Download - Breakdown in Commercial Liquids
Breakdown in Commercial Liquids
Factors affecting Breakdown
a. Nature and condition of Electrodes
b. Physical properties of liquid
c. Impurities present
d. Dissolved Gases
e. Geometry of Electrodes
f. Volume of Liquid in High Stressed Region
Theories Proposed for Breakdown
a. Suspended particle mechanism
b. Cavitation and bubble mechanism
c. Statistical or stressed oil volume theory
Suspended particle theory• Impurities or particles floating in the liquid align
• The force that drags them into the Electric Field is given by
• If r is large,
• The particles form conducting chain which leads to breakdown
23
22
1ErF
ls
ls
23
2
1ErF
Cavity and bubble breakdown theory
Liquids usually contain bubbles or cavities located
a. on electrode surfacesb. as dissolved gases in liquid volumec. dissociation of products by electron collision
giving rise to gaseous productsd. liquid vaporization by internal discharges
from points and irregularities at the electrode surfaces.
When a free gas bubble or a cavity exists and is immersed in a liquid subjected to uniform electric field E0, the field inside the
bubble is given by
If Eb is equal to the field at which the gas
inside the bubble ionizes, a discharge takes place within the bubble
2
3 0
lb
EE
Formation and Collapse of a Bubble
Instability of Bubble leading to Breakdown
Stressed oil volume theory
• The weakest link or particle in a large volume determines the breakdown strength ;of the liquid
• More the volume, larger will be the number of weak links or particles
• The breakdown strength is inversely proportional to the “stressed oil volume”
• The breakdown strength is highly influenced by gas content in oil, solid impurities present
Breakdown Stress as a function of Stress Oil Volume
Power Law for Breakdown Voltage
The breakdown of a spark gap under uniform
field conditions follows the power law
Vb = V0dn
where V0 = breakdown Voltage of 1 unit length
gap
n = a constant usually between 0.6 to 0.9 that
depends on properties of the liquid
Breakdown Strength of typical commerical oils
Oil Working Stress (kV/cm)
Breakdown Strength (kV/cm)
Transformer Oil 20 to 50 150 to 200
n-hexane (cable oil) 100 to 150 200 to 250
Polybutane (capacitor Oil)
100 to 200 ~1000