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