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Long-term ECG (electrocardiogram) measurement in patients with burns is a complicated problem since the overlapping of surface contact electrodes can lead to additional injuries. The possibility of ECG recording in patients with burns using capacitive electrodes was not proved, and there are no models of the electrode contact with a patient’s body while rehabilitation means are used. In this paper, the model of the contact between capacitive electrodes and the skin was modified and the circuit model of the contact: skin – bandages (saline solution) – film – active capacitive electrode, was described. The influence of the parameters of a capacitive electrode on the amplitude-frequency characteristics (AFC) of the contact of an electrode with skin was assessed. It was found that contact capacitance is crucial to obtain a high-quality ECG signal. The parameters of the impedance of bandages, saline solution, a dielectric film were calculated, and their effect on the AFC was studied. Based on the modified model, the AFC contact was modeled taking into consideration all the calculated parameters; it was found that the resulting AFC of the contact corresponds to the frequency range of the ECG signal. Analysis of the calculations proves the possibility of using capacitive electrodes when applying various rehabilitation means. It was found that at a change in the impedance of the saline solution from 0.1gigaohms to 1gigaohm, the changes in the AFC of the contact are not crucial for the final quality of the received signal. All calculations were carried out by modeling in the Qucs environment (ngspice SPICE). Simulation results can be used in the development of new types of capacitive electrocardiographic electrodes. The proposed model can be used to study other wound covers, as well as to model physiological processes when putting artificial skin and wound covers

Due to the breakneck switching speed, SiC mosfet is extremely sensitive to parasitics in the power device, circuit layout, and also measurement probe. It is not clear how the parasitics of measurement probes affect the transient stability of SiC mosfet, and it poses an unsolved challenge for the industrial field. This paper focuses to uncover the transient instability mechanism of SiC mosfet intruded by probes. Mathematical and circuit models of voltage and current probes are created, by considering the parasitics, input impedance, and bandwidth issues. To reveal the stability principles of SiC mosfet associated with probes, impedance-oriented and heterogeneity-synthesized models combining device with probes are proposed. Furthermore, an assessment methodology and root locus analysis are presented to demonstrate the transient stability schemes and the stable boundaries of SiC mosfet influenced by multiple factors, including probe parasitics, device parameters, gate resistances, and snubber circuits. Comparative experiments are presented to confirm the transient behaviors of SiC mosfet intruded by probe parasitics and regulated by control circuits. It is proven that, because of low bandwidth specifications, the large input capacitance of the voltage probe and coil inductance of the current probe degrade the transient stability of SiC mosfet. Due to the deteriorated stability margin of SiC mosfet intruded by the inserted parasitics of probes, instability may also be activated by using the small gate resistance. The snubber circuit is helpful to enhance the transient stability. Advanced probes with high bandwidth and high impedance are crucially needed for stable measurement of wide bandgap power devices like SiC mosfet. Ministry of Education (MOE) Nanyang Technological University This work was supported in part by the National Natural Science Foundation of China under Grant 51607016, in part by the National Key Research and Development Program of China under Grant 2017YFB0102303, in part by Singapore ACRF Tier 1 Grant RG 85/18, and in part by the NTU Startup Grant (SCOPES) for Prof Zhang Xin.

In this paper, impedance modeling of a DC microgrid system consisting of a source and load converter, including an input filter, is performed. Impedance-based modeling has been used to derive mathematical models of the output impedance of the source converter and the input impedance of the load converter. The correlation between the converter interaction and system stability is analyzed based on the mathematical model. An impedance-based stability analysis is used to determine the system stability by analyzing the interactions among the converters in the DC microgrid system. Middlebrook's stability criterion, which uses the impedance transfer function, is applied to determine system stability. Moreover, in this paper, a stability enhancement control algorithm is proposed to resolve the system instabilities resulting from interaction among the converters and the distortion caused by the harmonics emanating from the AC input. The proposed stability enhancement control algorithm consists of a feed-forward type virtual impedance (VI) and a proportional-resonant (PR) controller. The validity of the proposed method is demonstrated by the results of the response characteristics in the frequency domain, and the effectiveness of the proposed control algorithm is verified via simulations and prototype experimental models.

A modular multilevel converter (MMC) is an emerging converter technology widely used in wind power applications using high-voltage direct current (HVDC) transmission, and it has been largely investigated in medium-voltage solar harvesting as well as electric motor drives. Different from studies dealing with active circulating current control loops, this paper focuses on impedance modeling and harmonic stability studies for MMCs with passive circulating current filters (PCCFs). A power stage circuit including a PCCF is transformed and remodeled to obtain small-signal formulations. This transformation leads to additional high-order matrix computing complexity due to the added impedance subnetwork matrix from the PCCF as well as relevant crossed-frequency impacts. To simplify the computational burden, the proposed model aims to solve one arm equation instead of all six MMC arm equations. To achieve this result, additional challenges occur when the MMC is connected to a renewable energy source instead of the grid voltage, where low-level zero-sequence components might exist in the neutral point of three phases, especially in the case of no integrated advanced modular voltage balancing control, which prevents computational simplification. To overcome this challenge, further impedance matrix adjustments are conducted in this paper to theoretically suppress the impact of zero-sequence harmonics when obtaining small-signal impedance, taking into account the frequency coupling effect. Finally, simulations are carried out to validate the developed impedance model under different operating scenarios. Harmonic stability case studies of MMCs with PCCFs connected to renewable energy current sources are also presented, where frequency-domain stability analyses based on Bode diagrams are compared to time-domain simulation waveforms and FFTs, which validate the effectiveness of the proposed impedance model.

The doubly fed induction generator (DFIG) is the most widely applied wind turbine in practice and contains a dc-link capacitance in its back-to-back converter to deliver the generated power and to provide dc-link voltage for converters. In small-signal analysis, fluctuations in dc-link voltage can be caused by perturbations injected at the ac-terminals of DFIG system and the dc-link voltage fluctuations can affect the outputs of ac-terminals in reverse. It has been found in former publications that this dc-link dynamic behavior is able to shape the small-signal characteristics of DFIG system and influence the system stability. However, the modeling and analysis for the dc-link dynamic behavior is lacked, which makes it difficult to indicate the influencing factors of dc-link dynamics and how to suppress the negative influence of weakly-damped dc-link dynamics on the DFIG system. In this paper, the mechanism of dc-link dynamics and how dc-link dynamics affect the small-signal characteristics of DFIG system are described at first. Then, an indicator function that models the dc-link dynamic behavior is firstly defined and then obtained based on harmonic linearization method. The proposed indicator function will be applied to describe the impedance shaping effect of dc-link dynamics on DFIG system, indicate the influencing factors of dc-link dynamics and analyze the influence of weakly-damped dc-link dynamic behavior. Several experiments based on Control-hardware-in-loop (CHIL) platform will also be carried out to verify the analytical models and theoretical analysis in this paper.

Based on the complex vector theory, a more comprehensive doubly-fed induction generator (DFIG) impedance model under series compensated network, taking both outer loops and phase-locked loop (PLL) dynamic into consideration, is developed in stationary reference frame (αβ-frame). Then the complex vector-based impedance model is combined with the equivalent RLC circuit to intuitively and quantitatively explain the mechanism of sub-synchronous resonance (SSR) and its mitigation methods. The impact of the RSC controllers and series compensation level on the equivalent impedance and the SSR frequency of the DFIG system are analyzed. The mechanism of the two mitigating SSR methods, one is damping control and the other is virtual inductance control, is revealed based on the complex vector impedance model. Finally, the accuracy of the derived model and the theoretical analysis are verified by simulation.

Due to the advantages of superior harmonics attenuation ability and reduced size, the LCL filter has been widely adopted to interface between the inverter and the grid for improving the quality of injected grid currents. However, the high-order characteristics and various constraints of the LCL filter complicate the filter design. Moreover, the stability of the internal current control loop of the individual inverter is susceptible to the inherent LCL -filter resonance peak. Meanwhile, the overall system stability would be aggravated by the external interactions between the inverter and the weak grid as well as among the paralleled inverters. Both the LCL -filter resonance peak and two types of interaction would cause severely distorted grid currents. Motivated by the existing problems, a comprehensive review on the modeling and stability analysis of the LCL -type grid-connected inverters is conducted in this paper. Concretely, the generalized parameter constraints of the LCL filter are outlined to facilitate the passive components selection, and the magnetic integration techniques of filter inductors are also introduced to reduce the weight and size of filter for increasing the power density of the system. Then, the various damping methods for enhancing the individual internal stability and the relevant application issues are also discussed. Furthermore, the impedance-based method for evaluating the system-level interactive stability is subsequently reviewed, with the emphasis on different modeling methods of inverter output impedance and online impedance measurement techniques. Finally, the future research trends on the modeling and stability analysis of LCL -type grid-connected inverters are also presented.

Static synchronous compensator (STATCOM) is able to mitigate the resonance in wind farms by optimizing the controller parameters in a way to shape its impedance. However, doing so may be not always effective due to the limitations in the STATCOM design. Therefore, this paper proposes a STATCOM impedance shaping control scheme to damp the system resonance by adding a voltage-feedforward-based active control path. As the basis, impedance models of the STATCOM considering a general voltage feedforward control are developed. Then, the effect of the STATCOM with the voltage feedforward control on the stability of wind farm systems is analyzed, which leads to the development of the voltage-feedforward-based impedance shaping control. The proposed method can mitigate system resonance, while maintaining the performance of the STATCOM. The effectiveness of the proposed method has been demonstrated on a representative wind farm by simulation.

The stability impacts of the internal control dynamics of the modular multilevel converters (MMCs) have been discussed recently. Yet, the impact of zero-sequence circulating-current (ZSCC) dynamics on the ac-side dynamics of the MMCs is hitherto unaddressed. This article develops the ac impedance model for the grid-connected MMCs by means of complex vectors and harmonic transfer-function matrices, which allows separately characterizing the dynamics of the ZSCC. Then, based on the complex-valued model, a single-input-single-output closed-loop equivalent impedance is derived for grid-connected MMCs, considering both the frequency-coupling dynamics of the MMCs and the interactions with the grid impedance, which enables a design-oriented analysis on the stability impact of the ZSCC. It is revealed that the resonant peaks in the ac impedance of the MMC are yielded due to the absence of the ZSCC control, which tends to destabilize the system in weak grids. A systematic parameter-Tuning method of the ZSCC control loop is developed to guarantee the system stability. Case studies in time-domain simulations corroborate the theoretical analysis.

This paper presents the analysis and mitigation of sub-synchronous resonance (SSR) for doubly fed induction generators (DFIG) under virtual synchronous generator (VSG) control, based on impedance methods. VSGs are considered to have grid-supporting ability and good stability in inductance-based weak grids, and are implemented in renewable power generations, including DFIG systems. However, stability analyses of VSGs for DFIG connecting with series capacitor compensation are absent. Therefore, this paper focuses on the analysis and mitigation of SSR for DFIG under VSG control. Impedance modeling of DFIG systems is used to analyze SSR stability. Based on impedance analysis, the influence of VSG control parameters and the configuration of damping factor of reactive power are discussed. Next, a parameter configuration method to mitigate SSR is proposed. Finally, time-domain simulation and fast fourier transform (FFT) results are given to validate the correctness and effectiveness of the impedance model and parameter configuration methods.

This paper proposes a symmetrical phase-locked loop (PLL) that can eliminate the frequency-coupling terms caused by the asymmetric dynamics of conventional PLLs. In the approach, a concept of complex phase angle vector with both real and imaginary phase components is introduced, which enables to control the direct-and quadrature-Axis components with symmetrical dynamics. The small-signal impedance model that characterizes the dynamic effect of the symmetrical PLL on the current control loop is also derived, which, differing from the conventional multiple-input multiple-output impedance matrix, is in a single-input single-output (SISO) form based on complex transfer functions. This SISO representation allows for a design-oriented analysis. Moreover, the undesired sub-synchronous oscillation caused by the conventional asymmetrical PLL can be avoided, and the classical SISO impedance shaping can be utilized to cancel the negative resistor behavior caused by PLL; thus can greatly enhance the grid synchronization stability under weak grid conditions. The effectiveness of the theoretical analysis is validated by experimental tests.

The virtual synchronous generator (VSG) has been widely studied due to its ability in simulating synchronous generator. From the perspective of output impedance, the harmonic interaction between grid and VSG-based inverters are analyzed in this paper. Firstly, the dynamic response between the outer loop and the inner loop of the VSG is analyzed. It is found that the dynamic response of inner loop is much faster than that of the outer loop. Therefore, the VSG output impedance model neglecting outer loop is established, which reveals that there is still a harmonic resonance risk for the grid-connected inverter using the VSG control strategy. Secondly, considering the influence of the low-order harmonics generated by the dead-band effect on the grid-connected current, this paper establishes an accurate VSG model considering the dead-band effect. Based on the established VSG accurate model, the output impedance of the multi-VSGs cluster is derived. It is found that the output impedance decreases with the increase of the paralleled VSGs, and the dead-band effect has more obvious influence on the grid-connected current. Finally, the correctness of the proposed output impedance model of VSG is verified by simulation.

Subsynchronous oscillation (SSO) is a critical issue for the direct-drive permanent magnet synchronous generator (D-PMSG) based wind farm integrated to a weak onshore AC grid. To analyze the mechanism of the SSO phenomenon of D-PMSG based wind farm, widely used impedance-based stability analysis method is utilized in this paper. First, the impedance model based on the harmonic linearization theory of grid-connected D-PMSG is proposed, and the mechanism of sub/supsynchronous currents coupling is analyzed quantitatively for the first time. Then, based on the impedance model and relative stability criterion, the influence of wind farm operating parameters and grid impedance on stability is discussed. Simulations are carried out to verify the correctness of theoretical analysis.

Harmonic stability of double-fed induction generators (DFIGs) now has become a significant topic because of its harmful impact on power quality issues of the system. Since the double pulse width modulation (PWM) converter is one of the main harmonic sources in DFIGs, it may cause harmonic instability with increasing harmonic contents. Thus, the modeling and stability analyses of PWM converters in DFIGs are essential steps to assess the harmonic stability of DFIGs. Aiming at dual PWM converters, which include the grid side converter (GSC) and the rotor side converter (RSC), this paper divides converters into two parts: circuit modules and control modules. Closed-loop input impedance models of each module are then derived by means of transfer functions. Hence, the stability of the system can be readily predicted through Nyquist diagrams. The contributions of parameters to the system’s harmonic stability are also identified. Finally, time-domain simulations are conducted in a real-time digital simulation (RTDS) system. Simulation results confirm that the established impedance model can effectively reveal the stability of the DFIG-based system and can give critical conditions for the occurrence of harmonic instability.

This paper presents a parallel resonant inductive wireless power transfer system for medical implant applications. The aim of the transcutaneous charging system is to address a larger range of implant depth compared to the current state of the art technology. The impact on amplifier load impedance and with that, -design and -modeling, is shown from an analytical stand-point. The obtained model provides insight into component tolerance impact as well. An objective simulation comparison approach for rectifier topologies further ensures an efficient design. With a 0.5 W transmitter output power, a transferred current of 100 mA is measured over an implant depth of 10 to 50 mm and fitting well both a time- and frequency domain simulation model. The latter enabling complex analyses like class- mathrm{D} amplifier load pull combined with component tuning.

This paper proposes a complex-vector based phase-locked loop (PLL) that can eliminate the frequency-coupling terms caused by the asymmetric dynamics of the conventional PLL. In the approach, a complex phase angle with both real and imaginary components is introduced, which enables to control the direct- and quadrature-axis components symmetrically. As a result, the original multiple-input multiple-output (MIMO) system can be simplified to single-input-single-output (SISO) complex transfer functions, which greatly facilitates the system stability analysis. Moreover, the dangerous subsynchronous oscillation hazard caused by the frequency coupling of the conventional PLL can be avoided, and SISO impedance shaping technique can be utilized to enhance the grid synchronization stability under the weak grid condition. The complex-vector PLL is verified by both the theoretical analysis and experimental tests.

This paper presents a small-signal model for modular multilevel converters (MMCs) by using the harmonic state space (HSS) method. The ac output impedance of the MMC is then derived and compared with that of the commonly used two-level voltage-source converters (TL-VSCs). It turns out that the low frequency ac impedance of the MMC differs from that of the TL-VSC, due to the influences of internal dynamics, especially for the open-loop control and the voltage-mode control. In contrast, for the current-mode control, this difference can be minimized by the current regulator with a high-gain in the low frequency range. Moreover, despite the used control schemes, the high frequency ac impedance of the MMC is similar to that of the TL-VSC. The time-domain simulations are provided to validate the effectiveness of the theoretical analysis.

The electric vehicle fast-charging station is an important guarantee for the popularity of electric vehicle. As the fast-charging piles are voltage source converters, stability issues will occur in the grid-connected fast-charging station. Since the dynamic input admittance of the fast-charging pile and the dynamic output impedance play an important role in the interaction system stability, the station and grid interaction system is regarded as load-side and source-side sub-systems to build the dynamic impedance model. The dynamic input admittance in matrix form is derived from the fast-charging pile current control loop considering the influence of the LC filter. Similarly, the dynamic output impedance can be obtained similarly by considering the regional power grid capacity, transformer capacity, and feed line length. On this basis, a modified forbidden region-based stability criterion is used for the fast-charging station stability analysis. The frequency-domain case studies and time-domain simulations are presented next to show the influence of factors from both the power grid side and fast-charging pile side. The simulation results validated the effectiveness of the dq frame impedance model and the stability analysis method.

The output impedance of a power converter plays an important role in the stability assessment of the converter. The impedance can be expressed in different frames such as the stationary frame (phase domain) or in the synchronous frame ( dq domain). To treat the three-phase system like a single-phase system, the system can be divided into positive and negative sequences in the phase domain. This paper demonstrates that there exist couplings between the positive and negative sequences, even in a balanced system due to the PLL, which is important for synchronization. Further it will be shown that even though these couplings are very small in magnitude, they are important in the stability of the converter.

International audience; Supercapacitors (SCs) impedance modeling represents one important key to get information for SCs health monitoring and simulation. As a matter of fact, SCs behavior is dependent on various parameters such as pore size distribution of electrodes, electrolyte state, temperature, aging, etc. We propose in this study to model SC taking into account physical considerations. Basing on the theory on homogenous structure impedance, we first define the single pore model using the uniform size of pores distribution. In the second step, we attempt to generalize this circuit model considering an electrode with different pores sizes or different penetrabilities of the pores. This model called multipore (MP) model (or multipenetrability model) has been developed by our laboratories. This model allows analyzing the behavior of SC by modeling each group of electrode pores as a branch of an electrical equivalent circuit. This paper is the first in the literature to give a simple and reproducible method for MP model parameters identification. The validity of the identification method will also be verified at different SC aging rates thanks to experimental aging tests.

© 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. Aeronautical ground lighting (AGL) systems assist aircraft pilots in airport operations. These systems are singlephase series circuits where constant current regulators (CCRs) feed luminaires with an adjustable rms current. CCRs are voltage source converters (VSCs) which may cause harmonic resonance instabilities when their control interacts with poorly damped resonances in the 0.75 - 2.5 kHz range. The paper studies these instabilities from the impedance-based representation of AGL systems which are grid-connected VSC systems much simpler than the usual systems analyzed in the literature. This offers the possibility of contributing with a comprehensive stability assessment. Approximate expressions of the CCR and AGL circuit equivalent admittances are given to assess system stability. A comprehensive overview of the influence of AGL system parameters on harmonic resonance instabilities is presented and several recommendations on the AGL system components and CCR parameters are proposed to improve stability. Additionally, an active damping solution based on virtual conductances is analyzed. The expressions of the CCR admittance and some of the recommendations on CCR and AGL system parameters contribute to stability studies of grid-connected VSC systems. Matlab/Simulink and PSCAD/EMTDC simulations of a 68 luminaire AGL system illustrate the contributions of this paper. Field measurements support the stability study.

Electrical lines, also known as wirelines, are commonly used for well intervention techniques to increase flow of oil and gas wells that can be executed during a low service time. Previous studies show the importance of this electrical conductor for power delivery as well as communication purposes. Nevertheless, a validated model and its parameters of this channel are still missing. This papers outlines a geometrical approach to estimate the electrical dynamic behavior for wireline types used in the field with results supported by four different types of validation. The proposed method proves to capture the wireline dynamic behavior characteristics and is particular accurate in modeling the capacitive element within 5 %.