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The article presents the experimental results on the role of carbon quantum dots (CQD) covered silica nanofillers on the partial discharge (PD) properties of transformer oil insulation. The improvement in PD performance of nanofiller blend oil is tested with increased voltage gradient and nanofiller concentration. PD of nanoblend oils for various concentrations of modified silica ranging from 0 to 0.1%wt was measured. PD activity of the test samples is simulated in the laboratory with needle, rod and plane electrode geometry combinations. The facets of PD signals such as PD magnitude, PD inception and time duration of PD extracted from phase-resolved partial discharge patterns were analyzed. Detailed time-frequency domain evaluation of PD pulse is carried out. The enclosure of CQD-SiO2 nanostructures has considerably improved the inception voltage up to 23% than base transformer oil and SiO2 nanofluid. Magnitude of discharge is noticeably reduced by the influence of CQD-treated SiO2 nanoparticles in mineral oil irrespective of the electrode geometry. [ABSTRACT FROM AUTHOR]

There is a relative lack of research on the development of creepage discharge in aged oil-paper insulation subjected to AC-DC combined voltage in converter transformers. In this study, based on the physical and chemical properties of oil-paper, we propose a sphere-plate electrode model to study the creepage discharge parameters and characteristic spectra of aged oil-paper subjected to AC-DC combined voltage. The results show that the creepage discharge inception voltage of aged oil-paper is related to the AC voltage component of the AC-DC combined voltage. However, when the DC voltage component is 60 kV, the flashover phenomenon was found to occur in one of the severely aged samples before the AC voltage was applied. Despite being subjected to the same DC voltage, the flashover phenomenon did not occur in three other samples aged to different degrees. Large-amplitude discharges were found to appear earlier in the severely aged samples. At the same creepage discharge stage, as the aging time of the oil-paper increased, the number of large-amplitude discharges was found to rapidly increase, and the time interval between adjacent discharges was found to gradually decrease. At each stage of the creepage discharge, the small-amplitude discharge has a “hump” shaped distribution. The larger the DC voltage component, the shorter the endurance time of aged oil-paper, and the more severe the damage to the oil-paper. The DC voltage component was found to have an accelerating effect on the development of creepage discharge in aged oil-paper insulation.

This paper tries to relate Pedersen’s model on partial discharges and work carried out by Bruning and co-workers on the possibility of the existence of charging phenomena below inception voltage, which may eventually cause deterioration of polymeric insulation. Moreover, with the aid of the Electromagnetic theory, some aspects of the Pedersen’s model are tried to be clarified, especially those which are correlated with space charges, electric dipoles, charge distribution, charge dynamics, and partial discharge activity.

In this article, an experimental study of a corona discharge generated with a tip-plane geometry and negative polarity in air at atmospheric pressure, is presented. The effects of the active electrode material and its curvature radius on the characteristics of the corona discharge are studied. Two different materials, aluminum and copper, are used to produce active electrodes. The current–voltage characteristics show a greater intensity of the current for the aluminum electrodes as compared to the copper ones. On another side, the current is greater with the tip having the smallest curvature radius. The current–voltage characteristics have a shape well approximated by a parabolic equation. Trichel pulses are more regular for aluminum electrodes than for the copper ones. Eventually, the use of aluminum electrodes to have a stronger regular discharge would be better to make a cold plasma reactor. [ABSTRACT FROM AUTHOR]

Studying the discharge characteristics of transmission line insulators in the presence of pollution, particularly when the contaminated layer is wet by rain, fog or condensation, is necessary for selecting the proper insulation. Therefore, identifying the major air pollutants as well as the most effective ones on the discharge performance of outdoor insulators is mandatory. A systematic approach has been proposed to evaluate the impact of dominant air pollutants of an area on partial discharge (P.D) inception voltage of specimen insulators. Based on the suggested method, determining the pollution constituents, defining the dominant pollutant of the area, finding the most commonly used insulators for medium and above distribution voltages within the geographical boundaries of the Central Province of Iran, as well as examining the effect of dominant air pollutant of the region on partial discharge inception voltage of designated insulators by laboratory measurements, are the necessary steps toward a comprehensive study of the subject. [ABSTRACT FROM AUTHOR]

In this contribution, a mathematical model allowing for the prediction of the AC surface arc propagation on polluted insulators under non-uniform electric field is proposed. The approach is based on the experimental concept of Claverie and Porcheron. The proposed model, which makes it possible to reproduce the surface electric discharge, includes a condition for arrest of the propagating discharge. The electric field at the tip of the discharge is the key parameter governing its random propagation. A finite element approach allows for mapping of the electric field distribution while the discharge propagation process is simulated in two dimensions. The voltage drop along the arc discharge path at each propagation step is also taken into account. The simulation results are validated against experimental data, taking into account several electro-geometric parameters (distance between electrodes, pollution conductivity, radius of high-voltage electrode, length of the plane electrode). Good agreement between computed and experimental results were obtained for various test configurations.

Partial discharges in cavities are one of the main reasons of breakdown of insulating materials and insulating systems in general. The present paper aims at commenting some aspects of the relation between partial discharges and insulation damage. Some basic models explaining the workings and impacts of discharges in cavities are discussed. Discussions about the possibility of damage in the presence of multiple cavities as well as of the risks of minute discharges and/or charging phenomena below the so-called inception voltage are conducted. It is remarked that the question of the relationship between discharge parameters and insulation damage is still an important one, especially for composite insulating systems.

Improving the heat dissipation ability for light-emitting diode (LED) filament bulb is very difficult. A tri-needle/ring ionic wind generator was developed to improve the heat dissipation condition of bulbs. The operation characteristics of the ionic wind generator, such as the electrode gap, inception voltage, and discharge current with regard to the operation voltage, were studied by experiments. The ionic wind velocity within the bulb was investigated under different electrode gaps and applied voltages. The temperature drop achieved by the ionic was were tested with the consideration of many operation parameters for analysis. The experiments showed that ionic wind can provide efficient and stable cooling effect for LED filament bulbs. The temperature drop of LED junction can reach 30 °C at best. Good stability and adjustability of the ionic wind generator were demonstrated by tracing the temperature history of LED filaments in long-term tests. Finally, the light efficiency of LED filament bulbs with ionic wind cooling was studied. The experimental results showed that light efficiency was improved by 7.3% under the best cooling case. The experimental results indicate that the embedded ionic wind generator can provide an effective solution for cooling LED filament bulbs.

This paper tries to relate Pedersen’s model on partial discharges and work carried out by Bruning and co-workers on the possibility of the existence of charging phenomena below inception voltage, which may eventually cause deterioration of polymeric insulation. Moreover, with the aid of the Electromagnetic theory, some aspects of the Pedersen’s model are tried to be clarified, especially those which are correlated with space charges, electric dipoles, charge distribution, charge dynamics, and partial discharge activity. [ABSTRACT FROM AUTHOR]

Abstract The current-voltage curves in wire-to-cylinder arrangement are investigated for positive corona discharge varying the electrode gap and the emitter radius. Two well-known expressions are used to relate the corona current as a function of the voltage. Then, the dependencies of the equation parameters on geometrical characteristics of the electrodes are studied. The main results are as follows: 1) the dimensional parameter K shows a directly dependence with the emitter radius whereas an inversely dependence is observed with electrode distance; 2) the inception voltage V 0 increases with both emitter radius and electrode gap; 3) the exponent m varies inversely with emitter radius and it is proportional to the electrode gap. Based on the experimental results, a derived empirical formula for positive corona current discharge is proposed as a function of the applied voltage and the geometrical characteristics of the electrodes. Finally, the empirical equation is examined with data of previous studies to assess its validity. Highlights • The effect of wire radius and electrode gap on current-voltage characteristics are analysed. • We proposed a method to take into account the effect of cylinder radius. • An empirical equation based on applied voltage and geometrical factors of the electrodes to predict corona current. [ABSTRACT FROM AUTHOR]

Partial discharges in cavities are one of the main reasons of breakdown of insulating materials and insulating systems in general. The present paper aims at commenting some aspects of the relation between partial discharges and insulation damage. Some basic models explaining the workings and impacts of discharges in cavities are discussed. Discussions about the possibility of damage in the presence of multiple cavities as well as of the risks of minute discharges and/or charging phenomena below the so-called inception voltage are conducted. It is remarked that the question of the relationship between discharge parameters and insulation damage is still an important one, especially for composite insulating systems. [ABSTRACT FROM AUTHOR]

A cavity is a typical defect in a solid insulator which may cause the appearance of partial discharge (PD) activity in power equipment [1–4]. It is mainly caused by imperfections in the manufacturing process as well as by the creep during equipment operation [5]. In epoxy resin insulators, the creep brings irreversible deformations that increase with time at elevated temperatures and external stress [6,7]. Once a cavity has been formed, the PD can lead to insulation failure. Many studies on PDs in void defects, a type of gas-filled cavity, have been performed by pasting together three epoxy laminates and demonstrated factors that affect their intensity, including void size, shape and interface roughness [8–12]. However, such a test void is quite different in its structure and composition from what we further consider in this article, i.e., cavities appearing during actual insulator operation. Moreover, the pressure in a real cavity is usually very low, because of air leakage and polymerization shrinkage, whereas the test void typically remains under atmospheric pressure. Considering the above mentioned differences, we prepared experimental model insulators, containing various low-pressure cavities. The insulators were manufactured by means of automatic pressure gelation (APG) technique by changing the pressure and temperature during the resin pouring process, which allowed for simulating a more realistic operating environment. [ABSTRACT FROM AUTHOR]

Electrical tree is a serious threat to silicone rubber (SIR) insulation and can even cause breakdown. Electrical trees under alternating current (AC) and direct current (DC) voltage have been widely researched. While there are pulses in high-voltage direct current (HVDC) cables under operating conditions caused by lightning and operating overvoltage in the power system, little research has been reported about trees under combined DC-pulse voltage. Their inception and growth mechanism is still not clear. In this paper, electrical trees are studied under several types of combined DC-pulse voltage. The initiation and growth process was recorded by a digital microscope system. The experimental results indicate that the inception pulse voltage is different under each voltage type and is influenced by the combined DC. The initial tree has two structures, determined by the pulse polarity. With increased DC prestressing time, tree inception pulse voltage with the same polarity is clearly decreased. Moreover, a special initial bubble tree was observed after the prestressing DC.

This article deals with the problem of electrical discharge initiation in mineral oil in the setup of insulated electrodes. The results of laboratory experimental studies were collated with the results of the numerical calculations of electrical field stress. Both kinds of the research methods were applied to analyze the two model electrode setups immersed in mineral oil: setup with insulated HV electrode and setup with bare HV electrode having the same outer dimensions as the insulated one. The obtained similarities in the values of maximum inception electrical field stress and the equality in the experimentally evaluated initiation delay indicated that the most stressed oil volume and weak points included in it may be responsible for discharge initiation in paper-oil insulation setups with oil of technical purity. The same number of weak points included in oil may be, in both the cases considered, an equally productive source of initiation sites. [ABSTRACT FROM AUTHOR]

The AC (50 Hz) partial discharge (PD) characteristics of oil-impregnated pressboard (OIP) insulation are severely influenced by the DC voltage component in the converter transformer. Therefore, this study investigates the effect of different DC prestressing voltage (DCPV) amplitudes, prestressing time, and repetitions times of the applied AC voltage on the OIP insulation AC PD characteristics. Meanwhile, the space charge characteristics of distribution and dissipation are measured by pulsed electro-acoustic method. The experimental results show that the PD inception voltage (PDIV) and PD extinction voltage (PDEV) after DCPV were lower than that under AC voltage without DCPV. The PDIV decrease with higher DCPV at prestressing time from 30 to 60 min. But it increase with higher DCPV at prestressing time from 90 to 120 min. However, PDIV initially increase then decrease with the increasing repetition times of the applied AC voltage. The space charge distribution in 1 mm OIP insulation takes 60 min to reach a balanced condition. The residual space charge is relatively higher at a longer prestressing voltage time and higher DC voltage when the DC voltage was turn off. The PDIV and PDEV after DCPV is improved by Joule heating with longer prestressing time and higher DCPV. The space charge had negative effect on them. [ABSTRACT FROM AUTHOR]

This paper concerns the studies on streamer propagation in mineral oil, synthetic ester and natural ester at standard lightning impulse inception voltage in small point‐plane gaps. The inception voltages of both polarities were measured using step method and then analysed statistically. Simultaneously, streamer characteristics such as shape, emitted light and propagation velocity were compared between the liquids tested. The obtained results showed that the inception voltages were close to each other for all the three liquids tested. Concerning streamer characteristics, it was seen that the spatial shapes of the streamers were similar taking into consideration the given voltage polarity. The same may be said about the propagation velocities, indicating that the velocities of the positive streamers were twice as high as the negative ones. In the case of registered time courses of emitted light, the frequencies of the light pulses corresponding to the propagation of the streamers in both esters were a little bit higher than the pulses emitted by the streamers, which developed in mineral oil. In general, in all three liquids the streamers at inception voltages developed as so‐called 'stopping length' streamers. [ABSTRACT FROM AUTHOR]

In this contribution, a mathematical model allowing for the prediction of the AC surface arc propagation on polluted insulators under non-uniform electric field is proposed. The approach is based on the experimental concept of Claverie and Porcheron. The proposed model, which makes it possible to reproduce the surface electric discharge, includes a condition for arrest of the propagating discharge. The electric field at the tip of the discharge is the key parameter governing its random propagation. A finite element approach allows for mapping of the electric field distribution while the discharge propagation process is simulated in two dimensions. The voltage drop along the arc discharge path at each propagation step is also taken into account. The simulation results are validated against experimental data, taking into account several electro-geometric parameters (distance between electrodes, pollution conductivity, radius of high-voltage electrode, length of the plane electrode). Good agreement between computed and experimental results were obtained for various test configurations. [ABSTRACT FROM AUTHOR]

Abstract: In this study, structured MgO films were synthesized on stainless steel electrodes to reduce the glow discharge inception voltage in a diode-type discharge system in 6%Xe+94%Ne atmosphere. The relationship between the MgO surface configuration and the discharge inception voltage was investigated. We prepared two types of MgO film, namely, those with a flat surface and those with a structured surface. The structured surface appeared to have a desert-rose-like structure. The discharge inception voltage varied with the surface configuration of the MgO film. The inception voltages in the structured MgO-coated electrodes were lower than those in the MgO films with a flat surface at a pressure of 0.4–10kPa. One of the causes of the reduction in inception voltage in a structured MgO-coated electrode is considered to be the field enhancement effect at the tip of the MgO-coated electrode. [Copyright &y& Elsevier]

Electric discharge between the tip of an icicle and a plane electrode has been studied because of its relevance to the flashover of high voltage ice-covered insulators. A photomultiplier was used to scan the high voltage rod simultaneously with current measurement technique. The basic properties of streamers, namely the onset voltage, leakage current, mean propagation velocity and breakdown voltage, have been measured and compared with propagation from a metal rod as a reference. The effects of several experimental parameters such as freezing water conductivity, surrounding temperature and air gap length on the discharge characteristics have been examined. Possible mechanisms, which control discharges from ice points, have also been discussed and empirical relationships to predict breakdown voltage have been derived. [ABSTRACT FROM AUTHOR]

In inverter-fed motor coils, surge voltages with the rise time of several tens or hundreds of nano-second may cause partial discharge (PD) and degradation of electrical insulation performance of the inverter-fed motor coils. This paper discusses PD inception characteristics as well as PD propagation characteristics after PD inception for magnet wire of inverter-fed motor under surge voltage application. Experimental results firstly revealed that PD inception voltage (PDIV) decreased with the increase in the length of enamel-coated wire, which was evaluated in terms of the stressed wire contact length under surge voltage application, i.e. size effect. We proposed a regression line for the size effect on PDIV for the electrical insulation design of inverter-fed motor coils. Secondly, PD propagation characteristics were also investigated under the higher voltage application, and their mechanisms were discussed in terms of generation probability of initial electrons, space charge behavior in the wedge-shaped air gap, charging on the enamel surface and so on. [ABSTRACT FROM AUTHOR]

Partial-discharge (PD) characteristics in air and argon under low pressures down to 133 Pa (0.1 Ton) and 60 Hz ac energization are studied in an energized needle-plane electrode arrangement. The electrode configuration, vacuum chamber, facilities, and electrical connections for the experimental setup are described. Two cases are studied for each of two gases, air and argon, with 20 mm spacing between the two electrodes: (1) with and (2) without a Teflon® cap (dielectric barrier). Results for the four series of experiments and analysis of the discharge current pulse waveforms are presented. Topics discussed are the typical waveforms of the discharge current pulses at different pressures, and discharge current pulse rise time vs. pressure relationships. [ABSTRACT FROM AUTHOR]

This paper theoretically addresses the factors affecting the partial discharge activities due to metallic particles in hybrid gas-insulated transmission lines (HGITL) using the charge simulation method. There are two main reasons for using flexible HGITL. They are: (1) to overcome the technical problems and the huge cost associated with the on-site welding of large number of coaxial cylinders; and (2) to improve the utilization of the internal insulation system. The results are interpreted in terms of the basic principles of the streamer criterion in compressed gases, and also their trends are compared with those for other experimental works. The results reveal that HGITL have superior mitigation of the particle-initiated corona activities over the normal GITL, where the bundled HGITL have less improvement. The corona inception voltage decreases with the increase in the wire length, the surface charge density on the conductor insulation/spacers and the close proximity to charged or uncharged spacers, and with the decrease in pressure. On contrary, the number of occurrence of streamers and the integration of the effective ionization coefficient have opposite trends. Ribbed spacers are less sensitive to particle length and show a considerable improvement in the insulation reliability, especially when the rib becomes closer to the conductor insulation. [Copyright &y& Elsevier]

Improving the heat dissipation ability for light-emitting diode (LED) filament bulb is very difficult. A tri-needle/ring ionic wind generator was developed to improve the heat dissipation condition of bulbs. The operation characteristics of the ionic wind generator, such as the electrode gap, inception voltage, and discharge current with regard to the operation voltage, were studied by experiments. The ionic wind velocity within the bulb was investigated under different electrode gaps and applied voltages. The temperature drop achieved by the ionic was were tested with the consideration of many operation parameters for analysis. The experiments showed that ionic wind can provide efficient and stable cooling effect for LED filament bulbs. The temperature drop of LED junction can reach 30 °C at best. Good stability and adjustability of the ionic wind generator were demonstrated by tracing the temperature history of LED filaments in long-term tests. Finally, the light efficiency of LED filament bulbs with ionic wind cooling was studied. The experimental results showed that light efficiency was improved by 7.3% under the best cooling case. The experimental results indicate that the embedded ionic wind generator can provide an effective solution for cooling LED filament bulbs. [ABSTRACT FROM AUTHOR]

The problem of insulation damage due to partial discharges of low magnitudes has not yet attracted much attention. Indications of possible damage and long term effects of accumulated partial discharges of low magnitudes are evident from published literature. In this paper, the possibility of existence of such phenomena is investigated with different insulation systems namely small air gaps under point-plane electrode arrangement, gas-gap in a coaxial geometry consisting of a 10:90 gas mixture of SF6 and N2 gases and paper-oil insulation of a power transformer. It is observed that random discharges below inception voltage do exist. Such discharges are studied and their impact on insulation is discussed. Experimental evidence is offered to substantiate that discharges change from the pulse-type to a pulse less-type as the gap becomes larger in case of air. Phenomena of PD below or in the vicinity of inception voltages may help in better understanding of the mechanism of small current flow at relatively law voltages and may help in development of better dielectric materials and insulation systems for high voltage applications. [ABSTRACT FROM AUTHOR]

Investigates the partial discharge (PD) inception characteristics of a simplified inverter-fed motor-coil sample. Difference in the PD inception characteristics of a twisted pair under 60-Hz voltage and surge voltages with a steep wave front; Electric field analysis; PD inception voltage under alternating current.

Electrical tree is a serious threat to silicone rubber (SIR) insulation and can even cause breakdown. Electrical trees under alternating current (AC) and direct current (DC) voltage have been widely researched. While there are pulses in high-voltage direct current (HVDC) cables under operating conditions caused by lightning and operating overvoltage in the power system, little research has been reported about trees under combined DC-pulse voltage. Their inception and growth mechanism is still not clear. In this paper, electrical trees are studied under several types of combined DC-pulse voltage. The initiation and growth process was recorded by a digital microscope system. The experimental results indicate that the inception pulse voltage is different under each voltage type and is influenced by the combined DC. The initial tree has two structures, determined by the pulse polarity. With increased DC prestressing time, tree inception pulse voltage with the same polarity is clearly decreased. Moreover, a special initial bubble tree was observed after the prestressing DC. [ABSTRACT FROM AUTHOR]