EET 413 EET413 HIGH VOLTAGE ENGINEERING 1 CHAPTER 3 CONDUCTION - - PowerPoint PPT Presentation

EET 413

HIGH VOLTAGE ENGINEERING

1 EET413 HIGH VOLTAGE ENGINEERING

CHAPTER 3

EET413 HIGH VOLTAGE ENGINEERING 2

CONDUCTION & BREAKDOWN IN LIQUID DIELECTRIC

On completion of this lesson, a student should be able to:

EET413 HIGH VOLTAGE ENGINEERING 3

Ability to analyze the various breakdown mechanism and applications

• f vacuum, liquid, solid and composite

dielectrics

TOPIC OUTLINE

EET413 HIGH VOLTAGE ENGINEERING 4

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

6.1 INTRODUCTION

EET413 HIGH VOLTAGE ENGINEERING 5

 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.

Electrical Equipment Using Liquid Dielectric

EET413 HIGH VOLTAGE ENGINEERING 6

Power transformer Power capacitor Switchgear

6.2 CLASSIFICATION OF LIQUID DIELECTRIC

EET413 HIGH VOLTAGE ENGINEERING 7

 Transformer Oil (mineral oil)  Silicone Oil  Synthetic hydrocarbons  Chlorinated Hydrocarbon  Ester  Latest developments

Transformer Oil

EET413 HIGH VOLTAGE ENGINEERING 8

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.

Silicone Oils

EET413 HIGH VOLTAGE ENGINEERING 9

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.

Synthetic Hydrocarbons

EET413 HIGH VOLTAGE ENGINEERING 10

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.

Chlorinated Hydrocarbons

EET413 HIGH VOLTAGE ENGINEERING 11

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

Esters

EET413 HIGH VOLTAGE ENGINEERING 12

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)

Latest Developments

EET413 HIGH VOLTAGE ENGINEERING 13

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

6.3 CHARACTERISTICS OF LIQUID DIELECTRICS

EET413 HIGH VOLTAGE ENGINEERING 14

• 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

Electrical Properties

EET413 HIGH VOLTAGE ENGINEERING 15

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

1016 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 .

Electrical Properties

EET413 HIGH VOLTAGE ENGINEERING 16

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.

Heat Transfer Characteristics

EET413 HIGH VOLTAGE ENGINEERING 17

 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 K = thermal conductivity ; A = Coefficient of expansion C = specific heat per unit volume, v = kinematics viscosity ; n = 0.25 -0.33

n

v AC K f N ] / [

3



Chemical Stability

EET413 HIGH VOLTAGE ENGINEERING 18

Insulating liquid are subjected to thermal and

electrical stresses in the presence of materials like

• xygen, 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.

6.4 CONDUCTION AND BREAKDOWN IN PURE LIQUIDS

EET413 HIGH VOLTAGE ENGINEERING 19

Fig 3.3 shows the characteristic of

conduction current-electric field in a hydrocarbon liquid. The curve has three distinct regions.

Cont….

EET413 HIGH VOLTAGE ENGINEERING 20

 At very low fields, the current is due to the dissociation

• f 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

• f the cathode. In addition, the liquid viscosity, liquid

temperature, the density and the molecular structure of liquid also influence the breakdown strength of liquid.

Cont….

EET413 HIGH VOLTAGE ENGINEERING 21

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

• xygen evolved and the liquid hydrostatic pressure

will increase the breakdown strength in n-hexane as shown in Fig 3.4 and 3.5

Table 6.1

EET413 HIGH VOLTAGE ENGINEERING 22

Liquid Maximum BD MV/cm Hexane 1.1 – 1.3 Benzene 1.1 Transformer oil 1.0 Silicone 1.0 – 1.2 Liquid oxygen 2.4 Liquid nitrogen 1.6 – 1.9 Liquid Hydrogen 1.0 Liquid Helium 0.7 Liquid Argon 1.1 -1.42

CONDUCTION AND BREAKDOWN IN PURE LIQUIDS

EET413 HIGH VOLTAGE ENGINEERING 23

CONDUCTION AND BREAKDOWN IN COMMERCIAL LIQUIDS

EET413 HIGH VOLTAGE ENGINEERING 24

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.

6.5 CONDUCTION AND BREAKDOWN IN COMMERCIAL LIQUIDS

EET413 HIGH VOLTAGE ENGINEERING 25

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

a) Suspended Particle Mechanism

EET413 HIGH VOLTAGE ENGINEERING 26

The impurities will be present as fibres or as

dispersed solid particles.

The permittivity of this particle (ε2) will be

different from the permittivity of the liquid (ε1).

If we considered the impurities to be

spherical particles of radius r, the particles experience a force F

 

2 2 1 1 2 3

E grad 2 2 1        r F

Cont….

EET413 HIGH VOLTAGE ENGINEERING 27

 If the voltage is continously applied (d.c.) or the duration

• f the voltage is long (a.c.), then this force drives the

particles towards the areas of maximum stress. If the number of particles present are large, they becomes aligned due to these forces, and thus form a stable chain bridging the electrode gap causing a breakdown between the electrodes.

 If there is only a single conducting particle between the

electrodes, it will give rise to local field enhancement depending on its shape. If this field exceeds the breakdown strength of the liquid, local breakdown will

• ccur near the particle, and this will result in the

formation of gas bubbles, which may lead to the breakdown.

 The larger the size of the particles, the lower were the

breakdown strengths

Cont….

EET413 HIGH VOLTAGE ENGINEERING 28

The following processes have been responsible for the formation of the vapour bubbles. i) gas pockets at the surface of the electrodes. ii) electrostatic repulsive forces between space charges which may be sufficient to overcome the surface tension. iii) gaseous products due to the dissociation of liquid molecules by electron collisions. iv) vapourization of the liquid by corona type discharge

b) Culvitation and Bubble Theory

EET413 HIGH VOLTAGE ENGINEERING 29

The bubble will elongate in the direction of the electric field under the influence of electrostatic forces. Breakdown

• ccurs when the voltage drop along the length of the

bubble becomes equal to the minimum value on the Paschen’s curve, and the breakdown field is given as σ : surface tension of the liquid ε1 : liquid permittivity ε2 : gas bubble permittivity r : initial radius of the bubble (sphere shape) Vb : voltage drop in the bubble

 

2 1 2 1 2 1

1 2 4 2 2 1                               rE V r E

b

     

c) Thermal Mechanism

EET413 HIGH VOLTAGE ENGINEERING 30

Based on the experimental observations of

extremely large currents just before breakdown. The high current pulses originate from the tips of the microscopic projections on the cathode surface with densities of the order of 1 A/cm3. This high density current pulses give rise to localised heating of the oil which may lead to the formation of vapour bubbles.

When a bubble is formed, breakdown follows, either

because of its elongation to a critical size or when it completely bridges the gap between the electrodes.

The breakdown strength depends on the pressure

and the molecular structure of the liquid.

Example

EET413 HIGH VOLTAGE ENGINEERING 31

In an experiment for determining the breakdown strength of transformer oil, the following observations were made. Determine the power law dependence between the gap spacing and the applied voltage of the oil.

Gap spacing (mm) 4 6 8 10 Voltage at breakdown (kV) 88 135 165 212

Solution

EET413 HIGH VOLTAGE ENGINEERING 32

n

Kd V 

The relationship between the voltage and gap is normally given as Our objective is to find out values of K and n. Substituting values of two observations , we have

Solution

EET413 HIGH VOLTAGE ENGINEERING 33