Ac Capacitance Research Articles

Stand-Alone Wind Energy Conversion System with an Asynchronous Generator

This paper deals with a stand-alone wind energy conversion system (WECS) with an isolated asynchronous generator (IAG) and voltage and frequency (VF) control feeding three-phase four-wire loads. The reference generator currents are estimated using the instantaneous symmetrical component theory to control the voltage and frequency of an IAG system. A three-leg voltage source converter (VSC) with an isolated star/delta transformer is used as an integrated VSC. An integrated VSC with a battery energy storage system (BESS) is used to control the active and reactive powers of the WECS. The WECS is modeled and simulated in MATLAB using the Simulink and the Sim Power System (SPS) toolboxes. The proposed VF controller functions as a voltage and frequency regulator, a load leveler, a load balancer and a harmonic eliminator in the WECS. A comparison is made on the rating of the VSC with and without ac capacitors connected at the terminals of an IAG. Simulation and test results are presented to verify the control algorithm.

Bacterial Foraging Algorithm based Enhancement of Voltage Profile and Minimization of Losses Using Thyristor Controlled Series Capacitor (TCSC)

minimization in power system is an important research issue. Transmission line losses in a power system can be minimized by means of reactive power compensation. The continuous demand in electric power system network has caused it to be heavily loaded leading to voltage instability. This phenomenon has also led to voltage profile depreciation below the acceptable secure limit. The significance and use of Flexible AC Transmission Systems (FACTS) devices and capacitor placement is in order to alleviate the voltage profile decay problem. Identification of the optimal value of compensating devices requires proper optimization technique, easy to search the optimal solution with less computational burden. This paper presents an application of Bacterial Foraging (BF) algorithm in optimizing the optimal location and design of Thyristor Controlled Series Capacitor (TCSC) for voltage profile improvement and minimization of losses in a power system which utilized the TCSC as the control variable. The proposed approach has been evaluated with three different objective functions namely, loss minimization, voltage profile improvement and voltage stability enhancement. Voltage stability level of the system is defined on the L-index of the load buses. The IEEE 14bus and IEEE 30bus systems are used as test systems to demonstrate the applicability and efficiency of the proposed system. The proposed method is compared with Genetic Algorithms (GAs) and Non-dominated Sorting Particle Swarm Optimization (NSPSO). The test result shows that the location of TCSC improves the voltage profile of the system and also minimizes the transmission line losses. Keywords— Flexible AC Transmission Systems (FACTS), Thyristor Controlled Series Capacitor (TCSC), Multi-Objective function, Bacterial Foraging (BF) , L-index.

Fabrication of glass-coated electrodes with nano- and micrometer size by means of dissolution with HF

Platinum nano-electrodes were fabricated at the success rate 76% by the six processes; (i) etching a Pt wire in ethanol–water mixture, (ii) sealing it with a glass sheath, (iii) grinding roughly the glass tip, (iv) polishing the tip while monitoring the capacitive ac current flowing through the glass thin film on the Pt, (v) dissolving the glass film in HF solution until a part of Pt was exposed, and (vi) heating the tip at 85 °C for stabilization. A key of the process was to make a thin glass film on the Pt tip (iv) and to expose the Pt surface chemically by (v). 50 electrodes thus fabricated had diameters ranging from 1 nm to 5 μm, estimated from the steady-state diffusion-controlled current of ferrocene in acetonitrile and ferrocenyl derivative in aqueous solution. They exhibited reproducible and stable voltammograms without hysteresis, withstanding 6 hours’ continuous use and 15 hours’ iterative processes of heat and voltammetry. Not only the halfwave potentials but also slopes of the log-plots were independent of radii of the electrodes. No kinetic effect was revealed in the steady-state voltammograms.

Fractal simulation of the resistivity and capacitance of arsenic selenide

The temperature dependences of the ac resistivity R and ac capacitance C of arsenic selenide were measured more than four decades ago [V. I. Kruglov and L. P. Strakhov, in Problems of Solid State Electronics, Vol. 2 (Leningrad Univ., Leningrad, 1968)]. According to these measurements, the frequency dependences are R ∝ ω−0.80±0.01 and ΔC ∝ ω−0.120±0.006 (ω is the circular frequency and ΔC is measured from the temperature-independent value C0). According to fractal-geometry methods, R ∝ ω1−3/h and ΔC ∝ ω−2+3/h, where h is the walk dimension of the electric current in arsenic selenide. Comparison of the experimental and theoretical results indicates that the walk dimensions calculated from the frequency dependences of resistivity and capacitance are hR = 1.67 ± 0.02 and hC = 1.60 ± 0.08, which are in agreement with each other within the measurement errors. The fractal dimension of the distribution of conducting sections is D = 1/h = 0.6. Since D < 1, the conducting sections are spatially separated and form a Cantor set.

On the Distribution of AC and DC Winding Capacitances in High-Frequency Power Transformers With Rectifier Loads

In this paper, a method to adjust the ac winding capacitance of high-voltage high-frequency transformers by means of winding-rectifier integration is described. First, a theoretical background to the method is given. From the theory, an equivalent circuit describing the characteristics of the combination of the transformer and the rectifier is derived. The derived circuit introduces the concept of a dc capacitance. Finally, the equivalent circuit and the method itself are verified by means of experiments on a transformer-rectifier system from an industrial application with the ratings 70 kV, 30 kW, and 25 kHz. The results from the experiments show that it is possible to vary the ac component of the winding capacitance from a few percent up to 95% of the total winding capacitance. This means that it is virtually free to choose between ac and dc capacitances during the design stage. This is very important in applications such as resonant converters with transformers having secondary windings connected to rectifiers with capacitive output filters.

An Improvement On- Line Failure Diagnosis of DC Link Capacitor in PWM Power Converters

DC link electrolytic capacitors are widely used in various PWM power converter system, such as adjustable speed driver(ASD) or DC/DC converter. Electrolytic capacitors, which is the most of the time affected by aging effect, plays a very important role for the power electronics system quality and reliability. This objective of this paper is to propose a improvement method to detect the rise of equivalent series resistor(ESR) in order to realize the online failure prediction of electrolytic capacitor for DC link of PWM power converter. The ESR detection scheme is based on the determination of the electrolytic capacitor AC losses calculated from voltage/current measurement using AC coupling. Therefore, the preposed online failure prediction method has the merits of easy ESR computation and circuit simplicity compare with BPF method. Simulation results show the veridity of the proposed on-line ESR estimation method.

Modified static VAR compensator using a large value AC capacitor

To overcome some of the limitations of conventional static VAR compensators (SVCs) which are widely used for reactive power compensation, a modified SVC (MSVC) has been proposed. MSVC uses a large value AC capacitor. The large value AC capacitor is realized using unipolar DC capacitors and power semiconductor devices. Unlike the conventional SVCs, the proposed MSVC does not require additional shunt passive filters for harmonic filtering. MSVC has been verified through analysis and simulation. A practical implementation of MSVC has been realized and tested.

Bidirectional, High-Power DC Transformer

This paper studies a bidirectional dc-dc converter concept which is capable of achieving very high stepping ratios with MW level power transfers. The converter can find potential application in connecting high-power dc sources, interfacing to high-voltage dc transmission or to flexible ac transmission system elements. The converter is based on two resonant circuits which share a common ac capacitor. The topology is simple and utilizes thyristors with potentially all soft switchings. Complete analytical modelling is presented which enables systematic design procedure for the converter. The detailed digital simulation on PSCAD platform confirms satisfactory operation on a 5-MW test system, which connects 4-kV dc source to an 80-kV high voltage dc grid. The converter shows good responses to rapid changes in power magnitude/direction and it is concluded that robustness to terminal voltage disturbances is excellent. Since the highest-power phase-control thyristors are employed, the converter can potentially be used at much higher power levels. The passive components are of reasonable size.

A SOLUTION TO IMPROVE PERFORMANCE OF AC-CDI SYSTEMS USED IN MOTORCYCLES

This paper presents a solution to improve performance of AC capacitor discharge-ignition (AC-CDI) systems for most of motorcycles presently used in Vietnam. Due to low cost, these AC-CDI systems normally use very simple structures and electrical electronic-components, especially their electronic control units (ECUs). Their main disadvantages are high power loss and only one or several different constant ignition timings. With a low cost micro-processor and good control algorithms, a new design for AC-CDI ECU could reach both higher power efficiency for the system and highly variable ignition timing related to engine rpm. These advantages could lead to more powerful spark, better power, higher fuel economy, lower exhaust emissions and smoother driveability for engine.

Study of minority carrier injection phenomenon on Schottky and plasma deposited p–n junction diodes

Both temperature dependent ac capacitance ( C) together with ac conductance ( G) under various frequencies ( ω) and dc current ( I) measurements as functions of bias ( V) were employed to investigate majority as well as minority carrier injection phenomenon on chromium-p-type crystalline silicon (Cr/p-cryst. Si/ohmic contact) Schottky and aluminum/plasma deposited n-type hydrogenated amorphous silicon/p-c-Si (Al/n-a-Si:H/p-c-Si/ohmic contact) p–n structures. Ambipolar transport process was eventual for both diodes and behaved differently according to the mentioned techniques: for a well defined forward bias interval, there was an increase in capacitance towards maximum that interpreted as majority carrier injection from the back electrode. On the other side, minority carrier injection from front electrode begun when the bias went beyond the critical voltage (i.e., built-in voltage) that corresponded to the peak position and caused sharp decrease in measured capacitance, leading to observation of a hump in C– V measurement. Moreover, shape and peak position of the hump were frequency/temperature dependent. Remarkably, in the bias range where capacitance pronounced, space charge limited current (SCLC) was discerned as enrolled carrier flow mechanism according to I– V measurement, confirming further majority carrier injection. In addition, both dc conductivity and ac conductance followed the same dependence with bias voltage in forward direction. On the other side, reverse current seemed proportional to square root of reverse bias, implying generation current in the depletion region. These experimental identifications convinced us of the existence of conductivity modulation issue for the structures at hand.

Effect of Ag on the electrical properties of a-Ge20Se80 glasses

This article reports the effect of Ag impurity (low ~4 and ~6 at.% and high ~10 at.%) on the ac conductivity (σac) of a-Ge20Se80 glass. Frequency (ω)-dependent ac conductance and capacitance of the samples over a frequency range ~100 Hz to 50 kHz was taken in the temperature range ~268–358 K. At frequency 2 kHz and temperature 298 K, the value of σac increases at low as well as at higher concentration of Ag. The σac is proportional to ωs for undoped and doped samples. The value of frequency exponent (s) decreases as the temperature increases.

Electrochemical behavior of aluminum in 1-n-butyl-3-methylimidazolium tetrafluoroborate ionic liquid electrolytes for capacitor applications

This study compares two series of solvents for application in aluminum electrolytic capacitors: ethylene glycol (EG) and water mixtures, and ethylene glycol and 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI.BF4) ionic liquid (IL) mixtures. Electrochemical impedance spectroscopy and cyclic voltammetry were carried out with a previously anodized aluminum disk electrode. Comparative measurements of solution resistance, polarization resistance, AC capacitance, and passive current were made. The results show that EG–IL mixtures with low amounts of IL (10% IL–90% EG v/v) have a low solution resistance. Low values of solution resistance, high values of polarization resistance, small passive current, and uniform capacitance of anodized aluminum in EG–IL mixtures are favorable properties of the BMI.BF4 IL for use as an electrolyte in the construction of electrolytic aluminum capacitors.

Load-Commutated SCR Current-Source-Inverter-Fed Induction Motor Drive With Sinusoidal Motor Voltage and Current

Current source inverter (CSI) is an attractive solution in high-power drives. The conventional gate turn-off thyristor (GTO) based CSI-fed induction motor drives suffer from drawbacks such as low-frequency torque pulsation, harmonic heating, and unstable operation at low-speed ranges. These drawbacks can be overcome by connecting a current-controlled voltage source inverter (VSI) across the motor terminal replacing the bulky ac capacitors. The VSI provides the harmonic currents, which results in sinusoidal motor voltage and current even with the CSI switching at fundamental frequency. This paper proposes a CSI-fed induction motor drive scheme where GTOs are replaced by thyristors in the CSI without any external circuit to assist the turning off of the thyristors. Here, the current-controlled VSI, connected in shunt, is designed to supply the volt ampere reactive requirement of the induction motor, and the CSI is made to operate in leading power factor mode such that the thyristors in the CSI are autosequentially turned off. The resulting drive will be able to feed medium-voltage, high-power induction motors directly. A sensorless vector-controlled CSI drive based on the proposed configuration is developed. The experimental results from a 5 hp prototype are presented. Experimental results show that the proposed drive has stable operation throughout the operating range of speeds.

Optimal design of a self-excited capacitor in a dual-stator winding induction generator

The design requirement of the self-excited capacitor in a dual-stator winding induction generator (DSWIG) is different from that of the conventional induction generator, because there are numerous combinations between the reactive capacitance of two windings. The efficiency optimisation is taken as the design aim for a self-excited capacitor in this paper. Based on the equivalent circuit model of the induction generator-rectifier system, the characteristic analysis of a three-phase bridge rectifier with an AC capacitor is presented, especially the comprehensive analysis on the condition with neglecting commutation angle. Moreover, the relation between self-excited capacitor and corresponding stator loss is derived, and the value of self-excited capacitor at maximum efficiency is obtained. The coincidence between the experimental results and analytical results verifies the validity of the analytical method. The results obtained herein are of importance to the optimal design of the self-excited capacitor in DSWIG.

Four-Terminal-Pair Inductance Comparison Between INRIM and CTU

For 40 years, it has been known that a four-terminal-pair (4TP) definition achieves the highest accuracy in impedance measurement. While, for ac resistors and capacitors, a 4TP standard definition is commonplace, and 4TP commercial measurement systems are available on the market, for inductors, a two- or three-terminal definition is still the most common. This paper describes the first international intercomparison of inductance performed with 4TP measurement systems and a 4TP travelling standard. At the Istituto Nazionale di Ricerca Metrologica (INRIM), the measurement system is based on a 4TP implementation of the three-voltage method, and traceability is to calibrated ac resistors. At Czech Technical University (CTU), Prague, a 4TP double-balance transformer bridge is employed, and traceability is via calibrated capacitors, ac resistance, and ac resistance ratio. The comparison has been performed in the context of EUROMET Project 607, with a 4TP 100-mH temperature-controlled travelling standard constructed for the purpose. Despite the big differences in the measurement methods and traceability chains, the measurements reported by INRIM and CTU are fully compatible, within their relative expanded uncertainties of 7 times 10-6 and 6.2 times 10-5, respectively.

Evaluation of Levels of Defect Sites Present in Highly Ordered Pyrolytic Graphite Electrodes Using Capacitive and Faradaic Current Components Derived Simultaneously from Large-Amplitude Fourier Transformed ac Voltammetric Experiments

The level of edge plane defect sites present in highly ordered pyrolytic graphite (HOPG) electrodes has been evaluated via analysis of dc, ac fundamental, and higher-order ac harmonics available from a single large-amplitude Fourier transformed (FT) ac voltammetric experiment. Deliberate introduction of a low level of edge plane defect was achieved by polishing, with a higher level being introduced via electrochemical pretreatment. Kinetics regimes associated with fast electron transfer on the edge plane defect sites and slow electron transfer on the basal plane surface are resolved under ac conditions when using the surface-sensitive [Fe(CN)(6)](3-/4-) redox probe. However, because of their insensitivity to slow electron transfer, higher-order ac faradaic harmonics almost exclusively detect only the much faster processes that emanate from edge plane defect sites. Thus, detection of fourth- and higher-order ac Faradaic harmonic components that are devoid of background capacitive current is possible at freshly cleaved HOPG in the region near the reversible potential for the [Fe(CN)(6)](3-/4-) process. Under these circumstances, dc cyclic voltammograms exhibit only reduction and oxidation peaks separated by more than 1 V. The fundamental ac harmonic provides detailed information on the capacitive current, which increases with the level of edge plane defect sites. Apparent charge transfer rate constants also can be derived from peak-to-peak separations obtained from the dc aperiodic component. Estimates of the percentage of edge plane defect sites based on ac higher harmonics, capacitance, and dc aperiodic component that are available from a single experiment have been compared. The edge plane defect levels deduced from capacitance (fundamental harmonic ac component) and higher harmonic Faradaic currents are considered to be more reliable than estimations based on apparent rate constants derived from the dc aperiodic component or conventional dc cyclic voltammogram.

A novel large value ac capacitor using two dc capacitors

The paper proposes a novel power electronic circuit to realize bipolar ac capacitor using two unipolar dc capacitors. Experiments are successfully carried out to verify the validity of the proposed...

Gas indiffusion contribution to impedance in tin oxide thick films

The ac electrical resistance and capacitance of SnO2 thick films under vacuum and air atmosphere were analyzed using the Cole–Cole plot. To fit the experimental results, a simple circuit model that considers a capacitance and a resistance in parallel was employed. An explanation for the resistance variation considering spherical grains with different characteristics is proposed. A careful analysis of the resulting depletion layers and doping levels gives evidence for gas diffusion into the grains.

Studies on ac response of zinc oxide pellets

The ac responses of the ZnO pellets have been studied by ac measurements (impedance, capacitance and phase angle) over the temperature range 300–435 K. The ac conductivity of the ZnO pellets is observed to be proportional to ωs, where ω is the angular frequency and the exponent s is a temperature- and frequency-dependent parameter. Based on the existing theories of ac conduction, it has been concluded that for low frequency region (20 Hz–2 kHz), the dominant conduction mechanism in the ZnO pellets is multihopping at all temperatures, whereas for high frequency region (500 kHz–2 MHz), the small polaron tunneling model is the dominant mechanism in the pellets. Activation energies for ac conduction processes are estimated to be in the range of 0.028–0.277 eV which are found to vary with the frequency of the ac signal. These results are found to be consistent with the hopping model. The ac capacitance and the dielectric loss tangent are found to be dependent on both frequency and temperature. Such dependences have been explained taking into account the equivalent circuit model comprising a frequency-independent capacitive element in parallel with a temperature-dependent resistive element, both in series with a low value resistance. Impedance spectroscopy studies show single semicircular arcs in the complex impedance spectra at all temperatures in the range 300–435 K, with their centres lying below the real axis at a particular angle of depression indicating a multirelaxation behaviour in the pellets.

A novel large value ac capacitor using two dc capacitors

AbstractThe paper proposes a novel power electronic circuit to realize bipolar ac capacitor using two unipolar dc capacitors. Experiments are successfully carried out to verify the validity of the proposed circuit. Copyright © 2008 John Wiley &amp; Sons, Ltd.