Chapter 3
HV Insulating Materials: Gases
• Air is the most commonly used insulating material.
• Gases (incl. air) are normally good as electrical
insulating material.
• Under high E-field conditions, gases become ionized,
leading to: corona, sparks and flashover.
• Why?
Discharges on an insulator
• Why?
• How are these
discharges formed?
Ionisation processes
Photo-ionization
• Bohr model of an atom: electrons in fixed orbits.
• Photo-ionisation: Planck: W = hf (Joules).
Ionisation processes
Photo-ionization (cont.)
• Energy gained from light raises electrons to higher
energy level (orbit) or quantum band.
•
Energy is absorbed when moving to higher orbit.
•
Energy is emitted when falling back.
•
If energy gained exceeds the ionisation energy of the gas
the electron leaves the atom.
Ionisation processes
Orbits and Energy Levels
Ionisation processes
By collision
• Free initiating electrons always present (cosmic rays)
•
•
•
•
Initiating electrons accelerated by Lorentz force due to
the E-field.
Electron gains kinetic energy.
Collide against gas atoms - kinetic energy converted to
potential energy.
Ionisation occurs if this energy
E
+exceeds the ionisation energy of the atom, sets free more electrons and
leaves positive charge behind.
e
+
+
e
+
e
e
e
+
+
+
e
e
ini ti al e le ctron
+
Ionisation processes
By collision (cont.)
• Townsend’s primary ionisation coefficient: 
•
 : No. of ionising collisions for 1 mm length movement
•
•
by one electron.
Exponential growth: avalanche formation
n = n0exp( x) – number of electrons liberated at point x
d
+ ++ + + ---- ++++++++ + ++ ++ + ----------------+ ++
+ + + + + + + + ++++ --------- ++
x
Ionisation processes
By collision (cont.)
• Electrons are more mobile than (relatively heavy)
positive ions.
• Not a self-sustaining process (depends on initiating
electron)
• Typical application - Geiger counter
Avalanches really do exist
Wilson’s cloud chamber
Ionisation processes
By collision (cont.)
• Townsend’s secondary process
• An avalanche is not self-sustaining: process stops if
initiating electrons not available. Positive feedback thus
required.
• Pos. ions move back to cathode (-) and collide against
cathode, releasing more initiating electrons.
•  : new electrons gained at
cathode by (+) ion impact
• New avalanches form, plasma
column formed - higher current
leads to breakdown
• Thus a self-sustaining process.
C a th o d e (-)
A n o d e (+ )
O n e e le c tro n
a t c a th o d e
A va la n ch e
e  d -1 n e w e le c tro n s
a t a n o d e a n d e  d -1
Io n s le ft b e h in d
e  d -1 p o s itive io n s m o ve
b a ck to c a to d e a n d co llid e a g a in s t it
Im p a c t a t ca th o d e
( e  d -1 ) n e w e le c tro n s
Electronegative gases
• Some gases are electronegative: have electron
affinity.
•
•
•
Electrons attach to the molecules.
Thus lower mobility and collision ionization probability.
This raises the flashover voltage.
• Attachment process represented by the attachment
coefficient .
•
•
•
n  n0 e
(   ) x
Townsend’s first ionization coefficient () is effectively
lowered to (-).
If >, then ionization stops.
Electronegative gases
SF6
• Sulphurhexafluoride (SF6) is an electronegative gas
•
Flashover voltage roughly 4 times
higher than air.
• The following attachment
processes occur in SF6:
•
•
•
SF6+e  SF5+F+2e
SF6+e  SF6 –
SF6+e  SF5 – + F
Electronegative gases
SF6 Substations (GIS)
• Colourless, odourless, non-toxic, chemically inactive.
• 5 times heavier than air.
• Also arc quenching medium in circuit breakers.
F ig ur e 3 .13 : E s k o m ’s 8 00 k V Al ph a
s ub s ta ti on
Streamer discharges
• A self-sustaining discharge can develop from a single
avalanche.
• Space charge (ions) distort and enhance field.
• Photons cause further avalanches in high field
regions.
• Streamer discharges occur if
n  5 .108.
• Occurs for non-uniform long
gaps and at high pressures.
Flashovers
Avalanche with x = 20
Photons
E - Field
Dr WL Vosloo
Anode (+)
Cathode (-)
Streamer mechanism – Medium gaps (> 5 Bar.mm)
Paschen’s Law
• Sustained Townsend discharge leads to spark then arc
(flashover).
•
Formulated mathematically by Paschen (see p 52).
• The flashover voltage is a function of the product of
the gas pressure and the gap length for a uniform
field.
• Implications in practice:
•
•
•
Altitude effect
Compressed gases
Vacuum contactors
Paschen’s Law
• Approximation for curve:
V c  6 . 72
pd  24 . 36 pd
10000
Breakdown Voltage (kV)
1000
100
10
1
0.001
0.01
0.1
1
10
0.1
pressure x gap length (cm bar)
Empirical Formula (Eq. 3.11)
Paschen Equation ( Eq. 3.10)
100
Paschen’s Law
+
a) Low press ure
(f ew collisions:
low ionization)
-
+
b) H igh pr essur e
(low kinetic e nergy:
low ionization)
-
+
-
c) M edium pre ssure
( optim al: high
ionization)
Low gas density - more kinetic energy gained but less collisions
High gas density – more collisions but less energy gained
Paschen’s Law
3 1 .1
2 5 .5
U d  6 .72
pd  24 . 36  pd 
Townsend
Streamer
Asymmetrical, non uniform gaps
The polarity Effect
• Region of high field strength near the sharp point, in
both cases.
+
_
• Avalanches are formed in
these regions, leaving a
positive space charge in this _
+
region.
•
•
In the case of the positive tip the space charge has the
same polarity as the electrode and assists in increasing the
field.
In the case of the negative tip the space charge opposes
the polarity of the tip.
Asymmetrical, non uniform gaps:
The polarity Effect
• A lower flashover voltage is thus obtained for the
positive tip, compared to the negative one.
Long gaps
Leader mechanism
• For gaps > 1 m a different
flashover mechanism exists.
•
•
•
C a th o d e (-)
Corona at tip merges into
thermal leader channel, similar
to lightning.
Long gaps are vulnerable for
switching surges as leader
mechanism occurs.
Note minimum at pulse front time
of 100 s – typical for switching
surges
G o ro n a
s tre am e rs
A n o d e (+ )
L e a de r (p la sm a )
L e a de r (p la sm a )
G o ro n a
s tre am e rs
L e a de r (p la sm a )
Flashover in gases
Townsend vs. Streamer mechanism
Dr WL Vosloo
Flashover
When do the different mechanisms apply?
Townsend mechanism
Streamer mechanism
Leader mechanism
 Small uniform gaps at
 Medium sized uniform
 Large gaps on power
atmospheric pressure.
 Larger gaps at low
pressure (discharge
tubes).
 < 5 bar mm
 AC and DC
gaps.
 Medium sized nonuniform gaps.
 > 5 bar mm
 AC, DC and lightning
impulses
systems.
 Gaps > 1 m,
 Switching impulses and
AC.
Flashover
Non-Uniform Gap – Effect of voltage type
Corona, sparks and arcs
A bnorm al glow
E - IR
V
Vc
F las hover
E
G low
A rc
critica l a rc le n g th
A rc ch ar a cte ristics
V
In cre asin g le n gt h
T own send dis charges
I
I
Corona, sparks and arcs
Corona Discharges
• Non-uniform gaps
•
High E-field near electrode with smallest radius:
Er=V/(r ln(b/a))
• Ionisation threshold ( 30 kV/cm)
exceeded in purple region
• Partial discharge in this region:
no flashover
• Peek’s formula defines inception
surface gradient, E > 30 kV/cm:
O utside con duc tor
45
40
35
30
25
20
15
10
5
2
4
6
8
10
12
Distan ce fro m ce ntre lin e (c m )
E Peek  30 m  (1 
0 .3
r
)  
p ( 273  t 0 )
Co ron a
In side co ndu ctor
p 0 ( 273  t )
m < 1.0: surface roughness factor
Corona Discharges
• Corona inception if Emax > Epeek
• Critical disruptive voltage: Yield Emax > 30 kV/cm
• Visual critical corona voltage: Yield Emax > Epeek
k V /c m
Co-axial cylinders: E-max = f (a)
Outside radius = 10 cm
120
Corona
E - fie ld
in
100
E-max
80
Corona
inception
60
No
Corona
40
20
0
0.1
0.3
0.5
0.7
0.9
1.1
1.3
Conductor radius in cm
1.5
1.7
1.9
Corona Discharges
DC +
LAB DEMO
Dr WL Vosloo
LAB DEMO
Corona Discharge
DC -
LAB DEMO
Dr WL Vosloo
LAB DEMO
Corona Discharges
AC
•
•
•
•
•
Bluish luminous discharge, ozone formed
Causes Interference : 0,2 to 10 MHz (pulse corona)
Power losses (tens of MW on 500 kV line)
Corona increases during rain (water drops)
Use bundled conductors
AC Corona
(twins and quads) and corona
rings to curb corona
V olt ag e
C a p ac it ive cu r r en t
P os itive st r ea m e r s
C o n tin uo u s c or o n a
cu r r en t
T r ich e l p uls es
Corona Discharges
AC
LAB DEMO
Dr WL Vosloo
LAB DEMO
GASES – NON-UNIFORM GAPS – PARTIAL AIR BREAKDOWN – CORONA LOSSES
Corona Discharges
Corona Losses
Dr WL Vosloo
GASES – NON-UNIFORM GAPS – PARTIAL AIR BREAKDOWN – CORONA
Dr WL Vosloo
Corona Discharges
Effect of corona rings
A
Corona
Discharges
30 kV/cm
B
A
Corona
Discharges
30 kV/cm
No Corona
Discharges
B
B
Dr WL Vosloo
A