Breakdown In Dielectric Liquids Research Articles

Spectral and optic-metric methods of monitoring parameters of plasma channels caused by discharge currents between metals granules in working liquids

Introduction. Spark-erosion processing of metals and alloys granules in working liquids is the basis of a several technological processes. Efficiency of energy use in them and parameters of the resulting product largely depend on the accuracy of stabilization and regulation of pulse power in each plasma channel between the granules. To achieve this, until now only the voltage and current of the discharge pulses in the entire layer of granules have been controlled. Problem. The measurement methods, which are used, are not effective enough for monitoring the parameters of individual plasma channels and predicting the size distribution of eroded metals particles at the stage of their formation. The aim of the work is to develop a method for determining the volumes of components of plasma channels in layers of metals granules during their spark-erosion treatment to predict the size distribution of eroded metal particles at the stage of their formation, as well as to simplify the method of spectrometric analysis of the elemental composition of substances surrounding plasma channels for the operational prediction of the chemical composition of resulting products. Methodology. A series of experiments were carried out on spark-erosion processing of Al and Ag granules layers in distilled water. Using a digital camera, images of the plasma channels in them were obtained. Based on the theory of pulsed electrical breakdown of liquid dielectrics, an analysis of the components of plasma channels was carried out. Using the specialized ToupView program, the volumes of equivalent ellipsoids of rotation were determined, approximating the halos of colored radiation likely arising from streamers, as well as the spark cores of plasma channels emitting white light. The shades of the resulting radiation were studied for several metals and working liquids. The obtained data were compared with the known results of spectrometric studies for the same elements excited by similar mechanisms. Results. The theory of discharge-pulse systems for spark-erosion processing of granular conductive media has been developed in the direction of new methods for monitoring the parameters of discharge pulses and predicting the chemical composition and size distribution parameters of eroded metal particles at the stage of their production. An optic-metric method has been developed for determining the volumes of halos and cores of plasma channels. A simplified spectral method for determining the chemical composition of erosion particles based on the shade of the resulting radiation was proposed. Originality. The developed new optic-metric method makes it possible to obtain information about almost every plasma channel, which refines predictions of the size distribution of erosion particles. To implement the method, general-purpose hardware and specialized software that is freely available are used. The developed method of simplified spectral analysis of excited atoms makes it possible to make preliminary predictions of the chemical composition of the obtained erosion particles already at the stage of their formation without the use of expensive specialized equipment. Practical significance. The ratio of the volumes of halos and cores of plasma channels between Al and Ag granules in distilled water was measured. An analysis of the emission spectra of plasma channel halos between Al, Ag and Cu granules in distilled water, Fe in ethyl alcohol, Ni-Mn-Ga and Ti-Zr-Ni alloys in liquid nitrogen, and Ti-Zr-Ni in liquid argon was carried out. Based on spectrometry data, the resulting shades of these radiations were substantiated and their description in the RGB system is given. References 56, table 1, figures 4.

Bubbles in Transformer Oil: Dynamic Behavior, Internal Discharge, and Triggered Liquid Breakdown

It is generally accepted that the presence of bubbles severely degrades the electric strength of liquid dielectrics, but the influencing factors and mechanisms have yet to be elucidated. Dynamic behavior, partial discharges (PDs) inside bubbles, and their effects on the breakdown of transformer oil are investigated experimentally and theoretically in this study. The results show that, under a nonuniform electric field, the presence of bubbles does not significantly affect the PD inception voltage (PDIV) and breakdown voltage (BV) of the liquid, as bubbles migrate to the region with lower field strength under dielectrophoretic force. In a uniform electric field, conversely, bubbles greatly reduce PDIV and BV; an increase in the number of bubbles or a slower voltage rise rate appears to exacerbate this effect. The PDIV observed in the experiment is much higher than the theoretical value calculated by the Townsend theory or streamer criterion, which may be due to the lack of free electrons inside the bubble, thus resulting in scarce internal discharge. The addition of X-rays enhances the radiation ionization and then eliminates the inconsistency between the theoretical and measured values of PDIV, which supports our hypothesis. The bubble-triggered breakdown of liquid dielectrics, namely, once a discharge occurs inside the bubble, likely triggering the breakdown of the liquid gap, is observed. The electric field enhancement caused by surface charges and the instability of the gas–liquid interface may account for this.

Effect of voltage type and polarity on streamer dynamics in transformer oil

Mineral oil is one of the most important and widely used liquid as an insulation and cooling medium in large power transformers. Hence, maintaining the insulation strength of oil is the highest goal of all transformer manufacturers. The breakdown in liquid dielectrics involves various pre-breakdown stages. The electrical streamer is the main factor in these pre-breakdown processes. The positive streamer creates greater risk to oil insulated high voltage electrical equipment due to its high propagation velocity. A 2D-axisymmetry fluid model has been developed to investigate the phenomenon before the breakdown in transformer oil by applying the impulse voltage and direct current high voltage using needle-plane geometry with 1 mm gap. The electric field dependent molecular ionization technique is used to represent the streamer dynamics. The effect of voltage polarity and its type on streamer dynamics have been studied.

DIELECTRIC OIL`S TEST MACHINE WISH VOLTAGE RISE ELECTRONIC MODULE

For test operations according to the liquid dielectric breakdown voltage measurement method we use high voltage machines that consist of high-voltage step-up transformer, voltage rise block, test cell with electrodes and so on. Described dielectric oil's test machine UIM – 90 with electromechanical voltage rise block. Cause of hard requirements in specification documents about voltage sine wave form on cell's electrodes, we performed field tests for UIM – 90 that help to evaluate the mains voltage impact on the test voltage distortion and measurement accuracy. Was discovered that during usage of electromechanical voltage rise block voltage steps disrupt sine wave’s form proportionally to step-up transformer’s transformation coefficient. Performed analysis of this block’s construction and established that usage of ЛАТР and mechanical voltage controller could lead to additional sine’s wave disruption. Decided to develop electronic voltage rise block which will allow to get rid of mains influence on test data. Created the algorithm of wave shaping from microcontroller, which generates voltage ramp to the amplifier representing pulse width modulator, then to the step-up transformers cascade. Proposed to use additional transformer for level matching of amplifier’s output voltage and main high voltage transformer’s input voltage. Presented flow sheet for UIM – 90 with electronic voltage step-up block and cascading start ofstep-up transformers. Provided voltage oscillograph trace and it spectrograph on the main transformer’s primary side, received due to the implementation of developed electronic voltage step-up block, prove that voltage sine wave form doesn’t rely on mains quality. After upgraded UIM – 90 and it world analogues technical parameters analysis we could make a conclusion about it competitive capability on global level.

Electron generation and multiplication at the initial stage of nanosecond breakdown in water

Electrical breakdown of liquid dielectrics under nanosecond pulsed high voltage has been investigated extensively in the last decade. Prior studies have focused on either experimental characterization of the breakdown process and discharge plasma or formulation/verification of the electrostrictive cavitation mechanism of the breakdown initiation. There remain knowledge gaps toward a clear physical picture of how the first plasma is generated in a region saturated by nanoscale cavities created by electrostrictive forces in inhomogeneous fields at the nanosecond timescale. Initial plasma results from the multiplication of primary electrons that gain energy collisionlessly in the cavities to cause collisional ionization of water molecules on the cavity walls. This paper quantitatively discusses the possible sources of primary electrons that seed the plasma discharge. Electron detachment from hydroxide is shown to be the most probable and sustainable electron source. Using numerical modeling, this study demonstrates the plausibility of an electron multiplication mechanism involving two neighboring cavities. The drift of hydrated electrons from one cavity to the next is the rate-limiting step and sets the minimum electric field requirement. This work will inform subsequent experimental studies and have implications in various applications such as plasma sources in biomedical applications, cavitation study, and insulation of pulsed power equipment.

Systems Theory: Simulation Analysis of Space Charge Generation Mechanisms in Transformer Oil under High Electric Field

Simulation analysis plays an important role in Systems Theory nowadays. In order to reveal the mechanism of space charge injection and generation in the process of liquid dielectric breakdown, the discharge of transformer oil between needle-plane electrodes under high electric field is studied. Based on the 2-D hydrodynamic model and Poisson equation of electric field, the system simulation model of oil discharge by different charge generation mechanism is established, based on which the inception and propagation process of discharge is simulated by COMSOL. By system simulation, the temporal and spatial distribution of the electric field intensity, space charge density, electric potential and temperature of transformer oil is obtained and deeply analyzed. The simulation results prove that the space charge generated by metal field emission and ionic disassociation are neither of the major factor for streamer formation in transformer oil, while the Zener molecular ionization and impact ionization are the major factors. Our research improves the understanding of the inception, propagation and breakdown process for discharge in transformer oil, and also the ionization mechanism in the liquid dielectric

Electrical Discharge Micromachining in Strong Electric Fields Using Voltage Nanopulses and Streamer Formation of Breakdown Paths

This work examines electrical discharge micromachining in the electric breakdown of dielectric liquids. The formation of a breakdown channel using nano- and microsecond pulses is considered. Variants of incomplete and short-time breakdown of the gap are given. Thermal simulations of electrode heating to the melting and evaporation temperatures of the electrode materials are carried out.

Kinetic and electrical phenomena in gas–liquid systems

Disperse systems consisting of a liquid and gas bubbles located in it are considered. Two possible versions of evolution of bubbles under the conditions studied are assessed. In simple liquids, contact between two bubbles causes them to merge, as the separating film breaks. In the case of complex organic liquids, amphiphilic film is formed on the surface of bubbles, and the lifetime of bubbles in contact increases with their size. Under an external electric field, chains of bubbles are formed, lined up along the electric field potential lines. The presence of bubbles in liquid greatly lowers the breakdown threshold, as the critical parameters of the breakdown field in liquids are two to three orders of magnitude higher than those in gases at atmospheric pressure. Various breakdown mechanisms in liquids are discussed from the viewpoint of formation of the gas phase during the passage of an electric current through a liquid medium. The character of propagating a streamer in separate bubbles is studied with their random distribution in liquid and in the case of formation of some structures of bubbles; the critical parameters of disperse systems, that can lead to their electrical breakdown, are presented. Along with the general concepts of electrical breakdown in dispersed systems, experimental studies of these processes are considered, and the nature of electrical breakdown in liquid dielectrics, including transformer oil, is discussed.

Three-dimensional LBE simulations of a decay of liquid dielectrics with a solute gas into the system of gas–vapor channels under the action of strong electric fields

The three-dimensional simulations of an anisotropic decay of binary mixtures of a dielectric liquid with solute gas in a strong electric field are carried out. The Lattice Boltzmann Equation method (LBE) is exploited for computer simulations of the evolution of such systems with the newly arising interfaces between vapor and liquid phases. The parallel implementation of the LBE algorithm is realized on a large number of cores in the GPU. For the GPU programming, the CUDA technology is used.It is important that new regions of the low-density phase appear as thin quasi-cylindrical gas–vapor channels oriented along the electric field. The gas–vapor channels expand because of the diffusion of the solute gas from the mixture, evaporation of liquid into the channels and also due to the coalescence of channels with each other. The critical values of electric field necessary for such decay of a binary mixture are considerably lower than the critical electric field for pure dielectric liquids. Hence, if we take into account a solute gas, the electric fields for which the anisotropic mechanism of streamer channels generation and growth is operated, become considerably lower.Thus, at a breakdown of dielectric liquids in a strong electric field, the anisotropic instability is possibly the key mechanism of the generation of a gas phase, inception of conducting streamer structures, their fast growth in the form of thin filamentary channels, as well as branching of streamer structures during propagation.

Theoretical study of the initial stage of sub-nanosecond pulsed breakdown in liquid dielectrics

This paper presents a simple theoretical analysis of the initial stage of a sub-nanosecond pulsed high-voltage breakdown in liquid dielectrics. We show that a rapid breakdown is related to a rupture in the continuity of fluid induced by the electrostrictive forces in the inhomogeneous electric field in the vicinity of a needle electrode.

Dielectric liquids [Editorial

Summary form only given. The present issue of the IEEE Transactions on Dielectric and Electrical Insulation is principally devoted to papers dealing with dielectric liquids. The subject is of great importance in the electrical insulation area, because insulating liquids are extensively used in the area of liquid filled power apparatus such as transformers and capacitors. Over the past decade there have been considerable efforts to improve the electrical and thermal properties of existing liquid dielectrics as well as to introduce new dielectric liquids with better insulating properties as well as develop improved testing methods for their evaluation. The opening paper in this issue is of a tutorial nature and addresses the development of standards for partial discharge (PD) measurements on dielectric liquids. Such information should be useful for standards oriented organizations that are currently engaged in the preparation of testing methods for PD detection and measurement on insulating liquids. In many respects partial discharges in insulating liquids must be viewed as a precursor stage of breakdown streamers in these liquids. The latter have received much attention in the past and continue to sustain interest in studies dealing with the mechanism of breakdown in dielectric liquids. Over the past decade there has been considerable effort directed towards researching the effects of copper sulfide in mineral oils. The problem gained attention when a significant number of transformers and reactors, that had undergone failure, were found to contain corrosive sulphur in their mineral oil. Subsequent research and testing of these oils indicated that they contained corrosive sulphur, which after its reaction with copper conductors gave rise to the formation of semi-conducting copper sulfide, resulting in overheating and the further formation of hot spots. Thereafter the work volume on this subject diminished considerably, as is evident in the present issue. Finally, mention should be made of the natural and synthetic esters, which are now being used in transformers. They have received considerable attention, because both types of esters are biodegradable and thus benign to the environment. Their electrical properties are found to be comparable to those of mineral oils. Both types of liquids are principally used primarily in distribution transformers. Evidently, the number of published papers in the Special Issue on Dielectric Liquids reflects the activity in this field of endeavor. It is surprising that in the past issues on Dielectric Liquids, the number of papers published tended to hover in the vicinity of 20, as it also does in the present issue. The tentative date of the next Special Issue on Dielectric Liquids is planned four years hence.

Impulse breakdown delay in liquid dielectrics

Theoretical images of streamers, revealing the mechanisms behind impulse breakdown in liquid dielectrics, are presented. Streamers lead to electrical breakdown by forming paths, capable of carrying large current amplitudes between electrodes. Breakdown delays and terminal currents are calculated for various electrode geometries (40 μm needle and 6.35 mm sphere) and gap distances (up to 10 mm). Modeling results indicate that the breakdown in needle-needle electrodes requires higher impulse voltage amplitudes than in needle-sphere electrodes for the same gap distances. Streamers in needle-sphere geometries are about 50% thicker than streamers propagating in needle-needle geometries under similar impulse voltage amplitudes.

Lattice Boltzmann model for simulation of the electric breakdown in liquids

Abstract We investigate pre-breakdown hydrodynamic flows and initial stages of the electric breakdown in dielectric liquids. Three models are considered. The first one represents the purely thermal mechanism. Here, the liquid is simulated by a single-phase lattice Boltzmann equation (LBE) method. The temperature and the electric charge density are described by additional LBE components with zero mass. The permittivity is assumed to be constant. The conductivity increases with the increase of temperature. Electric force acting on a charged liquid is coupled with the hydrodynamics by the exact difference method [Kupershtokh, 2004; Kupershtokh & Medvedev, J. Electrostatics, 2006]. The last process in the model is the Joule heating. In the second model, a possible phase transition is included. To simulate a fluid with an arbitrary two-phase equation of state (such as van der Waals or Carnahan-Starling EOS), the method proposed by Kupershtokh is used [Kupershtokh, 2005; Kupershtokh et al. 2007]. The conductivity increases with the decrease of the fluid density. When the voltage is applied, the charge injection from the surface of electrode begins. The electric force acting on the charged fluid produces negative pressure near electrode leading to a phase transition (evaporation). Conductivity increases leading to enhanced evaporation and growth of a conducting bubble. Thus, the bubble mechanism of breakdown is realized. The last model includes the density-dependent permittivity. For nonpolar liquids, the dependence is given by the Clausius — Mosotti law. In this case, several additional processes are possible. First, dielectrics is pulled into regions with higher electric field which produces rarefaction waves. Second, an anisotropic instability [Kupershtokh & Medvedev, Phys. Rev. E, 2006] can develop producing low-density channels along the electric field. Since these channels can easily become conducting, another mechanism of the breakdown is realized.

Anisotropic instability of a dielectric liquid in a strong uniform electric field: Decay into a two-phase system of vapor filaments in a liquid

The linear stability analysis of dielectric liquid placed in uniform electric field with respect to perturbations of density of an initially uniform state was carried out. The electric field increases the instability increment for the stratification along the field and decreases it for the transversal stratification. Thus, anisotropic separation into liquid and vapor phases is possible in high electric fields for a liquid that is initially in unstable state, as well as in metastable or stable states. Computer simulations of electrohydrodynamics confirm the theoretical calculations. It is important that new regions of low density phase appear as narrow cylindrical channels oriented along the field. This mechanism of generation of gaseous phase in locally high electric field can play a key role in processes of inception and ultrafast propagation of streamers during breakdown of liquid dielectrics in nanosecond range.

Anisotropic instability of dielectric liquids and decay to vapor-liquid system in strong electric fields

The results of a linear analysis for the stability of a homogeneous dielectric liquid with respect to density perturbations in a uniform electric field are presented. The electric field increases the instability increment for the stratification along the field and decreases this increment in the transverse direction. Thus, a strong electric field can induce anisotropic decay into liquid and vapor phases for a fluid that is initially both in a labile state and in a metastable or stable state. Theoretical calculations are confirmed by the results of numerical modeling of the fluid dynamics. The new regions of a low-density phase have the form of narrow cylindrical channels oriented along the field. This new mechanism of gas phase formation in strong local electric fields probably plays a key role in the inception and ultra-fast propagation of streamers during the breakdown in dielectric liquids in a nanosecond range.

Percolative model of electric breakdown in liquid dielectrics

A percolative approach, useful in modeling liquid dielectric breakdown, is presented. The dielectric is treated as a network of resistors having random values and breakdown characteristics based on a specified statistical distribution. The method is quite general, and lends itself to the inclusion of internal fluctuations and localized heating. It should also be applicable to mixtures and facilitate the inclusion of air bubbles. Despite its simplicity, the model successfully characterizes fractal structure in dielectric breakdown. In particular, the fractal dimension for a two-dimensional (2-D) lattice as given by the exponent of a power law, agrees with the theoretical value. The dependence of critical external voltage on the internal disorder is also investigated. It is shown that the overall breakdown process consists of the successive breakdown of individual elements to finally form a percolation cluster. The clusters have a typical dendrite structure. Also, in keeping with qualitative expectations, it is shown that V/sub br/ decreases with the disorder and increases with resistor variance.

The sequence of events in mid-gap laser-triggered electrical breakdown in liquid dielectrics

Recent experiments on mid-gap laser-triggered electrical breakdown in liquid dielectrics have been scrutinised. The sequence of events that leads to the disruptive discharge is described and accompanied by electric field calculations.

Laser-triggered electrical breakdown in liquid dielectrics. Imaging of the process by the shadowing technique

Electric breakdown in transformer oil subjected to a quasi-uniform dc electric field has been triggered by a focused laser pulse and prebreakdown discharge phenomena were imaged by the shadowing technique. Extensive data were recorded of the prebreakdown phenomena, such as streamers and gas bubble growth, from the initiation of the laser plasma to the disruptive discharge. The data was used to qualitatively and semi-quantitatively describe these phenomena. The image material also was used to improve and modify the qualitative description of the laser-triggered breakdown process previously presented by Sunesson et al. [1994].

Growth model for filamentary streamers in an ambient field

We have simulated the fast streamer stage of liquid dielectric breakdown as stochastic growth of a branching fractal tree. Breakdown and threshold properties of the fluid are represented in the random filter procedure. A range of fractal densities, from sparse to bushy, is approximated by the choice of power-law (4th-power to linear). The choice of threshold (cutoff) voltage also significantly affects the growth form. These parameters combine with the shape and concentration of the electric field, to regulate the distribution and directedness of the local discharge growth pattern. Inclusion of a voltage gradient along the streamer tree produces a secondary narrowing effect on the growth. A large grid (128 cubed) is used for the discretization. Diagonal growth paths to neighbor-vertices are included, increasing the choice of available directions for each discharge event. We use a combination of data-parallel programming and three-dimensional visualization. Complete growth histories, evolving from the voltage distribution, can be displayed in animation or in color banding against the "trials" variable, which simulates a time tick. Side views of the structures provide comparison against sub-microsecond snapshots from experiment. Results include sparse, directed trees evolving from a 4th-power-law filter; also dense trees from a linear filter, whose conical upper-envelope boundary is strongly influenced by the choice of threshold (cutoff) potential.

Development of pulsed dielectric breakdown in liquids

A model is presented for the development of breakdown in liquids subjected to uniform high amplitude electric field stresses with duration of microseconds or less. In this model, breakdown proceeds in four stages: (i) formation of a low density site (nucleation) in the liquid near an electrode, (ii) growth and expansion of this site until the local density is reduced below a critical density for electron impact ionization to take place, (iii) growth of an electron avalanche and its transformation into an ionizing front, and (iv) propagation of the ionization front via a sequence of processes occurring in the region ahead of the front; namely, heating by electron injection, lowering of the liquid density and avalanche growth and retardation. Expressions for the duration of each of these stages are derived and their behaviour with pressure and field strength analysed for cathode initiated breakdown. From this, a critical energy criterion for breakdown is obtained. Comparison is made with experimental results for water, salt solutions, and the liquid noble gases and for pulse durations in the microsecond and sub-microsecond time scales. This model serves to elucidate the dynamics of pulsed breakdown of liquids.