System Impedance Research Articles

Impedance Modeling and Analysis of MMC-HVDC for Offshore Wind Farm Integration

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.

A New Filtering Scheme for HVDC Terminals Based on Damped High-Pass Filter

This paper presents the design and application of a recently developed damped high-pass (DHP) filter for the line-commuted converter HVDC terminals. The DHP filter can damp resonance at the noncharacteristic harmonic frequencies commonly encountered by multipulse HVDC links. A design method is developed to address two unique issues faced by HVDC filter design: a wide range of system impedances seen by a HVDC terminal and a variable reactive compensation configuration at the terminal. Both are challenging filter design problems even for traditional filter banks. Using realistic industry case and sensitivity studies, this paper has shown that the proposed filtering scheme and design method can result in saving in filter costs and space requirements without sacrificing performance in comparison with the common filtering schemes used at HVDC terminals. In addition, the proposed design concepts are also applicable to the design of other filters facing the same design issues.

Robust subsynchronous interaction damping controller for DFIG-based wind farms

This paper proposes a robust controller to improve power system stability and mitigate subsynchronous interaction (SSI) between doubly-fed induction generator (DFIG)-based wind farms and series compensated transmission lines. A robust stability analysis is first carried out to show the impact of uncertainties on the SSI phenomenon. The uncertainties are mainly due to the changes in the power system impedance (e.g., transmission line outages) and the variations of wind farm operating conditions. Then, using the µ-synthesis technique, a robust SSI damping controller is designed and augmented to the DFIG control system to effectively damp the SSI oscillations. The output signals of the supplementary controller are dynamically limited to avoid saturating the converters and to provide DFIG with the desired fault-ride-through (FRT) operation during power system faults. The proposed controller is designed for a realistic test system with multiple series capacitor compensated lines. The frequency of the unstable SSI mode varies over a wide range due to the changes in power system topologies and wind farm operating conditions. The performance of the proposed controller is verified through electromagnetic transient (EMT) simulations using a detailed wind farm model. Simulation results also confirm the grid compliant operation of the DFIG.

Impedance matching control design for the benchmark problem in real-time hybrid simulation

Abstract This paper presents an application of impedance matching to the benchmark control problem in real-time hybrid simulation (RTHS). Impedance matching is conceptually different from conventional approaches to designing controllers for RTHS. Rather than view the transfer system merely as a device to realize prescribed boundary conditions between the virtual and physical substructures, the controller is designed to match the impedance of transfer system as closely as possible to that of the virtual substructure. Some of the key features of impedance matching are—(i) it does not explicitly require a tracking controller, greatly simplifying the control design process, (ii) control design is decoupled from the physical substructure (demonstrated in this paper by introducing nonlinearity in the physical substructure), (iii) the controller is easy to evaluate and implement, (iv) performance is less sensitive to the choice of partitioning configuration compared to provided sample controller, and (v) exhibits robust stability. Overall, controllers designed based on impedance matching are found to result in stable and accurate RTHS.

An Adaptive Threshold for Robust System Impedance Estimation

The supply system impedances seen at a load bus is an important parameter for power system planning and operations. An existing method to measure the system impedance is to utilize the natural fluctuation of the load in the downstream. However, its performance may not be satisfactory when the system side has certain variations. To address this issue, this letter proposes an adaptive threshold to improve the accuracy of system impedance estimation. The performance of the proposed method has been verified using simulation studies and field data.

Analytical model for the equivalent impedances of the domestic induction heating system with rectangular cross‐sectional windings

This study proposes an analytical model to evaluate the equivalent impedances of domestic induction heating systems. The method of separation of variables is employed to determine the vector potential produced by a filamentary coil under a semi-infinite conductor. Then, based on the superposition principle of linear problems, the total vector potential of the coaxial, coplanar, connected-in-series rectangular cross-sectional multi-windings with same thicknesses, which are made of enamelled Litz wire, is derived by integrating the vector potential of filaments. Finally, the total voltage on the windings and the equivalent impedance of the induction heating system are obtained. The proposed analytical method is flexible to be used for calculating the equivalent impedance of multi-windings with any number of windings in an induction heating system with any mixture of windings with different lengths in the radial direction, and any distance between two adjacent windings in the radial direction. The proposed generally analytical method is successfully verified by comparison with the finite element method and shows improvement with respect to the conventional analytical method.

Electrical analysis and ultra-fast sequential imaging of surface barrier discharge with streamer-leader sequence generated with 100 kHz frequency at the water interface

A time resolved study of the electrical parameters and ultra-fast 2D imaging of the surface barrier discharge emerging from liquid electrodes in atmospheric pressure argon is reported. Analysing the electrical measurements and the charge-voltage (Q-V) plots, a resistive component of the electrical system impedance is revealed and described by the logical progression of the method to current-voltage (I–V) plots. The necessity to include a resistive component in the simplest equivalent circuit is demonstrated. Net discharge current, charge or effective gap voltage are linked to the light emission. A special nanosecond-gated camera enabling multiple expositions within a few hundreds of nanoseconds is applied and reveals the spatiotemporal development of the discharge luminosity. Propagation of the streamer-leader stepping sequence on the dielectric surface is observed and the mean velocities and axial light-emission development is quantified. A light emission of an excited gas prior to and after the discharge’s main current peak is detected revealing an increased activity between the subsequent discharges. This is caused by the high pre-ionisation of the gas volume and the intensive charging of the surface. The generation of subsequent streamers emerging from/between the surface charge domains is evidenced.

Sensorless Unbalance Modeling and Estimation as an Ancillary Service for LV Four-Wire/Three-Phase Power Converters

This paper describes a method to provide low-voltage four-wire three-phase power converters with the capability of unbalance estimation as an ancillary service to the main role that they play in the distribution system (distributed generator, energy storage system, drive, etc.). Typically, dedicated grid/load current sensors are needed to effectively comply with unbalance compensation tasks, increasing system cost and reducing reliability. This is due to the difficulties that arise in the extraction of the zero- and negative-voltage components from the voltages at the point of common coupling, such as the inadequate resolution of full-scaled voltage sensors and limited spectral separation. In this paper, the proposed method does not rely on additional sensors to those typically used in voltage-source converters, and in any case, those sensors are limited to the point of connection of the power converter. Impedance estimation only using converter-side current sensors is implemented by adding a high-frequency voltage excitation over the fundamental command. A new model approach is proposed for the real-time extraction of system impedance using a complex-valued compact form. Considering the voltage source at that frequency to be unique in the grid, it will be proved that the impedance and, thus, the resulting negative-sequence current, which is used for unbalance compensation, can be estimated. For the zero sequence, a special arrangement of the converter voltage sensors together with a repetitive controller is used.

Determination of the Impedance Parameters of Antennas and the Maximum Power Transfer Efficiency of Wireless Power Transfer

In this paper, we derive formulas that calculate the impedance parameters (Z-parameters) of multiple antennas near objects to analyze wireless power transfer. Furthermore, we propose a method to calculate the impedance parameters of antennas, the maximum power transfer efficiency, and the optimum load impedance of a wireless power transfer system using equivalent current that generate the same electromagnetic field as that generated by an antenna in transmitting mode.

A 2nd-Order Model of the Vertical Impedance of the Locomotor System During Human Walking

Objective: To design better shock absorbing prosthetic components, walking machines, or bionic legs, values and properties of the mechanical impedance of the locomotor system of able-bodied ambulators need to be determined. Methods: The representation of a simple 2nd-order shock absorption component (i.e., effective body mass, $M_{e}$ , supported by a stiffness, ${k}$ , and damping, ${B}$ , in parallel) was incorporated into a previously proposed rocker-based inverted pendulum model of walking to more accurately model the impedance associated with human walking, including the effects of shock absorption. Using system identification techniques and quantitative gait analysis data from able-bodied subjects ( ${n}\,\,=$ 7), the properties and values of the mechanical impedance for each subject’s trial were determined. Results: It was determined that stiffness, ${k}$ , increased linearly with walking speed while damping, ${B}$ , increased with the square root of walking speed. Furthermore, the locomotor system behaves like an optimal underdamped mechanical system (i.e., damping ratio $\zeta $ of approximately 0.5–0.7). Conclusion: The locomotor system seems to behave like an optimal and adaptive low-pass filter in which the cutoff frequency is adjusted based on the locomotor’s cadence. It could be hypothesized that this occurs due to the principle of conservation of the passenger’s well-being: the fact that one of the locomotor’s functional responsibilities is to prevent higher frequency vibrations that are harmful for the musculoskeletal system of the passenger. Significance: This is a holistic, simplified, and accurate model of the vertical leg mechanical impedance (locomotor system) that establishes parameters during walking over a range of speeds. These results can be used for the scientific design of better shock absorption prosthetic components and lower-limb prostheses, management of shock absorption related gait pathologies, or the design of human-like walking machines and rehabilitation robots.

A comprehensive analysis of the effect of frequency-dependent soil electrical parameters on the lightning response of wind-turbine grounding systems

The impact of frequency-dependent soil electrical parameters on the lightning response of typical wind turbine grounding systems is investigated. It is shown that the frequency dependence of soil parameters is responsible for decreasing the ground potential rise and, thus, the impulse impedance and impulse coefficient of the grounding systems. This effect is more pronounced for high-resistivity soils and fast current pulses. It is also shown that, considering typical dimensions of actual wind turbine grounding systems and soils with resistivity higher than 300Ωm, the impulse impedance seen by each single wind turbine is expected to be lower than the low-frequency grounding resistance.

Energy-Density Improvement in Li-Ion Rechargeable Batteries Based on LiCoO2 + LiV3O8 and Graphite + Li-Metal Hybrid Electrodes

We developed a novel battery system consisting of a hybrid (LiCoO2 + LiV3O8) cathode in a cell with a hybrid (graphite + Li-metal) anode and compared it with currently used systems. The hybrid cathode was synthesized using various ratios of LiCoO2:LiV3O8, where the 80:20 wt% ratio yielded the best electrochemical performance. The graphite and Li-metal hybrid anode, the composition of which was calculated based on the amount of non-lithiated cathode material (LiV3O8), was used to synthesize a full cell. With the addition of LiV3O8, the discharge capacity of the LiCoO2 + LiV3O8 hybrid cathode increased from 142.03 to 182.88 mA h g−1 (a 28.76% improvement). The energy density of this cathode also increased significantly, from 545.96 to 629.24 W h kg−1 (a 15.21% improvement). The LiCoO2 + LiV3O8 hybrid cathode was characterized through X-ray diffraction analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Its electrochemical performance was analyzed using a battery-testing system and electrochemical impedance spectroscopy. We expect that optimized synthesis conditions will enable the development of a novel battery system with an increase in energy density and discharge capacity.

Frequency‐coupled impedance model based subsynchronous oscillation analysis for direct‐drive wind turbines connected to a weak AC power system

Recently, the emerging subsynchronous oscillation (SSO) issue caused by interactions between power electronic converters and weak grids has provoked serious stability concerns. Impedance modelling based method has been widely used to analyse the emerging SSO phenomenon. In this paper, a frequency coupled impedance model (FCIM) is proposed to analyse this interaction. The FCIMs at multiple frequencies are measured and then the transfer function model is identified. The modelling method is then applied to type-4 wind turbine based farms connected to a weak AC grid. The aggregated impedance of the whole system is obtained by combining the FCIMs of all components in the power system. The system stability is then investigated by varying the online number of wind turbines. Time-domain simulations using PSCAD/EMTDC are carried out to verify the proposed methods of impedance modelling and stability analysis.

Maximum Efficiency Point Tracking for Multiple-Transmitter Wireless Power Transfer

We propose a maximum efficiency tracking for a multiple-transmitter system that optimizes both the current ratios of transmitter (Tx) coils and the effective Rx load. It is found that unlike the single-Tx case, the optimal load impedance of multiple-Tx system depends on the current ratios among Tx coils. Meanwhile, the Tx current ratios should be controlled to be the same with magnetic coupling ratios between each Tx and Rx. Hence, we propose a two-step tracking method that first optimizes the Tx current ratios and then optimizes the Rx load impedance. To match the Tx current ratio with the magnetic coupling ratio, it is analyzed that the reflected resistances at every Tx become identical to each other when the Tx coil current ratios are equal to the magnetic coupling ratios. It is also revealed that the reflected resistance at specific Tx is reduced (increased) when its Tx current is increased (reduced) relative to other Tx currents. The proposed feedback equalizes all reflected resistances based on this property. The second step of the proposed tracking is to optimize the Rx load while fixing the Tx current ratios by globally scaling up or down the strengths of whole Tx currents. Maximum of 478 W is delivered at 79% efficiency. The algorithm does not have a limit on the number of Txs except the small standby power consumption. The proposed theoretic analysis and control method enable the first-ever maximum efficiency tracking for multiple-Tx system, where the first step optimizes the relative Tx current ratios and the second step optimizes the load impedance.

Evaluation of fluctuating voltage topology with fuel cells and supercapacitors for automotive applications

To develop a single-stage power conversion topology in which energy storage devices can be directly coupled, a fluctuating voltage topology is applied, leading to lower cost and more compactness with the absence of DC/DC converters. This paper investigates such a topology for automotive applications where fuel cells are directly connected to the DC bus of the inverter, resulting in fluctuating voltage across the DC bus. Further, a supercapacitor pack is also introduced to maintain the power capacity and voltage stability. The hybridization principle and practical application of such a topology are then discussed in the time domain and frequency domain. Furthermore, the transient power requirement is decomposed to design the size of fuel cells and supercapacitors. Simulation results from the modeling of the fuel cell-supercapacitor powertrain demonstrate the feasibility and effectiveness of this topology. The supercapacitors can serve as a low-pass filter for the fuel cells. In conclusion, the peak power requirement can be successfully achieved because of the lowered system impedance, and the fuel cells only need to supply the average power.

Different contributions from lungs and chest wall to respiratory mechanics in mice, rats, and rabbits.

Changes in lung mechanics are frequently inferred from intact-chest measures of total respiratory system mechanics without consideration of the chest wall contribution. The participation of lungs and chest wall in respiratory mechanics has not been evaluated systematically in small animals commonly used in respiratory research. Thus, we compared these contributions in intact-chest mice, rats, and rabbits and further characterized the influence of positive end-expiratory pressure (PEEP). Forced oscillation technique was applied to anesthetized mechanically ventilated healthy animals to obtain total respiratory system impedance (Zrs) at 0, 3, and 6 cmH2O PEEP levels. Esophageal pressure was measured by a catheter-tip micromanometer to separate Zrs into pulmonary (ZL) and chest wall (Zcw) components. A model containing a frequency-independent Newtonian resistance (RN), inertance, and a constant-phase tissue damping (G) and elastance (H) was fitted to Zrs, ZL, and Zcw spectra. The contribution of Zcw to RN was negligible in all species and PEEP levels studied. However, the participation of Zcw in G and H was significant in all species and increased significantly with increasing PEEP and animal size (rabbit > rat > mice). Even in mice, the chest wall contribution to G and H was still considerable, reaching 47.0 ± 4.0(SE)% and 32.9 ± 5.9% for G and H, respectively. These findings demonstrate that airway parameters can be assessed from respiratory system mechanical measurements. However, the contribution from the chest wall should be considered when intact-chest measurements are used to estimate lung parenchymal mechanics in small laboratory models (even in mice), particularly at elevated PEEP levels. NEW & NOTEWORTHY In species commonly used in respiratory research (rabbits, rats, mice), esophageal pressure-based estimates revealed negligible contribution from the chest wall to the Newtonian resistance. Conversely, chest wall participation in the viscoelastic tissue mechanical parameters increased with body size (rabbit > rat > mice) and positive end-expiratory pressure, with contribution varying between 30 and 50%, even in mice. These findings demonstrate the potential biasing effects of the chest wall when lung tissue mechanics are inferred from intact-chest measurements in small laboratory animals.

Acute Effects of Oscillatory PEP and Thoracic Compression on Secretion Removal and Impedance of the Respiratory System in Non-Cystic Fibrosis Bronchiectasis.

Bronchiectasis is characterized by abnormal and permanent dilatation of the bronchi, caused mainly by the progression of inflammatory processes and loss of the ability to remove mucus. Techniques to clear the airways are essential for the treatment of these patients. In this study, we aimed to evaluate the acute effects of oscillatory PEP and thoracic compression on both the clearance of secretions and impedance of airways in subjects with bronchiectasis. This was a randomized crossover single-blinded study that involved both subjects with bronchiectasis and healthy subjects evaluated by using an impulse oscillometry system, which assessed resistance at 5 Hz and resistance 20 Hz, reactance at 5 Hz, reactance area, and resonant frequency, before, after, and 30 min after oscillatory PEP, chest compression, or control sessions. Dry and total weights, adhesiveness, purulence of the expectorated secretions, the dyspnea scale score, the acceptability and tolerance scale score, pulse oximetry, and difficulty in expectoration were also assessed. The dry and total weights of secretions were higher after the use of the oscillatory PEP technique than those in a control session (P = .005 and P = .039, respectively). In the bronchiectasis group, there was a decrease after oscillatory PEP in total airway resistance (P = .04), peripheral resistance (P = .005), and reactance area (P = .001). After compression, there was a decrease in peripheral resistance Hz (P = .001) and reactance area (P = .001). In the healthy group, there was an increase in resistance at 5 Hz (P = .02) after oscillatory PEP. There were no differences in acceptability and tolerance, dyspnea, and oxygen saturation. The oscillatory PEP technique was effective for the removal of secretions and in decreasing total and peripheral respiratory system resistance; thoracic compression had comparable positive effects on the peripheral resistance. Both techniques were safe and well tolerated by the subjects with bronchiectasis. ClinicalTrials.gov registration NCT02509637.).

A measurement study ear simulators being in use in Poland and the influence of their acoustic parameters on hearing tests and assessment

Ear simulators are used for traceable calibration and testing of audiometric equipment. They should be periodically calibrated because their parameters can change over time affecting the results of audiometric equipment calibration and, consequently, the results of hearing tests and assessment. During calibration two acoustic parameters are determined: the overall pressure sensitivity level of the ear simulator system and acoustical impedance of the ear simulator, which is additionally assessed in terms of compatibility with IEC 60318-1:2009. The sensitivity level is used for assuring traceability of measurements while acoustical impedance provides information on the condition of a given ear simulator. There are two objectives of this paper. The first is to show how a change in acoustical impedance influences the response of the ear simulator system to a signal from an audiometric earphone. The other is to present the results of a measurement study on acoustic parameters of ear simulators in use in Poland. The article presents the results of measurements of the sensitivity level and acoustical impedance of ear simulators, and the assessment of simulators in the term of their suitability for calibration of audiometric equipment.

Behavior of an Electrochemical Integrator Based on Solid Electrolyte in the Galvanoharmonic Charging Mode

The behavior of an electrochemical integrator based on a solid electrolyte is studied in the galvanoharmonic charging mode. The possibility of applying simpler and more graphic calculation techniques and separating the impedance of electrochemical systems into active and reactive components is shown. The plotting of the dependences of the active and reactive impedance components on the ac frequency is used to determine the parameters of the studied equivalent electric cuircuits.

Reactive Power Capability Model of Wind Power Plant Using Aggregated Wind Power Collection System

This article presents the development of a reactive power capability model for a wind power plant (WPP) based on an aggregated wind power collection system. The voltage and active power dependent reactive power capability are thus calculated by using aggregated WPP collection system parameters and considering losses in the WPP collection system. The strength of this proposed reactive power capability model is that it not only requires less parameters and substantially less computational time compared to typical detailed models of WPPs, but it also provides an accurate estimation of the available reactive power. The proposed model is based on a set of analytical equations which represent converter voltage and current limitations. Aggregated impedance and susceptance of the WPP collection system are also included in the analytical equations, thereby incorporating losses in the collection system in the WPP reactive power capability calculation. The proposed WPP reactive power capability model is compared to available methodologies from literature and for different WPP topologies, namely, Horns Rev 2 WPP and Burbo Bank WPP. Performance of the proposed model is assessed and discussed by means of simulations of various case studies demonstrating that the error between the calculated reactive power using the proposed model and the detailed model is below 4% as compared to an 11% error in the available method from literature. The efficacy of the proposed method is further exemplified through an application of the proposed method in power system integration studies. The article provides new insights and better understanding of the WPPs’ limits to deliver reactive power support that can be used for power system stability assessment, particularly long-term voltage stability.