Introduction of High Voltage Engineering:
In general, gases are good insulating media for electric voltage. For example a 0.5 cm layer of air can withstand a voltage of the order of 17 kV. There are gases having more electric strength than air. Sulphur hexafluoride has 2.5 times the strength of air. The electric strength of a gas increases with rise in pressure. However beyond certain voltage gases loose insulating property and they become a conducting medium. Under this special condition called "plasma" the greater part of the gas molecules get ionized.
TYPES OF IONISATION:
(i) Ionization by collision:
If a particle with a mass m (electron, ion or neutral molecule), moving with a velocity v, collides with a neutral atom or molecule, kinetic energy of the moving particle can be expended in bringing about the act of ionization if the inequality
(mv2)/2 >= Wi
holds goods.
(ii) Photo-ionisation:
Ionization of gas under the action of short wave radiations. It occurs when
hf >= Wi or λ<= ch / Wi
where f is frequency, c is velocity of light, X. is wavelength.
(iii) Thermal ionization:
It occurs at high temperatures caused by thermal condition of a gas.
PASCHEN LAW:
Paschen's Law states that, "At temperature remaining constant breakdown voltage in a uniform field is a function of the product of gas pressure (p) and distance between the electrodes."
Mathematically,
V0=f (p,s).
BREAKDOWN VOLTAGE:
The electric strength of a gap in a uniform field is same for direct and power frequency voltages and can be determined from the formula
V0= 6.66 √ δ S + 24.55 δS
where
S = distance between the electrodes in cm
δ = relative air density.
High voltage Engineering > CORONA DISCHARGE:
In high voltage systems corona discharges have to be taken into account. For a smoothly polished conductor of radius r situated along the axis of a cylinder R >>r, the critical intensity can be determined from the relation
Ee= 31 δ (1+ 0.308 / √(rδ))
and the corresponding critical voltage from
Vc = Ec r loge R/r
POWER LOSS DUE TO CORONA:
Pc = 244 ( 25+ f ) √(r/D) (E - E0)2 x 10-5 kW/phase/km
where f = frequency in Hz ,
E = voltage in kV to neutral (rms).
DISRUPTIVE VOLTAGE:
E0= m δ r g ln (D/r) kV to neutral
g = 21.1 kV (rms)
E0= 21.1 m δ r g ln (D/r) kV to normal
where
D = distance between conductors in cm.
High voltage Engineering > ELECTRIC FIELDS:
Knowledge of electric fields is necessary in many application in the design and operation of electrical/electronic equipment e.g. design of insulation and for assessing electrical stresses in high-voltage sources, machine windings and cables etc.
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