conduction and breakdown of pure liquids
TRANSCRIPT
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.