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I
2
26 High-Voltage Engineering
N
T
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O Conduction and
D Breakdown in Gases
U
C 2.1 GASES AS INSULATING MEDIA
T The simplest and the most commonly found dielectrics are gases. Most of the
electrical apparatus use air as the insulating medium, and in a few cases other
I gases such as nitrogen (N2), carbon dioxide (CO2), freon (CCl2F2) and sulphur
6) are also used.
O Various phenomena occur in gaseous dielectrics when a voltage is applied.
N and the insulation retains its electrical properties. On the other hand, if the ap-
sharply, and an electrical breakdown occurs. A strongly conducting spark formed
during breakdown practically produces a short-circuit between the electrodes.
Chapter Introduction provides a quick look
into the concepts that will be discussed in
the chapter.
W
WORKED EXAMPLES O
Example 4.1 A solid specimen of dielectric has a dielectric constant of 4.2,
R
and tan = 0.001 at a frequency of 50 Hz. If it is subjected to an alternating K
dielectric loss.
E
Solution Dielectric heat loss at any electric stress E [Eq. (4.5)]
D
E 2 f r tan
= W/cm3
1.8 1012 E
For the specimen under study, the heat loss will be X
50 50 10 6 50 4.2 .001
=
1.8 1012
A
= 0.291 mW/cm3 M
Example 4.2 A solid dielectric specimen of dielectric constant of 4.0
P
L
E
the voltage at which an internal discharge can occur. S
Every chapter contains several worked
out Examples which guide the student in
understanding the concepts and working
out the exercise problems.
xvi Contents
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T
Table 3.1 Dielectric properties of some liquid dielectrics
A Property Transformer Cable Capacitor PETEP Silicone
B Breakdown strength
oil
15
oil
30
oil
20
oil
> 15
oils
30–40
L at 20°C on 2.5 mm
standard sphere gap
kV/mm kV/mm kV/mm kV/mm kV/mm
E Relative 2.2–2.3 2.3–2.6 2.1 2.7 2–73
10–3 0.1 10–3 10–3
S Tan
10–3 0.5 10–3 10–4
Resistivity (ohm-cm) 1012–1013 1012–1013 1013–1014 > 1014 3 1014
at 20°C
20°C (CS)
Acid value (mg/gm Nil Nil Nil < 0.03 Nil
1.6000
Thermal expansion 7 10–4/°C 7 10–4/°C 7 10–4/°C 0.00075 5
(20–100°C) 10–4/°C
Max. permissible 50 50 50 200 < 30
water content (in ppm) (negligible)
Wherever necessary, Tables provide
accurate and extensive information on the
topic discussed.
F
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G
U
R
E
S
Fig. 4.2 Breakdown channels in perspex between point-plane electrodes.
Radius of point 0.01 in, thickness 0.19 in. Total number of impulses
190. Number of channels produced 16; (n) point indicates end of nth
channel. Radii of circles increases in units of 10–2 in.
Source: R. Cooper, International Journal of Elec. Engg. Education,
vol. 1, 241 (1963)
Well-labelled illustrations give a clear
understanding of the concepts.
Contents xvii
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M
C MULTIPLE CHOICE QUESTIONS
1. A small high-voltage laboratory usually will have
Q (a) ac, dc test sources with ratings less than 100 kV, 10 kVA./kW and
impulse of voltage 400 kV, 5 kJ
s (b) ac, dc test sources of 500 kV, 100 kVA/kW, and impulse of 1 MV,
10 kJ
(c) ac voltage sources of 300 kV, 10 kVA, and impulse voltage of
W 1 MV, 15 kJ
(d) ac, dc sources only.
I 2. Test sources required for testing power apparatus of 220 kV, 3-phase ac
system are
T (a) 500 kV ac, 1 MV impulse
(c) 300 kV ac, 500 kV impulse
(b) 800 kV impulse
(d) 250 kVA, 500 kV impulse.
H 3. The kVA rating of a testing transformer unit intended for test voltage and
test object capacitance ‘C’ (pF)
(a) C V2 (b) C V2 10–9
(c) C2 V2 109 (d) C V2 10–6.
A 4. The rating of an impulse voltage generator with generator capacitance
Cg and voltage rating V with n stages is (kJ)
N (a) 0.5 Cg V2 (b) (n/2) (Cg V2)
Cg V 2
S. (c) (d)
(Cg V 2 )
2n 2n 2
Multiple choice questions help the student
gain a quick overview of the important
topics in the chapter. Answers are also
provided at the end.
Q
REVIEW QUESTIONS U
6.1
high dc voltages.
E
6.2
proper voltage division between the valves ensured, if a number of tubes
S
T
6.3 Why is a Cockcroft–Walton circuit preferred for voltage multiplier
agram. I
6.4 Give the expression for ripple and regulation in voltage multiplier circuits.
O
N
PROBLEMS
S
6.21 An impulse generator has 12 capacitors of 0.12 F, and 200 kV
rating. The wave front and wave tail resistances are 1.25 k and 4 k
respectively. If the load capacitance including that of the test object is
impulse wave produced.
6.22 An 8-stage impulse generator has 1.2 F capacitors rated for 167 kV. What
is its maximum discharge energy? If it has to produce a 1/50 s waveform
wave tail resistances.
Review questions and problems help
students hone their problem-solving skills.
xviii Contents
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R
E REFERENCES
F 1. Alston, L.L., High Voltage Technology
E (1967).
2. Electromagnetic Fields, McGraw-Hill, New York (1960).
R 3. Kuffel E., Zaengl, W.S. and Kuffel, J., High Voltage Engineering
Fundamentals
E 4. Popovic, B.D., Introductory Engineering Electromagnetics, Addison-
Wesley (1971).
N 5. Finite Elements in Electrical and
C Magnetic Field Problems, Wiley-Interscience Publication, John Wiley and
Sons, New York (1980).
E 6. Silvester, P.P. and Ferrari, R.L., Finite Elements for Electrical Engineers,
S 7. Binns, K.J. and Lawrenson, P.J., Analysis and Computation of Electric
and Magnetic Field Problems
8. The Finite Element Method in Engineering Science,
McGraw-Hill, London (1977).
IEEE Transaction on power apparatus and systems, 97, 1156–
1166 (1977).
Extensive list of References helps in
selecting books for further study.
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Appendix Appendix 463
P
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Important Formulae D
I
X
Field Enhancement Factor
Emax
f=
Eavg.
Townsend current growth equation
I = I0 exp ( d)
Current growth in presence of Secondary Processes
I 0 exp ( d )
I=
1 exp ( d ) 1
Appendix at the end lists important
Formulae and Symbols.
