Conduction and Breakdown of Pure Liquids

Pure Liquids

A chemically pure liquid is one

• which does not contain any other compound

• other impurities are less than 1 in 109

• is very simple in chemical structure

Examples: n-hexane [C6H14]

n-heptane [C7H16]

Purification Process

1. Removing all solid impurities suspended

or dissolved in liquid

2. Separating other liquids like water

(moisture) and ionic impurities

3. Removing the dissolved gases

Methods employed in Purication process

• Filtration using mechanical filters, spray filters, centrifuging and electrostatic filtering.

• Chemical treatment: may consist of acid washing, treating with alkalies to remove excess acid, ion exchange process.

• Drying and degaussing: the processed oil is degaussed using vacuum pump and is dried so that all dissolved gases and moisture are completely removed.

Typical Liquid Purification System

Conduction in Liquids

1. Low Field Region ( < 100 KV/m)

2. Intermediate Fields (100 KV/m to 2 MV/m)

3. High Field Region (1 MV/m to 100 MV/m)

Conduction in liquids can be divided into three main regions

Conduction Current as a function of Pressure & Temperature

Conduction at High Fields

Factors affecting the Breakdown

1. test procedure

2. electrodes, their field configuration and surface condition

3. physical and chemical purity

4. temperature and pressure

5. nature of applied voltage DC, AC or pulse and its duration.

Breakdown Strength of Pure Liquids

Liquid Breakdown Strength (MV/cm)

n-Hexane 1.1 to 1.3

Benzene 1.1

Hydrocarbon Oils ~ 1.0

Silicone oils 1.0 to 1.2

Liquid Oxygen 2.4

Liquid Nitrogen 1.6 to 1.8

Liquid Hydrogen 1.0

Liquid Argon 1.1 to 1.4

At Cryogenic temperatures

Static Breakdown Voltage-Gap Characteristic for n-Hexane

Influence of Hydrostatic Pressure on Breakdown Strength of n-Hexane

Breakdown Strength of n-Hexane subjected to Pulse Voltage

Breakdown Processes

Electron emission from cathode with electron multiplication

via collision ionization leading to breakdown similar to the

“Townsend process” in gases

Excitation of molecular bond vibration due to electrons

emitted which leads to charge multiplication

Thermal processes – energy released from cathode or local

asperities, vapouring the liquid forming a bubble, which grows

to critical size causing breakdown

Suspended particles in a liquid either bridging the

electrodes or leading to bubble formation or cavities in the

liquid

Breakdown Strength of n-Hexane as a function of Electrode Separation

Breakdown Strength of n-Hexane as a function of Electrode Radius

Breakdown Volt-Time Characteristic of n-Hexane

Breakdown Criteria for Bubble or Cavity Instability that initiate Breakdown

Pc + Pvp = Pes + Pst + Ph where

Pc = coulomb pressure or electric stress developed

Pvp = vapour pressure in the bubble or cavity

Pes = electrostatic pressure

Pst = pressure due to surface or interfacial tension

Ph = hydrostatic pressure

No simple theory is available which fully explains the process of Breakdown in Liquids.