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Employment in Nigerian is pervasive and is further compound by the present economic recession in which many companies have downsized and increasing the number of unemployed. r millions of jobless and unskilled Nigerians. Driven by the hash economic c
commercial law cases
Breakdown in Commercial Liquids
Factors affecting Breakdown a. Natu Naturre and and condi conditi tion on of of Elec Electr trod odes es b. Physi Physica call prop proper erti ties es of of liqu liquid id c. Impu Impurritie itiess prese esent d. Diss Disso olved lved Gase Gasess e. Geome eometr try y of Ele Elect ctrrodes odes f. Volum olumee of Liq Liqui uid d in High High Str Stres esse sed d Reg Regio ion n
Theories Proposed for Breakdown a. Suspended particle mechanism b. Cavi Cavita tati tion on and bu bub bble ble mec ech hani anism c.
Stat tatisti istica call or or st stre ress ssed ed oil oil vol volu ume the theo ory
Suspended particle theory •
•
Impurities or particles floating in the liquid align The force that drags them into the Electric Field is given by F
1 2
•
•
If
3
r
e s e s
r is large,
e l
2
E
2e l
F
1
3
2
r E
2
The particles form conducting chain which leads to breakdown
Cavity and bubble breakdown theory
a. b. c. d.
Liquids usually contain bubbles or cavities located on el electrode surfaces as di dissol solved gases in in li liquid vo volume disso issoci ciat atio ion n of of pro produ duct ctss by by ele elecctr tro on col colli lisi sion on giving rise to gaseous products liqu liquid id vapo vapori rizzatio ation n by inte intern rnal al disc discha harg rges es 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 E b
3 E 0 e
l
2
If Eb is equal to the field at which the gas inside the bubble ionizes, a discharge takes place within the bubble
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 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)
