Needle-plane Configuration Research Articles

Breakdown Characteristics of Liquid Silicone Rubber in Needle-Plane Configuration Under Polarity Reversal

The results of breakdown voltages in liquid silicone rubber (LSiR) insulation under stepped polarity reversal voltages at a uniform temperature in a needle-plane electrode system are presented in this article. In the presence of defects such as sharp conductor projections or protrusions in the cable insulation stressed under polarity reversal direct current (dc) voltages, the electric field distribution is far more complex due to nonlinear insulation conductivity and time-dependent field distribution. The electric field distribution and space-charge accumulation are intricate functions of factors such as geometry and nonlinear conduction. In this article, a two-dimensional (2-D) axisymmetric model of a needle-plane geometry is developed using the finite-element method to compute the time-dependent field, space charge, and current density distribution under polarity reversal dc, which apparently did not get much attention until now. Interesting results on the effect of polarity transition time and applied polarity time step on the abovementioned distributions are addressed and presented. The pronounced effects of nonlinear conductivity on the time-dependent field distribution are also demonstrated. Furthermore, from the breakdown experiments conducted on tip-plane electrode system for different tip radii and applied time steps, the results put forward a rational and practical estimate of the breakdown field using the proposed model.

High field conduction in mineral oil based ZnO nanofluids prior to negative streamer inception

The electric conduction under intense electric fields (up to ∼ 109 V/m) in nanofluids using surface-modified ZnO–C18 nanoparticles dispersed in mineral oil as host, is investigated with both experiments and numerical simulations. The measurements are used to estimate unknown parameters necessary to represent the generation and loss of electrons in an electrohydrodynamic model for mineral oil with and without ZnO–C18 nanoparticles in a needle-plane configuration. The model suggests that ZnO–C18 nanoparticles induce an enhanced field emission from negative needles, explaining the significantly larger conduction currents measured in the nanofluid compared with those in the host liquid. It is also found that the scavenging of electrons by ZnO–C18 nanoparticles is a process which is negligible compared with the loss of electrons due to attachment in mineral oil. It is shown that ZnO–C18 nanoparticles hinder the streamer initiation process by reducing the effective electric field at the tip of the needle. This electric field reduction is caused by the combined effect of enhanced electron injection through ZnO–C18 nanoparticles and strong electron attachment in mineral oil. Thus, the electric field on the needle tip reaches the same threshold value when the streamer is incepted in the nanofluid as in mineral oil, although at a larger voltage. Solid evidence indicating that the additional electron scavenging and the reduced electron mobility introduced by nanoparticles has no effect in the conduction currents and in the negative streamer inception in the tested ZnO–C18 nanofluids is shown.

On the injection and generation of charge carriers in mineral oil under high electric fields

Charge injection and generation mechanisms under intense electric fields (up to 109 V m−1 ) in mineral oil are assessed experimentally and numerically. For this, current-voltage characteristics under positive and negative polarities are measured in a needle–plane configuration using sharp needles (with tip radius Rtip ≤ 1.1 μm). In addition, a state of the art electro-hydrodynamic (EHD) model is implemented to calculate the contribution of the different mechanisms on the high-field conduction currents in the liquid. In order to evaluate exclusively the contribution of field emission, experiments are also performed in vacuum. It is found that neither field emission nor field ionisation can explain the conduction currents measured in mineral oil. It is proposed that field molecular ionisation, as described by Zener tunnelling model for solids, and electron impact ionisation are the processes dominating the generation of excess electron-ion pairs in mineral oil under positive and negative polarity, respectively. It is also shown that Zener molecular ionisation alone grossly overestimates the measured currents when parameters previously suggested in the literature for mineral oil are used. Preliminary model parameters for these mechanisms that best fit the conduction currents measured in mineral oil are presented and discussed.

Development of industrial scale PVC nanocomposites with comprehensive enhancement in dielectric properties

This study aims to develop large-scale polyvinyl chloride (PVC) nanocomposites for industrial application with power cables. To achieve this goal, PVC/silicon dioxide and PVC/titanium dioxide nanocomposites were fabricated with two different loadings of nanoparticles: 0.3 and 0.6 wt.%, in the presence of a suitable coupling agent that was used to reduce the agglomeration of nanoparticles and enhance the compatibility with polymer matrix. The coupling agent used in this study was the amino silane, and the process followed in the nanocomposites preparation was the melt blending method. The dielectric properties of these synthesised nanocomposites were studied by measuring the AC dielectric breakdown strength under the uniform field, then compared with the simulation results. The relative permittivity (ɛr), loss tangent (tan δ), and DC electrical conductivity (σ) were also measured under frequencies ranging from 20 Hz to 1 MHz. In addition, the internal discharge measurements are performed using the traditional needle-plane configuration with the help of phase-resolved partial discharge (PD) analyser. This technique is used to analyse the PDs activity with respect to the phase angle of the applied voltage. It was found that the dielectric breakdown strength and PD resistance of the prepared samples are increased higher than that of the neat PVC; however, the ɛr, tan δ, and σ at 50 Hz are decreased.

Three-dimensional kinetic modeling of streamer propagation in a nitrogen/helium gas mixture

A fully resolved kinetic model (particle-in-cell and direct simulation Monte Carlo for particle/photon collisions) of a near atmospheric pressure ionization wave is presented here. Fully resolving the required numerical spatial (sub-μm) and temporal scales (tens of fs) for atmospheric pressure discharges in three-dimensions is still a challenging task on modern super computers. To keep the overall problem tractable, the total number of elements are reduced by only simulating a 10° wedge rather than a full 360° geometry. The ionization wave is generated in a needle-plane configuration with a gap size of 250 μm and a background of nitrogen and helium gas. A voltage of 1500 V is applied to the anode and an initial electron and ion density of 109 cm−3 is seeded in a region near the anode electrode tip and extending towards the cathode. As these initial electrons are swept away, photoionization and photoemission create new electrons and allow the ionization front to propagate towards the cathode. Results from the 90% N2, 10% He discharge indicate that photoionization has minimal impact on plasma formation processes and cathode photoemission is the dominant mechanism for new electrons. In the 90% He, 10% N2 discharge case, however, photoionization likely has an impact as the observed locations of photoionization occur far enough away from the ionization front to allow for sufficient avalanche processes that contribute to the propagation of the ionization wave. Additionally, the electron energy distribution functions in the 90% He, 10% N2 case indicate that there is less energy loss to the low lying molecular N2 electronic states as well as the vibrational and rotational modes. This leads to higher electron energies and faster plasma development times of ∼0.4 ns for the 90% He, 10% N2 case, and ∼1.5 ns for the 90% N2, 10% He case. In addition to analysis of the ionization wave results, the overall challenges associated with simulations near atmospheric pressure discharges in three-dimensions are discussed, including the limitations of the 10° wedge that produces, at least qualitatively, minimal 3D effects.

Analysis of electrical tree inception in silicone gels

This work assesses the initial and crucial part of electrical treeing degradation, the inception stage, focusing on its dependence on applied voltage waveform and frequency. Tests have been performed on needle-plane configuration samples in solids and gels. A physical model has been formulated through an adaptation of an established theory for solids in which electrical tree inception is related to damage-producing injection currents. The voltage rise time appeared to be the most important parameter influencing the tree inception in the gel, while in the solid material the frequency is more relevant. The analysis leads to the conclusion that tree inception in gels is due to a single high-energy event, in contrast to what is commonly known for solids where damage accumulation takes place. A tree inception model is proposed for the gel, in which initiation is driven by a pressure wave generated by the electric field and the space charge injected into the sample. The model fits the experimental data and may be used to predict the tree initiation for different waveforms and voltage values.

Numerical study of the effect of the needle tip radius on the characteristics of Trichel pulses in negative corona discharges

This paper presents a numerical study of the impact of the needle tip radius on the electrical characteristics of Trichel pulses in negative corona discharges for a needle-plane configuration in atmospheric air. The radius of curvature of the needle tip varies from 20 μm to 45 μm. The first current pulse, subsequent pulse train is discussed here based on the distributions of charged species and electric field. Three species continuity equations along with Poisson's equation are solved by the hydrodynamic drift-diffusion approach, in which the role of photoionization is considered. The increasing needle tip radius reduces the peak of the first pulse significantly and delays the start of the first pulse, but almost keeps the duration of the first pulse constant. At the instant of the first pulse peak, both the magnitude of the charged species densities and the electric field decrease with the needle tip radius, and the electric field is strongly distorted by the space charge field. For the subsequent current pulses, the current magnitude is weakly related to the needle tip radius, whereas the pulse period is proportional to the needle tip radius. The increasing needle tip radius reduces the positive ion and electron densities but increases the negative ion density at the instant of the current pulse peak, which diminishes the difference of the electric field for different needle tip radii.

Understanding Corona discharge activity in titania nanoparticles dispersed in transformer oil under AC and DC voltages

Titania (TiO2) nanoparticles dispersed transformer oil has high Corona Inception Voltage (CIV) and breakdown strength, and gets further enhanced on insertion of the barrier in the electrode gap. The volume of nanoparticles and the amount of surfactant have a strong influence on the dispersion of nanoparticles in transformer oil and were characterized through viscosity measurement, particle size analysis and by zeta potential measurements. It was also observed that with needle plane configuration, negative DC voltages have high breakdown voltage as compared to positive DC and AC voltages. Improvement in breakdown voltage with barrier can be obtained by placing the barrier at 20–80% of electrode gap from the tip of the high-voltage electrode. The breakdown voltage of needle plane electrode configuration was calculated based on normal distribution parameters. It was observed that the corona activity generates Ultra High Frequency (UHF) signal in the frequency range of 0.7–2 GHZ with its dominant frequency at 0.9 GHz and was characterized using a ternary plot. The discharge activity was studied by using a spectrum analyzer by operating it in zero span mode with 0.9 GHz as center frequency. It was observed that for a given voltage under AC and DC voltages, the magnitude of discharges is less with nanoparticles dispersed transformer oil. A pyrolysis study indicated no variation in the composition of the oil with the addition of titania. It was observed that the esters present in the oils were alkyl substituted oxalic acid and alkyl substituted sulfurous acid esters.

Partial discharge development in needle-plane configuration of oil-paper insulation under AC voltage

Condition of insulation materials can be obtained by analyzing partial discharge (PD) characteristics since features of PD are closely related to it. In this paper, electrical deterioration caused by partial discharge of oil-paper insulation under long-term AC voltage is studied. Feature characteristics of PD, such as phase-resolved partial discharge analysis and equivalent time-frequency, were recorded during the test process. Electrical deterioration is divided into four stages: initial, develop, stagnation and erupt stage. Four stages are separated by pulse repetition rate and discharge magnitude of PD. In different deterioration stage, statistical characteristics of PD change significantly. Surface properties of pressboard in different deterioration stage were studied. Test results indicate that the depolarization of cellulose in pressboard is the main reason for development of partial discharge, and discharges occur in different position of pressboard in different deterioration stages.

N2 Mole Fraction Dependence of Plasma Bullet Propagation in Premixed He/N2 Plasma Needle Discharge at Atmospheric Pressure**supported by National Natural Science Foundation of China (No. 11465013), the Natural Science Foundation of Jiangxi Province, China (No. 20151BAB212012), and in part by the International Science and Technology Cooperation Program of China (No. 2015DFA61800)

In this work, a computational modeling study on the mechanism of the acceleration behavior of a plasma bullet in needle-plane configuration is presented. Above all, in our model, two sub-models of time-dependent plasma dynamics and laminar flow are connected using a oneway coupled method, and both the working gas and the surrounding gas around the plasma jet are assumed to be the same, which are premixed He/N2 gas. The mole fractions of the N2 (NMF) ingredient are set to be 0.01%, 0.1% and 1% in three cases, respectively. It is found that in each case, the plasma bullet accelerates with time to a peak velocity after it exits the nozzle and then decreases until getting to the treated surface, and that the velocity of the plasma bullet increases at each time moment with the peak value changing from 0.72×106 m/s to 0.80×106 m/s but then drops more sharply when the NMF varies from 0.01% to 1%. Besides, the electron impact ionizations of helium neutrals and nitrogen molecules are found to have key influences on the propagation of a plasma bullet instead of the penning ionization.

Numerical investigation of the formation of Trichel pulses in a needle-plane geometry

This paper presents a numerical investigation of the formation of the Trichel pulses in negative dc corona discharge for a needle-plane configuration in atmospheric air. A 2D axisymmetric model of the problem considering three charged species and consisting of a hyperboloid needle with a tip radius of 35 μm and needle-plane spacing of 6 mm over the voltage range of −3.5 kV to −12 kV has been studied. A commercial finite element package COMSOL was used for simultaneously solving three convection-diffusion equations along with Poisson’s equation. In order to obtain a better understanding of the processes which lead to the pulse formation, a close look is taken at one of the pulses. The distributions of the major charged species and the timeline of peak-values of charged species densities and electric field are presented. Through tracing the evolution of the locations of the peak densities of the charged species some new insights have been provided. The configuration of the model was chosen so that the simulation results could be compared with the experimental data published by Lama and Gallo. The numerical results were in acceptable agreement with the experimental values. Some explanations are given for the discrepancies between experimental and simulation results. It is also shown, that as the frequency of the pulses increases with voltage, the transition from Trichel pulse discharge to glow discharge initiates and full glow discharge is reached at −12 kV.

Surface charge dynamics studied by the temporal evolution of the corona charging current

Abstract The decay of surface charges deposited on the dielectric material by the partial discharge (PD) activity has a great impact on the repetition of partial discharges. In this work, the effect of dielectric placed on the surface of ground electrode in a needle-plane configuration on the discharge activity was investigated, with the application of a periodic negative step voltage. The charge decay mechanisms on a corona charged dielectric surface were investigated based on a comparison between experiments and a FEM-based numerical model. The comparison indicates that the surface charges may decay due to different mechanisms depending on the applied stress.

PIC/MCC Simulation of Glow Discharge in N<sub>2</sub>

The simulation of glow discharge in N2 in a needle-plane configuration with PIC/MCC method is presented. In the discharge channel the electric field driven by space charges is solved by PIC method, and various collisions between particles are described with MCC method. The electric field intensity, the electric potential and the net charge distribution in the discharge area are obtained. The numerical results are consistent with the corresponding features of glow discharge. This work can provide references to further study of glow discharge.

Effect of Test Method and Needle Plane Configuration on Partial Discharge Inception Voltage Measurement of Mineral Oil based on Weibull Analysis

Abstract The discussion of test method and electrode configuration for Partial Discharge Inception Voltage (PDIV) measurement is continuously developed because the PDIV can be used as indicator to determine condition of insulating liquid. However, many researchers investigate the PDIV using different test method and electrode configurations. This paper discusses the effect of test methods and needle to plane electrode configuration on the PDIV measurement of mineral oil as insulating liquid. The mineral oil that used in experiment test was Nynas Nitro 4000x with water content less than 10 ppm. In experiment, the test circuit was set up according to IEC 60270. The first test method that was carried out in this experiment according to IEC 61294 with ramp rate of test voltage 1 kV/s until PDIV occurred. Meanwhile, a Combine Method as the second test method was used as comparative PDIV test technique for this experiment. With this combine method, the test voltage was applied to the test object with rate 1 kV/s until 70% of the PDIV value obtained from the first test method. Then the test voltage was increased in steps with 1 kV/step with step duration of 1 minute until PDIV obtained with PD charge ≥ 100 pC. The rest time between consecutive tests was 5 minutes. The needle tip radii that used as high voltage electrode were 10 μm and 20 μm, meanwhile brass plane electrode with diameter 50 mm was used as the grounded electrode. In this paper, the electric field stress of the needle to plane electrode was calculated using Finite Element (FEM) simulation with input voltage 100 volt. The test result was analyzed using Weibull distribution to calculate the Cumulative Distribution Function (CDF) and Probability Density Function (PDF) of PDIV value. The calculation results show that the test results agree with Weibull distribution.

Influence of needle tip distance on barrier discharge and ozone generation for multiple needle‐and‐plane electrode configuration

AbstractThe relationship between the barrier discharge characteristics and ozone generation on application of an AC voltage in a triple needle‐plane configuration has been investigated for various distances among the triple needle tips (d=0 to 7.0 mm) with a constant distance between the needle tip and the plane (g=3.0 mm) in dry air. The characteristics of the barrier discharge and ozone generation depend on the needle tip distance. It is considered that the influence is caused by the presence of space charge and accumulated charge, as suggested by discharge images taken by a still camera and a CCD camera. The ozone generation efficiency is also estimated from the power consumption and the ozone concentration. It is found that when the distance among the triple needle tips is small, the above influence is strengthened. In this case, the ozone generation efficiency is improved. © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 93(7): 32–41, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10289

The barrier effect in dielectrics: the role of interfaces in the breakdown of inhomogeneous dielectrics

The barrier effect in high voltage engineering is usually understood to mean an increase in the breakdown voltage of the insulation gap due to the use of additional insulation layer (a barrier) which is placed between two layers of the main dielectric. This effect has been known for more than 70 years and it is widely applied in high voltage engineering with a needle-plane configuration to increase the breakdown voltage. The noticeable progress in the barrier effect study was achieved over the last 15 years, when applying new composite polymeric materials with specific properties, the so-called nanomaterials, as the barrier materials. An analysis of existing models of the barrier effect is presented. The influences of various factors such as the barrier position in the gap, the ratio of permittivities and conductivities of the barrier and main dielectric, space charge and inhomogeneous polarization are demonstrated in this review.

Electric field calculation and the influence of water trees on insulation breakdown in needle–plane geometry

This paper presents analytical and numerical methods for the calculation of the electrical field distribution in the absence and in the presence of water trees in needle–plane geometry. The needle electrode is considered hyperboloidal and conical. A study of different analytical expressions used in literature for hyperboloidal needle–plane configuration reveals that the same values for the electric field are obtained. In order to determine the electric field in conical needle–plane configuration, a numerical method is proposed. The accuracy of this method is proven for the hyperboloidal needle–plane configuration. Using the same method, the electrical field distribution is determinated in conical plane configuration with and without water trees. Water trees generate a very important modification of the electric field distribution. Basically, the electric field decreases inside the polymer zones degraded by water trees and increases outside these zones. Finally, some results of experiments performed on needle–plane specimens with and without water trees are given. A very good correlation is remarked between experimental results and numerical calculations. These results can serve to understand the pre-breakdown behavior of a cable insulation containing needle–plane type defects in the presence of water trees.

Local corona behavior and creeping discharge on (needle‐dielectric, semiconductor) composite electrodes

AbstractThis paper describes local corona behavior and creeping discharge on composite electrode systems in N2. The composite electrode consisted of a needle contacted with a borosilicate glass or a ZnO disk on a needle–plane configuration. The discharge voltages increased when the needle was in contact with the glass with increasing creeping distance (L) from the needle tip to the bottom of the disk. The discharge voltages showed a reversed polarity effect beginning at L = 4 mm. In the case of a needle in contact with ZnO, the discharge voltage increased with distance until L = 2 mm, but decreased beyond L = 2 mm. Observation using an ultrahigh‐speed camera showed that the discharge behavior was different for the glass and for the ZnO. Corona photographs taken by a camera with an image intensifier showed that the corona was generated at the triple junction in the case of ZnO. The differences in discharge voltage characteristics and the discharge behavior between the glass and ZnO were associated with the corona properties. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 145(1): 1–9, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10208

(針-誘電体,半導体)複合電極における局部コロナの挙動と沿面放電

This paper describes local corona behavior and creeping discharge on composite electrode systems in N2. The composite electrode consisted of a needle contacted with a borosilicate glass or a ZnO disk on a needle–plane configuration. The discharge voltages increased when the needle was in contact with the glass with increasing creeping distance (L) from the needle tip to the bottom of the disk. The discharge voltages showed a reversed polarity effect beginning at L = 4 mm. In the case of a needle in contact with ZnO, the discharge voltage increased with distance until L = 2 mm, but decreased beyond L = 2 mm. Observation using an ultrahigh-speed camera showed that the discharge behavior was different for the glass and for the ZnO. Corona photographs taken by a camera with an image intensifier showed that the corona was generated at the triple junction in the case of ZnO. The differences in discharge voltage characteristics and the discharge behavior between the glass and ZnO were associated with the corona properties. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 145(1): 1–9, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10208

The role of polarization in the breakdownof an air gap with barrier

It is shown that the inhomogeneous polarization of an insulation gapwith barrier in an electrical field due to a sharp change in the permittivityand/or conductivity at the interface of `basic dielectric-barrier' is one ofpossible reasons of the so-called barrier effect in dielectrics. The influenceof the preliminary polarization of thin polyethylene terephthalate barriers onthe breakdown voltage of an air gap with a needle-plane configuration underac and dc voltage is determined.