Showing papers on "Electrical impedance published in 2011"

Impedance-Based Stability Criterion for Grid-Connected Inverters

Abstract: Grid-connected inverters are known to become unstable when the grid impedance is high. Existing approaches to analyzing such instability are based on inverter control models that account for the grid impedance and the coupling with other grid-connected inverters. A new method to determine inverter-grid system stability using only the inverter output impedance and the grid impedance is developed in this paper. It will be shown that a grid-connected inverter will remain stable if the ratio between the grid impedance and the inverter output impedance satisfies the Nyquist stability criterion. This new impedance-based stability criterion is a generalization to the existing stability criterion for voltage-source systems, and can be applied to all current-source systems. A single-phase solar inverter is studied to demonstrate the application of the proposed method.

Design and Analysis of the Droop Control Method for Parallel Inverters Considering the Impact of the Complex Impedance on the Power Sharing

Abstract: This paper investigates the characteristics of the active and reactive power sharing in a parallel inverters system under different system impedance conditions. The analyses conclude that the conventional droop method cannot achieve efficient power sharing for the case of a system with complex impedance condition. To achieve the proper power balance and minimize the circulating current in the different impedance situations, a novel droop controller that considers the impact of complex impedance is proposed in this paper. This controller can simplify the coupled active and reactive power relationships, which are caused by the complex impedance in the parallel system. In addition, a virtual complex impedance loop is included in the proposed controller to minimize the fundamental and harmonic circulating current that flows in the parallel system. Compared to the other methods, the proposed controller can achieve accurate power sharing, offers efficient dynamic performance, and is more adaptive to different line impedance situations. Simulation and experimental results are presented to prove the validity and the improvements achieved by the proposed controller.

Analysis, Design, and Implementation of Virtual Impedance for Power Electronics Interfaced Distributed Generation

Abstract: This paper presents a virtual impedance design and implementation approach for power electronics interfaced distributed generation (DG) units. To improve system stability and prevent power couplings, the virtual impedances can be placed between interfacing converter outputs and the main grid. However, optimal design of the impedance value, robust implementation of the virtual impedance, and proper utilization of the virtual impedance for DG performance enhancement are key for the virtual impedance concept. In this paper, flexible small-signal models of microgrids in different operation modes are developed first. Based on the developed microgrid models, the desired DG impedance range is determined considering the stability, transient response, and power flow performance of DG units. A robust virtual impedance implementation method is also presented, which can alleviate voltage distortion problems caused by harmonic loads compared to the effects of physical impedances. Furthermore, an adaptive impedance concept is proposed to further improve power control performances during the transient and grid faults. Simulation and experimental results are provided to validate the impedance design approach, the virtual impedance implementation method, and the proposed adaptive transient impedance control strategies.

A Printed Leaky-Wave Antenna Based on a Sinusoidally-Modulated Reactance Surface

Abstract: A simple procedure for designing a sinusoidally-modulated reactance surface (SMRS) that radiates at an arbitrary off-broadside angle is outlined. The procedure allows for nearly independent control of the leakage and phase constants along the surface. Printing an array of metallic strips over a grounded dielectric substrate is discussed as a way to practically implement the theoretical SMRS. A method of mapping the gaps between metallic strips to a desired surface impedance is presented as an efficient alternative to mapping methods used in the past. A printed leaky-wave antenna with a sinusoidally-modulated surface reactance is designed using the procedure mentioned above. The TM-polarized antenna radiates at 30° from broadside at 10 GHz, and exhibits an experimental gain of 18.4 dB. Theoretical, simulated, and experimental results are presented.

RF Generator System for Surgical Vessel Sealing

Abstract: Systems, methods, and apparatus for providing power to an electrosurgical instrument. In particular, a power supply is disclosed in which non-sinusoidal (e.g., pulsed) constant frequency voltage having a variable amplitude is passed to an LC circuit to produce a quasi-sinusoidal current in the LC circuit. The constant driving frequency can be one half the resonant frequency of the LC circuit allowing the LC circuit to operate as an impedance, and thus limit current spikes and arcing. The frequency and phasing of the driving voltage also enables the LC circuit to discharge energy back into a power provider of the power supply so that energy does not build up in the LC circuit. These features result in less severe current spikes and arcing, as well as reduced cutoff times.

Spiral Leaky-Wave Antennas Based on Modulated Surface Impedance

Abstract: Different kinds of spiral planar circularly polarized (CP) antennas are presented. These antennas are based on an interaction between a cylindrical surface-wave excited by an omnidirectional probe and a inhomogeneous surface impedance with a spiral pattern. The surface impedance interaction transforms a bounded surface wave into a circularly polarized leaky wave with almost broadside radiation. The problem is studied by adiabatically matching the local 2D solution of a modulated surface-impedance problem to the actual surface. Analytical expressions are derived for the far-field radiation pattern; on this basis, universal design curves for antenna gain are given and a design procedure is outlined. Two types of practical solutions are presented, which are relevant to different implementations of the impedance modulation: i) a grounded dielectric slab with a spiral-sinusoidal thickness and ii) a texture of dense printed patches with sizes variable with a spiral-sinusoidal function. Full wave results are compared successfully with the analytical approximations. Both the layouts represent good solutions for millimeter wave CP antennas.

Microwave resonator configured by composite right/left-handed meta-material and antenna apparatus provided with the microwave resonator

Abstract: A reflective impedance element is connected to a port of a composite right/left-handed transmission line, and operates at a predetermined operating frequency so that an impedance when the reflective impedance element is seen from the port becomes a pure imaginary number jB. A reflective impedance component is connected to a port of the composite right/left-handed transmission line, and operates at the predetermined operating frequency so that an impedance when the reflective impedance element is seen from the port becomes −jB.

Oscillation mechanics of the respiratory system.

Abstract: The mechanical impedance of the respiratory system defines the pressure profile required to drive a unit of oscillatory flow into the lungs. Impedance is a function of oscillation frequency, and is measured using the forced oscillation technique. Digital signal processing methods, most notably the Fourier transform, are used to calculate impedance from measured oscillatory pressures and flows. Impedance is a complex function of frequency, having both real and imaginary parts that vary with frequency in ways that can be used empirically to distinguish normal lung function from a variety of different pathologies. The most useful diagnostic information is gained when anatomically based mathematical models are fit to measurements of impedance. The simplest such model consists of a single flow-resistive conduit connecting to a single elastic compartment. Models of greater complexity may have two or more compartments, and provide more accurate fits to impedance measurements over a variety of different frequency ranges. The model that currently enjoys the widest application in studies of animal models of lung disease consists of a single airway serving an alveolar compartment comprising tissue with a constant-phase impedance. This model has been shown to fit very accurately to a wide range of impedance data, yet contains only four free parameters, and as such is highly parsimonious. The measurement of impedance in human patients is also now rapidly gaining acceptance, and promises to provide a more comprehensible assessment of lung function than parameters derived from conventional spirometry.

Voltage Stability Monitoring Based on the Concept of Coupled Single-Port Circuit

Abstract: This paper reveals that the impedance match (or the Thevenin circuit) based voltage stability monitoring techniques have problems to predict voltage stability limits when applied to multi-load power systems. Power system loads are nonlinear and dynamic. They cannot be simply represented as Thevenin circuit parameters for impedance match analysis. To overcome these difficulties, a new concept called “coupled single-port circuit” is proposed in this paper. The concept decouples a meshed network into individual single generator versus single bus network and, as a result, a modified version of the impedance match theorem can be used. This leads to a real-time voltage stability monitoring scheme without the need to estimate Thevenin parameters. The scheme can estimate voltage stability margin and identify weak areas in a system based on the SCADA and PMU data. Case studies conducted on several test systems have verified the validity of the proposed method.

Improved Instantaneous Average Current-Sharing Control Scheme for Parallel-Connected Inverter Considering Line Impedance Impact in Microgrid Networks

Abstract: A new control scheme for parallel-connected inverters taking into account the effect of line impedance is presented. The system presented here consists of two single-phase inverters connected in parallel. The control technique is based on instantaneous average current-sharing control that requires interconnections among inverters for information sharing. A generalized model of a single-phase parallel-connected inverter system is derived. The model incorporates the detail of the control loops that use a proportional-resonant controller, but not the switching action. The voltage- and current-controller design and parameters selection process are discussed. Adaptive gain scheduling is introduced to the controller to improve the current and power sharing for a condition, where the line impedance is different among the inverters. The simulation results show that the adaptive gain-scheduling approaches introduced improve the performance of conventional controller in terms of current and power sharing between inverters under difference line impedance condition. The experiments validate the proposed system performance.

Compact Disc Monopole Antennas for Current and Future Ultrawideband (UWB) Applications

Abstract: Circular disc monopole antennas are investigated for current and future ultrawideband (UWB) applications. The studied antennas are compact and of small size (25 mm × 35 mm × 0.83 mm) with a 50-Ω feed line and offer a very simple geometry suitable for low cost fabrication and straightforward printed circuit board integration. More specifically, the impedance matching of the classic printed circular disc UWB monopole is improved by introducing transitions between the microstrip feed line and the printed disc. Two particular designs are examined using a dual and single microstrip transition. By using this simple antenna matching technique, respective impedance bandwidths (|Sn | <; -10 dB) from 2.5 to 11.7 GHz and 3.5 to 31.9 GHz are obtained. Results are also compared to a classic UWB monopole with no such matching network transitions. Measured and simulated reflection coefficient curves are provided along with beam patterns, gain and group delay values as a function of frequency. The transient behavior of the studied antennas is also examined in the time domain.

Investigation Into the Application of Artificial Magnetic Conductors to Bandwidth Broadening, Gain Enhancement and Beam Shaping of Low Profile and Conventional Monopole Antennas

Abstract: The reflection coefficient phase is investigated for several different artificial magnetic conductors (AMCs) having canonical FSS-type shapes. Three of them are selected, each representing a different class, and fine tuned to exhibit identical resonant frequency. Polarization and angular dependence as well as the effects of losses on these structures are studied. Next, a low-profile inverted L-shape monopole antenna (ILSMA) is placed horizontally above the ground plane. Vertical monopole antenna (VMA) is also placed above them. It is shown that using some of the aforementioned AMCs, the input impedance of both ILSMA and VMA can not only be matched, but also the input impedance bandwidth enhancement as wide as 27% and 35% are obtained, respectively. The VMA study on AMC ground planes which reveals a counter-intuitive phenomenon has not been explored in the literature, previously. It is revealed that the broadband characteristics can also be achieved for smaller size of the AMC ground planes, which enables the antenna to be designed in compact size. It is also illustrated that reflection characteristics of the AMC is not sufficient to evaluate AMC performance when it is used as an antenna ground plane. This is illustrated through extensive simulation and measurement results.

Impedance mediated control of power delivery for electrosurgery

Abstract: A method of controlling electrosurgical power delivery based on a comparison of sensed tissue impedance to various impedance threshold values is provided. Energy is delivered to tissue in a sealing cycle as a series of pulses. An initial pulse has a profile with a preset energy starting value that increases at a ramping rate to a preset end value. Sensed impedance data are monitored throughout each pulse and compared to each of an impedance threshold value for RF setpoint, an impedance threshold value for cumulative time, and an impedance threshold value for energy cutback. Based on sensed impedance during a pulse, the profile of a subsequent pulse can be modified. In the event of a high impedance event that reflects low tissue presence, energy may be cutback. A sealing cycle is stopped when a cumulative amount of time with an impedance value over the impedance cumulative time threshold value reaches a sealing cycle duration limit.

Modeling of the Lung Impedance Using a Fractional-Order Ladder Network With Constant Phase Elements

Abstract: The self similar branching arrangement of the airways makes the respiratory system an ideal candidate for the application of fractional calculus theory. The fractal geometry is typically characterized by a recurrent structure. This study investigates the identification of a model for the respiratory tree by means of its electrical equivalent based on intrinsic morphology. Measurements were obtained from seven volunteers, in terms of their respiratory impedance by means of its complex representation for frequencies below 5 Hz. A parametric modeling is then applied to the complex valued data points. Since at low-frequency range the inertance is negligible, each airway branch is modeled by using gamma cell resistance and capacitance, the latter having a fractional-order constant phase element (CPE), which is identified from measurements. In addition, the complex impedance is also approximated by means of a model consisting of a lumped series resistance and a lumped fractional-order capacitance. The results reveal that both models characterize the data well, whereas the averaged CPE values are supraunitary and subunitary for the ladder network and the lumped model, respectively.

Substrate processing apparatus

Abstract: PROBLEM TO BE SOLVED: To uniformize the film thickness distribution, and to suppress the reflected power in a discharge electrode SOLUTION: A substrate processing apparatus includes high frequency power supply units 17a, 17b for outputting the high frequency power of the predetermined frequency through the phase modulation, an opposing electrode 3 supporting a substrate, a discharge electrode which is supplied with the high frequency power output from the high frequency power supply units to form plasma between the opposing electrode and itself by the high frequency power, a matching box 13 which has a plurality of matching circuits 20a-20d each of which being set to different impedance, and matches the impedance on the discharge electrode side to the impedance on the high frequency power supply unit side, and a selection means 22 for selecting a matching circuit in which the impedance on the discharge electrode side fluctuated by the phase modulation is matched with the impedance on the high frequency power supply unit side out of the plurality of matching circuits when supplying the high frequency power COPYRIGHT: (C)2011,JPO&INPIT

Method and circuitry to calculate the state of charge of a battery/cell

Abstract: The present inventions, in one aspect, are directed to techniques and/or circuitry to applying a current pulse to the terminals of the battery during a charge, measuring a voltage at the terminals of the battery, determining a relationship of an open circuit voltage to an amount of charge in the battery using data which is representative of a state of health of the battery, calculating an open circuit voltage of the battery using the voltage measured at the terminals of the battery, a current applied to or removed from the battery and an impedance of the battery, and determining a state of charge of the battery using (i) the calculated open circuit voltage and (ii) the relationship of the open circuit voltage to the amount of charge.

A fully parallel multi-frequency EIT system with flexible electrode configuration: KHU Mark2

Abstract: We report the development of a new multi-frequency electrical impedance tomography (EIT) system called the KHU Mark2. It is descended from the KHU Mark1 in terms of technical details such as digital waveform generation, Howland current source with multiple generalized impedance converters and digital phase-sensitive demodulators. New features include flexible electrode configurations to accommodate application-specific requirements, multiple independent current sources and voltmeters for fully parallel operations, improved data acquisition speeds for faster frame rates and compact mechanical design. Given an electrode configuration, we can design an analog backplane in such a way that both current injections and voltage measurements can be done without using any switch. The KHU Mark2 is based on an impedance measurement module (IMM) comprising a current source and a voltmeter. Using multiple IMMs, we can construct a multi-channel system with 16, 32 or 64 channels, for example. Adopting a pipeline structure, it has the maximum data acquisition speed of 100 scans s(-1) with the potential to detect fast physiological changes during respiration and cardiac activity. Measuring both in-phase and quadrature components of trans-impedances at multiple frequencies simultaneously, the KHU Mark2 is apt at spectroscopic EIT imaging. In this paper, we describe its design, construction, calibration and performance evaluation. It has about 84 dB signal-to-noise ratio and 0.5% reciprocity error. Time-difference images of an admittivity phantom are presented showing spectroscopic admittivity images. Future application studies using the KHU Mark2 are briefly discussed.

Passive and Active Elements Using Fractional ${\rm L}_{\beta} {\rm C}_{\alpha}$ Circuit

Abstract: This paper introduces a qualitative revision of the traditional LC tank circuit in the fractional domain. The paper can be divided into six major parts, aiming in turn to establish the various conditions under which LβCα impedance may act as a resistor, negative resistor, or a positive or negative pure imaginary inductor or capacitor, in accordance to new frequency definitions; illustrate the process by which the phase response chooses the shortest path from initial to final phase, and use this illustration to verify the cases discussed in part one; develop the generalized parameters for the bandpass filter response of the LβCα circuit, such as the resonance frequency and quality factor versus α- β plane; discuss sensitivity analyses with respect to the fractional orders, as well as the time domain analyses for the impulse and step responses with their analytical formulas; and lastly, to propose some possible applications for this generalized circuit. Mathematical and PSpice simulation results are included to validate the discussion.

New type LCCT-Z-source inverters

Abstract: Z-source inverters are suited for applications which require a large range of gain, such as in renewable energy. Recently proposed Trans-Z-source inverters (TZSIs) and T-source inverters (TSIs) characterize improved voltage gain thanks to the application of coupled inductors with turns ratio higher than one. The present paper extends the concept of Trans-Z-source (T-source) inverters. Two novel impedance networks: the LCCT (inductor-capacitor-capacitor-transformer) Z-source impedance network and the LCCT-qZ-source impedance network are presented. The former contains two built-in DC-current-blocking capacitors connected in series with transformer windings and therefore prevent the transformer core from saturation while the latter contains one built-in DC-current-blocking capacitor and combines the features of qZ-source and T-source impedance networks. The novel topologies of LCCT-impedance-source inverters: LCCT-Z-source inverter (LCCT-ZSI) and LCCT-quasi-Z-source inverter (LCCT-qZSI) proposed in the paper characterize available continuous input current which is the advantage compared to TZSIs and TSIs. Simulations and experimental results are provided to verify the analysis of two proposed topologies.

Detection of stator winding faults in induction motors using three-phase current monitoring.

Abstract: The objective of this paper is to propose a new method for the detection of inter-turn short circuits in the stator windings of induction motors. In the previous reported methods, the supply voltage unbalance was the major difficulty, and this was solved mostly based on the sequence component impedance or current which are difficult to implement. Some other methods essentially are included in the offline methods. The proposed method is based on the motor current signature analysis and utilizes three phase current spectra to overcome the mentioned problem. Simulation results indicate that under healthy conditions, the rotor slot harmonics have the same magnitude in three phase currents, while under even 1 turn (0.3%) short circuit condition they differ from each other. Although the magnitude of these harmonics depends on the level of unbalanced voltage, they have the same magnitude in three phases in these conditions. Experiments performed under various load, fault, and supply voltage conditions validate the simulation results and demonstrate the effectiveness of the proposed technique. It is shown that the detection of resistive slight short circuits, without sensitivity to supply voltage unbalance is possible.

RF energy harvesting system from cell towers in 900MHz band

Abstract: An experimental RF energy harvesting system to harvest energy from cell towers is presented in this paper. An electromagnetically-coupled square microstrip antenna is designed and fabricated for deployment in the presented system. Antenna gain of 9.1dB and bandwidth from 877 MHz to 998 MHz is achieved. A Schottky diode-based single stage voltage doubler and six-stage voltage doubler has also been designed and fabricated for DC voltage generation. Measured results show that a voltage of 2.78V is obtained at a distance of 10m from the cell tower and a voltage of 0.87V is obtained at a distance of 50m.

Advanced Dynamic Simulation of Supercapacitors Considering Parameter Variation and Self-Discharge

Abstract: In this paper, dynamic simulation of the equivalent circuit model of the supercapacitor, taking into account the parameter variations and self-discharge, is discussed. Self-discharge is modeled with equivalent impedance including a constant phase element (CPE), and the parameter variations depending on the voltage are reflected. Since it is difficult to directly simulate the ZARC element (R-CPE parallel circuit) with a circuit simulation tool such as the professional simulation program with integrated circuit emphasis (PSPICE), equivalent transformation to three R-C parallel circuits is introduced in the simulation. The accuracy of simulation with the model is then verified through a comparison with results of an experiment. The comparison shows that the model using a CPE is effective in representing the dynamic characteristics and self-discharge of supercapacitors. Accordingly, it proves that the method proposed in this study can be useful in developing systems that include supercapacitors, and can be applied in an integrated simulation of a supercapacitor and a power electronic system.

Experimental and theoretical investigation on the high frequency dielectric properties of Ag/Al2O3 composites

Abstract: The impedance and dielectric properties of Ag/Al2O3 composites are investigated experimentally in the frequency range from 100 MHz to 1 GHz. Besides, equivalent circuit analysis and numerical simulations were carried out. For the composites with sufficiently high silver loading, current paths were formed and negative permittivity appeared. The negative permittivity can be well described by lossy Drude model. Moreover, the negative permittivity sample manifests inductive characteristic and shunt inductors are added to its equivalent circuit. Numerical simulations show that the interconnection of silver particles results in negative permittivity, hence the serious attenuation of electromagnetic waves.

Wideband Artificial Magnetic Conductors Loaded With Non-Foster Negative Inductors

Abstract: We examine how the bandwidth of artificial magnetic conductors (AMCs) can be greatly increased when loaded with negative-inductance non-Foster circuits. This increase in bandwidth is achieved by enhancing the structural inductance of the AMC by combining it in parallel with a negative inductance, thus achieving a bandwidth that exceeds what is possible with passive approaches. A prototype VHF-UHF active AMC was fabricated and measured, which achieved a bandwidth greater than 80% at a resonant frequency of 263 MHz.

Carbon nanotube composite coating of neural microelectrodes preferentially improves the multiunit signal-to-noise ratio

Abstract: Extracellular metal microelectrodes are widely used to record single neuron activity in vivo. However, their signal-to-noise ratio (SNR) is often far from optimal due to their high impedance value. It has been recently reported that carbon nanotube (CNT) coatings may decrease microelectrode impedance, thus improving their performance. To tease out the different contributions to SNR of CNT-coated microelectrodes we carried out impedance and noise spectroscopy measurements of platinum/tungsten microelectrodes coated with a polypyrrole–CNT composite. Neuronal signals were recorded in vivo from rat cortex by employing tetrodes with two recording sites coated with polypyrrole–CNT and the remaining two left untreated. We found that polypyrrole–CNT coating significantly reduced the microelectrode impedance at all neuronal signal frequencies (from 1 to 10 000 Hz) and induced a significant improvement of the SNR, up to fourfold on average, in the 150–1500 Hz frequency range, largely corresponding to the multiunit frequency band. An equivalent circuit, previously proposed for porous conducting polymer coatings, reproduced the impedance spectra of our coated electrodes but could not explain the frequency dependence of SNR improvement following polypyrrole–CNT coating. This implies that neither the neural signal amplitude, as recorded by a CNT-coated metal microelectrode, nor noise can be fully described by the equivalent circuit model we used here and suggests that a more detailed approach may be needed to better understand the signal propagation at the electrode–solution interface. Finally, the presence of significant noise components that are neither thermal nor electronic makes it difficult to establish a direct relationship between the actual electrode noise and the impedance spectra.

Positional dependence of particles in microfludic impedance cytometry

Abstract: Single cell impedance cytometry is a label-free electrical analysis method that requires minimal sample preparation and has been used to count and discriminate cells on the basis of their impedance properties. This paper shows experimental and numerically simulated impedance signals for test particles (6 μm diameter polystyrene) flowing through a microfluidic channel. The variation of impedance signal with particle position is mapped using numerical simulation and these results match closely with experimental data. We demonstrate that for a nominal 40 μm × 40 μm channel, the impedance signal is independent of position over the majority of the channel area, but shows large experimentally verifiable variation at extreme positions. The parabolic flow profile in the channel ensures that most of the sample flows through the area of uniform signal. At high flow rates inertial focusing is observed; the particles flow in equal numbers through two equilibrium positions reducing the coefficient of variance (CV) in the impedance signals to negligible values.

Impedance Identification Procedure of Three-Phase Balanced Voltage Source Inverters Based on Transient Response Measurements

Abstract: Impedance-based models are commonly used to perform system-level stability analysis of distributed power systems. In this paper, a new impedance identification procedure for three-phase balanced voltage source inverters voltage source inverters (VSIs) is proposed. This procedure is based on the transient response of the inverter to simple passive load step tests. Transfer function models are obtained from the transient response by means of a well-established and easy to use identification algorithm. The performance of the proposal is validated through both simulation and experimental results obtained on VSIs for aircraft applications.