chapter 3 hv insulating materials: gases air is the most commonly used insulating material. gases...
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Chapter 3HV 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 processesOrbits and Energy Levels
Ionisation processesBy 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
exceeds the ionisation energy of theatom, sets free more electrons andleaves positive charge behind.
E -
e ee
ee
ee
+
++
+
+
++
initial electron
+
+-
Ionisation processesBy 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 processesBy 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 processesBy 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 atcathode by (+) ion impact
• New avalanches form, plasmacolumn formed - higher currentleads to breakdown
• Thus a self-sustaining process.
Cathode (-) Anode (+)
One electronat cathode
ed -1 new electrons at anode and ed -1Ions left behind
Avalanche
ed -1 positive ions move back to catode and collide against it
( ed -1) new electrons
Impact at cathode
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 .• • Townsend’s first ionization coefficient () is effectively
lowered to (-).• If >, then ionization stops.
xenn )(
0
Electronegative gasesSF6
• 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 gasesSF6 Substations (GIS)
• Colourless, odourless, non-toxic, chemically inactive.• 5 times heavier than air.• Also arc quenching medium in circuit breakers.
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 longgaps and at high pressures.
------
++ +++ +++
++++ +
+
++++
+++
+++
+++ ++
++++
+++ - -
-
-
-
-
--
----- --
--- -- ---- --
- ------
-
_ +E (applied)
photon
new avalanche
Dr WL Vosloo
Cat
hode
(-)
Ano
de (
+)
E - Field
Photons
Avalanche with x = 20
FlashoversStreamer 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
pdpdVc 36.2472.6 • Approximation for curve:
0.1
1
10
100
1000
10000
0.001 0.01 0.1 1 10 100
pressure x gap length (cm bar)
Bre
akd
ow
n V
olt
age
(kV
)
Empirical Formula (Eq. 3.11)
Paschen Equation ( Eq. 3.10)
Paschen’s Law
a) Low pressure(few collisions:low ionization)
b) High pressure(low kinetic energy:low ionization)
c) Medium pressure( optimal: highionization)
+ - + - + -
Low gas density - more kinetic energy gained but less collisions
High gas density – more collisions but less energy gained
Paschen’s Law
31.1 25.5
Townsend Streamer
pdpdUd 36.2472.6
Asymmetrical, non uniform gapsThe polarity Effect
• Region of high field strength near the sharp point, in both cases.
• Avalanches are formed inthese regions, leaving apositive space charge in thisregion.• 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 gapsLeader mechanism
• For gaps > 1 m a different flashover mechanism exists.• Corona at tip merges into
thermal leader channel, similarto 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
Cathode (-) Anode (+)
Goronastreamers
Goronastreamers
Leader (plasma)
Leader (plasma)
Leader (plasma)
Flashover in gasesTownsend vs. Streamer mechanism
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FlashoverWhen do the different mechanisms apply?
FlashoverNon-Uniform Gap – Effect of voltage type
Corona, sparks and arcs
Vc
Townsend discharges
GlowArc
V
I
Flashover
Abnormal glow
V
I
E - I R
E
Arc characteristics
Increasing length
critical arc length
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 inceptionsurface gradient, E > 30 kV/cm:
)3.0
1(30
r
mEPeek
p t
p t
( )
( )
2 7 3
2 7 30
0
m < 1.0: surface roughness factor
0
20
40
60
80
100
120 E
-fie
ld in k
V/c
m
0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9Conductor radius in cm
Co-axial cylinders: E-max = f (a)Outside radius = 10 cm
E-max
CoronainceptionCorona No
Corona
Corona Discharges
• Corona inception if Emax > Epeek
• Critical disruptive voltage: Yield Emax > 30 kV/cm
• Visual critical corona voltage: Yield Emax > Epeek
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LAB DEMO LAB DEMO
Corona DischargesDC +
Corona DischargeDC -
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LAB DEMO LAB DEMO
Corona DischargesAC
• 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
(twins and quads) and coronarings to curb corona
Voltage
Capac it ive current
Continuous corona current
Trichel pulses
Pos itive streamers
AC Corona
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LAB DEMO LAB DEMO
Corona DischargesAC
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GASES – NON-UNIFORM GAPS – PARTIAL AIR BREAKDOWN – CORONA LOSSES
Corona DischargesCorona Losses
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GASES – NON-UNIFORM GAPS – PARTIAL AIR BREAKDOWN – CORONA
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B
A
AB
AB
30 kV/cm
30 kV/cm
Corona Discharges
Corona Discharges
No Corona Discharges
Corona DischargesEffect of corona rings