HigH Voltage engineering - saadawi1.netsaadawi1.net/upLoadedFiles/extra_files/71a12n76j.pdf · The various solid dielectrics breakdown mechanisms are: (i) Intrinsic Breakdown (ii)

HigH Voltage engineering

Course Code: EE 2316

12/17/2017 Prof. Dr. Magdi El-Saadawi 1

Prof. Dr. Magdi M. El-Saadawi

www.saadawi1.net

E-mail : [email protected]

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ContentsChapter 1

Introduction to High Voltage Technology

Chapter 2

Generation of High Voltages and Currents

Chapter 3

Measurement of High Voltages and Currents

Chapter 4

Breakdown Mechanism of Gases, Liquid and Solid Materials

212/17/2017 Prof. Dr. Magdi El-Saadawi

Chapter 4

Breakdown Mechanism of Gases, Liquid and Solid Materials

4.1 Introduction

4.2 Breakdown of Gases

4.3 Breakdown in Liquid Dielectrics

4.4 Breakdown in Solid Dielectrics

4.5 Breakdown in Vacuum

4.6 Solved Examples


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Chapter 4 Part II

4.3 Breakdown in Liquid Dielectrics 4.3.1 Suspended Solid Particle Mechanism

4.3.2 Cavity Breakdown

4.3.3 Treatment of Transformer Oil

4.3.4 Testing of Transformer Oil

4.3.5 Application of Oil in Power Apparatus


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4.3.3 Treatment of Transformer Oil pp. 138

Prof. Dr. Magdi El-Saadawi

12/17/2017 6Prof. Dr. Magdi El-Saadawi

Methods used to remove oil impurities

(a) Filtration and Treatment Under Vacuum

(b) Centrifugal Method: الطرد المركزى

(c) Adsorption Columns: االمتصاص الكیمیائى

(d) Electrostatic Filters



(a) Filtration and Treatment Under Vacuum

Different types of filters have been used.

Oil is pre-dried before filtering

By suitable selection of the various components of the plant e.g., rate of flow of oil, degassing surface تحریر الغازات(سطح التفریغ( , vacuum pump

etc., a desired degree of purity can be obtained.

(b) Centrifugal Method

This method is helpful in partially extracting solid impurities and free water.

It is totally ineffective as far as removal of water and dissolved gases is concerned



(c) Adsorption Columns: االمتصاص الكیمیائى

Here the oil is made to flow through one or several columns filled with an adsorbing agent either in the form of grains or powder. Following adsorbing agents have been used:

(i) Fuller earth نوع من الطین االرضى (ii) Silica gel

(iii) Molecular sieves. المناخل الجزیئیة

(d) Electrostatic Filters:

The oil to be treated is passed between the two electrodes placed in a container. The electrostatic field charges the impurities and traces of water which are then attracted and retained by the foam coated

electrodes

The oil is poured in a container known as test-cell

The electrodes are polished spheres arranged horizontally

and a suitable gauge مقیاس is used to adjust the gap

The test-cell should be carefully cleaned and the moisture

and suspended particles should be avoided.

In Fig. 13, the voltmeter is connected to the primary side of

the H.V transformer but calibrated on the high voltage side.

The gap between the spheres is adjusted to 4 mm with the

help of a gauge and the spheres are immersed in oil to a a

prescribed depth.


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The voltage is increased gradually and continuously till a

flash over of the gap is seen or the MCB operates. This

voltage is known as rapidly-applied voltage.

Next bring the voltage back to zero and start with 40% of

the rapidly applied voltage and wait for one minute. See if

the gap has broken. If not, increase the voltage every time

by 2.1/2% of the rapidly applied voltage and wait for one

minute till the flash over is seen or the MCB trips. Note

down this voltage.


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Start again with zero voltage and increase the voltage to a

value just obtained in the previous step and wait for a

minute. It is expected that the breakdown will take place.

A few trials around this point will give us the breakdown

value of the dielectric strength.

The acceptable value is 30 kV for 4 mm applied for one

minute. And If it is less than 30 kV, the oil should be sent

for reconditioning.


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Chapter 4 Part II

4.4 Breakdown in Solid Dielectrics 4.4.1 Intrinsic Breakdown

4.4.2 Electromechanical Breakdown

4.4.3 Breakdown due to Treeing and Tracking

4.4.4 Thermal Breakdown

4.4.5 Electrochemical Breakdown


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The solid insulation not only provides insulation to the live

parts of the equipment from the grounded structures, it

sometimes provides mechanical support to the equipment.

The breakdown of solid dielectrics not only depends upon

the magnitude of voltage applied but also it is a function of

time for which the voltage is applied.

Roughly speaking, the product of the breakdown voltage

and the log of the time required for breakdown is almost a

constant i.e., Vb* ln tb = constant


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The dielectric strength of solid materials is affected by

many factors viz. ambient temperature, humidity, duration

of test, impurities or structural defects whether a.c., d.c. or

impulse voltages are being used, pressure applied to these

electrodes etc.

The various solid dielectrics breakdown mechanisms are: (i) Intrinsic Breakdown

(ii) Electromechanical Breakdown

(iii) Breakdown Due to Treeing and Tracking

(iv) Thermal Breakdown

(v) Electrochemical Breakdown


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If the dielectric material is pure and homogeneous, and a voltage is applied for a very short time of the order of 10–8 second, the dielectric strength of the specimen increases rapidly to an upper limit known as intrinsic dielectric strength.

It is affected by the ambient temperature as the structure itself might change slightly by temperature condition.

The intrinsic strength is reached when electrons in the valance band gain sufficient energy from the electric field to cross the forbidden energy band to the conduction band.

The process continues and finally may lead to formation of an electron avalanche similar to gases and will lead finally to breakdown if the avalanche exceeds a certain critical size.

The stresses required are of the order of 1 MV/cm for pure insulators and depends on insulation material

4.4.1 Intrinsic Breakdown


When a dielectric material is subjected to an electric field, charges of opposite nature are induced on the two opposite surfaces of the material and hence a force of attraction is developed and the specimen is subjected to electrostatic compressive forces and when these forces exceed the mechanical withstand strength of the material, the material collapses



When a dielectric material is subjected to an electric field, charges of opposite nature are induced on the two opposite surfaces of the material and hence a force of attraction is developed and the specimen is subjected to electrostatic compressive forces and when these forces exceed the mechanical withstand strength of the material, the material collapses



We know that the strength of a chain is given by the strength of the weakest link in the chain.

Similarly, whenever a solid material has some impurities in terms of some gas pockets or liquid pockets in it the dielectric strength of the solid will be more or less equal to the strength of the weakest impurities.

Suppose some gas pockets جیوب ھوائیة are found in a solid material during manufacture. The gas breaks down at a relatively lower voltage due to smaller relative permittivity.

The charges are concentrations at these voids within the dielectric lead and the breakdown starts step by step and finally a complete rupture of the dielectric is occurred.

This is called Breakdown due to Treeing


12/17/2017 Prof. Dr. Magdi El-Saadawi 20



When an electric field is applied to a dielectric, conduction current, however small it may be, flows through the material.

The current heats up the specimen and the temperature rises.

The heat generated is transferred to the surrounding medium by conduction through the solid dielectric and by radiation from its outer surfaces.

Part of the heat generated raises the temperature of the dielectric and another part is radiated to the atmosphere

Breakdown occurs when the heat generated becomes larger than the heat dissipated.

Thermal breakdown sets up an upper limit for increasing the breakdown voltage when the thickness of the insulation is increased.

4.4.4 Thermal Breakdown


In the presence of air and other gases some dielectric materials undergo chemical changes when subjected to continuous stresses.

The effects of electrochemical and chemical deterioration could be minimized by carefully studying and examining the materials.

The chemical and electrochemical deterioration increases very rapidly with temperature, and hence high temperatures should be avoided.



Some of the important chemical reactions that occur are:

• Oxidation: In the presence of air or oxygen, material such as rubber and polyethylene undergo oxidation giving rise to surface cracks تشقق السطح.

• Hydrolysis: When moisture or water vapor is present on the surface of a solid dielectric, hydrolysis occurs and the material loses their electrical and mechanical properties.

Electrical properties of materials such as paper, cotton tape, and other cellulose materials deteriorate very rapidly due to hydrolysis.



• Chemical Action: Even in the absence of electric fields, progressive chemical degradation of insulating materials can occur due to a variety of processes such as chemical instability at high temperatures, oxidation and cracking in the presence of air and ozone, and hydrolysis due to moisture and heat.

Since different insulating materials come into contact with each other in any practical reactions occur between these various materials leading to reduction in electrical and mechanical strengths resulting in a failure.


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HigH Voltage engineering - saadawi1.netsaadawi1.net/upLoadedFiles/extra_files/71a12n76j.pdf · The various solid dielectrics breakdown mechanisms are: (i) Intrinsic Breakdown (ii)