Impedance-based Analysis Research Articles

Impedance-Based Method for Predictive Stability Assessment

Impedance-based analysis methods enable a more specific and earlier foundation for assessing harmonic stability in decentralized converter-based power plants compared to the conventional compliance testing in the grid connection process. Essentially, they can be implemented as black-box model approaches without the necessity to disclose internal control models. Initially, only knowledge of the input impedances of the planned grid connection point and the planned PV system is required for application. For this purpose, the method of impedance spectroscopy for inverters has already been developed as a means to determine the effective impedance profile and the internal harmonic sources of inverters, allowing for the description of the frequency-dependent behavior of individual units. The time- and frequency-dependent grid impedance at the grid connection point (GCP) has also been successfully measured in several campaigns on medium and low-voltage grids. Through the coordinated application of both measurement methods, a predictive harmonic assessment is intended in the future, ensuring high planning reliability and grid quality even in grids with a high penetration of power electronics-coupled systems. This paper provides an overview of the current state of research on impedance-based stability criteria and presents measurement methods for practical implementation. Furthermore, it outlines remaining open questions until application in the field.

Impedance network-based oscillatory stability analyzer (INOSA) – A frequency domain tool for investigating IBR-grid interactions

Unstable subsynchronous oscillations (SSOs) may be excited due to adverse interactions between inverter-based resources (IBRs) and series-compensated or weak ac networks. The frequency-sweep impedance modeling and analysis method is widely used for scanning grid-tied IBRs against the risk of SSO. The stability of such a system is heavily influenced by system-wide parameters such as IBR dispatching conditions (input resource, and different dispatch scenarios), and network status (short-circuit and/or series-compensation levels). The variation in the operating conditions must be taken into account when conducting frequency-scanned impedance-based modeling and analysis. This paper proposes a tool, named impedance network-based oscillatory stability analyzer (INOSA), which integrates decades of research on impedance-based analysis to investigate the risk of SSO under varying operating conditions. The INOSA tool is developed as a MATLAB application package. It takes the available system parameters and frequency-scanned impedance data as input and processes it to carry out impedance network-based stability analysis under all operating conditions. The developed tool outputs key stability information such as damping and frequency of each oscillation mode, and a multi-dimensional security boundary/region plot that can be used to assess the risk of SSO and take appropriate measures. A case study on an actual power system is presented to demonstrate the features and capability of the analysis tool.

Towards an Impedance-Based Analysis of SOFC-Stacks

To understand the performance and durability limitations in SOFC stacks and systems, impedance spectroscopy is becoming increasingly important. However, in the analysis of entire stacks, gradients over the active cell area are averaged out, preventing access to local phenomena.To get better insight into the spatial distribution of performance losses in a stack layer, incremental cells and repeat units with an active electrode area of 1 cm² are investigated in this study.Impedance spectroscopy and the subsequent impedance data analysis by the distribution of relaxation times (DRT) is applied to deconvolute the polarization processes in the Ni/CGO fuel electrode and the LSM air electrode of the investigated electrolyte supported cell. To ensure an appropriate deconvolution of air and fuel electrode processes, next to full cells symmetrical anode and cathode cells are applied. In case of Ni/CGO fuel electrodes charge transfer and gas diffusion overlap in the spectrum and the assignment has to be done indirectly using inert gas mixtures [1].In Figure 1 impedance spectra and DRTs of fuel and air electrode obtained from symmetrical cells are compared with the full cell spectrum. This analysis enables a clear allocation of the processes in the single cell to Ni/CGO fuel electrode and LSM air electrode.Next to the cell testing in an inert, ideal environment namely an Al2O3 flow field with gold and nickel contact grids respectively, the study is expanded to stack like conditions. The additional losses due to contact resistances and gas supply limitations are studied similar to Kornely et al. [2] for single repeat units (single cell + contact layer + metallic interconnects). In this contribution a physio-chemical equivalent circuit model based on these measurements will be presented and the complex impedance behavior of single cells and repeat units will be discussed.

Towards an Impedance-Based Analysis of SOFC-Stacks

This proceeding aims to understand the performance of SOFC stacks using impedance spectroscopy. For the stack analysis, gradients over the active cell area are averaged out, preventing access to local phenomena. Hence, to get better insight into the spatial distribution of losses, single cells with an active electrode area of 1 cm² are characterized at 850 °C based on EIS and DRT-analysis. To ensure an appropriate deconvolution of processes, symmetrical anode and cathode cells are also tested. Next to the ideally contacted cells, metallic interconnecting repeat units are analyzed to study the impact of contacting and gas supply in the stack. The effect of fuel utilization was also investigated by mimicking different locations in the stack. In this contribution, the complex impedance behavior of the stack based on polarization losses will be discussed. The paper also provides disentangled information often lost or averaged out in stacks.

Oscillatory frequency characterization based on impedance analysis

The impedance-based analysis has been extensively used for frequency domain stability analysis in power systems with power electronics devices. Also, such analysis is used to determine the oscillatory mode frequencies. The impedance ratio analysis is a conventional approach employed to determine both stability conditions and oscillatory mode frequencies. In particular, oscillatory mode frequencies can be approximated from the gain and phase crossover of the impedance ratio. Such an assumption has been considered in the harmonic frequency range, but in the low frequency range, approximately under 100 Hz, this might not be valid. Low frequency interactions are mainly caused by voltage source converter outer loops and synchronous generator circuits and controls. This paper presents an alternative impedance-based analysis to identify mode frequencies and the participation of impedance components to the interactions. This method is compared to the approximation from the gain and phase crossover of the impedance ratio. Also, the simplification of synchronous generators as Thévenin equivalents is explored for low frequencies based on the impedance frequency response. An essential power system model with a voltage source converter, a synchronous generator, and a load is presented to show the advantages of this alternative impedance-based analysis compared to the gain and phase crossover of the impedance ratio.

An Improved Q-Axis Current Control to Mitigate the Low-Frequency Oscillation in a Single-Phase Grid-Connected Converter System

An electric railway system is a typical single-phase grid-connected converter system, and the low-frequency oscillation (LFO) phenomenon in electric railway systems has been widely reported around the world. Previous research has indicated that the LFO is a small-signal instability issue caused by impedance mismatching between the traction network system and electric trains. Therefore, this paper proposes an improved q-axis current control method to reshape the train’s impedance. The proposed method can be implemented easily by relating a reverse q-axis reactive current directly to the reference of the q-axis current under the dq current decoupled control. Moreover, considering the additional q-axis reactive current control, a small-signal impedance model of a train–network system is built. The impedance-based analysis results indicate that the proposed q-axis reactive current feedback control can increase the magnitude of the train’s impedance, which is beneficial to enhancing the system’s stability. Finally, this paper employs experimental results to verify the effectiveness of the proposed method.

Flagellar motility mediates biofilm formation in Aeromonas dhakensis

Aeromonas dhakensis possesses dual flagellar systems for motility under different environments. Flagella-mediated motility is necessary for biofilm formation through an initial attachment of bacteria to the surface, but this has not been elucidated in A. dhakensis. This study investigates the role of polar (flaH, maf1) and lateral (lafB, lafK and lafS) flagellar genes in the biofilm formation of a clinical A. dhakensis strain WT187 isolated from burn wound infection. Five deletion mutants and corresponding complemented strains were constructed using pDM4 and pBAD33 vectors, respectively, and analyzed for motility and biofilm formation using crystal violet staining and real-time impedance-based assays. All mutants were significantly reduced in swimming (p < 0.0001), swarming (p < 0.0001) and biofilm formation using crystal violet assay (p < 0.05). Real-time impedance-based analysis revealed WT187 biofilm was formed between 6 to 21 h, consisting of early (6–10 h), middle (11–18 h), and late (19–21 h) stages. The highest cell index of 0.0746 was recorded at 22–23 h and biofilms began to disperse starting from 24 h. Mutants Δmaf1, ΔlafB, ΔlafK and ΔlafS exhibited reduced cell index values at 6–48 h when compared to WT187 which indicates less biofilm formation. Two complemented strains cmaf1 and clafB exhibited full restoration to wild-type level in swimming, swarming, and biofilm formation using crystal violet assay, hence suggesting that both maf1 and lafB genes are involved in biofilm formation through flagella-mediated motility and surface attachment. Our study shows the role of flagella in A. dhakensis biofilm formation warrants further investigations.

Settling Angle-Based Stability Criterion for Power-Electronics-Dominated Power Systems

Impedance-based analysis methods are widely applied to the small-signal stability analysis of grid-connected converters. As the initial step to carry out the impedance-based stability analysis, it is critical to select whether to represent the converter as a Norton or Thevenin equivalent circuit. However, there is lack of definition on the representation of converters, and inappropriately selected impedance model choice leads to improper stability analysis. Moreover, existing impedance-based stability analysis methods, e.g., return-ratio matrix (RRM)-based method, only apply to minimum phase systems where the RRM does not contain right-half-plane poles. In complex and power-electronics-dominated power systems, interactions between multiple converters lead to nonminimum phase power systems, where current methods can no longer be directly applied. To address these problems, a stability criterion is proposed by introducing the concept of settling angle. As the main contribution of this article, the settling angle of total admittance is proven as the intrinsic hallmark of power systems and directly reflects the system stability. The proposed settling-angle-based stability criterion is validated by control hardware-in-the-loop experimental results for a three-terminal system that integrates grid-forming and grid-following converters through a common bus.

Generalized impedance model and interaction analysis for multiple grid-forming and grid-following converters

The growing use of power electronics converters in the power system leads to emerging stability issues. However, existing impedance models are insufficient for analysis of the converter-induced stability issues in the converter-congregated local area, especially faced with the hybrid integration of both grid-following (GFL) and grid-forming (GFM) converters. In this paper, a generalized impedance model in the d q frame is developed to facilitate the impedance-based analysis of any grid node with both GFL and GFM converters. With this model, closed-loop, internal and external stability performance can be accurately described and predicted. This paper shows that the internal stability of power converters has the feature of transferability. Therefore, to facilitate the internal stability analysis of multiple-input and multiple-output systems, this paper proposes a stability analysis method that utilizes the determinant of total admittance (matrices in high dimension) for direct stability demonstration, based on the recognition of instability pattern. Finally, the procedure for integrating power converters is considered to help with the stability-oriented design. The established impedance model, the proposed stability analysis method and the converter integration procedure are validated with control hardware-in-the-loop results in a study case of a multi-terminal network with current-controlled converters and virtual synchronous generators. • Impedance model describing any local area with GFL and GFM converters is proposed. • The determinant of total bus admittance is used for internal stability demonstration. • Impacts of critical system parameters on internal and external stability are studied. • A three-step procedure is adopted to facilitate the stability-oriented design.

A broadband active damping method for high-frequency resonance suppression in MMC-HVDC system

Negative damping in modular multilevel converter (MMC) impedance may lead to high-frequency resonance (HFR) in MMC-high-voltage direct current systems. The existing solution is based on amplitude attenuation by digital filters. However, filters have weak amplitude attenuation and obvious phase shifts in the attenuation band, resulting in the transfer of negative damping. The system still has a risk of HFR when grid condition is changed. This study proposed a broadband active damping method based on phase compensation to address this limitation. The embedding position and scheme of the phase compensation controller are optimally designed through impedance-based analysis. By tuning parameters correctly, the negative damping of MMC is eliminated over a wide frequency range (500–2000 Hz). Thus, the proposed method performs better in HFR suppressing under varying grid conditions. In addition, considering the harmonic amplification problem caused by the differential link, an inertia link and a proportional link are further employed to increase high frequency impedance magnitude. Time-domain simulations in MATLAB/SIMULINK verify the effectiveness of the proposed method.

Low-Cost ITO Microelectrodes for Bio-Sensing and Impedance Based Cellular Assays

Excellent electronic and optical properties make Indium tin oxide (ITO) an attractive electrode substrate.1 It is an alloy of indium oxide (In2O3) and tin oxide (SnO2) in a weight ratio of 90:10. It is transparent as thin film, so, it is also good to be viewed under different types of microscopes used in cell-culture studies. Despite the commercial availability of finest quality ITO and some low-cost methods for direct deposition being in use by now, definition of patterns on them is still a concern. Placing its popularity and extensive usage aside, the manufacturing of ITO electrodes so far lacks a rapid, proficient, highly reproducible, flexible, cost-effective, easy patterning process that could surpass non-trivial, time consuming techniques as lithography. A promising advancement towards achieving these limitations was the use of lasers for the fabrication of the ITO electrodes and related structure. Even though several laser techniques are explored, with the exception of CO2 lasers, none of the others provides the benefit of being widely available and comparatively inexpensive means for fabrication of microelectrode sensors. Presently, these are the highest-power continuous wave lasers available. With their high efficiency, they are usually employed in laser cutting, welding, drilling and surface treatment.2 A cost-effective method based on CO2 laser irradiation for the preparation of ITO microelectrodes to find applications in the bio-sensing, impedance-based analysis, and more, is presented. Electrodes of different sizes and shapes were examined to identify the performance of the samples. They were later optimized to suit our applications for studies in three dimensional cell-cultures. For rectangular shaped electrodes, the smallest sizes optimized were 25, 50 and 100 µm wide. The figure attached shows cyclic voltammetry results illustrating the reproducibility of 100 µm electrodes with ferrocenedimethanol as the redox probe in potassium nitrate. However, smaller electrodes are not always desirable. For the cell-culture work, electrodes with area 1.75mm2 were fabricated and characterized to get reproducible results. Initial measurements of impedance based cellular assays have been conducted first in DMEM medium and then in the cell culture medium to understand the parameters and study the adhesion of the cells on to the electrodes. Analysis of glucose sensing properties of the system have also been attempted. Further modifications of the electrode structure and characterizations for validation will be discussed.References Stadler A. et al. J.Materials, 2012, 5, 661-683Wang Y. et al. J Energy Chem. 2021, 59, 642-665 Figure 1

The potential of remdesivir to affect function, metabolism and proliferation of cardiac and kidney cells in vitro

Remdesivir is a prodrug of a nucleoside analog and the first antiviral therapeutic approved for coronavirus disease. Recent cardiac safety concerns and reports on remdesivir-related acute kidney injury call for a better characterization of remdesivir toxicity and understanding of the underlying mechanisms. Here, we performed an in vitro toxicity assessment of remdesivir around clinically relevant concentrations (Cmax 9 µM) using H9c2 rat cardiomyoblasts, neonatal mouse cardiomyocytes (NMCM), rat NRK-52E and human RPTEC/TERT1 cells as cell models for the assessment of cardiotoxicity or nephrotoxicity, respectively. Due to the known potential of nucleoside analogs for the induction of mitochondrial toxicity, we assessed mitochondrial function in response to remdesivir treatment, early proteomic changes in NMCM and RPTEC/TERT1 cells and the contractile function of NMCM. Short-term treatments (24 h) of H9c2 and NRK-52E cells with remdesivir adversely affected cell viability by inhibition of proliferation as determined by significantly decreased 3H-thymidine uptake. Mitochondrial toxicity of remdesivir (1.6–3.1 µM) in cardiac cells was evident by a significant decrease in oxygen consumption, a collapse of mitochondrial membrane potential and an increase in lactate secretion after a 24–48-h treatment. This was supported by early proteomic changes of respiratory chain proteins and intermediate filaments that are typically involved in mitochondrial reorganization. Functionally, an impedance-based analysis showed that remdesivir (6.25 µM) affected the beat rate and contractility of NMCM. In conclusion, we identified adverse effects of remdesivir in cardiac and kidney cells at clinically relevant concentrations, suggesting a careful evaluation of therapeutic use in patients at risk for cardiovascular or kidney disease.

Performance degradation analysis of solid oxide fuel cells using dynamic electrochemical impedance spectroscopy

The commercial application of solid oxide fuel cells (SOFCs) is now facing the challenge caused by limited lifetime and durability. More in-depth understanding and scientific evaluation of the complex and coupled performance degradation mechanisms become a priority. In this study, a quantitative evaluation method is established based on dynamic electrochemical impedance spectroscopy (DEIS) measurements under different DC biases. The contributions of ohmic resistance and different polarization resistances to the overall performance degradation are separately determined through coupled distribution of relaxation time (DRT) analysis, equivalent circuit model (ECM) fitting and integral voltage loss calculation. This method is used to investigate the performance degradation of a single cell which is operated under galvanostatic mode for more than 1000 h. Precise contributions of individual processes to the overall degradation are quantitatively determined. Afterwards, post-test characterization on electrode microstructures is provided to support the impedance-based analysis results. In addition, the results are compared with our previously reported analysis of initial-stage rapid performance degradation. Since the proposed method is based on DEIS measurements, it can be further applied to performance degradation analysis of practical stacks as well as other electrochemical devices.

Mitigating High-Frequency Resonance in MMC-HVDC Systems Using Adaptive Notch Filters

Recently, high-frequency resonance (HFR) incidents have occurred in several MMC-HVDC projects. The resonance frequency highly depends on system conditions, making mitigation methods designed for specific scenarios ineffective once the grid condition changes. To address the issue, this paper proposes a novel HFR mitigation method based on adaptive notch filters (ANFs). The embedding positions of ANFs in the MMC control are optimally determined through impedance-based analysis. With the HFR frequency estimated by the interpolated discrete Fourier transform, the parameters of ANFs are adaptively tuned. The ANFs improve the damping performance of the captured HFR mode without affecting the dynamic characteristics at other frequencies. As a result, the HFR under varying grid conditions can be mitigated effectively. The effectiveness of the proposed method is verified through electromagnetic transient (EMT) simulations of an actual MMC-HVDC system.

A Multi-Frequency DQ Impedance Measurement Algorithm for Single-Phase Vehicle-Grid System in Electrified Railways

Dq impedance measurement is of great importance as a part of the impedance-based analysis. At present, impedance of objects in single-phase vehicle-grid system are mainly obtained by injecting the voltage or current disturbance of one single frequency into the system circularly. However, as the number of measurement frequency points increases, there will produce much time-consuming of measurement. In this paper, a quick dq impedance measurement algorithm for single-phase vehicle-grid system based on multi-frequency sweep is proposed. First, pseudo-random binary sequences (PRBS) and ternary sequences are modified as the driving signals rather than directly injecting into the control link. Then three types of disturbance with specific frequency characteristics are introduced into the system and their parameters setting principle are given for the first time. According to the different characteristics of measurement objects, the proposed algorithms are also different to ensure the maximum specificity and sensitivity. Finally, for different objects in vehicle-grid system like traction network, single-phase converter and dual-interleaved converter, simulation and experimental results based on the hardware in the loop real-time simulation platform both show the correctness of the proposed algorithm, and the possible causes of errors are also analyzed in detail. Moreover, the proposed algorithm makes it possible to quickly measure the dq impedance of the single-phase vehicle-grid system, and the correctness of the theoretical and measured value can be judged and used for the stability analysis of electrical systems.

Evaluation of CAR-T cell cytotoxicity: Real-time impedance-based analysis.

This chapter describes the most common method for evaluating cytotoxicity of chimeric antigen receptor (CAR) T cells, the xCELLigence real-time cell analysis (RTCA) platform (Agilent Technologies, Inc., Santa Clara, CA). Though there are a variety of assays used to evaluate conventional and engineered T cell cytotoxicity, the benefit of the xCELLigence platform is the depth of real-time data collected. This chapter begins by providing information on the conceptual basis underlying the xCELLigence assay, followed by a detailed protocol for the application of this assay to evaluate your own CAR-T cells, as well as specific insight and helpful tips for assay design, usage, and data analysis. Application of the information and methods discussed within this chapter will provide a greater understanding for evaluating cytotoxicity of CAR-T cells using this in vitro model system.

Analysis of Stationary- and Synchronous-Reference Frames for Three-Phase Three-Wire Grid-Connected Converter AC Current Regulators

The current state of the art shows that unbalance and distortion on the voltage waveforms at the terminals of a grid-connected inverter disturb its output currents. This paper compares AC linear current regulators for three-phase three-wire voltage source converters with three different reference frames, namely: (1) natural (abc), (2) orthogonal stationary (αβ), and (3) orthogonal synchronous (dq). The quantitative comparison analysis is based on mathematical models of grid disturbances using the impedance-based analysis, the computational effort assessment, as well as the steady-state and transient performance evaluation based on experimental results. The control scheme devised in the dq-frame has the highest computational effort and inferior performance under negative-sequence voltage disturbances, whereas it shows superior performance under positive-sequence voltages among the reference frames evaluated. In contrast, the stationary natural frame abc has the lowest computational effort due to its straightforward implementation, with similar results in terms of steady-state and transient behavior. The αβ-frame is an intermediate solution in terms of computational cost.

Electrical Impedance of Upper Limb Enables Robust Wearable Identity Recognition against Variation in Finger Placement and Environmental Factors.

Most biometric authentication technologies commercialized in various fields mainly rely on acquired images of structural information, such as fingerprints, irises, and faces. However, bio-recognition techniques using these existing physical features are always at risk of template forgery threats, such as fake fingerprints. Due to the risk of theft and duplication, studies have recently been attempted using the internal structure and biological characteristics of the human body, including our previous works on the ratiometric biological impedance feature. However, one may still question its accuracy in real-life use due to the artifacts from sensing position variability and electrode–skin interfacing noise. Moreover, since the finger possesses more severe thermoregulatory vasomotion and large variability in the tissue properties than the core of the body, it is necessary to mitigate the harsh changes occurring at the peripheral extremities of the human body. To address these challenges, we propose a biometric authentication method through robust feature extraction from the upper-limb impedance acquired based on a portable wearable device. In this work, we show that the upper limb impedance features obtained from wearable devices are robust against undesirable factors such as finger placement deviations and day-to-day physiological changes, along with ratiometric impedance features. Overall, our upper-limb impedance-based analysis in a dataset of 1627 measurement from 33 subjects lowered the classification error rate from 22.38% to 4.3% (by a factor of 5), and further down to 2.4% (by a factor of 9) when combined with the ratiometric features.

Real-Time Analysis of Antiproliferative Effects of Mouthwashes Containing Alcohol, Sodium Fluoride, Cetylpyridinium Chloride, and Chlorhexidine In Vitro.

Objectives In this study, the cytotoxic responses of six different over-the-counter mouthwashes on L929 cells were analyzed by two different techniques: the traditional colorimetric tetrazolium-based reduction assay (MTT) and the modern impedance-based real-time cell analysis (RTCA) system to investigate their biocompatibility in vitro. Thus, the investigation of the antiproliferative effects of the specified materials via different techniques is vital to reach this goal. Materials and Methods First, L929 mouse fibroblasts were exposed to the dilutions of mouthwashes for 2 minutes. After incubation, the tetrazolium reduction method was used to assess the metabolic viability of cells measured by colorimetric MTT assay and morphological inspection of cells was performed via phase-contrast microscopy. Furthermore, the effect of each mouthwash on the proliferation, morphology, and adhesion of L929 cells was monitored continuously by a noninvasive and label-free RTCA system for 140 h. Results Our data showed that all of the mouthwashes had varying cytotoxic effects on fibroblasts compared to the control group in MTT assay. In addition to that, RTCA technology has provided the growth kinetic profiles that can be used to analyze if the treatment is causing antimitotic or DNA-damaging effect on cells. Thus, analysis via this system can tell us the mechanism of toxicity behind the cell growth inhibition in vitro. Here, we found that only mouthwash 1 moderately maintained the viability of the L929 cells, yet displaying antimitotic effects and the other mouthwashes (mouthwash 2-mouthwash 6) showed toxicity via DNA-damaging effects. Conclusions Of the six types of mouthwash tested, the most biocompatible result was obtained from a mouthwash containing alcohol (i.e., mouthwash 1). On the other hand, sodium fluoride- (NaF-) and cetylpyridinium chloride- (CPC-) containing mouthwash (i.e., mouthwash 2) showed the most cytotoxic effect.

Model and stability analysis of grid-connected PV system considering the variation of solar irradiance and cell temperature

In recent years, solar photovoltaic (PV) power technology has been applied widely. The interaction between the PV generator and the grid, however, may cause the stability issue and oscillation in practical applications. Hence it is very important to analyze the stability of grid-connected PV system. Due to the fluctuation of solar irradiance, the operating characteristics of the grid-connected system alter, and the grid-connected system shows different stability at the altered operating points. Therefore, it is vital to analyze the grid-connected PV system stability at all operating points in the entire operation range. The impedance-based analysis method is a useful tool for stability analysis of the system. However, since the operating point of the system often alters continuously, it is difficult to use conventional impedance/admittance model to characterize the system performance. To overcome this drawback, this paper attempts to incorporate the solar irradiance into the conventional PV generator admittance model and develops a multivariable small signal admittance model for PV generator. Moreover, the PV cell temperature influence is also considered in the model. Further, a new analysis method is proposed to enable the stability analysis. Simulation and experiment were conducted to demonstrate the effectiveness of the model and the stability analysis method.