Prebreakdown phenomena in dielectric liquids

The aim of this paper is to make a synthesis of a number of characteristic features of prebreakdown phenomena in liquids, e.g. those events which precede arc development. The subject is restricted to short term processes in the bulk, more specifically of hydrocarbons. First, the basic physical properties involved in the prebreakdown mechanisms are considered, with special emphasis on the influence of pressure and temperature on the physical and electronic properties of liquids and their vapor. Following this, the advantages of using a point-plane electrode geometry to study separately cathodic and anodic prebreakdown stages are demonstrated. To begin with, there is injection of carriers which consequently produces avalanches in the liquid followed by bubble generation. These mechanisms are now well characterized. Eventually, discharge channels are developed, streamers or leaders. The initiation and the propagation of these channels depend on many experimental conditions which are discussed. Characterization and understanding of these phenomena are in progress. Finally, prebreakdown phenomena in uniform fields are considered, either at short (μm) gaps or at large gaps, in the presence of large protrusions or free moving particles     

Pulse measurements using LED in dielectric liquids under impulsevoltages

Current pulses, in the form of partial discharges, are known to be precursors of electrical breakdown in dielectric liquids. The precise measurement of these pulses is of great value for the understanding of the breakdown mechanism. The measurement of current pulses under impulse voltage is very difficult when using conventional electrical measurement systems. To overcome this difficulty, a novel measuring system using fiber optics and a light emitting diode (LED) has been developed. In this paper, the improvements in time response and charge sensitivity of such a LED current-pulse measurement system are described. Using this system and a high speed shadow-graph assembly under impulse conditions, the current pulse and the streamer initiation in dielectric liquids are measured simultaneously     

Early stages of negative PD development in dielectric liquids

The pulse shapes in the early stages of partial discharge (PD) development, well before liquid streamer onset, and obtained with a 500 MHz wide band detection system using needle-to-metallic plane electrodes submerged in a mineral oil, were compared to those measured with needle to mineral oil plane electrodes in air under negative dc potential. All measurements were carried out ≈50% below the breakdown field value, using gap separations between 3 and 25 mm. Whereas the needle to plane discharge behavior in the mineral oil was characterised by discrete sporadic pulses or burst of several pulses of increasing negative amplitude and oscillatory form, the needle to mineral oil plane gap in air gave rise to primarily dense bursts of unidirectional negative pulses with sequentially decreasing amplitude. The pulses had rise times typically of the order of 1 to 2 ns, indicating the involvement of rapid PD mechanisms     

High field conduction and prebreakdown phenomena in dielectric liquids

The present paper is mainly devoted to phenomena occurring in point-plane electrode geometry, where breakdown is the result of the initiation and propagation of prebreakdown phenomena called "streamers". In this configuration, an investigative study of the streamer initiation processes, requiring very high electric field strengths (/spl sim/ MV/cm), and of propagation (requiring low electric field, /spl sim/ kV/cm) can easily be carried out for negative streamer development as well as for the positive case. From analysis of experimental results in pure liquids the physical processes connected with streamer initiation and propagation, particularly the electronic ones, are presented and discussed. Estimations of the main parameters of slower subsonic streamers and of the faster filamentary ones (such as field strength at the streamer tip, field inside the channel, charge density, etc.) have been obtained from qualitative considerations and compared to experimental data.     

Arcing resistance of high fire point dielectric liquids

High fire point (HFP) dielectric liquids have been in use since the demise of Askarels (PCB/TCB) in the mid 1970s. Having fire points of at least 300°C, their main application has been in the role of transformer impregnants particularly in units located in, on, or near buildings. This study was aimed at evaluating the suitability of such liquids for application in arcing environments. At present, most HFP liquids are produced from one of three different chemical bases. These are: (a) polyol ester; (b) high molecular weight hydrocarbon (HMWH); and (c) dimethyl silicone. Samples of these liquids were evaluated in a typical oil switch primarily utilized in capacitor switching applications. Although not an HFP liquid, transformer oil was included in the test matrix to provide a base line to which the other liquids could be compared. Each liquid was exposed to 250 cycles of switching 8000 V at 50 A. The results show that both polyol ester and HMWHs compare favorably to transformer oil in this application. However, the dielectric breakdown value of dimethyl silicone was so rapidly degraded that its use under such conditions requires considerable caution. Further analysis of the particulate arc by-products formed in the dimethyl silicone revealed that this material had an extremely high dielectric constant. Through computer modeling, it was shown that the great disparity in dielectric constants between the fluid and the particulate would greatly intensify the electrical stress on the fluid and cause dielectric breakdown     

Phase relationship of PD pulses in dielectric liquids under acconditions

Partial discharge pulse phase analysis carried out on a mineral oil and a perfluoropolyether with point-to-plane electrodes provided further and additional evidence that discharge epochs tend to be concentrated in the vicinity of the alternating voltage peaks at discharge inception in lieu of the voltage zeros characteristic of normal cavities occluded in solid and liquid-impregnated solid insulating systems. Relatively large positive polarity discharge pulses, having an associated charge transfer of ⩾15 pC with a recurrence rate of at least one per every ten consecutive cycles, appear initially on the positive half-cycle; with further increases in applied voltage both their number and magnitude continue to exceed those of negative polarity over the negative half-cycle. This behavior is to be distinguished from that commonly observed with high sensitivity measurements, which clearly indicate that early discharge onset in liquids is characterized by the occurrence of minute highly intermittent negative polarity pulses. The increase in charge transfer of the discharge pulses with applied voltage is suggestive of the development of longer more intense streamers in the dielectric liquids at the more elevated electrical fields. Also the results infer that discharges or streamers form and propagate more readily in the mineral oil than in the electronegative perfluoropolyether     

Dynamics and linear stability of charged jets in dielectric liquids

The physical system to be considered is a blade-plane configuration in a dielectric liquid. For high electric fields, injection from the blade takes place with ions of the same polarity. The Coulomb force acting upon the injected charges originates an electrohydrodynamic (EHD) flow, referred in what follows as the charged jet. A laminar solution of this EHD jet is obtained using similarity analysis. If transport of charge is dominated by convection, i.e., neglecting molecular diffusion and ion drift, and the electric field is assumed constant, the problem is mathematically equivalent to the bidimensional thermal plume in the limit of large Prandtl numbers. The authors examine the stability of this EHD jet using linear theory and parallel-flow approximations. Neutral stability curves are computed numerically in terms of a nondimensional parameter which is the electrical analogous to the Grashof number. Finally, some experimental observations are presented, followed by a short discussion. The role played by the viscosity correlates reasonable well with the theoretical analysis     

Tunable pattern structures in dielectric liquids under high dc electric fields

This work focuses on the abrupt changes that the application of large enough electric fields provokes in the internal structure of hematite/silicone oil suspensions. Experimental results reflect the existence of two well-defined structural patterns according to the strength of the field and the concentration of particles. At low electric fields, columns of particles between the electrodes can be observed when the concentration of solids exceeds a critical volume fraction. However, at higher fields, electrohydrodynamic convection and eventually electrophoretic migration take place, reflecting the relevance of the particle charge. A complete theoretical discussion is given to explain the origin of these so different behaviors. While the mismatch in the electrical properties (particularly, conductivity) of the solid and liquid phases, that is the Maxwell-Wagner polarization, can justify the chain-like structures of particles, it is necessary to take into an account the process of charge injection at the electrode/suspension interface to support the electrophoretic migration and deposition. The experimental conditions for which polarization or current effects predominate are elucidated in terms of the conductivity of the solid phase and the magnitude of the applied electric field.     

Fire test methods for electrical and dielectric materials

Methods intended for the measurement of combustibility are briefly reviewed. The need for fire safety assessment to address parameters that extend beyond a material's tendency to burn is discussed. Emphasis is placed on recent developments in fire testing which provide a more complete characterization of a material's fire behavior and associated fire hazards. Fire test apparatus and toxicity and corrosivity of combustion products are considered     

Insulating properties of some liquids after an electrical arc

The insulating properties of a liquid are commonly studied before a breakdown occurs. In HV industrial apparatus such as high energy plastic metallized and liquid impregnated films capacitors, some local breakdowns, named self-healing, occur and the ability of the liquid to remain insulating after electrical degradation is of interest. To understand the influence of the liquid on the behavior of such capacitors, the post-arc behavior of the liquid alone has to be studied in a range of energy (~mJ) close to the self-healing process. The present work deals with the study of the ability of different liquids to restore in the millisecond range their insulating property after an electrical arc. Using an experimental apparatus based on the repetitive charging with a dc HV generator and discharging through an arc of a resistor-capacity system, different kind of liquids were tested. The liquids we used were alcane, esters, alkyl benzene and polyaromatic. We showed as expected that liquids with the smallest ratio of carbon versus hydrogen in their molecular formula present the best restoration property after an electrical arc, property that is crucial for liquids impregnating metallized capacitors     

Influence of molecular structure on propagation of positive streamer discharge in dielectric liquids

In this paper, streamer discharges in dielectric liquids which have various molecular structures, such as a double bond or a triple bond in the molecular formula, are observed in detail for positive point polarity under the application of an impulse voltage to the point electrode. As test liquids, four straight‐chain hydrocarbon liquids—n‐hexane, n‐heptane, n‐octane, and n‐decane—are selected and in addition the influence of a double or triple bond included in their molecular formula on the propagation of positive streamers is investigated in detail by simultaneously using a high‐speed schlieren method and an LED current measurement system. Some differences in the mean propagation velocity and the propagation shape of positive streamers are evident. Their shape is mainly filamentary in test liquids including a double or triple bond. Also, positive streamer propagation is promoted in these liquids. It is thought that this is due to the effect of π bonds which have smaller bond energies than σ bonds. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 149(1): 15–21, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10366     

Charging characteristics on dielectric surface by differentcharging processes in vacuum

This paper describes charging characteristics on a dielectric surface in vacuum by electron irradiation and field emission by a triple junction under negative and positive DC HV applications. The authors measured the 2-D distribution of electrostatic charging on a dielectric surface in situ. Experimental results revealed that the negative charge distribution caused by the electron beam had a conical shape over the whole surface. On the other hand, for a triple junction, it was an acute distribution around the triple junction. Moreover, they quantitatively investigated the difference of the 2-D charging distribution, using certain shape parameters, between the two charging processes mentioned above. In addition, they examined the time decay characteristics of the surface potential on the dielectric in vacuum     

一种介质阻挡放电型负载的等效电气模型

针对现有的介质阻挡放电(DBD)型负载等效电气模型存在的诸如只能适用特定激励波形、无法准确描述DBD型负载动态工作过程及模型系数确定过程繁琐等不足,本文在分析气隙内带电粒子的电离与复合过程的基础上,推导出一种DBD型负载等效电气模型,该等效电气模型由分别描述气隙被击穿时带电粒子的电离过程和气隙产生电离后带电粒子复合过程...     

A study of three‐dimensional observation of prebreakdown phenomena in dielectric liquids

This paper presents a new method which visualizes the high‐speed three‐dimensional images of prebreakdown streamers in dielectric liquids. A new optical system, having two crossed light axes to observe the needle tip from different directions, was designed. The shadowgraphic images from these two directions were combined through a beam splitter and focused on the photo‐cathode of an image converter camera. The photograph taken by this high‐speed shadowgraph system was reconstructed to three‐dimensional images using an image scanner and a computer. Using this system, the growth and decay processes of the streamer in cyclohexane were investigated. © 1999 Scripta Technica, Electr Eng Jpn, 128(4): 9–15, 1999     

Ageing of impregnating liquids with different chemical structure for polypropylene capacitor dielectric

The results of an ageing test of power capacitor dielectrics consisting of polypropylene film and impregnating liquids with different chemical structure (phenylxilylethan, monobenzyltoluol/dibenzyltoluol, polybuten, dodecylbenzol and perfluorcarbon) are presented in the paper. The testing was carried out on the specimens that modelled capacitor dielectric systems. The effects of aromaticity of synthetic hydrocarbon liquids and substitution of hydrogen atom by fluor atom in a perfluorcarbon liquid molecule, on critical partial discharge, rate of gas generation and gas absorption were established. The testing was carried out by means of a pulse height analyser.     

The discontinuity problem of a cylindrical dielectric post in a waveguide and its application on the dielectric constant measurements of liquids

Contents An exact solution of the discontinuity problem of a circular cylindrical dielectric post centered in a rectangular waveguide is presented. The waveguide is divided in three regions where the field is expressed in cylindrical waves or in waveguide modes. By point matching, a system of linear equations determining the reflection and transmission coefficients is found. — The structure of our interest includes a liquid column surrounded by a cylindrical glass tube. For this case, curves have been made showing the relationship between the coefficients of reflection and transmission and the permittivity of the rod. Such curves are used for measuring the dielectric properties of liquids.

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Das Problem der Diskontinuität einer zylindrischen dielektrischen Probe in einem Wellenleiter und seine Anwendung auf Messungen der dielektrischen Konstanten von Flüssigkeiten

Übersicht Für das Problem einer durch einen dielektrischen Stab in einem rechteckigen Wellenleiter verursachten Diskontinuität wird eine exakte Feldlösung angegeben. Der Wellenleiter wird dabei in drei Bereiche eingeteilt, in denen das Feld in Form von Zylinderwellen oder Wellenleitermoden ausgedrückt wird. — Durch punktförmige Anpassung wird ein System linearer Gleichungen gefunden, aus dem sich Reflexions- und Transmissionskoeffizienten bestimmen lassen. — Als Anwendungsbeispiel wird eine Flüssigkeitssäule in einem zylindrischen Glasrohr untersucht. Für diesen Fall werden Diagramme angegeben, aus denen die Beziehungen zwischen den Transmissions- und Reflexionsfaktoren zu den dielektrischen Konstanten der Probe entnommen werden können. — Solche Kurven werden zur Messung der dielektrischen Eigenschaften von Flüssigkeiten benutzt.

     

Influence of electro-mechanical stress on electrical properties of dielectric polymers

The influence of high electrical fields on water treeing, electrical treeing, relaxation, conductivity and charge mobility in various polymers is reviewed within the context of a molecular model. The real value of the AC field acting on water trees is questioned and it is shown that the strain induced by very large fields may affect the water tree growth in solutions with large dielectric constant. A model based on simple electrostatic and molecular parameters describes most water treeing results. Light emitted during electrical tree growth could be associated with the field-induced strain at the tip of the electrode. The strain in PET is also related to the Maxwell stress due to high DC field. The log of the charge stored in some polymers varies with the square of the field suggesting that there same phenomenon might influence charge formation under high fields. The significance of a constant mobility value is discussed in light of experiments showing that it is not constant at moderate fields in various polymers. It is shown that the only constant parameter under varying high fields is the activation volume typical of a given sample. Future work is pointed out, especially regarding the nature of the trapping sites and the influence of sample size on the activation volume.