenvalue-based method, being global, can determine the stability of the entire system; however, it cannot unambiguously predict sustained harmonic oscillations in voltage source converter (VSC) based high voltage dc (HVdc) systems caused by pulse-width modulation (PWM) switching. To generalize the observations, the two methods have been applied to dc-dc converters. To illustrate the difference and the relation between the two-methods, the two stability analysis methods are then applied to a two-terminal VSC-based HVdc system as an example of power electronics-based power systems, and the theoretical analysis has been further validated by simulation and experiments.
213 citations
TL;DR: In this article, an alternative impedance model is proposed for the single-loop current control, and damping injection based on the discrete derivative controller is proposed to enhance the passivity of individual converters and thereby stabilizing the paralleled converters.
Abstract: This paper addresses the harmonic stability resulting from the current-control interactions of the multiparalleled, LCL -filtered voltage-source converters. First, an alternative impedance model is proposed for the single-loop current control. The control output admittance of the converter is decomposed into a passive filter output admittance in series with an active admittance, which is dependent on the current controller and the time delay. The frequency-domain passivity theory is then applied to the active admittance for system stability analysis. It reveals that the stability region of the single-loop grid current control is not only dependent on the time delay, but affected also by the resonance frequency of the converter-side filter inductor and filter capacitor. Further on, the damping injection based on the discrete derivative controller is proposed to enhance the passivity of individual converters and thereby stabilizing the paralleled converters. Finally, simulation studies and laboratory tests validate the effectiveness of theoretical analysis and controller design.
160 citations
TL;DR: This paper presents the operating principles and analysis of continuous input current configuration enhanced-boost qZSI with two-switched impedance networks and compares with ZSI, switched inductor Z SI, DA/CA-qZSI, and enhanced-Boost ZSIs.
Abstract: In this paper, two topologies are presented for the enhanced-boost quasi-Z-source inverters (qZSI), namely continuous input current configuration and discontinuous input current configuration of enhanced-boost qZSI with two-switched impedance networks. Similar to enhanced-boost impedance-source inverters (ZSIs), these proposed inverter topologies possess very high boost voltage inversion at low shoot-through duty ratio and high modulation index to provide an improved quality output voltage. Compared to enhanced-boost ZSIs with two-switched Z-source impedance networks, these proposed inverter topologies share common ground with source and bridge inverter, overcome the starting inrush problem, and draw continuous input current and the lower voltage across the capacitors. Moreover, the input ripple current is negligible. This paper presents the operating principles and analysis of continuous input current configuration enhanced-boost qZSI with two-switched impedance networks and compares with ZSI, switched inductor ZSI, DA/CA-qZSI, and enhanced-boost ZSIs. The theoretical analysis is done and is validated through simulation and experimental results.
150 citations
TL;DR: A capacitive virtual impedance loop was implemented to improve the harmonic current sharing and attenuate the voltage harmonics at the PCC during islanded operation of the microgrid.
Abstract: Optimization of the islanded and grid-connected operation of microgrids is important to achieve a high degree of reliability. In this paper, the authors consider the effect of current harmonics in single phase microgrids during both modes of operation. A detailed analysis of the effect of the output impedance of the considered primary control loops on the harmonic output of the considered voltage source inverters is initially carried out. A virtual admittance loop is proposed to attenuate the current harmonic output in grid-connected operation that is generated due to the grid voltage distortion present at the point of common coupling (PCC) and due to local non-linear loads. This paper also considers the harmonic current sharing and resulting voltage harmonics at the PCC during islanded operation of the microgrid. A capacitive virtual impedance loop was implemented to improve the harmonic current sharing and attenuate the voltage harmonics at the PCC. Experimental results are given to validate the operation of the proposed algorithms.
132 citations
TL;DR: A previously developed surface polarization model based on the assumption of large electric and ionic charge accumulation at the external contact interface is adopted and able to quantitatively describe exotic features of the perovskite solar cell and provides insight into the operation mechanisms of the device.
Abstract: The analysis of perovskite solar cells by impedance spectroscopy has provided a rich variety of behaviors that demand adequate interpretation. Two main features have been reported: First, different impedance spectral arcs vary in combination; second, inductive loops and negative capacitance characteristics appear as an intrinsic property of the current configuration of perovskite solar cells. Here we adopt a previously developed surface polarization model based on the assumption of large electric and ionic charge accumulation at the external contact interface. Just from the equations of the model, the impedance spectroscopy response is calculated and explains the mentioned general features. The inductance element in the equivalent circuit is the result of the delay of the surface voltage and depends on the kinetic relaxation time. The model is therefore able to quantitatively describe exotic features of the perovskite solar cell and provides insight into the operation mechanisms of the device.
125 citations
TL;DR: In this article, a frequency-dependent virtual impedance approach is proposed to shape the high frequency impedance of the dc bus voltage control units, improving the system stability and mitigating the oscillations, and detailed design guidelines on virtual impedance to achieve good stability and transient performance are also provided.
Abstract: This paper focuses on the stability analysis and damping performance improvement of dc microgrids. The small-signal model of a dc microgrid has been derived. Eigenvalue analysis results reveal the relationship between the system stability and different factors of dc microgrids, including types of dc load, the droop coefficient, line parameters, etc. It shows that the poorly damped LC circuits in dc microgrids reduce the system damping and bring in high frequency oscillations. To improve the damping performance, a frequency-dependent virtual impedance approach is proposed, which can effectively shape the high frequency impedance of the dc bus voltage control units, improve the system stability, and mitigate the oscillations. Detailed design guidelines on virtual impedance to achieve good stability and transient performance are also provided. Simulation and experimental results are obtained to confirm the validity of the proposed approach.
124 citations
TL;DR: In this paper, the authors describe the use of a frequency domain finite-difference scheme to simulate the impedance spectra of diffusion in porous microstructures, which is relevant to battery electrode design as the techniques for nano-scale fabrication become more widespread.
119 citations
TL;DR: The multiple inner current loop damping schemes are presented, including the virtual positive, virtual negative, and variable harmonic sequence impedance loops for reactive and harmonic power sharing purposes, and the proposed active damping scheme using capacitor current feedback loop of the LCL filter, which shows enhanced damping characteristics and improved inner-loop stability.
Abstract: In this paper, the modeling, controller design, and stability analysis of the islanded microgrid (MG) using enhanced hierarchical control structure with multiple current loop damping schemes is proposed. The islanded MG consists of the parallel-connected voltage source inverters using inductor-capacitor-inductor (LCL) output filters, and the proposed control structure includes the primary control with additional phase-shift loop, the secondary control for voltage amplitude and frequency restoration, the virtual impedance loops which contain virtual positive- and negative-sequence impedance loops at fundamental frequency and virtual variable harmonic impedance loop at harmonic frequencies, and the inner voltage and current loop controllers. A small-signal model for the primary and secondary controls with additional phase-shift loop is presented, which shows an over-damped feature from eigenvalue analysis of the state matrix. The moving average filter-based sequence decomposition method is proposed to extract the fundamental positive and negative sequences and harmonic components. The multiple inner current loop damping scheme is presented, including the virtual positive, virtual negative, and variable harmonic sequence impedance loops for reactive and harmonic power sharing purposes, and the proposed active damping scheme using capacitor current feedback loop of the LCL filter, which shows enhanced damping characteristics and improved inner-loop stability. Finally, the experimental results are provided to validate the feasibility of the proposed approach.
TL;DR: In this paper, a general transmission line equivalent-circuit model was used to exemplarily analyze the impedance of a porous high-voltage LiNi0.5Mn1.5O4 cathode.
Abstract: A key for the interpretation of porous lithium ion battery electrode impedance spectra is a meaningful and physically motivated equivalent-circuit model. In this work we present a novel approach, utilizing a general transmission line equivalent-circuit model to exemplarily analyze the impedance of a porous high-voltage LiNi0.5Mn1.5O4 (LNMO) cathode. It is based on a LNMO/graphite full-cell setup equipped with a gold wire micro-reference electrode (GWRE) to obtain impedance spectra in both, non-blocking conditions at a potential of 4.4 V cell voltage and in blocking configuration achieved at 4.9 V cell voltage. A simultaneous fitting of both spectra enables the deconvolution of physical effects to quantify over the course of 85 cycles at 40°C: a) the true charge transfer resistance (RCT), b) the pore resistance (RPore), and c) the contact resistance (RCont.). We demonstrate that the charge transfer resistance would be overestimated significantly, if the spectra are fitted with a conventionally used simplified R/Q equivalent-circuit compared to our full transmission line analysis.
TL;DR: In this paper, the authors provided an extensive theoretical analysis of DC-link voltage ripple for full-bridge (H-bridge) inverters, with simulation and experimental verifications, considering a DC source impedance (nonideal DC voltage source).
Abstract: Direct current (DC)-link voltage ripple analysis is essential for determining harmonic noise and for DC-link capacitor design and selection in single-phase pulse-width modulation (PWM) inverters. This paper provides an extensive theoretical analysis of DC-link voltage ripple for full-bridge (H-bridge) inverters, with simulation and experimental verifications, considering a DC source impedance (non-ideal DC voltage source). The DC voltage ripple amplitude is theoretically estimated as a function of the output current, both amplitude and phase angle, and the modulation index. It consists of a switching frequency component and a double-fundamental frequency component (i.e., 100 Hz), thereby both components are considered in the analysis. In particular, the peak-to-peak distribution, maximum amplitude, and root mean square (RMS) values of the voltage switching ripple over the fundamental period are obtained. Based on the DC voltage requirements, simple and effective guidelines for designing DC-link capacitors are obtained.
TL;DR: The presented method utilizes closed-loop control operation for the EIS measurement functionality, which allows for better control of the output voltage and for upgrading the concept to be able to achieve no added perturbation ripple at the output of the system.
Abstract: This paper presents a method for an online real-time electrochemical impedance spectroscopy (EIS) measurement of batteries using closed-loop control of power converter. Unlike the previously proposed method which allows the measurement of the ac impedance for a single frequency, the presented method in this paper allows for obtaining the EIS for a spectrum of frequencies by using the information included in a single perturbation cycle, or a few cycles of perturbation to obtain a more accurate EIS with a very wide frequency range. This will result in faster EIS measurement for a spectrum of frequencies and under the same battery operating conditions. The presented method utilizes closed-loop control operation for the EIS measurement functionality, which allows for better control of the output voltage and for upgrading the concept to be able to achieve no added perturbation ripple at the output of the system. The presented online real-time EIS measurement method utilizes a power converter with closed-loop control in order to create an output voltage step-function perturbation at a given frequency to generate battery voltage and current responses. By applying Fourier analysis to these responses, an EIS can be obtained for a range of frequencies equal or higher than the perturbation frequency of the step function. In addition, this paper presents a method to eliminate the added perturbation ripple when two or more power converters are used. The theoretical basis and experimental prototype results are provided to illustrate and validate the presented method.
TL;DR: In this paper, the authors presented a theoretical study with experimental validation of a circulating-current suppression method for parallel operation of three-phase voltage-source inverters (VSI), which may be suitable for modular parallel uninterruptible power supply systems or hybrid ac/dc microgrid applications.
Abstract: This paper presents a theoretical study with experimental validation of a circulating-current suppression method for parallel operation of three-phase voltage-source inverters (VSI), which may be suitable for modular parallel uninterruptible power supply systems or hybrid ac/dc microgrid applications. The basic concept of the proposed circulating-current suppression method is to modify the original current references by using the current difference among the parallel inverters. In the proposed approach, both cross circulating current and zero-sequence circulating current are considered, and are added into the conventional droop plus virtual impedance control. In the control architecture, the reference voltages of the inverters are generated by the primary control loop, which consists of a droop control and a virtual impedance. The secondary control is used to compensate the voltage drop on the virtual impedance. Furthermore, a circulating-current control loop is added to improve the average current-sharing performance among parallel VSIs. Experimental results are presented to show the effectiveness of the proposed control method to suppress both cross and zero-sequence circulating currents.
TL;DR: In this article, it is shown that the modified sequence domain model can be derived from the dq-domain model, and vice versa, and that the stability analysis will be identical in these two two domains.
Abstract: Small-signal analysis of power electronic converters and systems is often carried out by impedance-based methods. At the core of these methods lies the impedance modeling, which can either be obtained through analytical calculations, simulations, or measurements. The impedance models can be obtained into two main domains: the dq -domain and the sequence domain. In the dq -domain, the impedance model is a $2\times 2$ matrix, while in the sequence domain, it is composed by the positive and negative sequence impedance. Recently, a third domain called the modified sequence domain was defined as an extension to the sequence domain, but also with clear similarities to the dq -domain. The objective of this paper is to unambiguously relate to each other the impedances in these three domains, and to show how this equivalence translates into their respective stability assessments. It is also proven that the sequence domain impedance has the same marginal stability condition as the dq -domain impedance matrix. The three-phase voltage source converter is used as an example converter in this paper, as its impedance model in all three domains is well established and reported by previous research. The results in this paper show that the modified sequence domain model can be derived from the dq -domain model (and vice versa), and that the stability analysis will be identical in these two domains. It is also shown how the original sequence domain model can be derived from the two other models through a model reduction. However, a small discrepancy between the two Nyquist plots is observed in the presence of components such as phase-locked loop or dc-link control.
TL;DR: In this paper, a novel Cartesian space impedance control method is proposed for hydraulic robotic manipulators to address the highly nonlinear dynamic behavior of the hydraulic manipulator, the system control is designed according to the subsystemdynamics-based virtual decomposition control (VDC) approach.
Abstract: In challenging robotic tasks, high-bandwidth closed-loop control performance of the system is required for successful task completion. One of the most critical factors inhibiting the wide-spread use of closed-loop contact control applications has been the control system stability problems. To prevent unstable system behavior, the need for rigorously addressed manipulator dynamics is substantial. This is because the contact dynamics between a manipulator and its environment can be drastic. In this paper, a novel Cartesian space impedance control method is proposed for hydraulic robotic manipulators. To address the highly nonlinear dynamic behavior of the hydraulic manipulator, the system control is designed according to the subsystem-dynamics-based virtual decomposition control (VDC) approach. The unique features of VDC ( virtual power flow and virtual stability ) are used to analyze the interaction dynamics between the manipulator and the environment. Based on the desired impedance parameters and stability analysis, an explicit method to design the control gains for the proposed impedance control law is developed. The $\boldsymbol {L}_2$ and $\boldsymbol {L}_\infty$ stability is guaranteed in both free-space motions and constrained motions. Experimental results demonstrate that the hydraulic robotic manipulator is capable of adjusting its dynamic behavior accurately in relation to the imposed target impedance behavior. This provides compliant system behavior, which is needed in many dynamically challenging robotic tasks.
TL;DR: It is concluded that decreasing the empirical parameter of a general class-B/J voltage equation with increasing frequency leads to a clockwise trajectory on the Smith chart of the second harmonic at the package plane.
Abstract: This brief explores the design space for realizable solution of a broadband class-B/J continuous mode of power amplifier (PA). The PA is initially designed at the current-source reference plane with the correct voltage and current waveforms. The intrinsic impedances are then projected to the package reference plane using the model-based nonlinear-embedding technique. An insight is provided into engineering the extrinsic harmonic impedance to rotate clockwise on the Smith chart to be able to match it using a Foster circuit. It is concluded that decreasing the empirical parameter $ {\alpha }$ of a general class-B/J voltage equation with increasing frequency leads to a clockwise trajectory on the Smith chart of the second harmonic at the package plane. In order to validate the advantage of this analysis, the PA is implemented using a 15 W gallium nitride high electron mobility transistor in the frequency range of 1.3 to 2.4 GHz and drain efficiency between 63% and 72% in measurement was achieved over the entire bandwidth.
TL;DR: The effects of grid-connected three-phase systems with different front-end topologies: conventional, small dc-link capacitor, and electronic inductor are presented.
Abstract: Harmonic emissions have been changed in distribution networks, with respect to frequency range and magnitude, due to the penetration of modern power electronics systems. Two new frequency ranges 2–9 and 9–150 kHz have been identified as new disturbing frequency ranges affecting distribution networks. This paper presents the effects of grid-connected three-phase systems with different front-end topologies: conventional, small dc-link capacitor, and electronic inductor. A power converter with a small dc-link capacitor can create a resonant frequency with the line impedance below and above 1 kHz depending on the grid configurations. The resonant effects depend on many factors, such as load power levels, filter types, and the number of parallel drives. These issues can affect the grid current harmonics and power quality of the distribution networks. Analyses and simulations have been carried out for three different topologies and the results have been verified by experimental test at system level. Current harmonic emissions have been considered for 0–2, 2–9, and 9–150 kHz frequency ranges.
TL;DR: In this paper, the effect of the AC (alternating current) pulse heating method on battery SoH (state of health) for large laminated power lithium-ion batteries at low temperature is investigated experimentally.
TL;DR: A novel adaptive algorithm for the PCC voltage feedforward to work well with the varied grid impedance is proposed and does not have to sacrifice the dynamic or the harmonics rejection performance, or to use the on or offline grid impedance estimation.
Abstract: The performance of the grid-connected inverter was affected by the uncertainty of the grid conditions including the background distortion and the grid impedance. Typically, the feedforward of the grid voltage at the point of common coupling (PCC) highly suppressed the grid current harmonics caused by the grid voltage distortion; however, the PCC grid usually had a nonnegligible grid impedance, and the PCC voltage feedforward aroused serious grid current harmonics or instability. This study proposes a novel adaptive algorithm for the PCC voltage feedforward to work well with the varied grid impedance. In the proposal, the band-pass filters at the harmonic frequencies are used to detect the variation of the grid impedance as well as to facilitate the adaptive PCC voltage feedforward. It is not necessary to inject an additional harmonic to estimate the grid impedance. The basic principles as well as the realization and logic of the proposed algorithm are detailed, and some selected waveforms are provided to verify the superior performance. Compared with the typical robust design or adaptive control, the proposed algorithm does not have to sacrifice the dynamic or the harmonics rejection performance, or to use the on or offline grid impedance estimation.
TL;DR: In this paper, the authors proposed a new approach using harmonic decomposition of the bridge switching patterns, which gives an explicit theoretical solution under all operating conditions, while also accommodating more complex ac coupling structures, practical impedance nonidealities, and the switching impact of dead-time and device capacitance.
Abstract: Dual active bridge (DAB) converters offer an unmatched capability to transfer energy in either direction between two dc sources, while also providing galvanic isolation and high conversion efficiency However, to operate at higher efficiencies, the bridges must operate with zero voltage switching (ZVS) over as wide an operating range as possible The conventional approach to determine ZVS operation uses time domain analysis with ideal ac coupling inductances, which only approximately identifies the ZVS boundaries This paper proposes a new approach using harmonic decomposition of the bridge switching patterns, which gives an explicit theoretical solution under all operating conditions, while also accommodating more complex ac coupling structures, practical impedance nonidealities, and the switching impact of dead-time and device capacitance The methodology is confirmed by matching analytical predictions with experimental results for selected DAB systems
01 Nov 2017
TL;DR: In this paper, the analytical and numerical equivalence of impedance expressions for Voight, matryoshka, and hybrid circuits is evaluated for perovskite solar cells (PSCs).
Abstract: Impedance spectroscopy is a powerful electrochemical small-perturbation technique that provides dynamic electrical data in solar cells. This technique has been widely used to characterize dye-sensitized solar cells and perovskite solar cells (PSCs). Physical parameters are normally obtained by fitting to an equivalent circuit, composed of electrical elements which theoretically correspond to physical processes involved in the photoconversion process. A variety of equivalent circuits to model the impedance spectra of PSCs are commonly used by different research groups. In this work, we evaluate their performance and adequacy. We demonstrate the analytical and numerical equivalence of impedance expressions for Voight, matryoshka, and hybrid circuits, which are used to fit a typical impedance spectrum of a PSC and compare the resulting parameters to the empirical values obtained without any equivalent circuit. The numerical equivalence can be demonstrated by using two- and three-component impedance spectra. In contrast, Maxwell-type equivalent circuits reveal parameters that have a more complex relation to empirical values. The presence of inductive effects such as “loops” and “negative tails” in impedance spectra are also discussed in terms of negative values of resistances and capacitances. We propose a general protocol to analyze impedance data of PSCs and to extract useful information from them.
TL;DR: In this article, the second-order generalized integrator frequency-locked loop (SOGI-FLL) was introduced for realizing grid synchronization in the stationary reference frame, and the influences of the voltage perturbation on the estimated phase and further on the output current were revealed.
Abstract: The latest research has pointed out that the phase-locked loop (PLL) plays an important role in shaping the impedance of grid-connected converters, yet most of the works so far merely focus on the synchronous reference-frame PLL. Alternatively, this letter presents the impedance analysis of the second-order generalized integrator frequency-locked loop (SOGI-FLL), which has been introduced for realizing grid synchronization in the stationary reference frame. The influences of the voltage perturbation on the estimated phase and further on the output current are revealed for the grid-connected converter using the SOGI-FLL. The frequency-coupling effect of the SOGI-FLL is also identified and verified in the experimental tests.
TL;DR: The inverter output impedance is passivized for solving the harmonic and the stability problems in the multiparallel inverters and the dynamic performance is enhanced with an additional current reference generation scheme.
Abstract: In this paper, the inverter output impedance is passivized for solving the harmonic and the stability problems in the multiparallel inverters The harmonics are easily aroused because of the disturbances, and the system stability is challenged by the grid impedance Based on the simplified equivalent impedance model, the two problems are analyzed in the low-frequency (LF) band and the high-frequency (HF) band, respectively Aiming at improving the LF performance, the weighted-proportional grid voltage feedforward and the harmonic quasi-resonant controller with phase compensation are proposed The dynamic performance is enhanced with an additional current reference generation scheme In order to improve the HF performance, a novel digital phase lead filter which brings the system back to a minimum-phase case is proposed By the proposed control method, the high modulus of each inverter output impedance $Z_{o}$ is guaranteed, while the phase angles of $Z_{o}$ over the entire frequency band are avoided to be lower than −90° The experiments based on four-parallel inverters have been conducted to validate the effectiveness of the proposed control method
TL;DR: In this paper, a flexible neutral grounding method based on electromagnetic hybrid Petersen coil (EHPC), which consists of a magnetically controlled reactor and an active power compensator (APC), is proposed.
Abstract: This paper proposes a flexible neutral grounding method based on electromagnetic hybrid Petersen coil (EHPC), which consists of a magnetically controlled reactor and an active power compensator (APC). Under normal conditions, the system's neutral point grounds through a reactor of high reactance. When a single line-to-earth fault occurs, the EHPC can be controlled flexibly to suppress the recovery voltage of faulty phase and to reduce the ground-fault current to almost zero. This paper expounds the two operating modes and corresponding characteristics, the measuring method of insulation parameters of the system (mainly including capacitance to earth and leakage resistance), and the full compensation principle for arc suppression based on the equivalent negative impedance of EHPC. This paper also proposes a compound control method of neutral-to-earth impedance based on neutral-point displacement voltage and a compensating method for the harmonic component of the ground-fault current. Compared to traditional grounding methods, this method can be fast and reliably make the residual current approach almost zero without arc reignition while greatly reducing the output capacity of APC and, thus, decreasing the cost of this device. Simulations and experiments verify the correctness of this measuring method and feasibility of this flexible grounding control strategy.
TL;DR: In this article, an overview of existing impedance control solutions for series elastic joints and derive the passivity characterizations that are still missing in the literature, thus providing a complete and coherent overview of the existing solutions, highlighting the advantages of impedance control based on positive acceleration feedback showing improved stability robustness and impedance accuracy with respect to existing solutions.
TL;DR: In this article, a broadband parametric amplifiers based on the kinetic inductance of superconducting NbTiN thin films in an artificial (lumped-element) transmission line architecture is presented.
Abstract: We present broadband parametric amplifiers based on the kinetic inductance of superconducting NbTiN thin films in an artificial (lumped-element) transmission line architecture. We demonstrate two amplifier designs implementing different phase matching techniques: periodic impedance loading and resonator phase shifters placed periodically along the transmission line. Our design offers several advantages over previous CPW-based amplifiers, including intrinsic 50 Ω characteristic impedance, natural suppression of higher pump harmonics, lower required pump power, and shorter total trace length. Experimental realizations of both versions of the amplifiers are demonstrated. With a transmission line length of 20 cm, we have achieved gains of 15 dB over several GHz of bandwidth.
TL;DR: In this article, a new nonlinear current control strategy based on backstepping control and high-order sliding mode differentiator is proposed in order to employ distributed generation unit interfacing converters to actively compensate harmonics/interharmonics of local loads.
Abstract: This paper presents a new nonlinear current control strategy based on backstepping control and high-order sliding mode differentiator in order to employ distributed generation (DG) unit interfacing converters to actively compensate harmonics/interharmonics of local loads. The converter-based DG unit is connected to a weak grid (with uncertain impedance) and local load (that can be parametrically uncertain and topologically unknown) through an LCL filter. The proposed strategy robustly regulates the inverter output currents and delivers pure sinusoidal, three-phase balanced currents to the grid. The new controller demonstrates the robust performance and robust stability of the DG unit system with respect to the filter parameters uncertainties, grid impedance, grid frequency, and grid voltage as well as the unknown load dynamics that include unbalanced loads and nonlinear loads with harmonic and interharmonic currents. We should remark that the local compensation of the loads with interharmonic current using a DG unit system is first proposed in this paper. When compared with the popular parallel proportional resonant control technique, the proposed controller offers smoother transient responses and a lower level of current distortion. The performance of the proposed control strategy is verified in MATLAB/SimPowerSystems toolbox.