HIGH VOLTAGE ENGINEERING EET 413 EET413 HIGH VOLTAGE ENGINEERING 1
CHAPTER 3 CONDUCTION & BREAKDOWN IN LIQUID 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 6.1 Introduction 6.2 Classification of Liquid Dielectric 6.3 Characteristics Of Liquid Dielectrics 6.4 Conduction And Breakdown In Pure Liquid 6.5 Conduction And Break Down In Commercial Liquids EET413 HIGH VOLTAGE ENGINEERING 4
6.1 INTRODUCTION Liquid dielectrics are used mainly as impregnants in hv cables and capacitor, and for filling up of transformers, circuit breakers etc. Act as heat transfer agents (in transformer), and as arc quenching media (in circuit breakers). The most important factor that affects the electrical strength of an insulating oil, is the presence of water in the form of fine droplets in the oil. Dielectric strength of oil reduces more sharply if it contains fibrous impurities in addition to water. In practice, the choice of a liquid dielectric is made mainly on the basis of its chemical stability. EET413 HIGH VOLTAGE ENGINEERING 5
Electrical Equipment Using Liquid Dielectric Power capacitor Switchgear Power transformer EET413 HIGH VOLTAGE ENGINEERING 6
6.2 CLASSIFICATION OF LIQUID DIELECTRIC Transformer Oil (mineral oil) Silicone Oil Synthetic hydrocarbons Chlorinated Hydrocarbon Ester Latest developments EET413 HIGH VOLTAGE ENGINEERING 7
Transformer Oil The cheapest and the most commonly used in power apparatus Almost colorless liquid consisting of a mixture of hydrocarbons Gradually ageing process, when the liquid in a transformer is subjected to prolonged heating at high temperature of about 95 °C With time the oil becomes darker due to formation of acid and resins, or sludge in the liquid. EET413 HIGH VOLTAGE ENGINEERING 8
Silicone Oils As alternative to polychlorinated biphenyl (PCB) but they are expensive At a temperature 150 °C they exhibit high long-term thermal stability Resistant to chemicals and oxidation, even at higher temperature Acceptable substitute for PCBs in transformer despite their slightly inferior nonflammable properties. EET413 HIGH VOLTAGE ENGINEERING 9
Synthetic Hydrocarbons Polyolefins are the dielectrics of choice for application in power cables Over 55% of synthetics hydrocarbons produced worldwide today are Polyolefins. The most commonly use olefins are polybutylene and alkylaromatic hydrocarbons. The general characteristics are very similar to mineral oil. EET413 HIGH VOLTAGE ENGINEERING 10
Chlorinated Hydrocarbons Askarels and Polychlorinated biphenyl (PCB) Posses high fire point and excellent electrical properties but in recent years their has been banned throughout the world, because they pose serious health hazards EET413 HIGH VOLTAGE ENGINEERING 11
Esters Natural ester (castor oil) has been used as a capacitors impregnant for many years, but currently two types of synthetic esters are being used i.e Organic ester and phosphate ester. Organic ester have: high boiling point, high fire point, good viscosity-temperature relationship, used extensively in capacitors. Phosphate ester have: high boiling point, low flammability, used in transformers (in hazardous areas) EET413 HIGH VOLTAGE ENGINEERING 12
Latest Developments High Temperature Hydrocarbon (HTH) have: Good electrical insulating Adequate heat transfer properties Chemically similar to transformer oil, but posses higher boiling point higher fire points Higher viscosity reduces heat transfer capability. Tetra chloroethylene (Cl2Cl4) Nonflammable insulating fluid Very low viscocity, gives excellent heat transfer EET413 HIGH VOLTAGE ENGINEERING 13
6.3 CHARACTERISTICS OF LIQUID DIELECTRICS 1. Electrical Properties 2. Heat Transfer Characteristic 3. Chemical Stability The electrical properties are essential in determining the dielectric performance of liquid dielectric are: Capacitance per unit volume or relative permittivity Resistivity Loss tangent ( tan d) or power factor Ability to withstand high dielectric stresses EET413 HIGH VOLTAGE ENGINEERING 14
Electrical Properties Permitivities of most the petroleum oils vary from 2.0 to 2.6 and Silicone oils from 2.0 t0 73. Resistivities used high voltage application more than 10 16 ohm-meter Power factor, determine power loss and is an important parameter in cable and capacitor. In case transformer, the dielectric loss in the oil is negligible when compared to copper and iron losses. Pure and transformer oil, power factor varying 10 -4 at 20°C and 10 -3 at 90°C . EET413 HIGH VOLTAGE ENGINEERING 15
Electrical Properties Dielectric Strength is the most parameter. Its depends on the atomic and molecular properties of the liquid itself. In practical the dielectric strength depends on • The material of the electrodes • Temperature • Type of applied voltage • Gas content in the liquid. EET413 HIGH VOLTAGE ENGINEERING 16
Heat Transfer Characteristics The main factors that control the heat transfer are thermal conductivity ( K ) and Viscosity ( v) Heat transferred mainly by convection. Under natural atm cooling condition convection (N) is given by 3 n N f [ K AC / v ] K = thermal conductivity ; A = Coefficient of expansion C = specific heat per unit volume, v = kinematics viscosity ; n = 0.25 -0.33 EET413 HIGH VOLTAGE ENGINEERING 17
Chemical Stability Insulating liquid are subjected to thermal and electrical stresses in the presence of materials like oxygen, water, fibers and etc. These will cause degradation of the liquid which can result in corrosion, impairment of heat transfer, deterioration of electrical properties, increased dielectric losses, discharge and arcing. EET413 HIGH VOLTAGE ENGINEERING 18
6.4 CONDUCTION AND BREAKDOWN IN PURE LIQUIDS Fig 3.3 shows the characteristic of conduction current-electric field in a hydrocarbon liquid. The curve has three distinct regions. EET413 HIGH VOLTAGE ENGINEERING 19
Cont…. At very low fields, the current is due to the dissociation of ions. With intermediate fields, the current reaches a saturation value, and at high fields the current generated because of the field-aided electron emission from the cathode gets multiplied in the liquid medium by a Townsend mechanisms. The current multiplication also occurs from the electrons generated at the interfaces of liquid and impurities. The breakdown voltage depends on the field, gap separation, cathode work-function and the temperature of the cathode. In addition, the liquid viscosity, liquid temperature, the density and the molecular structure of liquid also influence the breakdown strength of liquid. EET413 HIGH VOLTAGE ENGINEERING 20
Cont…. Typical maximum breakdown strength of some highly purified liquids and liquefied gases are given in Table 6.1. The breakdown strength is more if the dissolved gases are electronegative in character (like oxygen). Similarly the increase in the partial pressure of oxygen evolved and the liquid hydrostatic pressure will increase the breakdown strength in n-hexane as shown in Fig 3.4 and 3.5 EET413 HIGH VOLTAGE ENGINEERING 21
Table 6.1 Liquid Maximum BD MV/cm 1.1 – 1.3 Hexane Benzene 1.1 Transformer oil 1.0 1.0 – 1.2 Silicone Liquid oxygen 2.4 1.6 – 1.9 Liquid nitrogen Liquid Hydrogen 1.0 Liquid Helium 0.7 Liquid Argon 1.1 -1.42 EET413 HIGH VOLTAGE ENGINEERING 22
CONDUCTION AND BREAKDOWN IN PURE LIQUIDS EET413 HIGH VOLTAGE ENGINEERING 23
CONDUCTION AND BREAKDOWN IN COMMERCIAL LIQUIDS Commercial insulating liquids are not chemically pure and have impurities like gas bubbles, suspended particles etc. These impurities reduce the breakdown strength. When breakdown occurs in these liquids, additional gases and gas bubbles are evolved and solid decomposition products are formed. EET413 HIGH VOLTAGE ENGINEERING 24
6.5 CONDUCTION AND BREAKDOWN IN COMMERCIAL LIQUIDS The breakdown mechanism depends on the nature and condition of the electrodes, the physical properties of the liquid and the impurities and gases present in the liquid. In general the breakdown mechanisms are classified as follows a ) Suspended Particle Mechanism b) Cavitation and Bubble Mechanism c) Thermal Mechanism EET413 HIGH VOLTAGE ENGINEERING 25
a) Suspended Particle Mechanism The impurities will be present as fibres or as dispersed solid particles. The permittivity of this particle (ε 2 ) will be different from the permittivity of the liquid ( ε 1 ). If we considered the impurities to be spherical particles of radius r, the particles experience a force F 1 2 2 1 F grad E 3 2 r 2 1 2 EET413 HIGH VOLTAGE ENGINEERING 26
Recommend
More recommend
