Needle Tip Radius Research Articles

Streamer and Breakdown Characteristics of Dielectric Liquids Under Impulse Waveforms With Different Tail-Times

Impulse waveforms have been widely used in the studies of streamer and breakdown phenomena in dielectric liquids. In this article, both experiments and analyses were carried out to investigate the effects of impulse tail-time on streamer and breakdown characteristics of a mineral oil and a synthetic ester liquid. A range of positive impulse waveforms with the same front-time of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.8~ \boldsymbol {\mu } \textrm {s}$ </tex-math></inline-formula> but different tail-times from 8 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3200~ \boldsymbol {\mu } \textrm {s}$ </tex-math></inline-formula> were considered. Needle-plane electrodes with a needle tip radius of 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {\mu } \textrm {m} $ </tex-math></inline-formula> and a gap of 10 mm were used. It was found that reduction of tail-time leads to an increase of 50% impulse breakdown voltage, while the instant breakdown voltage and the time-to-breakdown remain constant. On this basis, an analytical method is developed to estimate the 50% impulse breakdown voltage under waveforms with various tail-times. The method is valid when streamers remain of second mode and in divergent fields. It was also observed that some “self-regulation” by branching explains the constancy of streamer propagation velocity when the applied voltage is increased.

Pre-breakdown Phenomena and Influence of Aging Byproducts in Thermally Aged Low Pour Point Ester Fluids Under AC Stress

Application of low pour point esters in transformers insulation serving in cold climatic regions is still a challenge. To address this concern, the pre-breakdown and breakdown analyses of thermally aged low pour point ester fluids are investigated. The breakdown measurements are performed at different aging conditions under AC stress. Needle-plane electrode geometry (with varying radii of the tip) is employed since this configuration is necessary for any practical diagnostic based on partial discharge (PD). This allows studying the effect of the chemical impurities (aging by-products) as assessed by some physicochemical properties (acidity, particles count, turbidity, and dissolved decay products) on the partial discharge and breakdown characteristics. The complete analysis is carried out for liquids having the pour points less than -50 °C, including a mineral oil (MO) and two synthetic ester liquids (SE1 and SE2). Changes in the streamer properties, such as partial discharge inception voltage (PDIV), breakdown-streamer inception voltage, and breakdown voltage are reported as a function of the needle tip radius and aging condition. It is observed that the breakdown properties of SE1 are better than that of SE2 and are comparable to MO. To understand the influence of aging on the PD, correlations between decay products, acidity, BSIV, and PDIV are reported. It is observed that the acidity and concentration of decay products directly impact the PD characteristics.

Comparison of discharge mechanism of negative corona between hyperboloid and hemispherical electrodes

Electrostatic precipitators based on corona discharge are a research hotspot in recent years. It is of great significance to investigate the characteristics of corona discharge under different system parameters for improving the dust removal efficiency. In this manuscript, the continuity equations coupled with Poisson’s equation are solved by the finite element method (FEM) to explore the different mechanisms of the generation of Trichel pulses when hyperboloid and hemispherical electrodes are used in negative corona discharges for a needle-plate configuration under atmospheric pressure. The fitting curve of the Trichel pulse frequency versus systematic macro parameters, including the applied voltage, the needle tip radius and the needle-plate distance, are given and their mechanisms are analyzed. Subsequently, the features of the Trichel pulses of the hyperboloid electrode and the hemispherical electrode at similar power are compared. From precipitation point of view, the hemispherical electrode is obviously better than hyperboloid electrode.

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.

Electrostatic field calculation in air gaps with a transverse layer of dielectric barrier

Abstract In this paper the influence of transverse layer of dielectric barrier to the electrostatic field strength of air gaps using a hyperbolic needle-to-plane configuration is investigated. A three dimensional field problem is presented for simulating the electrostatic field in the air gap. This is achieved by using the charge simulation method. Group of ring charges are used to simulate the needle surface. A genetic algorithm is used for optimum number, location, dimension and value of simulating ring charges. The used needle tip radius was 8 μm while the gap spacing varied from 0.3 cm to 50 cm. The optimum barrier position minimizes the maximum electrostatic field and the electric energy density at the needle tip and consequently maximizes the required breakdown value. The simulation result has been compared with available experimental observations. A good agreement is found between the numerical and experimental data.

The Implementation of Needle-Plane Electrode Configuration and Test Methods for Partial Discharge Inception Voltage Characteristic Measurement of Mineral Oil

The investigation of the Partial Discharge Inception Voltage characteristic is performed by many researchers using the needle-plane electrode configuration and various test methods. This was conducted due to the capability of Partial Discharge Inception Voltage characteristic to represent or determine the condition of insulating liquid. This paper discusses the implementation of the needle-plane electrode configuration and two test methods to find out the characteristics of Partial Discharge in Partial Discharge Inception Voltage level of mineral oil. For this purposes, the mineral oil which used was Nynas Nitro 4000x with water content less than 10 ppm. In experiment, the test circuit was set up according to IEC 60270. The Ramp Method as the first test procedure was conducted in this experiment according to IEC 61294 with ramp rate of rise test voltage 1 kV/s until Partial Discharge Inception Voltage occurred. Meanwhile, a Combine Method as the second test procedure was used as comparative Partial Discharge Inception Voltage test technique. With this combine method, the test voltage was applied to the test object with rate of rise test voltage 1 kV/s until 70% of the Partial Discharge Inception Voltage value obtained from the ramp method. Then the test voltage was increased in steps with 1 kV/step with step duration of 1 minute until Partial Discharge Inception Voltage obtained with apparent charge ≥ 100 pC. The rest time between consecutive tests was 5 minutes. The needle tip radius that used as high voltage electrode were 10 µm and 20 µm, meanwhile brass plane electrode with diameter 75mm and 50mm was used as the grounded electrode. The gap distance between the needle electrodes toward ground electrode was fixed at 50 mm. The calculation of electric field stress of the needle-plane electrode configuration was done by using simulation of Finite Element Method (FEM). Based on the Partial Discharge Inception Voltage test result, the needle electrodes with tip radius 10 µm will produce electric field stress that higher than other needle tip, meanwhile the Partial Discharge Inception Voltage is lower which tested using both test method. By using the combine method, the Partial Discharge Inception Voltage value, charge quantity and the number of Partial Discharge will be lower than the ramp method. The results also showed that the Partial Discharge pulse currents using both test method was about 0.2 - 0.85 mA. The time duration of Partial Discharge pulse was in the range of 1.01 - 8.38 µs and the rise time were between 68 - 301 ns. It was found that the Partial Discharge Inception Voltage test result agree with the Weibull distribution

A numerical model of electrical tree growth in solid insulation

A new model for investigating the electrical tree growth in solid insulation using a hyperbolic needle-to-plane gap is presented. The needle is embedded in the insulation medium. Classification of tree shape depends on the electric field value is presented. Then, accurate computation of the electric field is a pre-requisite for calculating electrical tree growth. The electric field distribution is obtained from Laplace's equation by treating the tree structure as an extension of the stressed electrode i.e., conducting medium. The electric field is redistributed during each growth of the electrical tree. This is achieved by using the charge simulation method. The charge at the needle surface is simulated by a group of ring charges. To determine the appropriate arrangement of simulating ring charges inside the needle, a genetic algorithm is used. A number of series finite line charge is used for simulating the charge over each branch and sub-branch during the treeing progress. The presented model for simulating electrical tree growth is a three dimensional field problem. The used needle tip radius was 3 ?m while the gap spacing varied from 0.3 to 15 mm. The results have been assessed through comparison with available analytical and experimental data.

Study about the physical processes leading to partial discharges in insulating fluids

This paper deals with the partial-discharge (PD) behaviour of some insulating fluids used in transformers. The PD are generated on a needle-plate electrode configuration. At first the influence of different parameters such as needle-tip radius and gap distance on the PD inception voltage and other relevant PD characteristics of mineral-based transformer oil are discussed. It follows a comparison of the PD behaviour of a mineral-based transformer oil and a synthetic fluid, used as insulating liquid in transformers. Based on the results it is shown that the initiation and the propagation of PD phenomena depends on different parameters.

Growth of needle and plate shaped particles: theory for small supersaturations, maximum velocity hypothesis

A solution to the diffusion controlled growth of needle and plate shaped particles is presented as their shape approaches respectively a paraboloid of revolution or a parabolic cylinder, under small supersaturation values, when capillarity and interface kinetic effects are present. The solutions show that as supersaturation decreases, the growth rate and needle tip radius approach a common value regardless of interfacial kinetics effects as capillarity is the main factor that retards particle growth. Simple asymptotic expressions are thus obtained to predict the growth rate and tip radius at low supersaturations, assuming a maximum velocity hypothesis. These represent the circumstances during solid state precipitation reactions which lead to secondary hardening in steels.

Factors affecting electrical tree testing

In an attempt to produce definitive experimental procedures for the assessment of treeing properties of polyester and epoxy resins, the influence of a variety of mechanical, electrical and geometrical conditions are investigated. The investigations are performed using point-plane specimens and the influence of the frequency of the applied voltage, the method of specimen preparation and the needle tip radius were examined. Consideration must be given to the correct preparation of the insulating resin and to the needle geometry in standard pin-plane assemblies. The cure and post-cure of the resin must be optimised to produce the most stable and reproducible specimens. Both undercured and overcured resin specimens have a lower resistance to electrical treeing. The presence of high internal mechanical strain in a specimen reduces the electrical tree initiation and growth times. Acceleration of 50 Hz treeing tests by the application of 1 kHz voltages should not be undertaken. Residual trapped charge in previously formed channels causes the mechanism of tree development at 1 kHz to be different to that at 50 Hz. Interpretation of such accelerated ageing procedures is unlikely to be of practical use.

Systematic study of the electrical characteristics of the ``Trichel'' current pulses from negative needle-to-plane coronas

When a negative dc voltage is applied to a needle in an electronegative gas such as atmospheric air, highly regular corona current pulses (Trichel pulses) are observed under some conditions. Systematic variation of the macroscopic experimental parameters has yielded the functional dependence of the pulse frequency, charge per pulse, and time-averaged corona current on the applied voltage, needle-tip radius, and needle-to-plane spacing. In combination with a theoretical analysis of the motion of the charge clouds, these results yield new insight into the physical mechanism of this gas-discharge phenomenon. For example, above threshold voltage, it appears that several negative-ion-charge clouds are simultaneously in transit across the gap.