Mechanism of ice accretion on insulator surface 

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Due to the continuous increase of energy demand, long distance power transmission lines and high voltage substations have been widely used in power networks. Hence, the performance of outdoor insulators as one of the main and essential devices in the power network is playing a vital role for the safe operation of overhead power transmission lines and related substations. From the electrical viewpoint, the main function of insulators is to withstand electrical stress with a low failure probability under the environmental and meteorological conditions to which they are subjected. Over recent decades, atmospheric ice accretion has been recognized to be a serious threat to power transmission lines. Therefore, it has been the subject of many investigations with the aim of improving understanding of the icing phenomenon and the atmospheric factors influencing the electrical performance of insulators under icing conditions. Therefore, a review of the literature pertinent to this thesis is presented in this chapter. The first part of this chapter provides a brief review of the theoretical studies and experimental investigations on the mechanism of ice accretion on an insulator surface, flashover of icecovered surfaces and the effects of atmospheric parameters on insulator performance. Then, the present chapter continues with the review of basic flashover modeling of insulators under pollution conditions. Lastly, the current dynamic and static models developed to predict flashover parameters of ice-covered insulator surfaces are presented.

Mechanism of ice accretion on insulator surface

Natural atmospheric ice deposits on insulators mainly result from a variety of conditions including hoarfrost caused by condensation of water vapor on cold surfaces, in-cloud icing involving the freezing of suspended super-cooled droplets in clouds or fog, and precipitation icing resulting from freezing rain and drizzle, as well as wet and dry snow .

The mechanism of ice accretion on various insulator types depends on the environmental conditions, such as air temperature, wind velocity, water droplet size, and liquid water content. Based on the investigations carried out by Imai , Oguchi and Kuroiwa , three types of ice, namely hard rime, soft rime and glaze, under different atmospheric conditions were formed on the insulator surface and other energized equipment of the power network. Hard rime is opaque and has a density between 0.6 and 0.9 g/cm³ . Soft rime is white and opaque, with a density less than 0.6 g/cm³ . Glaze is transparent and has a relatively high density of about 0.9 g/cm³.

The studies of the icing phenomenon have also been extended to the investigations of electrical performance of insulators. With the aim of investigating the flashover of ice covered insulators, a variety of ice types was formed in the climate chamber of the high voltage laboratory of CIGELE, UQAC. Two regimes of icing, dry and wet, were used by Farzaneh et al to investigate the performance of ice-covered insulators. The ice grown process is referred to a dry regime when the ice deposit temperature remains below 0°C. Both hard rime and soft rime ice were grown under a dry regime . Under the dry regime, all the water droplets impinging on an insulator surface were completely frozen. In contrast, glaze is grown under a wet regime at a temperature of 0°C where icicles can be formed around the insulator sheds . This type of ice deposition simulated ice accretion during freezing rain precipitation.

Conductivity of ice surface

It is well acknowledged that flashover characteristics of an insulator are affected by the ice surface properties. Over the last few decades, ice surface properties have been the subject of many scientific studies from chemical and mechanical point of views. Faraday observed that the surface layers of ice crystals, at temperatures near melting point, have physical properties which seem to differ considerably from those of the bulk. Much of the research in recent decades has been carried out on the topic of ice surface conductivity. Experimental results showed that ice surface has electrical conductivity which varies in the range of 10⁻¹¹ to 10⁻⁸ S, even with a small amount of impurities in ice . Another approach to studying ice surfaces is through observing the thickness of the liquid-like layer on the surface at temperatures close to the melting point of ice. Over the past few decades, several techniques such as semi-quantitative thermodynamic modeling , proton backscattering , optical ellipsometry , X-ray diffraction , molecular dynamics calculations , atomic force microscopy, photoelectron spectroscopy and infrared spectroscopyhave been used to measure the thickness of the liquid-like layer. The thickness measurements of this thin surface melting layer reveal a remarkable variation up to 200 nm depending on the experimental condition, the nature of the ice samples and environment temperature.

The presence of a water film on the ice surface accreted on an insulator string is one of the main conditions for flashover to occur. The water film on the ice surface of an energized insulator submitted to icing condition can be produced not only by a rise in air temperature, wet ice accretion, condensation or the effect of sunshine but also by many other factors, including the heating effect of leakage current and partial arcs . The high conductivity of the water film is caused by the rejection of ionic impurities from the bulk ice toward its surface during solidification/freezing process. This increase in conductivity can also be related to pollution of the ice surface by corona discharge by-products. As a consequence, the voltage drop across the air gaps increases . If the electric field across the air gaps is high enough, an arc propagating along the ice surface of the insulator may lead to flashover. Experimental results obtained by P. G. Buchan et al. showed that wet ice can be considered to have both volume and surface conductivity . H. T. Bui noticed that the value of the electrical resistance of ice during the icing period is higher than that of the de-icing period . Sugawara et al. showed that the average conductivity of the water film on the bottom of ice plates is approximately 2.5 times higher than the applied conductivity . Over the last years, the CIGELE research group has investigated leakage current distribution and the corresponding surface conductivity using a triangular ice sample. It was observed that at 0°C, water film thickness and leakage current are not uniformly distributed. However, when the air temperature is higher than 2.5°C, the distribution of leakage current is uniform. Also, it was found that the current flows through both the inner and the outer interface of the ice . The range of the conductivity of dripping water was reported to be 5 to 11 times greater than that of applied water conductivity . Moreover, it was observed that surface conductivity of the ice increases rapidly with an increase in applied voltage .

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Table of contents :

1.1. Overview.
1.2. Research objectives
1.2.1. Experiments
1.2.2. Simulations
1.3. Methodology
1.4. Statement of the originality of the thesis
1.5. Structure of the thesis »
2.1. Introduction
2.2. Mechanism of ice accretion on insulator surface
2.3. Conductivity of ice surface
2.3.1. Electrical surface conductivity of ice
2.4. Flashover mechanisms of ice-covered insulators
2.4.1. Effect of ice type and thickness
2.4.2. Effect of applied water conductivity
2.4.3. Effect of dry arc distance
2.4.4. Effect of air gaps
2.4.5. Effect of insulator type and configuration
2.4.6. Effect of voltage polarity
2.4.7. Effect of air pressure
2.5. Flashover modeling of insulators surfaces
2.5.1 Basic models of polluted surface flashover
2.5.2. Modeling of flashover on ice-covered surfaces Static models Dynamic models
2.6. Conclusion
3.1. Introduction
3.2. Test facilities
3.2.1. High-voltage equipment
3.2.2. Climate Chamber
3.2.3. Physical Objects
3.2.4. Current and voltage measurement devices
3.2.5. High-speed camera
3.3. Test Procedures.
3.3.1. Ice test preparation prior to flashover
3.3.2. Evaluation sequence
3.3.3. Water film conductivity and volume flow rate measurement procedure
3.4. Conclusion
4.1. Introduction
4.2. Withstand voltage measurement of a string of five standard insulator units under icing
4.3. Withstand voltage measurement of post insulator units under icing conditions
4.3.1. Experimental results and discussion under DC Voltage
4.3.2. Experimental results and discussion under AC Voltage
4.4. Arc propagation on the ice surface
4.4.1. Arc propagation velocity on the ice surface
4.5. Volume flow rate and water film conductivity
4.6. Discussions
4.6.1. Effect of voltage polarity on minimum flashover voltage
4.6.2. Effect of water film conductivity on arc velocity
4.6.3. Effect of freezing water conductivity and dry arc distance on maximum withstand
4.7. Flashover results presentation using Icing Stress Product (ISP)
4.7.1. Comparison of flashover results from different laboratories under icing conditions
4.8. Conclusion
5.1. Introduction
5.2. Model description
5.3. The equivalent circuit components
5.3.1. Arc channel characteristics
5.3.2. Ice section characteristics Electrical surface conductivity Variation of water film thickness along the ice-covered insulator
5.3.3. Propagation criterion and arc velocity
5.4. Two —arc model of flashover
5.5. General description of the proposed dynamic model
5.6. Conclusion Ill
6.1. Introduction
6.2. Validation of the equivalent surface conductivity mathematical model
6.3. Validation of the proposed two-arc model
6.3.1. Validation of the DC two-arc model
6.3.2. Validation of the AC two-arc model
6.4. Numerical simulations of potential and electric-field distributions along an ice-covered post insulator
6.4.1. Potential distribution along the insulator under DC voltage
6.4.2. Potential and electric-field distributions along the insulator under AC voltage
6.5. Conclusion

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