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This paper introduces a time–frequency analytic method for the exact identification of wideband DQ impedance models oriented to harmonic stability of converter-driven systems with transmission line models with distributed and frequency-dependent parameters. Wideband models are of great importance in the analysis of harmonic stability over a broad range of frequencies. The transmission line is a crucial component in this regard, but it is not straightforward to incorporate it into the impedance model of AC power-electronics-based power systems. This has led to the use of lumped parameter models or models based on rational approximations. This article proposes a method to include this important element to the impedance model directly from its frequency domain model. Salient features of the proposed modeling approach include the frequency telegrapher's equations with distributed and frequency-dependent parameters, exact delay models of PWM, control implementations of power electronics, and an exact steady-state computation using a balance method based on a hybrid time–frequency approach. The proposal avoids time-domain simulations, rational approximations, or the fast Fourier transform. A converter-driven system with transmission lines is used to validate the proposal and the results of the frequency scanning method conducted in OPAL-RT (ARTEMiS/EMTP-RV) and PSCAD/EMTDC support the effectiveness, speed, and accuracy. • Accurate D Q impedance model of AC systems with frequency-dependent lines. • Impedance model, including lines, avoids rational and approximate models. • Steady state accurately computed using exact equations for lines and converters. • Characterization avoids time-domain simulations, FFTs, and analytical modeling. [ABSTRACT FROM AUTHOR]

• The frequency characteristics of SRF-PLL, DSOGI-FLL and DDSRF-PLL are modeled respectively. • The impedances of the inverter are modeled with the PLLs displayed, and the influences of different PLLs on the inverter impedance are compared. • The effects of the PLLs on the stability of the system are studied to discuss the stability boundary conditions of the system. • Based on the equivalent impedance model of the PLL, the optimized design method of the PLL is proposed to improve the system stability. For grid-connected inverters, phase-locked loop (PLL) is an indispensable part for grid currents to track grid voltages. Hence, PLL will have a nonnegligible effect on the stability of the interconnected system involving a grid-connected inverter and a power grid. Therefore, it is necessary to model the frequency characteristic of PLL and analyze its effect on the system stability. In this paper, three kinds of the commonly used PLL circuits are firstly employed to model and compare their frequency characteristics. Second, the impedances of the inverter are modeled with the equivalent impedance model of PLL displayed, and the influences of different PLLs on the impedance characteristics of the inverter are compared. Then, the effects of PLLs on the stability of the interconnected system are studied to discuss the stability boundary conditions of the system. Based on the equivalent impedance model of the PLL, the optimized design method of the PLL is proposed from the perspective of optimizing the inverter impedance characteristics to improve the system stability. Finally, the simulation and experimental results are presented to verify the correctness of the PLL frequency characteristics and the inverter impedance models, and confirm the effectiveness of the optimized design method. [ABSTRACT FROM AUTHOR]

• The impedance models of power loop, voltage and current loop with different time scales are considered. • The generalized Nyquist curve criterion is applicable to multi-inverter systems. • The source-load division and stability of loads and multi converters are researched. • A novel strategy is proposed for high-frequency oscillation in multi-inverter systems. • The hardware in the loop experiment is designed for verification. In the context of the energy crisis and environmental pollution, microgrid technology has developed rapidly. There are grid-forming micro-sources and loads in the islanded microgrid, whose interaction can easily lead to the instability of the microgrid, and the inverter, as the power electronic interface device of the power generation unit, plays an important role in the reliable operation of the microgrid. The micro-sources, made up of three grid-forming inverters, are used as the research object in this paper. The small signal dq impedance modeling method is used to construct the full band impedance model based on the droop control inverter, whose model covers the power loop, voltage loop, and current loop at different time scales. A source-load partitioning method suitable for multi-inverter is designed. The relationship between parameter sensitivity and stability of the multi-inverter parallel operation system is analyzed from the perspective of impedance, and the parameter adjustment method for system stability is obtained. Based on the harmonic instability causes obtained by the impedance model, a novel impedance reshaping high-frequency oscillation suppression strategy is proposed that enhances the stability margin of microgrid islanding operations. Finally, the correctness of the theory in this paper is verified through simulation and experiments. [ABSTRACT FROM AUTHOR]

The large number of power electronic converters in hybrid DC ship microgrids (SMGs), with increasingly complex control strategies, has led to highly complicated stability analysis and parameter adjustments after system instability. In response to these challenges, this paper developed a system-level impedance model, including an asynchronous generator in the hybrid DC SMG. Subsequently, a variance-based Sobol parameter sensitivity analysis method was employed to quantitatively assess the impact of both system circuit parameters and control parameters on the system's stability, and the dominant parameters that exert the most significant influence on stability have been analyzed. Furthermore, the interaction among the dominant parameters is further discussed by varying these parameters and observing the evolution pattern of the system impedance ratio Nyquist curve, leading to the identification of 2-D and 3-D stable regions for the dominant parameters. Finally, the simulation results and hardware-in-the-loop experiments demonstrate that applying impedance modeling and stability analysis to the system and utilizing Sobol sensitivity analysis to optimize dominant parameters can expand the stability margin of the hybrid DC SMG. • A system-level impedance model of the hybrid DC ship microgrid, including an asynchronous generator, is established. • Using variance-based Sobol parameter sensitivity analysis method to quantify the influence of parameters on system stability. • The stable region of the system's dominant parameter operation is obtained through the evolution law of the impedance ratio Nyquist curve. • The parameter optimization steps in this paper can also be extended to other types of microgrids. [ABSTRACT FROM AUTHOR]

Lithium-ion batteries experiences impedance rise and therefore power fade during aging. Current understanding toward the impedance rise is yet qualitative, leaving quantification of multiple contributions a challenging gap. To fill in this gap, we combine the distribution of relaxation times method and physics-based modeling to analyze the electrochemical impedance spectroscopy of lithium-ion batteries aged by cycling at 45 °C. We find that the oft-neglected low-frequency diffusion is the largest sole source to the impedance rise. Furthermore, thanks to the advanced physics-based impedance model, we distinguish electrolyte- and solid-phase diffusion, identifying that the former dominates over the latter. This piece of understanding implies novel insights into improving battery lifetime, and the methodology developed is flexible to other battery chemistries. • DRT quantifies different polarization losses in a wide frequency range. • Causes of impedance rise during aging are identified. • The oft-neglected diffusion polarization is emphasized. • Model enables separation of electrolyte- and solid-phase diffusion. • Electrolyte-phase diffusion dominates over solid-phase diffusion. [ABSTRACT FROM AUTHOR]

In recent years, the exploitation of over-track construction in populated Chinese cities has resulted in buildings being subjected to potentially disturbing vibrations from rail lines in metro depots. Building owners and engineers are concerned about occupants’ complaints related to structure-borne noise and vibration related to human comfort. Similar concerns exist for laboratory buildings and manufacturing plants that use vibration-sensitive equipment. In Chinese metro depots, the subway trains usually run on the bottom floor of the metro depot and the vibrational energy they generate transmits through the primary building support structures, including columns and load-bearing walls, to the upper floors of the over-track buildings. In order to predict train-induced vibration levels in over-track buildings, a new impedance model is presented. The model accounts for the propagation of axial and bending waves through the columns and load-bearing walls from the foundation into the upper floors. The primary mode of transmission is through higher impedance axial waves across impedance discontinuities at the floors; while the transmission due to bending waves is attenuated more rapidly by the floors. The input for axial wave transmission is the measured vertical vibration levels at the base of the column at the foundation. The contributions from multiple columns/walls into a given floor have been found experimentally to be statistically independent so that the total response of a floor is obtained from a simple energy summation of contributions from all columns/walls where the inputs to each may vary depending, in part, on their distance from the train track. Measured train-induced vibrations at the foundation level and on upper floors in 14-story and 25-story buildings in Shenzhen metro depot were used to validate model predictions of vibration levels at upper floors relative to the foundation vibration. The support structures for both buildings include both columns and load bearing walls where the transmission involves both in-plane shear and dilatation at various angles within the wall though a conclusion from this study is that axial propagation in the vertical direction dominates. The impedance model predictions are in good agreement with the measured floor vibration levels. [ABSTRACT FROM AUTHOR]

For almost a decade, we have measured intracochlear sound pressures evoked by air conducted (AC) sound presented to the ear canal in many fresh human cadaveric specimens. Similar measurements were also obtained during round window (RW) mechanical stimulation in multiple specimens. In the present study, we use our accumulated data of intracochlear pressures and simultaneous velocity measurements of the stapes or RW to determine acoustic impedances of the cochlear partition, RW, and the leakage paths from scala vestibuli and scala tympani, as well as the reverse middle ear impedance. With these impedances, we develop a computational lumped-element model of the normal ear that illuminates fundamental mechanisms of sound transmission. To calculate the impedances for our model, we use data that passes strict inclusion criteria of: (a) normal middle-ear transfer function defined as the ratio of stapes velocity to ear-canal sound pressure, (b) no evidence of air within the inner ear, and (c) tight control of the pressure sensor sensitivity. After this strict screening, updated normal means, as well as individual representative data, of ossicular velocities and intracochlear pressures for AC and RW stimulation are used to calculate impedances. This work demonstrates the existence and the value of physiological acoustic leak impedances that can sometimes contribute significantly to sound transmission for some stimulation modalities. This model allows understanding of human sound transmission mechanisms for various sound stimulation methods such as AC, RW, and bone conduction, as well as sound transmission related to otoacoustic emissions. [ABSTRACT FROM AUTHOR]

• Based on impedance model of two-stage PV inverter in frequency domain, the passive equivalent impedance network of PV inverter connected to power grid is built. • The harmonic amplifying characteristic curve of PCC in full frequency range is established, and the influence of inverter parameters, reactive power compensation device and distributed transmission line model on harmonic characteristics is deeply analyzed. • A harmonic mitigation control strategy with superimposed multi-current resonance controllers and active damping controllers in synchronous rotating coordinate system is proposed, and the effectiveness of this control strategy is verified by simulation. To investigate the harmonic characteristics of a photovoltaic (PV) system connected to the weak grid, a passive impedance network is constructed using the impedance model of a PV inverter in the positive and negative sequence coordinate system. By analyzing the series resonance of the impedance network, the amplification coefficient of the harmonic voltage at the point of common coupling (PCC) is obtained. In addition, the effects of different PV inverter parameters, different reactive power compensation capacities, and different lengths of distributed transmission lines on the harmonic amplification are analyzed in detail. To solve the problem that the output harmonics exceed the standard under the background harmonic condition of the weak grid, a harmonic mitigation control strategy is implemented. This strategy is designed based on multiple resonant current controllers and active damping feedback to improve the ability of the PV inverter to suppress harmonics under the grid background harmonic conditions. The effectiveness of the harmonic mitigation control strategy is demonstrated by the simulation example of the inverter connected to the grid with symmetric and asymmetric background harmonics, respectively. [ABSTRACT FROM AUTHOR]

• An MMC-HVDC system model is established at medium and high frequencies. • The resonance influencing factors are analyzed by strict analytical calculations. • Reduced-order models of MMC impedances are proposed for different situations. • This article completely reveals the resonance mechanism of an MMC-HVDC system. • The resonance mechanism of DC and AC systems show different characteristics. The modular multilevel converter-based high-voltage direct current (MMC-HVDC) transmission system can arouse resonance that threatens the safety and stability of the power grid. In this paper, the impedance model and resonance mechanism of an MMC-HVDC transmission system are investigated at medium and high frequencies. First, an MMC-HVDC transmission system equivalent circuit model is proposed based on the partitioning impedance modeling method. Second, the concept of normalized sensitivity index is proposed to quantify the influential effects of crucial factors on the MMC equivalent impedance, including the control gains, control link delay, and circuit parameters; subsequently, the influencing factors are graded, and reduced-order impedance models are proposed for different situations. Then, the resonances in the MMC AC- and DC-side systems are analyzed considering both active and passive resonance, i.e., with or without a harmonic source, to systematically reveal the resonance mechanism of the MMC-HVDC system. Finally, a two-terminal MMC-HVDC simulation model is built to verify the proposed model and resonance analysis. [ABSTRACT FROM AUTHOR]

This paper presents electrochemical impedance simulation of a solid oxide fuel cell anode. The model takes in to account the gas-phase transport processes both in the gas channel and within the porous electrode and couples the gas transport processes with the electrochemical kinetics. The gas phase mass transport is modeled using the transient conservation equations (mass, momentum and species equations) and Butler–Volmer equation is used for the anode electrochemistry. In order to solve the system of non linear equations, an in-house code based on the finite volume method is developed and utilized. Results show a depressed semicircle in the Nyquist plot, which originates from gas transport processes in the gas channel, in addition to a Warburg diffusion impedance originates from gas transport in the thick porous anode. The simulation results are in good agreement with published experimental data. [ABSTRACT FROM AUTHOR]

• Establish a complete impedance model of the two-stage PV inverter. • Evaluate impedance characteristics and stability issues in case of a weak grid. • The increase of output capacity results in high frequency oscillations. • The increase of PLL bandwidth also causes oscillations in PV inverters. • The stability analysis is verified by the simulation results using PSCAD/EMTDC. In order to obtain impedance characteristics of the photovoltaic (PV) inverter and reveal potential stability issues of the PV inverter connected to a weak grid, a complete impedance model of the two-stage PV inverter is established in this paper. The model includes the PV arrays, front-end Boost converter, and rear-end inverter with output LCL filter. The impedance modeling of the PV inverter is derived at different interfaces of DC-link and point of common coupling (PCC), where the DC impedance characteristic at DC-link and d -q axis impedance feature at AC side are both investigated. It is worth noting that the DC output impedance of front-end circuit behaves as a first-order low-pass filter, and it is mainly influenced by the DC-link capacitor. Moreover, a significant feature is found that a reduced integral gain in the outer voltage loop may cause a negative resistance and capacitive impedance in d -axis impedance component. In addition, based on the impedance analysis method, the oscillation stability issues of the grid-connected PV inverter in case of a weak grid are evaluated by different divisions of source-load subsystems. It is demonstrated that the increase of the PLL bandwidth and the decrease of the integral gain of the outer voltage loop will both lead to the oscillation of the PV inverter. Moreover, the increase of the output capacity will deteriorate the stability margin, resulting in the high frequency oscillation of the system. Finally, the impedance model and the stability analysis are verified by the simulation results implemented in PSCAD/EMTDC platform. [ABSTRACT FROM AUTHOR]

The Advanced Light Source Upgrade (ALS-U) is a 4th generation diffraction-limited soft x-ray radiation source, consisting of a new accumulator ring (AR) and a new storage ring (SR). In both rings coupling-impedance driven instabilities need careful evaluation to ensure meeting the machine's high-performance goals. This paper presents the workflow followed in building the impedance models and the beam-stability analysis based on those models. We follow the commonly accepted approach of separating the resistive-wall and the geometric parts of the impedance; the former is obtained by analytical formulas, the latter by numerical electro-magnetic codes (primarily CST Studio software) with perfectly-conducting boundary conditions. Impedance budgets are established and pseudo-Green functions calculated to be used in beam dynamics studies. We also present various ways to cross-check simulation results for reliable impedance modeling. Finally, the crucial single-bunch instability current thresholds for various operation modes are determined and discussed. • Presents the basic/general workflow to build impedance models for accelerator rings. • Presents alternative ways to cross-check the results for reliable impedance models. • Presents systematic results for the 4th generation radiation source of ALS-U project. [ABSTRACT FROM AUTHOR]

Buildings located close to transportation corridors experience structure-borne sound and vibration due to passing traffic which can be disruptive to operation of sensitive equipment in manufacturing, and medical facilities. Structure-borne sound and vibrations, when high may also be annoying to human occupants in residential, office, and commercial buildings. Hence, there is a growing need for cost effective sound and vibration predictions to evaluate the need for mitigation. The research focuses on in-situ testing of a full-scale building for verification of a previously developed impedance-based methodology and to create a prediction model to study ground-borne vibrations in the test building. A mitigation methodology was also examined using the verified prediction model. Impedance modeling involves the propagation of axial waves through columns combined with the impedance of the intermediate floor slabs. The vibration transmission in the building was characterized and predicted using a single column model with attached floors. Train-induced floor vibrations in an existing four-story building in Boston were measured and compared with predictions of the impedance model. The impedance model predictions closely matched with the measured floor responses. A previously suggested mitigation method was investigated analytically using the impedance model. A thickened floor referred as the “blocking floor” was used on the lower elevation of the building and the reduction in vibration at the upper floors of the building was compared for various thicknesses of the blocking floor, to study its efficiency. The blocking floor has high impedance and reflects a major portion of the vibration transmitting in the columns preventing it from reaching the upper floors. The blocking floor was found to mitigate the transmission of ground-borne vibrations to upper floors. [ABSTRACT FROM AUTHOR]

Highlights: [•] The 2D simulation shows another feature in concentration impedance. [•] The channel gas transport causes a capacitive behavior. [•] Anode polarization variation has a significant influence on velocity distribution. [•] The influence of 2D simulation is important for channel height bigger than 2mm. [ABSTRACT FROM AUTHOR]

Abstract: It has been assumed that cation electron coupled charge transfer can be conducted in accordance with two mutually coupled reactions. Using a differential method an expression was introduced which describes the impedance of a cation electron coupled transfer reaction. Also, an electrical equivalent circuit has been defined. The total faradaic impedance is the sum of electron transfer impedance and cation transfer impedance. Issues of potentiostatic and potentiodynamic impedance measurements have been discussed. The relationship between polarization resistance and impedance has been explained. Polarization resistance as well as direct current depend on potential scan rate. The electrode potential influence on impedance has been determined. [Copyright &y& Elsevier]

Abstract: A finite transmission line is proposed for proton exchange membrane fuel cell reaction layer, when the faradic current is absent due to purging of Inert gas at the back of cathode and anode. Also a finite transmission line is presented when a charge transfer accrued among catalyst and electrolyte interface. The electrochemical impedances of finite transmission lines are computed using MATLAB software. Relative to the orders and types of the evaluated impedances, some relations to determine and identify the parameters of the proposed models are derived. In first model, it is shown that the electrical elements of transmission line can be extracted explicitly from the Nyquist and Bode diagrams whereas for the second one, some of the parameters cannot directly be investigated. However, using a numerical procedure, some valuable results about parameter variations are obtained. [Copyright &y& Elsevier]

Abstract: Planar rectangular coils provide some interesting features different from those of pancake coils, such as good performance in conductivity measurement at high frequencies, high sensitivity to scratches and other shallow imperfections and the possibility of inspecting complex surfaces. The impedance of these coils has been modeled by different authors. A brief discussion of some analytical approaches in the literature is presented and calculations using the second-order vector potential approximation are introduced, aimed at the study of the influence of coil shape and size on the sensitivity to electrical conductivity of the substrate. Three cases were modeled, (A) coil in free space—inductance L 0 as a function of number of turns (n) and width of the spiral conductive lanes (c) for different coil inner shape factor (rri ); (B) coil on a conductive substrate (Zircaloy-4)—impedance change, ΔZ, as a function of f·σ, the product of the test frequency (f) and the substrate electrical conductivity (σ) for different rri and (C) coil sensitivity to small changes in σ(Δσ). L 0=L 0(c, n) increased as expected with n and/or coil area and decreased with shape factor. Normalized ΔZ depends strongly on inner rri , the curves (or surfaces) for smaller values of rri enclosing those for larger values. Another shape factor, the outer shape factor (rro ), was introduced. Strong dependence of the sensitivity to Δσ on n was observed, as well as the existence of an optimal theoretical frequency. Because the sensitivity to surface conditions also increases with coil size, it could be established that for conductivity assessment it is better to use coils with the smallest n and c, compatible with the particular application, and that the resolving power for this type of measurements is not greatly affected by the shape factor. [Copyright &y& Elsevier]

In this work, acoustic metamaterials based on the concept of extended tube are investigated. The metastructure consists of an array of units, each featuring a thin tube protruding from a perforation into a backing cavity to form a Helmholtz resonator. First, an impedance model for a single-layer extended tube structure is developed using the equivalent circuit method. For the tube in which the viscothermal effect can not be ignored, an equivalent fluid model is used to characterize its acoustic property and end corrections are introduced to take into account the flow distortion effect at the two ends. Then, two cost-effective samples are fabricated and tested using the impedance tube where good accuracy between the prediction and measurement is found. The structural simplicity and low-frequency absorption property allow the extended tube acoustic metamaterials to be utilized in scenarios where installation space for acoustic absorption materials is constrained. As an example, we demonstrate its use on a commercial kitchen hood. Standard acoustic power measurement shows its superior noise reduction performance over conventional porous materials of the same thickness. [ABSTRACT FROM AUTHOR]

This paper describes a multivariable modeling and control strategy that increases the bandwidth of isotropic force transmission in multi-degree of freedom haptic interfaces. The controller structure leads to a straightforward model identification procedure and yields a simple control law that can be easily implemented. Experimental results show that the multivariable closed-loop control significantly improves performance on a five degree-of-freedom in-parallel haptic interface. [Copyright &y& Elsevier]

Increasingly dense microprocessors will require high-density passive components that can enable integrated power delivery with larger conversion ratios and higher output currents. Of these, nanoparticle-based tantalum capacitors can provide some of the highest volumetric densities compared to other capacitor technologies, due to the high surface area-to-volume ratio. Additionally, their temperature-stable dielectric can handle higher current ratings. However, the complex nanoparticle-based electrode structure results in long conduction paths, resulting in higher ESR and reduced frequency stability. A model is needed that can accurately predict the relationship between capacitor nanostructure and electrical performance so that the nanostructure can be optimized to meet the theoretical limits of device performance. This paper develops such a model, and demonstrates its accuracy by characterizing a real tantalum capacitor device. The model is then used to study the effect of capacitor materials and device geometry on the device performance, including capacitance density, frequency stability, and ESR. [Display omitted] • Characterization of capacitors based on sintered tantalum nanoparticle technology. • Developed a model that accurately correlates capacitor performance to nanostructure. • Effect of materials on capacitance density, ESR, and frequency stability is studied. • Capacitor frequency stability can be improved by using thinner nanoporous films. [ABSTRACT FROM AUTHOR]

The subsynchronous control interaction (SSCI) occurs when the wind turbine converter (WTC) controls interact with the series-compensated or weak AC network. The mechanism and attributes of the emerging interaction phenomenon invilving wind turbine generators (WTGs) are quite different from the traditional subsynchronous resonance or oscillation (SSR/SSO) phenomenon in steam turbine-generators (STGs). The SSCI is characterized by various system-wide parameters, including the wind speed and its uneven distribution in a wind power plant, varying number of WTGs connected to the grid, type of the WTGs, parameters of the WTC controls, level of series compensation, stiffness of the grid, etc. Such system-wide parameters change over time and thus make the modeling and analysis rather challenging to conduct SSCI studies. An ideal modeling approach for the SSCI is expected to preserve the system topology as well as valid for a wide range of operating conditions and parameters of the system. On the other hand, an ideal stability analysis method is supposed to give key quantitative information, such as the magnitude, frequency, and origin of the oscillation, as well as the component level participation indices. This paper provides a comparative insight into the application of existing and emerging modeling and stability analysis approaches for SSCI investigations in large-scale wind farms. It highlights the strengths and weaknesses of various modeling methods and analysis criteria. Finally, the paper underlines the recent advancements and points out towards the research directions in small/large-signal impedance modeling and stability approaches for accurate and quantitative investigation of the SSCI. • SSCI is a system-wide phenomenon thus the modeling and analysis is challenging. • Strengths and weaknesses of the existing modeling and stability analysis methods in the time and frequency-domain are discussed. • An modeling method should preserve system topology and remains valid under a wide range of operating conditions. • An ideal stability analysis method should give key quantitative information: magnitude, frequency and source. [ABSTRACT FROM AUTHOR]

The dc microgrid with multiple types of loads is considered to be a promising solution for integration of distributed generations (DGs). This paper presents the small signal impedance modeling of dc microgrid with multiple types of loads (including resistive loads, constant power loads and induction motor loads) which consists of source-side output impedance model and load-side input impedance model. Afterward, the Nyquist criteria based stability analysis is performed on the deduced small signal impedance models to investigate the interaction influence of different load states on the damping performance and stability boundaries of the dc microgrid system. Besides, a damping enhancement control strategy for dc microgrid is proposed in this paper based on the dual virtual inductive impedances. Since the dual inductive impedance can perform high-frequency attenuation feature during transients or oscillations, enhanced damping performance of the dc microgrid system is presented with the proposed control. Theoretical analysis and simulation cases show the validity and effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

• A detailed impedance model of the DFIG system considering the induction generator and all its controllers (e.g. PLL, inter and outer controllers) is built in the d-q frame. • The GNC proves that the stability of the DFIG system under weak grid can be improved by increasing the rotor inductance. • An improved virtual inductance method considering the PLL dynamics is proposed to improve the stability the DFIG system under weak grid condition. Impedance-based method is an effective method to analyze the system stability. Based on the impedance stability theory, an improved virtual inductance control method considering PLL dynamic is presented for the stability of doubly-fed induction generator (DFIG) under weak grid condition. Firstly, equivalent input admittance model of the DFIG system including all its controller is developed in the synchronous reference (d-q) frame. Secondly, the generalized Nyquist criterion (GNC) proves that the stability of the DFIG system can be improved by increasing the rotor inductance. Then, a virtual inductance term is introduced into the RSC current controller to emulate additional inductance in the rotor. However, the virtual inductance method may be ineffectual due to the influence of the PLL dynamic under weak grid condition. Therefore, an improved virtual inductance control method considering the PLL dynamic is implemented to improve the stability of the DFIG system under weak grid condition, and the simulation is used to verify the effectiveness of the improved virtual inductance control strategy. [ABSTRACT FROM AUTHOR]

The size of electrodes in bio-chemical sensors has shrunk quickly in recent years. The high impedance from polarizable microelectrodes brings new challenges to the sensor design. On the other hand, non-polarizable electrodes, such as Ag/AgCl electrodes, could have lower impedance at the low bio-signal frequency band. Thin-film Ag/AgCl electrode is commonly used as reference electrode. In this work, the electrochemical processes governing the impedance profile of thin film Ag/AgCl microelectrodes are systematically studied. New models are established for circuit parameters of thin film Ag/AgCl microelectrodes based on the new findings. The models are verified extensively with impedance measurements. [ABSTRACT FROM AUTHOR]