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Power system (11) - NOTES10
Power Electronics (203122357)
Parul University
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of go value of go is directly proportional to air density. Thus the breakdown strength of air at
a barometric pressure of b cm of mercury and temperature of tºC becomes δg 0 where
δ = air density factor = 273 t
3. 92
Under standard conditions, the value of δ = 1.
Critical disruptive voltage,
r
d Vc g 0 rln
Correction must also be made for the surface condition of the conductor. This is accounted for
by multiplying the above expression by irregularity factor mo.
Critical disruptive voltage,
r
d Vc g 0 m 0 rln kV/phase
where
mo = 1 for polished conductors
= 0·98 to 0·92 for dirty conductors
= 0·87 to 0·8 for stranded conductors
(ii) Visual critical voltage
It is the minimum phase-neutral voltage at which corona glow appears all along the line
conductors.
It has been seen that in case of parallel conductors, the corona glow does not begin at the
disruptive voltage Vc but at a higher voltage Vv, called visual critical voltage. The phase-
neutral effective value of visual critical voltage is given by the following empirical formula
r
d
r
V v mvg r )ln
0. 3
0 ( 1
kV/phase
where mv is another irregularity factor having a value of 1·0 for polished conductors and 0·72 to
0·82 for rough conductors.
(iii) Power loss due to corona Formation of corona is always accompanied by energy
loss which is dissipated in the form of light, heat, sound and chemical action. When
disruptive voltage is exceeded, the power loss due to corona is given by:
5 25 2
241 10 V Vc d
f r P
kw/km/phase
Advantages and Disadvantages of Corona
Corona has many advantages and disadvantages. In the correct design of a high voltage
overheadline, a balance should be struck between the advantages and disadvantages.
Below are the Advantages and disadvantages of Corona.
Advantages
Due to corona formation, the air surrounding the conductor becomes conducting and hence
virtual diameter of the conductor is increased. The increased diameter reduces the electro-
static stresses between the conductors.
Corona reduces the effects of transients produced by surges.
Disadvantages
Corona is accompanied by a loss of energy. This affects the transmission efficiency of the
line.
Ozone is produced by corona and may cause corrosion of the conductor due to chemical
action.
The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal
Voltage drop occurs in the line. This may cause inductive interference with neighboring
Communication lines.
Methods to reduce Corona Discharge Effect
Corona can be avoided
- By minimizing the voltage stress and electric field gradient.: This is accomplished by
using utilizing good high voltage design practices, i., maximizing the distance between
conductors that have large voltage differentials, using conductors with large radii, and
avoiding parts that have sharp points or sharp edges.
- Surface Treatments: Corona inception voltage can sometimes be increased by using a
surface treatment, such as a semiconductor layer, high voltage putty or corona dope.
- Homogenous Insulators: Use a good, homogeneous insulator. Void free solids, such as
properly prepared silicone and epoxy potting materials work well.
Power system (11) - NOTES10
Course: Power Electronics (203122357)
University: Parul University
