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