Positive Impulse Voltage Research Articles

Effects of Absolute Humidity on Flashover Voltage of Insulators

Effect of humidity on flashover voltage of four kinds of porcelain and one polymer insulators was studied experimentally for several years under natural humidity condition in Japan. It was found that the existing IEC humidity correction seems to be proper for most insulators regardless of the kind of applied voltage but that change may be necessary for pin type insulator under positive lightning impulse voltage application and for unit suspension disk and station post insulators under AC voltage application. Key-Words: Insulators, Flashover voltage, Absolute humidity, Humidity correction

Impulse breakdown characteristics of the plane-to-plane electrode system with a needle-shaped protrusion in SF6

Abstract This paper presents the impulse pre-breakdown and breakdown characteristics of the plane-to-plane electrode system with a needle-shaped protrusion in SF 6 gas. The breakdown voltage–time ( V – t ) characteristics and the breakdown voltage–gas pressure ( V – p ) characteristics of a highly non-uniform SF 6 gas gap under positive and negative lightning impulse voltages are investigated in the pressure range between 0.1 and 0.5 MPa. The pre-breakdown developments are examined by the corona current and light emission measurements with high time resolution. As a result, the dielectric strengths versus time-to-breakdown of SF 6 gas gap under positive lightning impulse voltages were nearly independent of the gas pressure. The first streamer corona was initiated at the tip of the needle electrode, and the streamer corona pulses developed with a stepwise propagation. The discharge paths were zigzag, and the branches of the discharge channel for positive polarity were created. On the other hand, the leader channel in the negative polarity was thicker and brighter than that in the positive polarity.

Lightning impulse breakdown characteristics of SF6 alternative gases for gas-insulated switchgear

Investigations of the lightning impulse breakdown characteristics of a SF6/N2 gas mixture, N2 gas and CO2 gas have been carried out. In the case of the SF6/N2 gas mixture the 50% breakdown (electric) field strength for the positive impulse voltage was slightly lower than that for the negative impulse voltage, and the pressure of the SF6/N2 gas mixture must be increased to approximately twice the pressure of SF6 gas to obtain the same dielectric strength as that of SF6 gas. For CO2 gas, the 50% breakdown field strength for the negative impulse voltage was lower than that for the positive impulse voltage in the case of SF6 gas. However, the dielectric strength of CO2 gas, even at twice the pressure of SF6 gas, was approximately 60% of the strength of SF6 gas, so that the CO2 gas-insulated busbar may be approximately 1.6 times as large as the SF6 gas-insulated busbar. In the case of N2 gas, the 50% breakdown field strength for the positive impulse voltage was lower than that for the negative impulse voltage, and the dielectric strength of N2 gas, even at twice the pressure of SF6 gas, was less than 50% of the strength of SF6 gas, so that the N2 gas-insulated busbar may be approximately 2.1 times as large as the SF6 gas-insulated busbar. A remarkable difference between CO2 and N2 gas was found from the observation of discharge forms and partial discharge properties.

Impulse partial discharge characteristics and their mechanisms under non-uniform electric field in N/sub 2//SF/sub 6/ gas mixtures

We have investigated impulse partial discharge (PD) and breakdown (BD) characteristics of a needle-plane gap in N/sub 2//SF/sub 6/ gas mixtures under positive lightning impulse voltage application, and discussed their physical mechanisms. The 50% probability PD inception voltage (PDIV/sub 50/), leader discharge onset voltage (LOV) and BD voltage (BDV/sub 50/) were measured and analyzed as a function of gas pressure and SF/sub 6/ content. Experimental results revealed the stepwise propagation process of the impulse PD and enabled us to classify the impulse PD in N/sub 2//SF/sub 6/ gas mixtures into two types, the streamer discharge and the leader discharge. We also discussed the impulse PD propagation mechanisms in terms of PD parameters such as propagation length, time interval and current pulse magnitude, and suggested a sequential relationship in the PD propagation process under non-uniform electric field.

Negative and positive impulse corona development along cylindrical insulator surfaces

Streamer propagation studies in a non-uniform gap under both positive and negative impulse voltages are presented, in the presence of cylindrical insulators and air alone. It is shown that the negative corona is highly constrained in all cases in comparison to positive corona, at comparable voltage levels in relation to breakdown voltages. Under both polarities the presence of PTFE promotes streamers to the largest extent. As evinced by photographs, positive corona in all cases studied show marked branching compared to negative corona where hardly any branching occurs. The presence of glass retards the growth of corona under negative polarity, whereas for positive polarity corona growth in the presence of glass is comparable to that in air.

The Aiming of the Bolt: How a Flashover Finds the Weak Spot

Purpose: To find how a spark from a positive or negative rod locates an object to connect to, or finds a weak spot in a cable insulation. Experimental: 1-2-m gap in ambient air between a rod and a horizontal wire suspended 0.25-0.5 m above ground, subjected to fast-rise (1.2 /spl mu/s) and slow-rise (150 /spl mu/s) positive and negative impulse voltages, and observed with photomultipliers, streak/framing camera, and open-shutter cameras. Main result : A leader channel from the rod feels its way by spraying a conical volume in front with streamers. When these connect to object(s) the leader follows either the direction of the highest streamer density or the most conductive streamer channel (our cameras cannot decide). If the streamers do not connect the influence of the missed object(s) is small. Additional : Evidence has been found for the slow increase with time of the field-to gas density ratio E/n in streamer channels due to channel heating and subsequent gas density reduction.

Insulation properties of a CO2/N2 50% SF6 gas mixture in a nonuniform field

AbstractThis paper describes partial discharge (PD) inception and breakdown voltage characteristics of a CO2/N2/SF6 gas mixture in a nonuniform field. These voltage characteristics were investigated with ac high voltage by changing the mixture rate of each gas of CO2, N2, and SF6 gas and the gas pressure from 0.1 MPa to 0.6 MPa. It was found that adding a small amount of CO2 gas into a N2/SF6 mixture causes a drastic increase in the breakdown voltage. For instance, when the mixture rate of SF6 in N2/SF6 gas mixture is 50%, with the addition of 1% CO2 the maximum breakdown voltage becomes 1.31 and 1.15 times higher than that of a 50% N2/50% SF6 gas mixture and pure SF6 gas, respectively. Moreover, those voltage characteristics of a CO2/N2/SF6 gas mixture were also investigated by changing the electric field utilization factor as well as by applying positive and negative standard lightning impulse voltages in order to discuss the corona stabilization effect, which seems to be one reason for the drastic increase in the breakdown voltage. These results and breakdown mechanism of the CO2/N2/SF6 gas mixture are discussed on the basis of the corona stabilization effect and the dissociation energies of the component gases by observing PD light images, PD light intensities through a blue and red filter, and PD current waveforms. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 140(3): 34–43, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10019

Breakdown sequence in SF6 at pressures up to 8 bar in a geometryrepresentative of a circuit-breaker

Breakdown sequence in SF6 of an electrode arrangement whichrepresents the contacts of a circuit-breaker is investigated at pressures upto 8 bar under positive impulse voltages. The Schlieren frames point out therole played by the cathode as a source emitting electrons by field effectemission. Photomultiplier records demonstrated, at Pd = 2 bar, that thedischarge's formative time strongly decreases: from 1 µs at pressure lowerthan Pd, down to 150 ns at pressure higher than Pd. A change in thedischarge regime is associated with this transition. An experimentalinvestigation, using photomultipliers and Schlieren devices, allows us todescribe both breakdown sequences. At low pressures, there is a criticalvolume at the anode. Nevertheless, it is a negative discharge which isresponsible for the breakdown although the geometrical electrical field at thecathode boundary is about 60 kV cm-1. At high pressures, there is nocritical volume. However, a positive discharge develops across the gap.Physical considerations taking into account the electron energy are proposedto explain the mechanisms involved in both types of discharge.

Streamer current in a three-electrode system

A study has been conducted on positive streamer discharges in air at atmospheric conditions for a three-electrode system, The electrode system consisted of two parallel planes (one grounded and one supplied with a negative dc voltage) and a small, insulated needle, sticking out from the center of the grounded plane. A triggering positive square impulse voltage of 5 /spl mu/s duration was applied to the insulated needle and the currents associated with the streamer discharge were measured simultaneously on all three electrodes. During the streamer propagation, the current measured at the needle was the conduction current while the other two were the displacement (or capacitive) currents generated by the movement of charge in the electrode gap. The objective of this study is to identify the three currents and to investigate if simple representations of the streamer can reproduce the displacement currents measured at the plane electrodes. Two models for the streamer were applied: (1) a charged sphere moving in the background field and (2) a channel with a constant voltage gradient extending in the gap. In both models it was assumed that the streamer propagated with a constant velocity, which was estimated from the measurements. The motion of the streamer was simulated by a series of electrostatic calculations, using a field calculation program. Comparison of the measurements with the simulations indicates that the charge of the streamer is confined to a spherical region (i.e. streamer head) and it is increasing continuously during its advancement in the electrode gap. A discussion on advantages and disadvantages with the two investigated models (sphere vs. channel with potential gradient) is conducted, and a possible hybrid model is suggested. In the proposed model, features from both considered streamer representations are included.

Tests of the ‘early streamer emission’ principle for protection against lightning

Experiments are described which are designed to test two devices based on the 'early streamer emission' (ESE) principle, for lightning protection, against the traditional Franklin rod. In all three cases, the device was subjected to a steady negative electric field from a sphere, simulating the field beneath a thundercloud, prior to application of a superimposed negative impulse field, simulating the field due to the downward leader. The first device consisted of a vertical rod to which a subsidiary 1/50 /spl mu/s positive impulse voltage, variable up to 40 kV peak, could be applied with varying delays from the start of the negative impulse field. Energising of the rod was thus independent of the applied negative field. The second device was a commercial product, energising of which was controlled by its own power supply. Sparkover voltages in the sphere/device gaps and times to breakdown were measured. It is shown that the ESE devices showed a small advantage, in time to breakdown, over the Franklin rod.

Sparkover in the rod–plane gap under combined direct and impulse voltages

Experiments are described in which composite voltages, produced by superimposing positive and negative switching impulse voltages on pre-stressing positive or negative direct voltages, are applied to a rod–plane gap of lengths up to 1 m. Changes in 50% sparkover voltages are described over a range of positive and negative pre-stressing voltages; it is shown that negative pre-stressing always reduces the composite sparkover voltage, but that positive pre-stress increases the positive composite sparkover voltage and reduces the negative one. Recordings of photomultiplier observations and ‘still’ photography of pre-breakdown corona have been made at the 50% sparkover level and the results related to the sparkover characteristics. It is shown that the latter depend on the way in which leader initiation is affected by the preceding DC corona.

Effects of negative direct voltage pre-stressing on the breakdown of conductor–rod gaps under positive impulse voltages

The sparkover of a 50 cm conductor–rod gap under positive impulses is investigated when a pre-stressing negative direct voltage is applied to the rod. Under lightning impulses the pre-stress has a small effect on the breakdown voltage of the gap. For long-front impulses, a considerable increase in the dielectric strength of the gap can be observed. Several parameters affect this behaviour, like the value of the pre-stressing direct voltage, the diameter of the energised conductor, the position of the rod with regard to the conductor and the waveshape of the applied impulse.

Prebreakdown Light Emission and Current Pulses in Liquid Nitrogen

Prebreakdown light emission phenomena and current pulses in liquid nitrogen under positive and negative impulse voltages of both polarities were simultaneously observed. Two structures of light emission, bush-like and filamentary, were observed in both polarities. The light pulses could be classified into a few patterns. A close relation between these patterns and the structures of the light emission was found. From the streak photographs using the image converter camera, it was found that light emission appears intermittently and propagates in a stepwise manner. The prebreakdown current pulses occur prior to the corresponding light pulses, and the time difference between them is 40 ns for both polarities. From the results of these observations, the mechanisms that cause the difference of the light emission structures is considered. For the negative polarity, the structure is determined by the pulse activity after the initiation of the phenomena. In contrast, the structure is already determined at the instance of the initiation for the positive polarity.

A simplified model for the simulation of positive-spark development in long air gaps

This paper presents a new modelling of positive-discharge development, based on new simplifying assumptions. It presents these assumptions, which are deduced from the existing theoretical models and concern mainly the coupling between the leader corona and the corona region. This model has been validated through the comparison of simulations with experimental results in the reference configuration of a rod - plane gap subjected to various positive impulse voltages. It has also been used to simulate the discharge behaviour with perturbations of the applied potential form.

SF<sub>6</sub>ガス中トリプルジャンクションに起因する沿面放電の部分放電発生機構

The authors deal with an initial stage of partial discharge inception phenomena at the triple junction formed in GIS. Partial discharge inception voltage (Vc) was measured by optical method during the application of positive lightning impulse voltage as parameters of the surface condition of electrode, and SF6 gas pressure. As a result, the followings were obtained. (1) Vc is not affected by the surface condition of the electrode. It seems that initial electrons are supplied from negative ion, SF6-, in the space charges formed by electron avalanche due to high field strength near the triple junction. (2) Covering the electrode with dielectric film is effective to the relaxation of the electric field near the triple junction. Therefore Vc for the dielectric coated electrode is higher than that for the bare electrode.

Influence of insulating barrier on the creepage discharge in transformer oil

The propagation aspects of a creepage discharge in transformer oil are observed simultaneously and in detail under a positive impulse voltage condition, by means of a high-speed Schlieren optical system, an LED current measurement system and a charge measurement system using a capacitor. In the present paper, the influence of the material type and the thickness of insulating barriers on the propagation of the impulse creepage discharge are discussed for a point to plane electrode geometry. In addition, the influence of a barrier surface to the electric field direction on the propagation of the impulse creepage discharge is discussed for a parallel plane electrode gap with a protruding point.

乾燥空気の正極性インパルスコロナ開始遅れに与える人工金属管壁効果

乾燥空気の正極性インパルスコロナ開始遅れに与える人工金属管壁効果

架空配電線への雷撃距離特性に関する検討

To clarify characteristics of lightning stroke distance to power distribution lines, experimental studies were done on discharge characteristics of a rod-ground wire (open wire) and a rod-conductor (insulated wire). The experiments were done by using the 12MV impulse generator at Shiobara testing yard of CRIEPI. 50% flashover voltage of a rod-insulated wire gap is higher than that of a rod-open wire gap under negative impulse voltages (2/50_??_250/1700μs) and a positive lightning impulse voltage (2/50μs). These effects become so notable as an insulated part of the insulated wire is thicker. Measurements by using ALPS (Automatic Lightning-Discharge Progressing-Feature Observation System) show that development of an upward leader from an insulated wire is suppressed by the insulated part of the wire. Therefore it is clarified that an insulated wire is more unlikely to attract lightning hits than an open wire. From these experimental results, characteristics of lightning stroke distance to power distribution lines were studied in the case that a ground wire is removed. Attractive area of direct lightning hit to a conductor (insulated wire) becomes narrower as the peak value of lightning stroke current becomes higher.

Influence of humidity on the breakdown mechanism of medium length rod-plane gaps stressed by positive impulse voltages

To better understand the influence of atmospheric humidity on the breakdown mechanism of medium length rod-plane gaps when stressed by positive impulse voltages, emphasis was given to the initial prebreakdown phenomena (the first and second coronas) as well as the dark period. The salient events have been measured and assessed, especially the variation of their statistical distribution. Results were also taken and assessed to determine the influence of humidity on the breakdown probability curves, on the 50% breakdown voltages and on the distribution of the times to breakdown. During this investigation the main parameters, in addition to humidity, were the gap length, the geometry of the end of the rod and the front duration of the impulse voltage. Humidity was found to have a great influence on the breakdown mechanism, especially its streamer region. It influences not only the 50% breakdown voltage but also its standard deviation. It was also found to cause changes in the shape of the U-curves. The necessary correction for humidity was found to vary considerably with the front duration of the applied impulse.

Breakdown and prebreakdown phenomena in liquids under positive impulse voltages

We examine the breakdown and prebreakdown behavior of two insulating liquids: benzyl-toluene and mineral oil in a nonuniform electric field at large distances (up to 20 cm) under a positive impulse voltage. We evaluate and compare the breakdown strengths of both liquids in these conditions as a function of interelectrode distance via Weibull type plots, a method commonly used when dealing with insulating gases and solids. The comparative behaviors of the liquids change greatly when the cumulative probability of breakdown is decreased from 100%. The second part of this study concerns streamer stopping lengths and the corresponding current and light emission signals. >