Ac Capacitance Research Articles

Isolated ZVS High-Frequency-Link AC-AC Converter With a Reduced Switch Count

This paper introduces an isolated soft-switched three-phase ac–ac converter with a high-frequency link and reduced switch count for low- and medium-power applications. A small air-gapped high-frequency transformer acts as the key component by providing galvanic isolation as well as transferring power using its magnetizing inductance. Two small ac capacitors are placed on both sides of the transformer to realize zero-voltage switching for the semiconductor devices in addition to passive clamping. Therefore, the proposed topology is expected to be highly efficient and reliable due to soft-switching operation and exclusion of limited-lifetime electrolytic capacitors. Furthermore, the proposed converter has a high input power factor, step-up/down ability, and bidirectional power flow with arbitrary output voltage amplitude and frequency. The basic operation of the converter includes eight modes and is described in detail. Theoretical analysis and design are also given to illustrate its operation at various operating points. Simulation and experimental results are provided to verify the performance of the proposed power converter.

A Soft-Switched Three-Phase AC–AC Converter With a High-Frequency AC Link

This paper introduces a three-phase ac-ac resonant converter with a high-frequency ac link, soft-switching operation, almost unity input power factor, step-up/down capability, and bidirectional power flow. The converter's link is composed of an ac inductor and a small ac capacitor. The link inductor transfers power by charging through the input and discharging to the output, and the link capacitor creates partial resonances to realize zero-voltage switching for all the switches of the converter. Therefore, the proposed topology is expected to have high reliability and long lifetime due to the soft-switching operation and exclusion of short-life electrolytic capacitors. The total number of switches in this novel topology is as low as 12, which is suitable for low- and medium-power applications. The basic operation of the converter includes eight modes, which are described in detail. The full analysis and optimal design of the proposed topology are provided as well. Results of several simulations and experiments at different operating conditions are also presented to show the validity of the proposed converter.

Step‐up/down three‐phase resonant high‐frequency ac‐link inverters

A new family of three-phase resonant inverters with a high-frequency (HF) ac-link and zero-voltage switching is introduced. In these inverters, the maximum output voltage is not limited to the dc-bus voltage, and the converters can conveniently operate in the step-up and step-down modes in both the forward and the reverse directions. The HF ac-link of the converters is composed of an ac inductor and a small ac capacitor. The link inductor is the main component for transferring power, and the link capacitor provides partial resonances for the zero voltage switching of all the power switches. The link inductor can be replaced by a small HF transformer to achieve galvanic isolation. This transformer is substantially smaller and more efficient than the conventional line-frequency three-phase isolation transformers. The total number of switches of the proposed unidirectional dc–ac converter is as low as seven, suitable for the low power applications. By adding three more switches, the converter can transfer power in both directions. The basic operation of the new converters includes six modes and is described in detail. A detailed analysis of the introduced topology is also given to reveal its behaviour at various operating points. Several simulation and experimental results are given to demonstrate the effectiveness of the proposed power converters.

Doping behaviors of yttrium, zinc and gallium in BaTiO3 ceramics for AC capacitor application

The doping behaviors of yttrium, zinc and gallium and their effects on the dielectric properties and microstructures of BaTiO3 were investigated. Y3+ dissolved in the lattice of BaTiO3, replacing both Ba2+ site and Ti4+ site; while Zn2+ and Ga3+ tended to occupy Ti4+ site. Compared with Y2O3 and Ga2O3, ZnO suppressed grain growth of BaTiO3 more effectively and promoted greater uniformity of grains, thus reducing the dielectric loss. The addition of Ga2O3 inhibited the appearance of second phase Y2Ti2O7 and enhanced the sinterability, which was ascribed to the compensation mechanism and synergistic effect. Proper amount of Y2O3, Ga2O3 and ZnO significantly improved the dielectric temperature characteristics due to the formation of the core–shell structure in the codoped BaTiO3 ceramics. High performance dielectrics with er of 2,690, tanδ of 1.0 % (at 1 kHz), and alternating current breakdown voltage E > 3.73 kV/mm, were achieved.

Fed an Induction Motor Drive Using an Advanced Current Source Inverter

The thyristor-based current source inverter (CSI) fed ac machine drives are often employed in high power applications. However, the thyristor-based CSI has intrinsic drawbacks because the thyristors cannot be turned off by the gating signals. Therefore, external circuits are required to turn the thyristors off by using reverse-biased voltages, as well as transfer reactive energies of inductive loads after turning off the thyristors. This paper presents a hysteresis modulated CSI drive which overcomes all these drawbacks and results in sinusoidal motor voltage and current even with CSI switching at fundamental frequency. The proposed CSI drive uses a voltage source inverter as an auxiliary circuit replacing the bulky ac capacitors used in the conventional drive. Then an indirect field oriented controlled CSI drive based on proposed configuration is developed. The simulation results are presented. Simulation results show that the proposed drive has stable operation even at low speeds

Simulation of cathodically protected pipeline with capacitive AC mitigation devices for interpretation of misleading instant Eoff readings

A cathodically protected buried pipeline is usually connected to earthing electrodes via several capacitive AC mitigation devices placed at various locations along the pipeline in order to decrease the AC voltage interference. During periodic switching of the cathodic protection circuit for the instant Eoff measurements, the capacitive AC mitigation devices tend to be discharged through the soil provoking an error in the instant Eoff readings, thus resulting to a possible false evaluation of the cathodic protection effectiveness. The aim of the present work is to investigate the role of the main factors, which influence this error. An electric circuit simulation of the pipeline with connected earthed capacitors is proposed. The circuit analysis has shown that the key determining factors of the aforementioned capacitive discharge time constant comprise the capacitance value of the capacitive AC mitigation device at every earthing site, the number of earthing sites where the capacitive AC mitigation devices are installed, the earthing electrodes resistance and the pipeline (or coating) resistance to remote earth. Measures to minimise this misleading error of Eoff readings through the control of the abovementioned factors and the modification of the time of the Eoff measurement are suggested. In this context, the Eoff should be measured at prolonged time after every switching, contrary to the usual practice. Furthermore, the capacitive AC mitigation devices should not be disconnected from the pipeline in order to prevent the adverse effects of AC interference on cathodic protection monitoring. The present work contributes to the understanding of the capacitive AC mitigation devices effect on pipeline cathodic protection monitoring, which has been rarely analysed in the past. Moreover, by recommending novel approaches, the paper can stand as a reference to the cathodic protection operators for a simple, reliable and economic monitoring of the cathodic protection effectiveness in presence of earthed capacitive AC mitigation devices.

Boost‐type inverter‐less shunt active power filter for VAR and harmonic compensation

Harmonics in power system caused by non-linear loads degrade the power quality. As a result, shunt active power filters are widely applied to deal with current harmonics and poor power factor. However, the traditional voltage-source inverter-based active filters, which usually employ large electrolytic capacitors as the energy-storage elements, cannot get much market penetration. An emerging technique that adopts the direct AC–AC conversion and even harmonic modulation can help to eliminate the bulky energy-storage elements while realising the harmonic compensation and power factor correction functions. A three-phase three-wire boost-type inverter-less active power filter for power factor correction and harmonic compensation is presented. The proposed solution eliminates the bulky electrolytic capacitors. Meanwhile, smaller amount of AC capacitors are needed compared with the buck-type AC–AC converters. Furthermore, continuous grid-side current is obtained, which implies that no extra grid-side filters are needed. Numerical calculation is applied to build the model, and the closed-loop control algorithm employing the synchronous reference frame for power factor correction and multi-harmonics compensation is also developed. The proposed topology and control algorithm is simulated in MATLAB/Simulink and validated with a 15 kVA experimental prototype.

Parameters Optimization Design of Four-Switch Three-Phase Shunt Active Power Filter

The working capability and cost of four-switch three-phase (FSTP) shunt active power filter (APF) are determined by the parameter selection. Based on mathematical model of FSTP inverter, the effect of key parameters, such as dc-bus voltage level and capacitor voltage transform is analyzed. On account of ac output filter inductance, voltage and capacitance on dc side, output filter in the design of four-switch three-phase APF, this paper expounds the optimal selection approach of parameters, sets up simulation model in PSIM, correctness of the method is verified by simulation on the basis of analysis about law of effect.

Modification of electronic properties of top-gated graphene devices by ultrathin yttrium-oxide dielectric layers

We report the structure characterization and electronic property modification of single layer graphene (SLG) field-effect transistor (FET) devices top-gated using ultrathin Y(2)O(3) as dielectric layers. Based on the Boltzmann transport theory within variant screening, Coulomb scattering is confirmed quantitatively to be dominant in Y(2)O(3)-covered SLG and a very few short-range impurities have been introduced by Y(2)O(3). Both DC transport and AC capacitance measurements carried out at cryogenic temperatures demonstrate that the broadening of Landau levels is mainly due to the additional charged impurities and inhomogeneity of carriers induced by Y(2)O(3) layers.

On the voltage‐dependent series resistance of a planar Schottky barrier diode

The series resistance of a planar Schottky barrier diode fabricated on p‐type silicon is investigated by analysing the current–voltage characteristics of the device. Different characterisation techniques have been applied to obtain the value of the series resistance of the device. It is found that the existing techniques are either not applicable for the present device or yield unreliable value for the series resistance. A numerical analysis of the I–V data reveals unusual voltage dependence of the series resistance of the device. The anomaly has been resolved by postulating a potential barrier at the ohmic contact and drawing analogy to serially connected high‐ and low‐barrier diodes in a back‐to‐back configuration. It is found that the voltage dependence of the series resistance of the device can be described by certain empirical law, which also applies to device on GaN. The measured voltage behaviour of the ac resistance and capacitance of the device at different frequencies have been found to be consistent with those of a serial combination of diodes considered to verify the postulate made in interpreting the I–V data.

Evaluation of slim-edge, multi-guard, and punch-through-protection structures before and after proton irradiation

Planar geometry silicon pixel and strip sensors for the high luminosity upgrade of the LHC (HL-LHC) require a high bias voltage of 1000V in order to withstand a radiation damage caused by particle fluences of 1×1016 1MeVneq/cm2 and 1×1015 1MeVneq/cm2 for pixel and strip detectors, respectively. In order to minimize the inactive edge space that can withstand a bias voltage of 1000V, edge regions susceptible to microdischarge (MD) should be carefully optimized. We fabricated diodes with various edge distances (slim-edge diodes) and with 1–3 multiple guard rings (multi-guard diodes). AC coupling insulators of strip sensors are vulnerable to sudden heavy charge deposition, such as an accidental beam splash, which may destroy the readout AC capacitors. Thus various types of punch-through-protection (PTP) structures were implemented in order to find the most effective structure to protect against heavy charge deposition. These samples were irradiated with 70MeV protons at fluences of 5×1012 1MeVneq/cm2–1×1016 1MeVneq/cm2. Their performances were evaluated before and after irradiation in terms of an onset voltage of the MD, a turn-on voltage of the PTP, and PTP saturation resistance.

Application of UPFC for enhancement of voltage profile and minimization of losses using Fast Voltage Stability Index (FVSI)

Application of UPFC for enhancement of voltage profile and minimization of losses using Fast Voltage Stability Index (FVSI)Transmission line loss minimization in a power system is an important research issue and it can be achieved by means of reactive power compensation. The unscheduled increment of load in a power system has driven the system to experience stressed conditions. This phenomenon has also led to voltage profile depreciation below the acceptable secure limit. The significance and use of Flexible AC Transmission System (FACTS) devices and capacitor placement is in order to alleviate the voltage profile decay problem. The optimal value of compensating devices requires proper optimization technique, able to search the optimal solution with less computational burden. This paper presents a technique to provide simultaneous or individual controls of basic system parameter like transmission voltage, impedance and phase angle, thereby controlling the transmitted power using Unified Power Flow Controller (UPFC) based on Bacterial Foraging (BF) algorithm. Voltage stability level of the system is defined on the Fast Voltage Stability Index (FVSI) of the lines. The IEEE 14-bus system is used as the test system to demonstrate the applicability and efficiency of the proposed system. The test result showed that the location of UPFC improves the voltage profile and also minimize the real power loss.

Excess current/capacitance observation on polymer–fullerene bulk heterojunction, studied through I–V and C/G–V measurements

Excess current/capacitance behavior was observed on the Ag:Ca/P3HT:PCBM/PEDOT:PSS/ITO structure and the associated mechanism(s) were identified by performing both dc current–voltage (I–V) and ac capacitance–conductance–voltage (C/G–V) measurements in dark and light exposure, respectively. The excess current/capacitance issue, originated owing to a minority carrier injection, has manifested itself as conductivity modulation in I–V and decrease in capacitance in C–V at a sufficiently large forward bias. Satisfactory analysis in I–V as transport property and C(G)–V as storage feature were carried out on Ag:Ca/P3HT:PCBM/PEDOT:PSS/ITO structure.

Cu2O–PEPC Composite Based Pressure Transducer

In this study, Al/Cu2O–PEPC/Al pressure transducer was designed and fabricated by drop-casting the blend of Cu2O–PEPC microcomposite thin films of copper oxide (Cu2O) micropowder, (3 wt.%) and poly-N-epoxypropylcarbazole, PEPC, (2 wt.%) in benzol on Al substrates. The thicknesses of the Cu2O–PEPC films were in the range of 30–100 μm. The AC resistance and capacitance at 120 Hz of the transducer was decreased by 1.1–1.4 and increased by 1.2–1.8 times, respectively, as the pressure was increased up to 100 kPa. The resistance-pressure and capacitance-pressure relationships were simulated.

AC electrical properties of FeIn2S4

The frequency and temperature dependences of the ac capacitance and resistivity of FeIn2S4 semiconductors are studied. Resonances are observed at certain temperatures in the frequency range (2.5–5.0) × 105 Hz. The permittivity of the crystals and the activation energy of charge carriers are determined. It is found that electrical conduction in the given temperature interval is governed by an activation mechanism. The activation energy is frequency-dependent, because the relaxation time of barrier layers decreases with rising frequency.

Life-Cycle Monitoring and Voltage-Managing Unit for DC-Link Electrolytic Capacitors in PWM Converters

A novel life-cycle monitoring and voltage-managing device for dc-link electrolytic capacitors in pulsewidth modulation converters is presented. The system performs online identification of the capacitor's equivalent series resistance (ESR) in order to detect the life-cycle status and permit preventive maintenance. The ESR detection is based on the capacitor's ac losses calculated from voltage/current measurements using a simple low-cost microcontroller. The unit is designed as small printed circuit board located directly at the capacitor's screw terminals in order to simplify the required temperature sensing and to minimize wiring effort. The minimized energy consumption allows a power supply taken out of the capacitor to be tested. Besides life-cycle monitoring, the unit further facilitates energy-efficient voltage balancing for capacitors in series arrangements avoiding any power resistors typically used in balancing circuits. Instead, the unit controls the capacitor voltage by influencing its power consumption. Each individual monitoring unit (one per each power capacitor of the converter) transfers the ESR results to the converter's main controller and receives control commands via a common optoisolated fieldbus. Alternatively, this data transfer is performed using wireless near-field communication leading to a completely autonomous monitoring unit without any wiring for power supply and data transmission.

VSC with a T-connected transformer-based electronic load controller for an isolated asynchronous generator feeding three-phase four-wire loads in a micro hydro system

This paper deals with a novel solid state controller (NSSC) for isolated asynchronous generator (IAG) feeding three-phase four-wire loads driven by constant power prime movers, such as uncontrolled micro hydro turbines. AC capacitor banks are used to meet the reactive power requirement of the asynchronous generator. The proposed NSSC is realised using a set of IGBTs (insulated gate bipolar junction transistors) based current controlled voltage source converter (CC-VSC) and a DC chopper at its DC bus, which keeps the generated voltage and frequency constant in spite of changes in consumer loads. The neutral point of the load is created using a T-configuration of the transformers. The IAG system is modelled in MATLAB along with Simulink and PSB (power system block set) toolboxes. The simulated results are presented to demonstrate the capability of isolated generating system consisting of NSSC and IAG driven by uncontrolled micro hydro turbine and feeding three-phase four-wire loads.

Capacitive level meter for liquid hydrogen

A capacitive level meter working at low temperatures was made to use in magnetic refrigerator for hydrogen liquefaction. The liquid level was measured from the capacitance between parallel electrodes immersed in the liquid. The meter was tested for liquid nitrogen, hydrogen, and helium. The operation was successful using an AC capacitance bridge. The estimated sensitivity of the meter is better than 0.2 mm for liquid hydrogen. The meter also worked with pressurized hydrogen.

The resistive and capacitive Cu2O–PEPC composite-based displacement transducer

In this paper, the Ag/Cu2O–PEPC (poly-N-epoxypropylcarbazole)/Al displacement transducer was designed and fabricated by drop-casting the blend Cu2O–PEPC microcomposite thin films consisting of copper oxide (Cu2O) micropowder (3 wt%) and PEPC (2 wt%) in benzol on glass substrates with pre-deposited silver electrodes of thickness 100 nm. The thicknesses of the Cu2O–PEPC films were in the range of 50–60 μm. The dc resistance of the transducer decreased 1.56 times as the displacement was changed over the range 0–0.6 mm, and accordingly, the ac capacitance at 120 Hz increased 2.31 times as the displacement was changed over the range 0–1.3 mm. The resistance–displacement and capacitance–displacement relationships were simulated. With respect to initial values, the maximum deviations of simulated resistance–displacement and capacitance–displacement graphs from experimental curves were in the range of 3 and 5%, respectively.

Interacting and noninteracting dipole systems in ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer

Ultrathin films of poly(vinylidene fluoride-trifluoroethylene) copolymers are prepared by a Langmuir–Blodgett technique. These films show ferroelectric properties like hysteresis loops of the polarization and butterfly curves of the ac capacitance versus the applied field. Small signal measurements exhibit a power law of the dielectric permittivity in the frequency domain as well as a power law for the depolarization current in the time domain, respectively. This behavior can be explained by a distribution of relaxation times in the dipole system. From these measurements alone it cannot clearly be distinguished whether this distribution results from the interaction between the dipoles or if the dipoles relax independently from each other with individual relaxation times. To this aim the field reversal experiment which is sensitive to the coupling of dipole systems is introduced. As final result we get that the small signal nonswitching relaxational response resulting in the dielectric permittivity is coupled to the large signal ferroelectric behavior resulting in the hysteresis loops.