AC Voltage Stress Research Articles

An Active Power Decoupling-Integrated Reduced-Switch Current-Fed Switched Inverter

In recent literature, high-gain dc–ac inverters are employed in grid-connected photovoltaic (PV) systems. These inverters can have several features, such as shoot-through immunization and buck/boost capabilities achieving a wide range of operations with reliability and minimum distortion. One such example is the single-phase reduced-switch current-fed switched inverter (RSCFSI). This article proposes the enhanced boost control-based pulsewidth modulation (EBC-PWM) strategy for RSCFSI to improve the inverter’s dc–ac gain. Being a single-phase inverter, RSCFSI suffers from the double-line frequency ripple problem. However, a low-frequency (LF) ripple analysis reveals that the RSCFSI is also plagued with fundamental frequency ripple, which poses additional challenges for the active power decoupling (APD) integration and corresponding closed-loop controllers. Although both LF ripples can be alleviated using markedly large passive elements, the power density and reliability of the inverter worsen. In this article, an APD-integrated RSCFSI (APDRSCFSI) is introduced as a solution that deflects the aforementioned LF ripple energy to an auxiliary capacitor without adding more active switches. As a result, the inverter can employ smaller passive elements. In addition, the EBC-PWM strategy is also extended for APDRSCFSI, improving its dc–ac voltage gain and voltage stress. A closed-loop control technique that combines output voltage control and APD functionality is also illustrated. A hardware prototype is used to corroborate the advantages of APDRSCFSI.

Partial Discharge Behavior of Epoxy-Mica Insulation System under Superimposed AC and DC Voltage Stress

Recent technology advances in wind energy conversion systems include a new type of permanent magnet synchronous generator with a segmented stator that allows lightweight concepts with high efficiency and superior controllability. Each stator segment is driven by a separate power converter. The various cascaded power converters can be connected in series and in parallel, which ultimately leads to the superposition of a DC and an AC voltage stress in the insulation of the stator coils. The resulting DC-AC mixed voltage is an entirely new electrical stress for the epoxy-mica insulation system that is typically utilized as main insulation of rotating electrical machines. In order to resemble this stress, typical dielectric tests such as partial discharge (PD) measurements and long-term aging of model insulation systems have to be be carried out under DC-AC mixed voltage stress. The mentioned investigations are presented in this contribution. The results indicate that the AC voltage is dominant with regard to the PD inception and influences the PD parameters such as the apparent charge and the repetition rate substantially when compared to the DC voltage proportion. The accelerated aging tests with significantly higher AC-DC mixed voltage stress than is expectable under operating conditions revealed no measurable PD deterioration of the investigated insulation systems.

Partial Discharges in HVDC Cables - The Effect of the Temperature Gradient During Load Transients

The paper investigates the role of the space charge accumulation phenomena in the inception of Partial Discharges (PD) in a defect within the bulk of the dielectrics of a High Voltage Direct Current (HVDC) cable. It is widespread accepted that the number of PD occurring in an HVDC cable during its lifetime is largely less than that under AC voltage stress. This is essentially due to higher values of the voltage between the conductor and the screen needed to trigger PD under a steady DC voltage than the same value under AC stress. Nevertheless, due to the dependency of the electrical conductivity of the insulation on the electric field and temperature, PD in HVDC cables is strongly influenced by the temperature distribution and, consequently, on the load current. Thus, the main goal of this research is to evaluate the main parameters of the PD occurring in an HVDC cable during a load transient. Numerical simulations by using Comsol Multlphysics® are validated by the results obtained with an ad hoc experimental setup. A model DC cable, subjected to transient phenomena due to a current step and with an air void defect in the insulation material, is simulated and setup. On the other hand, an experimental setup is used to carry out PD measurements on the same case study. The results of this research show the strong dependency of the PD parameters on the temperature distribution varying over time during a load transient.

Evaluation of Erosion Discharge Characteristics in Inclined Plane Tracking and Erosion Tests on Silicone Rubber under AC and DC Voltages

This paper investigates partial discharge (PD) characteristics that lead to the erosion of silicone rubber (SR) polymer under AC stress and DC stress of both polarities. The experiments are performed on high temperature vulcanized (HTV) SR material samples. The inclined plane test apparatus, constructed in accordance with IEC 60587 requirements, is employed to produce the surface partial discharges and the resulting accelerated aging of the specimens. Two commercial instruments are used to obtain the PD data. A concurrent analysis of visual observations of discharge and PD data is performed to classify discharges based on the severity of material degradation that each type causes. Three types of PD are identified and characterized using diagrams of charge magnitude versus time and a signal processing technique called time-frequency mapping. Individual pulse waveshapes of each type of discharge are also analyzed. PD pulse waveforms are analyzed according to their amplitude, energy, pulse width, and frequency spectrum. These pulse waveform parameters are evaluated and compared for the eroding discharge pulses under AC and DC voltage stresses. It is found that the stages of material degradation during IPT are related to the variations in discharge magnitude and the location of pulse clusters on the time-frequency maps.

Microscopic analysis of polyimide nanocomposites aging mechanism exposed to partial discharges under high frequency voltage stress

For the sake of investigating the influence of nanoparticles on the reduction in structural damage of neat polyimide (PI) and PI nanocomposites, scanning electron microscope (SEM) based microscopic study was conducted on unaged and aged pure PI and PI/TiO2 nanocomposite samples exposed to partial discharge activity under high frequency ac voltage stress. PI/TiO2 nanocomposites with 2, 3 and 5 wt.% were fabricated adopting in-situ polymerization technique. Partial discharge (PD) aging measurements were made and PD characteristic parameters were obtained to analyze the electrical performance of both neat PI and PI nanocomposites. Based on microscopic structural analysis, it was found that PI nanocomposite sample experienced less structural damage in contrast with neat PI. Smaller and lesser number of voids were found in PI/TiO2 nanocomposites than that of neat PI in result of PDs reduction in PI nanocomposite.

Studying the Aging Evolution of Oil-Paper Insulation Comprising of a Gas Cavity Under the Stress of AC and DC Voltage Depending on Partial Discharge and Dissolved Gases Measurements

The oil-paper insulation system is widely used for the insulation purposes of power transformers as a result of its ability to withstand high thermal and electrical stresses. The presence of a gas cavity between impregnated papers layer is a serious problem. When localized breakdown takes place within this cavity, partial discharge (PD) occurs and ages the rest of the insulation system. This aging phenomenon declines the expected life time of the insulation system and subsequently the life time of the power transformers. Dissolved gas analysis is a recommended technique that is used in order to assess the state of the insulation system inside the transformers. In this paper, PD is recorded in the case of gas cavity presence between the layers of the impregnated papers under the effect of AC and DC voltage stress. This record is extended from the instance of PD inception until the breakdown of the whole insulation system. Furthermore, the concentration of the gases that dissolve in the insulating oil due to the aging of the impregnated paper is measured eight times during the record of the PD. Finally, the degradation of the insulating papers is studied optically through the use of a microscope at different stages of the aging test. Depending on PD measurements, dissolved gas analysis (DGA) results, and the optical study of the paper aging, the evolution of the aging of oil-paper insulation system as a result of internal PD is declared when AC and DC voltage stresses are applied. It is obtained that the aging evolutionary process totally differs with the alteration of the applied voltage.

Charge generation and electrical degradation of cross‐linked polyethylene

Cross‐linked polyethylene is a chemically stable polymeric dielectric material with high dielectric strength, low dielectric constant and a small dissipation factor. Because of these excellent dielectric properties, cross‐linked polyethylene is used as an insulating material for electric power cables. Based on the results of much research working as an engineer of a cable manufacturer and in academia, this article describes the outline of charge generation under DC voltage stress and electrical degradation occurring in the material under AC voltage stress. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Raether–Meek criterion for prediction of electrodeless discharge inception on a dielectric surface in different gases

Electrodeless streamer inception on an epoxy surface under AC voltage stress was investigated for different gas compositions and pressures, with a focus on the pressure region below 1 bar. For this purpose, we used a set-up with cylindrical electrodes embedded out-of-axis in a cylindrical epoxy rod. Experiments were performed in N2, SF6, ambient air, Ar and CO2. The discharge inception voltage was measured, from which the critical value K of the ionization integral was reconstructed assuming a non-disturbed Laplacian field distribution. We have validated that for electropositive gases Ar an N2 the generally assumed value of K = 10 is in good agreement with our measurements. For electronegative gases, however, the experimentally obtained values turned out to be considerably higher. We attribute this discrepancy mainly to the statistical time delay of the first electron; to increase the probability of discharge inception in a critical region, it was necessary to extend the critical area by means of applying an overvoltage to the system.

Influence of DC Stress Superimposed with High Frequency AC on Water Tree Growth in XLPE Insulation

Power electronics used for HVDC converters will stress cable insulation with a DC voltage with superimposed transients. The effect these transients have on the performance of the polymeric cable insulation is yet not clear. The main purpose of this work has been to investigate the effect of such transients on XLPE cable insulation when exposed to moisture. Laboratory experiments were performed on Rogowski shaped test objects with an insulation thickness of 1.3 mm. At one of the semi-conductors, 20 sodium chloride (NaCl) particles were placed in order to facilitate initiation of vented water trees. The test objects were conditioned with water at 20 °C for two months ensuring saturation of water inside the insulation system before testing.The test objects were aged with an AC voltage simulating the transients from a HVDC converter. Experiments were performed using the AC voltage with and without DC stress to investigate the influence of the DC level on the water tree growth. The ageing was done at 30 °C.Test objects were taken out and inspected for water tree growth regularly. The results show a rapid ageing caused by water treeing when exposed to the DC voltage overlaid AC voltage with a frequency of 5 kHz and a pure AC voltage stress.

Investigation on the Applicability of the Time Domain Analysis of Discharges in Gases for the Defect Identification at AC Voltage

The partial discharge diagnosis is an established instrument for the condition assessment of high voltage insulations and equipment. Under AC voltage stress the phase resolved pattern is of great significance in order to become aware of the type of fault. As a result of the inapplicability for DC voltage stress, approaches for alternative interpretation techniques such as the time domain analysis of partial discharges were identified in recent investigations. In these different types of fault are taken into account as well as different insulating media. The purpose of this paper is to investigate whether an analysis of the pulse shape is also applicable for the defect identification under AC voltage stress. By focussing on gaseous insulating media, contact noise and surface discharges are emulated in ambient air, whereas corona discharges are emulated in ambient air and oxygen. A method for analysing discharges, occurring in the negative and the positive half-wave of the test voltage, is proposed and discussed.

Statistical investigation of AC breakdown voltage of nanofluids compared with mineral and natural ester oil

In this study, three different insulating liquids, natural ester, mineral oil which is currently used by the Public Power Corporation of Greece and a nanofluid of surface coated Fe3O4 nanoparticles into a natural ester matrix, were subjected to AC voltage stress and their statistical breakdown voltage was measured and compared. Three different electrode configurations were used during the measurements in order to study the effect of non-uniform fields on the breakdown voltage distribution. Four different statistical distributions were used for the statistical processing of the experimental results. The two of them were the frequently used normal and Weibull distributions; in addition, Gumbel and generalised extreme value (GEV) function distributions were studied. It was shown that, in most cases, the experimental data followed GEV and Weibull functions rather than normal or Gumbel. Therefore, GEV statistics fits indiscriminately better to the experimental results concerning breakdown voltage, for the three different electrodes configuration and the three understudied insulating oils. The experimental results indicated that nanofluid has better performance, in terms of dielectric breakdown voltage, compared with natural ester oil and mineral oil, hence nanofluid is considered as a potential substitute of the conventional dielectric liquids, with enhanced properties and prospects.

Failure Analysis of Metal Oxide Arresters under Harmonic Distortion

The probability of accelerated degradation or reduced time to failure of metal oxide arresters (MOA), when continuously exposed to distorted ac conduction, is analysed in this study. Metal oxide-based surge arresters of similar size and electrical characteristics are tested using accelerated degradation at elevated temperature and distorted ac voltage stress. The three-parameter Weibull probability method is applied to analyse the obtained time to failure distribution. The Fourier series’ expansion is also relied upon in order to evaluate the content of the harmonic resistive components of the measured leakage current. The results obtained indicate that for 6.24% and 5.58% content of the 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> and the 5 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> harmonic component, respectively embedded in the applied voltage stress, the probability of reduced time to failure or accelerated degradation is found to be 58.93 %, and the mean life reduction obtained is just above 40%. These results correlate with the pronounced shift of the U – I curve as well as the increase in the magnitude of the respective harmonic resistive current components of the arrester samples.

Encapsulation of high frequency organic Schottky diodes

Both operational lifetime and shelf-life of copper phthalocyanine (CuPc) based high frequency Schottky diodes have been studied in this work. Thin films of amorphous Si (a-Si) and polychloro-(para-xylylene) (parylene C) are deposited as a barrier layer using low temperature chemical vapor deposition to improve the lifetime of the CuPc Schottky diodes. The CuPc Schottky diodes encapsulated with the barrier layer exhibit at least 6× longer operational lifetime under both DC and AC voltage stress. Encapsulation does not significantly improve the shelf-life of the organic Schottky diodes.

Al2O3-HfO2-Al2O3와 SiO2-HfO2-SiO2샌드위치 구조 MIM 캐패시터의 DC, AC Stress에 따른 특성 분석

In this paper, reliability of the two sandwiched MIM capacitors of <TEX>$Al_2O_3-HfO_2-Al_2O_3$</TEX> (AHA) and <TEX>$SiO_2-HfO_2-SiO_2$</TEX> (SHS) with hafnium-based dielectrics was analyzed using two kinds of voltage stress; DC and AC voltage stresses. Two MIM capacitors have high capacitance density (8.1 fF/<TEX>${\mu}m^2$</TEX> and 5.2 fF/<TEX>${\mu}m^2$</TEX>) over the entire frequency range and low leakage current density of ~1 nA/<TEX>$cm^2$</TEX> at room temperature and 1 V. The charge trapping in the dielectric shows that the relative variation of capacitance (<TEX>${\Delta}C/C_0$</TEX>) increases and the variation of voltage linearity (<TEX>${\alpha}$</TEX>/<TEX>${\alpha}_0$</TEX>) gradually decreases with stress-time under two types of voltage stress. It is also shown that DC voltage stress induced greater variation of capacitance density and voltage linearity than AC voltage stress.

A finite weakest-link model of lifetime distribution of high- k gate dielectrics under unipolar AC voltage stress

This paper presents a new probability distribution function for the breakdown lifetime of high- k gate dielectrics under unipolar AC voltage stress. This function is derived from a finite weakest-link model, where the gate oxide layer is considered to consist of many potential breakdown cells. Each potential breakdown cell is modeled as a series coupling of several subcells, which is analogous to the fiber-bundle model for the strength statistics of structures. The present model indicates that the type of lifetime distribution varies with the gate area and the dependence of the mean lifetime on the gate area deviates from the classical Weibull scaling law. It is shown that the model agrees well with the observed lifetime histograms of HfO 2 based gate dielectrics under unipolar AC voltage stress.

Vibration of a water droplet on a polymeric insulating material subjected to AC voltage stress

The behavior of a water droplet on a silicone rubber plate was observed with a high-speed camera under ac voltage application. It was confirmed that a water droplet vibrated and that a remarkable deformation from the original droplet form occurred at the mechanical resonance frequency. Vibration frequency of an uncharged water droplet was double the frequency of the applied voltage. On the contrary, in the case of an artificially charged droplet, vibration frequency was the same with that of the applied voltage. A macroscopic mechanical model was devised, to which the experimental data were well correlated. Flashover voltage via the water droplet was also measured. Lower flashover voltage was obtained at the mechanical resonance frequency.

Laboratory Investigations of the Electrical Performance of Ice-covered Insulators and a Metal Oxide Surge Arrester

This review paper summarizes the main results of the laboratory investigations conducted at Tampere University of Technology (Finland) on ice-covered insulators and a metal oxide surge arrester (MOA). The most important factors affecting the dielectric strength of high voltage (HV) insulators in freezing rain conditions are the resistivity of the freezing water, the length and the number of icicles between insulator sheds and electrodes and the state of the ice deposit (i.e. how wet it is). The studies verify that most of the commercially available HV insulators may flashover even with a normal phase-to-ground ac voltage in rather light salty (or otherwise slightly contaminated), but long-duration freezing rain conditions. The ac strength of a clean insulator string under rime ice during melting period was approximately 75% of the ac strength in dry conditions. Accordingly, the ac strength of the contaminated (ESDD ap 0.3 mg/cm2 NaCl) insulator strings during the melting period of rime was on average 65% of the AC strength in dry conditions. The icing of an MOA consisting of two or more units in series may have harmful effects on the electrical performance of the MOA. With ac voltage stress an unevenly ice-coated MOA may be thermally stressed due to the leakage current transition from the ice covering of one unit to the interior of another unit not covered with so much ice. With switching impulse current surges the residual voltage across an ice-covered unit rather easily causes an external flashover. This leads to a sudden increase in the surge current: with the MOA of two units studied the current peak reached as much as twice the peak value in normal operation without ice covering.

14 MHz organic diodes fabricated using photolithographic processes

Organic semiconductor-based Schottky diodes operating at 14MHz, fabricated using conventional photolithographic and etching processes, have been demonstrated. Copper phthalocyanine is the semiconductor, with gold and aluminum as the Ohmic and Schottky contacts, respectively. The organic diode based rectifier circuit generated a dc output voltage of approximately 2V at 14MHz, using an input ac signal with a zero-to-peak voltage amplitude of 5V. These devices showed little degradation under continuous ac voltage stress when operated in vacuum.

Reliability characteristics of high- k dielectrics

In this paper, recent results of Weibull slopes, area scaling factors, and breakdown behaviors observed for both soft breakdown and hard breakdown are discussed. These results would help to shed light on the breakdown mechanism of HfO 2 gate dielectrics. The Weibull slope β of the hard breakdown for both the area dependence and the time-to-dielectric-breakdown distribution was found to be β=2, whereas that of the soft breakdown was about 1.4 (EOT=14 Å). We also integrated the time-to-breakdown characteristics of HfO 2 under unipolar AC voltage stress on MOS capacitors. The results show that longer lifetime of HfO 2 has been observed when compared to constant voltage stress. Higher frequency and lower duty cycle in the AC stress resulted in longer lifetime. As thickness decreases, the amount of lifetime enhancement decreases. The enhancement of unipolar t BD is attributed to less charge trapping during the “on time”, t on and charge detrapping during the off time, t off. It is proposed that time ( τ in) for charge to be trapped in HfO 2 is longer than t on of unipolar stress under high frequency. In addition to experimental results, possible solutions are discussed.

SILC dynamics in MOS structures subject to periodic stress

This work investigates stress-induced leakage current (SILC) in thin-oxide MOS capacitors subject to (quasiperiodic) ac voltage stress, under the condition of fixed charge fluence through the oxide. It shows that both trap creation and spontaneous trap annealing play a significant role when the duration of, and the time between, high-voltage pulses are comparable with characteristic times of trap dynamics. A phenomenological model is introduced that is able to accurately represent the main physical phenomena due to pulsed voltage stress under conditions of interest for unconventional programming schemes for fast programming nonvolatile memories (NVMs) with acceptable oxide degradation.