Mechanisms of discharge propagation over an electrolyte surface .

Mechanisms of discharge propagation over an electrolyte surface .

Statement of originality, and contribution to knowledge

To the best of our knowledge, no fundamental work has yet been done on the process of arc propagation over a one-meter ice surface. Previous researches [19, 25, 26] were conducted on small setups (a few centimeters) that are not representative of foil-scale insulators and in which thermal ionization does not play a significant part. Those investigations concentrated on the initiation and propagation of streamers. Moreover, the flashover phenomenon has been observed with streak and framing cameras, which only captures visible light. Employing a photomultiplier tube (PMT)5 which operates in the 185 nm to 900 nm range9 only produces an electrical output signal corresponding to the intensity of light.

It could not be used to observe the whole arc channel with spatial resolution (as opposed to video cameras). This study is the first to use an ultra-high-speed camera (above 20,000 fps and up to 675,000 ips) on a large-scale ice surface. Moreover, employing an image intensifier along with an ultraviolet (UV) band-pass filter enabled us to observe UV activity during the discharge propagation. Besides the originality of employing high-technology equipment, this work is the first research to emphasize close-up observations of arc root and channel, as well as the propagation pattern of discharge. Based on the new findings, this fondamental work proposes physical mechanisms contributing to arc propagation along an ice surface and involving forces and ionization activities.

Mechanisms of discharge propagation over an electrolyte surface

The pioneering work of Hampton [34] can be considered as the first attempt todescribe flashover propagation, although his work was based on the quantitative theory developed by Obenaus in 1958 [35], followed by Neumarker in 1959 [36]. Hampton [34] showed that as surge current heats up the pollution layer, its resistance falls in the normal manner of an electrolyte. Further heating boils off water in the layer, but the overall resistance does not change markedly until a saturated electrolyte has been formed, since a decrease in the thickness of electrolyte is offset by an increase in conductivity. When the electrolyte is saturated, farther loss of water results in a rapid increase in resistance. He noted that the growth of a discharge on a polluted surface usually took several voltage cycles to build up into flashover, and that this growth was markedly affected by the current going through zero at each half cycle.

Hence, with the aim of reducing the number of variables affecting discharge propagation, he used a direct-current supply for his work. He also confined his investigation to the behavior of an arc discharge rooted on the surface of a column of electrolyte having a constant resistance per unit length to overcome the complexities induced by the varying resistance of a wet polluted layer. These requirements were met by using as a resistance a jet of water flowing from a nozzle. A uniform flow of water was ensured by a constant-head tank and the resistivity of the water was adjusted by mixing in a small portion of saturated salt solution. The resistance of the water column was maintained at the required value by injecting salt solution at a controlled rate into the water flowing to the nozzle.

Effect of voltage polarity

Regarding the effect of voltage polarity on discharge propagation, experimental research resulted in finding the differences in arc constants and voltage-current characteristics for positive and negative cases [71]:
E=84.6r0772
for negative arcs
E=208.9r0449
for positive arcs The other difference was found to be the electrode voltage drop value for positive and negative polarities. For negative and positive arcs, the measured values of electrode voltage drop are 526 V and 799 V, respectively.

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Table des matières

ABSTRACT
RÉSUMÉ
ACKNOWLEDGMENTS
TABLE OF CONTENTS
LIST OF FIGURES
LIST OF TABLES
CHAPTER 1
INTRODUCTION
1.1. Overview
1.2. Problem description
1.3. Research objectives
1.4. Methodology
1.5. Statement of originality and contribution to knowledge
1.6. Thesis outline
CHAPTER 2
LITERATURE REVIEW
2.1. Introduction
2.2. Mechanisms of discharge propagation over an electrolyte surface .
2.3. Discharge propagation on an ice surface
2.4. Effect of voltage polarity
2.5. The effect of humidity
2.6. Conclusions
CHAPTER 3
FACILITIES AND PROCEDURES
3.1. Introduction..
3.2. Test facilities .
3.2.1. High-voltage equipment
3.2.2. Current and voltage measurement devices
3.2.3. Climate chamber
3.2.4. High-speed camera
3.2.5. Image intensifier
3.2.6. PMT.
3.2.7. Corona detection camera .
3.2.8. Data acquisition system .
3.3. Procedure.
3.3.1V Physical test setup preparation.
3.3.2. Setup alignments
3.3.3. Voltage application.
3.3.4. Effects of humidity on test procedure.
3.3.5. Water film thickness test procedure
3.3.6. Water film thickness measurement procedure.
3.3.7. Synchronization between electrical and optical measurements
CHAPTER 4 .
EXPERIMENTAL RESULTS ON ARC PROPAGATION FEATURES
4.1. Introduction
4.2. Glow to arc transition threshold
4.3. Arc radius variation along the arc channel
4.4. Arc radius and leakage current
4.5. Arc propagation featur
4.6. Major and minor collapse
4.6.1. DC-positive.
4.6.2. DC-negative
4.6.3. Physical appearance of the channel and contact surface
4.6.4. Anode and cathode current jumps
4.7. Effect of propagation direction on velocity
4.8. V-I characteristics of DC-positive arc on ice surfaces
4.9. AC discharge propagation
4.9.1. Subsidiary discharge along AC arcs.
4.9.2. Final jump
4.9.3. Effect of applied water conductivity.
4.10. Conclusion .
CHAPTER 5
EFFECT OF WATER FILM AND AIR HUMIDITY
5.1. Introduction
5.2. Effects of water film
5.2.1. Effect of residual resistance on discharge initiation.
5.2.2. The effect of water film thickness on leakage current variation
5.2.3. Effect of water conductivity .
5.3. Humidity effect.
5.3.1. Effect of humidity on visible discharge initiation .
5.3.2. Effect of humidity on discharge propagation .
5.4. Conclusion.
CHAPTER 6
INVOLVED MECHANISMS IN ARC PROPAGATION OVER AN ICE SURFACE.
6.1. Introduction .
6.2. General appearance of arc column
6.2.1. Glow-to-arc transition threshold
6.2.2. Arc radius and leakage current
6.2.3. Space charge distribution around the channel
6.2.4. Arc radius variation along the arc channel .
6.2.5. Arc channel luminosity
6.3. Arc propagation pattern and features
6.3.1. General pattern
6.3.2. Major and minor collapses
6.4. Anode and cathode current jumps
6.5. Water film.
6.6. Effect of humidity
6.7. General discussion on the mechanisms of arc development
6.8. Conclusion
CHAPTER 7..
CONCLUSIONS AND RECOMMENDATIONS .
7.1. Conclusions and Contributions of the Thesis
7.2. Recommendations for future work.
REFERENCES