HIGH VOLTAGE ENGINEERING EET 413 EET413 HIGH VOLTAGE ENGINEERING 1
CHAPTER 4 CONDUCTION & BREAKDOWN IN SOLID DIELECTRIC EET413 HIGH VOLTAGE ENGINEERING 2
On completion of this lesson, a student should be able to: Ability to analyze the various breakdown mechanism and applications of vacuum, liquid, solid and composite dielectrics EET413 HIGH VOLTAGE ENGINEERING 3
TOPIC OUTLINE 4.1 INTRODUCTION 4.2 INTRINSIC BREAKDOWN 4.3 ELECTROMECHANICAL BREAKDOWN 4.4 THERMAL BREAKDOWN 4.5 BREAKDOWN OF SOLID DIELECTRICS IN PRACTICE 4.6 BREAKDOWN IN COMPOSITE DILECTRICS 4.7 SOLID DILECTRICS USED IN PRACTICE EET413 HIGH VOLTAGE ENGINEERING 4
4.1 INTRODUCTION Solid dielectric materials are used in all kinds of electrical apparatus and device to insulate one current carrying part from another when they operate at different voltages. Solid dielectric have higher breakdown strength compared to liquids and gases A good dielectric should have : • Low dielectric loss • High mechanical strength • Should be free from gaseous inclusions and moisture • Resistant to thermal and chemical deterioration Types of Solid insulating materials: • Organic materials (paper, wood and rubber) • Inorganic materials (Mica, glass and porcelain and synthetic polymers) EET413 HIGH VOLTAGE ENGINEERING 5
Solid Insulating Material Porcelain XLPE Paper EET413 HIGH VOLTAGE ENGINEERING 6
Cont…. Breakdown in solid dielectric occurs, if solid dielectric strength less than electric stress. Breakdown Mechanism in solid dielectric depend on the time of application of voltage, and can be classified as follows: 1. Intrinsic or ionic breakdown 2. Electromechanical breakdown 3. Failure due to treeing and tracking 4. Thermal Breakdown 5. Electrochemical Breakdown, and 6. Breakdown due to internal discharges EET413 HIGH VOLTAGE ENGINEERING 7
Cont…. EET413 HIGH VOLTAGE ENGINEERING 8
4.2 Intrinsic Breakdown Intrinsic Breakdown occurs if the applied on solid dielectric increases to 10 6 Volt/cm in short duration in order 10 -8 sec. This breakdown depends upon the presence of free electrons which are capable of migration through the lattice of the dielectric. Based on experiment the maximum electrical strength recorded is15 MV/cm for Polyvinyl at -196 0C. The maximum strength usually obtainable ranges from 5 MV/cm to 10 MV/cm There are two types of intrinsic breakdown mechanism e.g Electronic Breakdown and Streamer Breakdown (avalanche ). EET413 HIGH VOLTAGE ENGINEERING 9
4.2.1 Electronic Breakdown Assumed to be electronic in nature (occurs in time 10-8 s) Initial density of conduction (free) electrons assumed to be large and electron-electron collisions occurs. When electric field is applied, electrons gain energy and cross the forbidden gap from the valency to the conduction band. This process repeated, more and more electrons available in conduction band, eventually leading to breakdown. EET413 HIGH VOLTAGE ENGINEERING 10
4.2.2 Avalanche or Streamer Breakdown Similar to breakdown in gases due to cummulative ionization. Conduction electrons gain sufficient energy above a certain critical electric field and cause liberation of electrons from the lattice atom by collisions. Motion of electron from cathode to anode will gain energy from the field and losses it during collisions. When the energy gained by an electron exceeds the lattice ionization potential, an additional electron will be liberated due to collision of the first electron. This process repeats itself resulting in the formation of an electron avalanche, and breakdown will occur when the avalanche exceeds a certain critical size. In practice, breakdown does not occur by the formation of a single avalanche, but occurs as a result of many avalanches formed and extending step by step through the entire thickness of the material as shown in Figure 4.2. EET413 HIGH VOLTAGE ENGINEERING 11
Cont…. EET413 HIGH VOLTAGE ENGINEERING 12
4.3 Electromechanical Breakdown When solid dielectrics are subjected to high electric fields, failure occurs due to electrostatic compressive forces which Can exceed the mechanical compressive strength. If the thickness of the specimen is d o and is compressive to a thickness d is under applied voltage V, then the electrically developed compressive stress is in equilibrium if, or 2 V d 2 Y d 2 2 0 0 V d Ln ( ) Y Ln 0 r d 2 d 2 d o r Y = the Young’s modulus Max. Electric stress before BD Mechanical instability occurs d/d o 1 / 2 V Y = 0.6 or d o /d = 1.67 E 0 . 6 max d EET413 HIGH VOLTAGE ENGINEERING o o r 13
4.4 Thermal Breakdown 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. Equilibrium is reached when the heat used to raise the temperature of the dielectric, plus the heat radiated out, equals the heat generated . EET413 HIGH VOLTAGE ENGINEERING 14
Equilibrium is reached when the heat used to raise the temperature of the dielectric, plus the heat radiated out, equal the heat generated. The heat generated under dc stress E is given as, W dc 2 E W/cm 2 = dc conductivity of the specimen The heat generated under a.c fields, 2 E f tan r W/cm 2 W ac 12 1 . 8 x 10 loss angle of the dielectric f = frequency (Hz), material E = rms value 15 EET413 HIGH VOLTAGE ENGINEERING 15
The heat dissipated (W T ) is given by dT W C div ( K grad T ) T V dt C v = Specific heat of the specimen T = temperature of the specimen, K = thermal conductivity of the specimen t = time over which the heat is dissipated BD occurs when W dc > W T for dc W ac > W T for ac 16 EET413 HIGH VOLTAGE ENGINEERING 16
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Example 4.1 A solid specimen of dielectric has a dielectric constant of 4.2, and tan 0.001 at a frequency of 50 Hz. If it is subjected to an alternating field of 50 kV/cm, calculate the heat generated in the specimen due to the electric loss. Using eq. r 2 E f tan W ac 12 1 . 8 10 2 3 50 10 50 4 . 2 0 . 001 12 1 . 8 10 3 0 . 291 mW/cm EET413 HIGH VOLTAGE ENGINEERING 18
4.5 Breakdown of Solid Dielectric in Practice There are certain type of breakdown which do not come under either intrinsic breakdown or thermal breakdown but occur after prolonged time, eg. breakdown due to tracking in which dry conducting tracks are formed on the surface of the insulation. These tracks act as conducting path - leading to gradual breakdown. Another type in this category is electrochemical breakdown caused by chemical transformation such as electrolysis, formation of ozone etc. Failure also occurs due to partial discharges which are brought about in the air pockets inside the insulation. This breakdown is very important in the impregnated paper insulation used in HV cables and capacitors. EET413 HIGH VOLTAGE ENGINEERING 19
4.5.1 Chemical and Electrochemical Breakdown In presence of air and other gases, dielectric materials undergo chemical changes when subjected to continuous electrical stresses. Some of the important chemical reactions that occur are: Oxidation : In the presence of air or oxygen, materials such as rubber and polyethylene undergo oxidation giving rise to surface cracks. Hydrolysis : When moisture or water vapour is present on the surface of the solid dielectric, hydrolysis occurs and the materials lose their electrical and mechanical properties. Materials like paper, cotton tape and other cellulose materials deteriorate very rapidly due to hydrolysis. Chemical Action : Progressive chemical degradation can occur due to a variety of processes such as chemical instability at high temperature, oxidation, cracking and hydrolysis. EET413 HIGH VOLTAGE ENGINEERING 20
4.5.2 Breakdown Due to Treeing and Tracking When solid dielectric subjected to electrical stresses for a long time, two kinds of visible marking are observed. a) The presence of a conducting path across the surface of insulation b) Mechanism whereby leakage current passes thru the conducting path, finally leading to the formation of spark. EET413 HIGH VOLTAGE ENGINEERING 21
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