Showing papers on "Electrical impedance published in 2020"
TL;DR: In this article, a passive power management circuit without any active components is designed based on Pulsed-TENG, which has promising applications in the self-powered electronic devices and the Internet of Things.
131 citations
TL;DR: In this article, the same authors used differential voltage analysis (dV/dQ) and alternating current (AC) impedance to investigate battery degradation mechanisms quantitatively, and showed that active cathode loss and loss of lithium inventory (LLI) are the dominating degradation factors.
121 citations
TL;DR: The main contribution of this paper is to detect loosening of the multi-bolt connection through the PZT-enabled active sensing method, which has several merits including easy-to-implement, low cost, and good ability of anti-environment disturbance.
104 citations
TL;DR: A mixed high-order compensation networks-based wireless power transfer system is proposed to generate multiple constant current outputs and current balance can be achieved, which is very suitable for driving multistring LEDs.
Abstract: Current imbalance in multistring light-emitting diodes (LEDs) is a critical issue. It may cause overcurrent in one or more LED strings, leading to rapid degradation. In this paper, a mixed high-order compensation networks-based wireless power transfer system is proposed to generate multiple constant current outputs. It is composed of an LCC resonant network in the transmitting side, a series resonant network, and multiple CLC resonant rectifiers in the receiving side. The CLC resonant rectifiers are connected in parallel to form multiple independent output channels, and each channel is then connected to an LED string. Based on the analysis of the T resonant circuit and the modeling of coupling coils, multiple constant output currents can be derived. As a result, current balance can be achieved, which is very suitable for driving multistring LEDs. The proposed system also offers a modular, scalable, and maintenance-free design, which can significantly reduce the construction cost and the control complexity. In addition, the inverter in the transmitting side can achieve zero phase angle. A laboratory prototype with dual independent output currents is built to verify the proposed method. The experimental results agree well with the theoretical analysis.
94 citations
TL;DR: In this article, a number of three-electrode cell designs have been compared and compared with those obtained from conventional two-terminal impedance complex plane plots, and the results of the separate contributions of anodic and cathodic impedances can be assessed.
76 citations
TL;DR: A new approach is demonstrated that uses multi-frequency impedance measurements to determine the complete intrinsic electrical properties of thousands of single cells at high throughput.
Abstract: Single-cell impedance cytometry is a label-free analysis technique that is now widely used to measure the electrical properties of a cell and to differentiate different subpopulations. Current techniques are limited to measuring the impedance of a single cell at one or two simultaneous frequencies. Also, there are no methods that extrapolate the intrinsic electrical properties of single cells. We demonstrate a new approach that uses multifrequency impedance measurements to determine the complete intrinsic electrical properties of thousands of single cells at high throughput. The applicability of the method is demonstrated by measuring the properties of red blood cells and red cell ghosts, deriving the unique values of conductivity and permittivity of the membrane and cytoplasm for each individual cell.
73 citations
TL;DR: The capability of the Nyquist-based analysis in identifying the system’s weak point and in facilitating better network design and planning is presented and a clarification of three commonly employed impedance-based stability criteria is achieved.
Abstract: Impedance is an intuitive and effective way for dynamical representation of power electronics devices [e.g., voltage source converters (VSCs)]. One of its strengths toward others is the natural association with circuits. However, impedances of VSCs are locally evaluated via linearization, a process dependent on the angle of the reference frame; thus, the reference frame transformation (i.e., rotation) is required before connecting them in circuits for the purpose of network analysis. Although this issue was properly treated in the state-space modeling, a counterpart for the impedance-based analysis, particularly the stability impacts of this rotation, has not been thoroughly discussed and worth being clarified. On the other hand, there are fundamental differences in applying the impedance-based stability criteria of a single-VSC system to an interconnected one. Several restrictions as revealed (e.g., sensitivity to partition points of the Nyquist-based analysis), if not properly considered, may lead to inaccurate stability assessments. In this respect, a clarification of three commonly employed impedance-based stability criteria is achieved. At last, the capability of the Nyquist-based analysis in identifying the system’s weak point and in facilitating better network design and planning is presented. All the models and analyses are verified by frequency-scanning and time-domain simulations in PSCAD/EMTDC.
72 citations
TL;DR: In this paper, a symmetrical phase-locked loop (PLL) is proposed to eliminate the frequency-coupling terms caused by the asymmetric dynamics of conventional PLLs.
Abstract: 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.
70 citations
TL;DR: In this article, a dc impedance model of an MMC is developed by harmonic transfer function method that considers the internal dynamics and typical controls of MMCs, while typical controls consist of dc voltage control, positive-negative sequence separation-based phase current control, circulating current control and some other linear controls.
Abstract: The dc impedance model of a modular multilevel converter (MMC) is the basis for analyzing harmonic resonances of MMC-based dc systems. As an MMC typifies a multiple harmonic response system, its internal dynamics and controls significantly influence its external characteristics. In this paper, a dc impedance model of an MMC is developed by harmonic transfer function method that considers the internal dynamics and typical controls of MMCs. The internal dynamics mainly include capacitor voltage fluctuation and multi-harmonic response characteristics, while typical controls consist of dc voltage control, positive–negative sequence separation-based phase current control, circulating current control, and some other linear controls. As a result, the proposed impedance model can be used not only to analyze the harmonic stability of an MMC-based dc system, but also to investigate the influence of additional controls in an MMC on system stability. Furthermore, the proposed model makes up for the deficiencies in harmonic stability analysis of MMC-based dc systems. The results of both the hardware-in-the-loop RT-LAB digital simulation and the physical experimentation validate the proposed impedance models and analyses.
68 citations
TL;DR: It is determined that a Rogowski coil could be constructed with adequate bandwidth to cover the relevant frequency band of 1 kHz–1 MHz and found that the amplitude of the pulse and the difference in the integrated fast Fourier transform of the current signal are well correlated with the series fault.
Abstract: Series arc faults can cause fire hazards in dc distribution systems. During a series arc fault, the line impedance usually increases rapidly and the arc current includes high-frequency components. To capture the arc-induced high-frequency signals, parallel capacitors are added to the circuit. The characteristics of the currents through these capacitors permit fault detection and localization. This paper determined that a Rogowski coil could be constructed with adequate bandwidth to cover the relevant frequency band of 1 kHz-1 MHz, found that the amplitude of the pulse and the difference in the integrated fast Fourier transform of the current signal are well correlated with the series fault, described experiments to demonstrate that, over the range tested, the two parameters are robust with respect to the pressure of the atmosphere in which the arc forms, the electrode material, the speed at which the electrodes separate to initiate the arc, the switch operation, the load-type and the load change, and demonstrated that within the well-known limits of traveling wave localization, such localization can be used based on the capacitor current pulse information after it has been validated by the spectrum analysis as a fault and not a spurious signal. The proposed approach was demonstrated in two low-voltage dc cable circuits.
64 citations
TL;DR: In this paper, the authors derived the ac-side impedance matrices (IMs) of the MMC with the open-loop and closed-loop voltage control and their relationship is also explicitly given.
Abstract: This paper addresses the small-signal stability of voltage-controlled modular multilevel converters (MMCs) feeding linear ac systems. By using the harmonic state-space (HSS) modeling method, the ac-side impedance matrices (IMs) of the MMC with the open-loop and closed-loop voltage control are derived and their relationship is also explicitly given. It is revealed that the ac voltage regulator has the same effect on the centered diagonal element of the IM of the MMC as that of two-level voltage-source converters (VSCs). Moreover, when the MMC is feeding linear ac loads, the return-ratio matrix of the cascaded system has an eigenvalue that is equal to the ratio between the centered diagonal element of the IM of the MMC and the load impedance, which, consequently, facilitates the stability evaluation by checking that single-input single-output impedance ratio as a necessary condition. These findings also provide physical insights into the subsynchronous oscillation (SSO) of the voltage-controlled MMC with the proportional-resonant (PR) regulator, and a proportional-integral-resonant (PIR) regulator is further introduced to mitigate the SSO. Finally, time-domain simulations verify the effectiveness of the theoretical analysis.
TL;DR: In this article, an ultrawideband and single-layer circuit analog absorber with a low profile and small unit size is proposed, which is composed of a Rogers 4003 dielectric with two square loop arrays, respectively, printed on the top and bottom, and a metal ground below.
Abstract: In this letter, an ultrawideband and single-layer circuit analog absorber with a low profile and small unit size is proposed. The absorber is composed of a Rogers 4003 dielectric with two square loop arrays, respectively, printed on the top and bottom, and a metal ground below. Lumped resistors are embedded on the four edges of the square-loop arrays to introduce resistance loss. Equivalent circuit, input impedance, and current distribution are then presented to provide great insight into the existence of three resonances and wideband. The absorber offers a fractional absorption bandwidth of 132.3% (4.91:1) with at least 10 dB reflection reduction. The thickness and the unit size are, respectively, reduced to $0.075\lambda _{L}$ (wavelength at the lowest frequency) and $0.13\lambda _{L}$ , leading to a good performance under oblique incidence. A good agreement between the calculated, simulated, and measured results validates the proposed design.
TL;DR: The proposed model overcomes the deficiencies in harmonic resonance analysis of MMC-HVdc based offshore wind power integration system and can be used to reveal the coupling between offshore offshore ac system and dc system.
Abstract: The offshore ac side impedance model of the modular multilevel converter (MMC) based high voltage direct current (HVdc) system is essential for analyzing the interaction stability between MMC-HVdc and the offshore wind power plants. This paper develops the offshore ac side impedance model of an MMC-HVdc system for wind power integration taking the effects of offshore station, dc cable and onshore station into consideration. The dc impedance model of the onshore station is first derived which includes the effects of dual closed loop dc bus voltage control and circulating current control. Then, the dc impedance of the onshore station and dc cable impedance are used to derive the ac side impedance of the offshore station under open loop control. In addition, the influence of the dual closed loop based ac bus voltage and frequency (VF) control on the offshore ac impedance is analytically derived. As a result, the proposed model can be used to reveal the coupling between offshore offshore ac system and dc system as well as to investigate the influence of the dc system and VF controller in the offshore ac impedance of the MMC-HVdc system. Furthermore, the proposed model overcomes the deficiencies in harmonic resonance analysis of MMC-HVdc based offshore wind power integration system. The results of the simulation in PSCAD/EMTDC validate the proposed models and analyses.
TL;DR: In this article, a harmonic-domain single-input single-output (SISO) equivalent modeling technique for the impedance modeling and stability analysis of a single-phase grid-connected voltage-source converter (VSC) is presented.
Abstract: This article presents a harmonic-domain single-input single-output (SISO) equivalent modeling technique for the impedance modeling and stability analysis of a single-phase grid-connected voltage-source converter (VSC). The basis is a conversion technique that transforms a harmonic transfer function (HTF)-based model into a SISO equivalent model while preserving all the information of frequency couplings. The proposed SISO modeling concept is useful for understanding the meaning and consequence of SISO impedance measurement of an interconnected system with frequency couplings, which further enables a simpler impedance measurement and impedance-based analysis. Applications of this method for the VSC model reduction and stability characteristic analyses are presented. From these results, useful conclusions regarding the accuracy of three types of reduced-order VSC impedance models and the stability effects of the VSC control with and without compensation for dc voltage variation are obtained. The presented examples of applications demonstrate how the proposed SISO modeling technique facilitates a simpler and efficient impedance-based analysis. Finally, experimental results verify the validity of the proposed VSC-SISO admittance and corresponding analyses.
TL;DR: A self-adjusting nominal voltage based modification in the $Q-V$ droop control has been proposed to improve the reactive power sharing amongst distributed generation sources in an islanded microgrid by coupling the nominal voltage calculation with the system frequency.
Abstract: In this paper, a self-adjusting nominal voltage based modification in the $Q-V$ droop control has been proposed to improve the reactive power sharing amongst distributed generation sources in an islanded microgrid. This is achieved by coupling the nominal voltage calculation with the system frequency, which has an effect of driving the voltage references of all the distributed generators in such a manner so as to improve the network-wide reactive power sharing. An appreciable improvement in reactive power sharing is achieved by using adaptive nominal voltage as compared to fixed nominal voltage. The proposed method neither depends on data exchange among sources nor does it need feeder impedance information. In addition, it is able to improve the sharing in cases of loads located locally or at a distance. The improvement in reactive power sharing does not compromise on voltage magnitude. The viability of the controller has been confirmed from detailed simulation studies and experimental validation on a laboratory prototype.
TL;DR: A controlled wide-bandwidth impedance measurement approach is presented in this paper, in which, a butterfly-type disturbance circuit and chirp pulsewidth modulation signal model are incorporated to generate the desired controlled-band width harmonics with a high aggregation as well as the average amplitude.
Abstract: The accurate information of the wide-bandwidth impedance versus the frequency is urgently needed for evaluating the system resonances, instabilities, and operations of the railway traction power system (TPS), and to avoid/control the harmonic resonance and oscillation issues. As the system topology and detailed parameters of the TPS are not fully known even timely varying, we have to obtain the detailed wide-bandwidth impedance information through exciting the harmonic disturbance into the system, and then, calculating the response information. Therefore, a controlled wide-bandwidth impedance measurement approach is presented in this paper, in which, a butterfly-type disturbance circuit and chirp pulsewidth modulation signal model are incorporated to generate the desired controlled-bandwidth harmonics with a high aggregation as well as the average amplitude. Impedance measurement results of the proposed approach have been validated through both simulation and experiment. Considering the measured errors, the proposed method is efficient in testing the wide-bandwidth impedance of the single-phase railway traction system.
TL;DR: The results indicated that the developed system can distinguish the objects and reconstruct medium distribution with high image contrast and frequency resolution and show advantages in imaging the local electrical properties with high-frequency resolution and obtaining the FD images in the sensitive BIS bandwidth.
Abstract: A wideband electrical impedance tomography (EIT) system based on bioimpedance spectrum (BIS) analysis was proposed to image biological objects. The system integrated 16 electrodes to realize driving and measurement over 1 kHz-1.1 MHz. A modified Howland current source and voltage measurement channels were designed to obtain complex impedance. Peripheral component interconnect interface-based serial data acquisition module transferred the measurement data to a host PC. The system has the least signal-to-noise ratio of 40 dB in each channel and good consistency. The system utilized a two-step strategy, based on short-time Fourier transform and Hilbert transform, the wideband chirp excitation signal was employed to analyze the BIS first, from which the sensitive bandwidth, i.e., the frequency range in which the amplitude (and phase angle) of the object changes rapidly, is obtained. Then, the discrete frequency points among the sensitive bandwidth were selected to compose a multisinusoidal signal for impedance tomography with time-difference and frequency-difference (FD) methods. An impedance model was tested to evaluate the performance of BIS analysis. Experiments on phantoms consisting of carrot cylinder and nylon rod were designed to verify the working strategy of the system. The results indicated that the developed system can distinguish the objects and reconstruct medium distribution with high image contrast and frequency resolution. Compared with the traditional multifrequency EIT, the proposed method and system show advantages in imaging the local electrical properties with high-frequency resolution and obtaining the FD images in the sensitive BIS bandwidth.
TL;DR: The constant phase element (CPE) is introduced to the traditional time domain circuit model by analyzing the electrochemical impedance spectra of lithium-ion batteries and the polarization resistance is replaced by Butler-Volmer (BV) equation.
Abstract: The lithium-ion batteries in the electric vehicles are nonlinear systems with complex electrochemical dynamics, and estimation of battery state-of-charge (SOC) is affected by factors such as environmental temperature and battery current. Considering the above problems, the accurate estimation of battery SOC has always been a difficult and the critical issue of battery management system (BMS). In this paper, the constant phase element (CPE) is introduced to the traditional time domain circuit model by analyzing the electrochemical impedance spectra of lithium-ion batteries. Accordingly, an equivalent circuit model based on electrochemical impedance is constructed by using fractional order theory, which has specific physical significant, leading to the improved estimation accuracy to represent battery voltage. Moreover, the polarization resistance in the model is replaced by Butler-Volmer (BV) equation, which can solve the problem caused by large current and temperature variation during the actual operation of electric vehicles. Next, based on the model, an $\text{H}\infty $ observer is designed for battery SOC estimation, and the proposed SOC observer is tested by real-time experimental data of battery. The efficiency of the proposed model and observer are validated by some simulations and experiment tests.
TL;DR: The model analysis results demonstrate that the MMC impedance is coupled with the grid impedance due to the internal harmonics, and the proposed model not only consists of the ac current and the circulating current control, but also incorporates dc voltage outer loop and PLL with a clear physical meaning.
Abstract: Harmonic state space (HSS) is seen as an effective impedance modeling method to precisely characterize the internal harmonic features of the modular multilevel converter (MMC). However, the existing MMC impedance models assume the ideal grid, ignoring the grid impedance, and they also do not incorporate the widely used dual-loop control and phase-locked loop (PLL). In this article, a complete MMC impedance model based on HSS is proposed to reveal the grid impedance coupling effect of MMC. The model analysis results demonstrate that the MMC impedance is coupled with the grid impedance due to the internal harmonics. This coupling causes MMC to be affected by the grid impedance and may cause instability. On the other hand, the proposed model not only consists of the ac current and the circulating current control, but also incorporates dc voltage outer loop and PLL with a clear physical meaning. Based on the proposed model, this article illustrates the factors that will enhance the coupling, which shows that the proposed model has to be used to improve the accuracy of the analysis. Finally, effectiveness of the proposed model is verified by simulation and experimental results.
TL;DR: In this article, a modified mutual inductance circuit model of an underwater WPT system was proposed to analyze the eddy current loss (ECL) and the detuning effect caused by the seawater.
Abstract: Due to the conductivity of the seawater, the traditional mutual inductance circuit model in the air cannot be used directly to describe wireless power transfer (WPT) systems in seawater applications. This paper proposes a modified mutual inductance circuit model of an underwater WPT system to analyze the eddy current loss (ECL) and the detuning effect caused by the seawater. The time-harmonic electromagnetic field in the seawater and the air near the coil that carries a sinusoidal alternating current is analyzed. The root-mean-square (rms) value and phase angle of the induced voltage on the secondary coil can be obtained by the integral of the electric field intensity along the coil path. By introducing the equivalent ECL impedance at both the primary and secondary sides, a modified mutual inductance circuit model of an underwater WPT system was obtained. Through adding a compensation inductance to the primary circuit, the detuned system in the seawater is turned back to be resonant at the same frequency as in the air. A seawater WPT prototype was built and the experimental results verified the theoretical analysis.
TL;DR: A nonlinear disturbance observer (NDO)-based sliding-mode control algorithm has excellent robustness, low computational burden, and no extra hardware cost and a generalized average state-space model is proposed to facilitate the control design.
Abstract: The interleave boost converter (IBC) has been used as the interface converter between the low-voltage electrical energy sources, such as lithium-ion battery banks, solar panels, and fuel cells, and the dc bus of dc microgrids. With increasing penetration of tightly regulated power electronic loads, which behave as constant power loads, the stability of microgrid dc-bus voltage that is fed by the IBC is threatened by the loads’ negative incremental impedance feature. To ensure the stability of the bus voltage, this article proposes a nonlinear disturbance observer (NDO)-based sliding-mode control algorithm. The proposed algorithm has excellent robustness, low computational burden, and no extra hardware cost. A generalized average state-space model is proposed to facilitate the control design. In addition, an NDO is employed to estimate the output power of IBC rapidly and accurately. To verify the effectiveness of the proposed algorithm, simulation and experimental results are presented.
TL;DR: This article proposes a novel MET control method of the WPT system with optimized T-type impedance matching network under the real-time identification of a coupling coefficient to realize the MET control.
Abstract: Transfer efficiency (TE) is an important research topic in the practical application of a wireless power transfer (WPT) system. WPT system is a loosely coupled system; the coupling coefficient changes drastically due to the relative movement of the transmitting and receiving sides. It is difficult to achieve maximum efficiency tracking (MET) only from the research of frequency tracking, impedance matching, and system parameters under a fixed coupling coefficient. This article proposes a novel MET control method of the WPT system with optimized T-type impedance matching network under the real-time identification of a coupling coefficient to realize the MET control. This method synthetically analyzes the nonlinearity of the rectifier circuit, the identification of coupling coefficient, the variation of load resistor, and the controllability of the output voltage, involving the main factors of the MET control. The simulation and experimental results show that the WPT system has a good performance on MET and constant output voltage control; the validity and accuracy of the proposed MET control method are consistently verified.
TL;DR: In this paper, the Pearson correlation coefficient (PCC) was used to detect the correlation characteristics of current derivatives of a long-distance LCC-HVDC transmission line to detect internal and external faults.
TL;DR: In this paper, Pearson's correlation coefficient (r) method was used to analyze the correlation between the phase angles in different frequencies and the impedance at low frequency end (|Z|0.90-0.95) for epoxy coatings.
10 Jun 2020
TL;DR: A virtual impedance control is proposed to eliminate the negative output resistance till half of the sampling frequency, which improves the system stability for grid-forming converters in grid-connected applications.
Abstract: This article presents a systematic evaluation on the impedance passivity of voltage-controlled voltage-source converters. The commonly used single- and dual-loop control structures with different linear controllers are compared extensively, considering the effect of the time delay involved in the control loop. A virtual impedance control, co-designed with different voltage control schemes, is then proposed to eliminate the negative output resistance till half of the sampling frequency, which improves the system stability for grid-forming converters in grid-connected applications. Both frequency-domain analysis and experimental results validate the theoretical findings.
TL;DR: The proposed fault-line-impedance calculation method is immune to the fault resistance and insensitive to power angle and fault location variations, and can work well under various types of faults, e.g., AG, BC, BCG, and ABC.
Abstract: Performance of distance protection based on the traditional fault-line impedance calculation method is adversely affected by fault resistance, which may cause distance protection to operate incorrectly, threatening the safe operation of power system. To improve immunity to the fault resistance, this paper presents a novel fault-line-impedance calculation method. The fault-line impedance, supplementary impedance, and measured impedance are rotated simultaneously in the complex plane until the supplementary impedance coincides with the positive direction of the real axis. Following the geometrical relationship between the rotated fault-line impedance and the rotated measured impedance in the complex plane, the fault distance and fault-line impedance are solved. The proposed method is immune to the fault resistance and insensitive to power angle and fault location variations. Furthermore, the proposed method can work well under various types of faults, e.g., AG, BC, BCG, and ABC. The simulation results show that the proposed method can calculate the actual fault distance accurately and identify in-zone and out-of-zone faults correctly.
TL;DR: Considering both the dynamic characteristics and stability of the dc system, a specific design method of controller parameter optimization and additional virtual damping controller is proposed and a detailed time-domain simulation model of MMC-based dc grid is established to prove the accuracy of the proposed impedance modeling method and the validity of the control strategy.
Abstract: This article proposes an accurate dc-side impedance modeling method of modular multilevel converter (MMC) and a control strategy for improving the stability of the dc system in MMC-based dc grid. First, the impedance modeling method based on harmonic linearization is adopted to establish the dc-side small-signal impedance model of MMC, which considers the internal multiharmonic coupling characteristics and the complete control system. Second, an equivalent model is established to analyze the frequency impedance characteristics of the dc ports in MMC-based dc grid, and the stability of dc ports is further analyzed based on the impedance-based stability criterion. Then, considering both the dynamic characteristics and stability of the dc system, a specific design method of controller parameter optimization and additional virtual damping controller is proposed. Based on MATLAB/Simulink, a detailed time-domain simulation model of MMC-based dc grid is established. The simulation results prove the accuracy of the proposed impedance modeling method and the validity of the proposed control strategy.
TL;DR: An online electrochemical impe-dance spectroscopy (EIS) technique for a Li-Ion battery using a frequency sweeping method and a precise impedance measurement algorithm that fulfills the precision level required for industrial Li- Ion battery is presented.
Abstract: This article presents an online electrochemical impe-dance spectroscopy (EIS) technique for a Li-Ion battery. A frequency sweeping method and a precise impedance measurement algorithm are used for the proposed online EIS technique. The proposed algorithm consists of fast Fourier transformations, peak amplitudes, and phase difference estimations. Additional instrumentation with offset clipping and amplifying circuits expand the accuracy of the signal measurement. The offset clipping circuit increases the ripple to measurement voltage ratio without phase delay. The amplifying circuit gives more resolution for the ripple voltage. The proposed instrumentation ensures the precise measurement of ripple voltage and current. The proposed impedance measurement algorithm fulfills the precision level required for industrial Li-Ion battery. To validate the proposed approach, a 40-Ah 13.6 V Li-Ion battery charger was built using a synchronous buck converter and an Opal-RT based controller. The proposed spectroscopy technique will be useful to track and estimate internal electrochemical conditions of a battery.
TL;DR: This work demonstrates that frequency-domain method can be used for oscillator-based converters in the high-frequency range, specifically, for harmonic mitigation in the converter output current, and proposes a virtual impedance-based selective harmonic current suppression method.
Abstract: Virtual oscillator control (VOC) is a nonlinear grid-forming controller that simultaneously achieves the functionality of output voltage control and primary control layers in a power electronics interfaced distributed generation (DG) unit. Unlike conventional phasor-based primary control methods such as the droop control and virtual synchronous machine control, VOC is a time-domain controller that can guarantee almost global asymptotic synchronization. However, the high-frequency dynamics has largely been ignored in the analysis of VOC in prior art; as a result, VOC-based DGs fail to suppress harmonic current in the presence of harmonic distortion in the network-side voltage. In this work, we demonstrate that frequency-domain method can be used for oscillator-based converters in the high-frequency range, specifically, for harmonic mitigation in the converter output current. We propose a virtual impedance-based selective harmonic current suppression method, and demonstrate that it is better to use the network-side current feedback rather than that of the converter-side current for VOC implementation with virtual impedance control. Established harmonic rejection strategies for grid-following and conventional grid-forming converters are compared with the proposed method for VOC. Through impedance-based analysis, we demonstrate that the proposed method augments the passivity range of the converter terminal response with a much simpler implementation. The proposed harmonic suppression strategy is validated through hardware experiments using a single-phase inverter prototype.
TL;DR: Comparison between the electrical properties of different devices should be made using high frequency impedance measurements performed in the steady-state voltage regime in which the cell is expected to operate, and it is shown that the high frequency response contains all the key information relating to the Steady-state performance of a PSC.
Abstract: Interpreting the impedance response of perovskite solar cells (PSCs) is significantly more challenging than for most other photovoltaics. This is for a variety of reasons, of which the most significant are the mixed ionic-electronic conduction properties of metal halide perovskites and the difficulty in fabricating stable, and reproducible, devices. Experimental studies, conducted on a variety of PSCs, produce a variety of impedance spectra shapes. However, they all possess common features, the most noteworthy of which is that they have at least two features, at high and low frequency, with different characteristic responses to temperature, illumination and electrical bias. The impedance response has commonly been analyzed in terms of sophisticated equivalent circuits that can be hard to relate to the underlying physics and which complicates the extraction of efficiency-determining parameters. In this paper we show that, by a combination of experiment and drift-diffusion (DD) modelling of the ion and charge carrier transport and recombination within the cell, the main features of common impedance spectra are well reproduced by the DD simulation. Based on this comparison, we show that the high frequency response contains all the key information relating to the steady-state performance of a PSC, i.e. it is a signature of the recombination mechanisms and provides a measure of charge collection efficiency. Moreover, steady-state performance is significantly affected by the distribution of mobile ionic charge within the perovskite layer. Comparison between the electrical properties of different devices should therefore be made using high frequency impedance measurements performed in the steady-state voltage regime in which the cell is expected to operate.