Physics of Corona and Gap Physics of Corona and Gap DischargesDischarges

AC and DC Transmission Line AC and DC Transmission Line Corona EffectsCorona Effects

UV Inspection User’s Group MeetingUV Inspection User’s Group MeetingFebruary 11-13, 2004February 11-13, 2004

ORLANDO, Florida, USAORLANDO, Florida, USAByBy

Dr. P. Sarma MaruvadaDr. P. Sarma MaruvadaNotas en español agregadas por Ing. Ariel Lichtig Notas en español agregadas por Ing. Ariel Lichtig

exclusivamente para curso teoría de Campos-FIUBAexclusivamente para curso teoría de Campos-FIUBA

Introduction• Electrical Design, Operation & Maintenance of HV

Transmission Lines Requires Consideration of:

- Air Insulation

- Corona

- Insulators

• All Three Aspects Require Knowledge of Electrical Discharges in Air, Which May Comprise:

- Partial Breakdown (Corona)

- Complete Breakdown (Gap Discharges)

Corona & Gap DischargesCorona & Gap Discharges

• Corona is an electrical discharge (i.e. partial breakdown of air insulation) occurring in the high electric field region, generally in the vicinity of conducting surfaces, but sometimes also near insulating surfaces, due to ionization processes in air.

Resulta de procesos de avalanchas de electrones bajo condiciones de campo no uniforme que produce que la avalancha cese antes de llegar a tierra.

• Complete electrical breakdown of air insulation between two electrodes separated by a very small gap is known as a micro-gap discharge or simply as Gap Discharge.

Basic Ionization ProcessesBasic Ionization Processes• Ionization and excitation by electron impact

A: molécula neutra A*: excitada e inestableA + e A* + e (excitation)A* A + hfp (photo-emission)A + e A+ + e + e (ionization, si E es >, e no sólo sube de órbita sino que se separa)

• Photo excitation & ionizationA + hfp A* (photo-excitation)A + hfp A+ + e (photo-ionization,

con más energía)

Basic Ionization ProcessesBasic Ionization Processes• Electron attachment

A + e A-

• RecombinationA+ + B- A + B + hfp (radiative recombination, only with electrons)

The energy of photons, or the frequency of light, depends on the difference in the orbital energies of the electron

El nitrógeno no tiene afinidad con electrones

Discharge in Uniform FieldsDischarge in Uniform Fields• Field intensified ionization & electron avalanche+ Ө: iones, más lentos - : electrones libres

electric field

Discharge in Uniform FieldsDischarge in Uniform Fields• Discharge development & breakdown Los iones positivos golpean el cátodo y liberan más electrones. Si hay

suficiente campo, el proceso se autosostiene

H V

iA

Cathode

Anode

Breakdown and CoronaBreakdown and Corona• Excitation of molecules and photon emission

occur simultaneously with ionization.

• Secondary ionization processes, due to impact of ions or photons, play a crucial role in breakdown.

• In non-uniform fields, such as in a conductor-plane gap, only partial breakdown or corona occurs (al disminuir el campo).

Modes of Corona in AirModes of Corona in Air• Negative DC Corona

Modes: Se crea una carga espacial que

cambia la distribución de campo.

- Trichel Pulse- Negative Glow- Negative streamer

Depende se los constituyentes (N2, O2), generación de fotones, carga espacial.

Modes of Corona in Air: Visual Modes of Corona in Air: Visual Appearance Appearance ofof Negative DC ModesNegative DC ModesPara punta de d=0,8 cm sobre esfera de D=7cm, gap=19 cm, exposición ¼ segundoPara punta de d=0,8 cm sobre esfera de D=7cm, gap=19 cm, exposición ¼ segundo

Trichel

pulse

Glow

Streamer

Modes of Corona in AirModes of Corona in Air

• Positive DC Corona Modes:

- Onset Streamer (el más importante)

- Positive Glow

- Breakdown Streamer

Modes of Corona in Air: Visual Modes of Corona in Air: Visual Appearance of Appearance of Positive DC ModesPositive DC Modes Para punta de d=0,8 cm sobre esfera de D=7cm, gap=19 cm, exposición ¼ segundoPara punta de d=0,8 cm sobre esfera de D=7cm, gap=19 cm, exposición ¼ segundo

Onset streamer Glow

Modes of Corona in Air: Modes of Corona in Air: AC ModesAC ModesEscalas: 50 microA/div - 1 ms/divEscalas: 50 microA/div - 1 ms/div

Se observan varios modos de corona durante cada ciclo al cambiar Se observan varios modos de corona durante cada ciclo al cambiar el campo continuamente en amplitud y polaridadel campo continuamente en amplitud y polaridad

Breakdown

Glow

Glow

Gap Discharges in AirGap Discharges in Air

Gap Discharges may Occur:

• Between metallic hardware parts of transmission and distribution lines;

• Between metallic and insulating surfaces;

• On the surface of polluted insulators

Gap Discharges in AirGap Discharges in Air

General Mechanism

g

1

U

Gap

Z

Z U2

Divisor

capacitivo

Gap Discharges in AirGap Discharges in Air

Typical Current Pulse Produced

rT

Td

Cur

rent

I cr

Time

ns ó μs

Light Emission from DischargesLight Emission from Discharges• Excitation: A + e A* + e

• Photo-emission: A* A + hfp

with hfp = (E2 – E1)

where E2 is the energy of the excited state and E1 is the energy of the ground state to which the molecule returns.

• Light spectrum emitted in air is mainly that of molecular nitrogen.

• Excitation potentials of N2 = 6.3 eV and of

O2 = 7.9 eV

La mayoría de los fotones está producida por N2

Diagram of the Electronic and Vibrational Diagram of the Electronic and Vibrational Energy Levels of the Nitrogen MoleculeEnergy Levels of the Nitrogen Molecule

Distintos tipos de fotón según el

salto de energía

Light Emission from DischargesLight Emission from Discharges• The frequency band of light emitted is in the

UV range, with the stronger emissions having wavelengths in the range of 300 nm to 500 nm and the weaker emissions in the range of 80 nm to 200 nm.

• The excitation coefficient (i.e. number of molecules excited by an electron drifting 1 cm in the field direction) depends on the composition of air and is a function of E/p (cociente campo eléctrico/presión)

Light Emission from DischargesLight Emission from Discharges

• Presence of any trace gases such as argon, carbon dioxide etc., can change the light spectrum emitted by discharges in air.

• Spectroscopic data in air suggest that sparks (breakdown) produce more intense light than streamers (corona).

PhotoabsorptionPhotoabsorption• Photons emitted during the avalanche

development in air are absorbed:a) partly by other gas molecules;b) partly by the negative oxygen molecules in the gas, leading to photo-detachment;

O2- + hfp O2 + e

• Other mechanisms leading to the loss of photons are:photoionization, step ionization, dissociation and dissociative ionization

PhotoabsorptionPhotoabsorption• Overall photoabsorption may be characterized by I

(intensidad de fotones):

where μ is the absorption coefficient.

Typical values of μ at atmospheric pressure are:

For N2, μ = 0.3 cm-1 ,

O2, μ = 30 cm-1 ,

Air, μ = 5 cm-1

A menor μ, se propaga mayor distancia

The presence of moisture in air reduces μ by about 25%.

xe0II

Radiation from a Corona DischargeRadiation from a Corona Discharge

Radiation from SunRadiation from Sun

Corona Onset Gradient Corona Onset Gradient (en kV pico/cm)(en kV pico/cm)

c

0c r

K1EmE

• E0 and K are empirical constants (for positive dc, E0=33.7 & K=0.24, for negative dc & ac, E0 =30.0 kV/cm & K = 0.30)

= (273+t0).p/(273+t)p0 is the relative air density; t is the temperature and p the pressure of ambient air and t0 and p0 are reference values; (t0 = 25 C and p0 = 760 mm)

• rc is the conductor radius in cm • m conductor surface irregularity factor, depende de la

rugosidad superficial del conductor

Corona Effects on AC and DC Corona Effects on AC and DC Transmission LinesTransmission LinesFor both ac and dc lines:

• Corona (power) Loss (CL)

• Electromagnetic Interference (EMI) (Includes RI, TVI, etc.,)

• Audible Noise (AN)

• Ozone, NOx etc.

For dc lines:

• Space Charge Effects

AC Space Charges and Corona LossAC Space Charges and Corona Loss

+++

+++ +

++++

+++

+ + ++

------

--- - --

++

++++

+

++ + +

+

- - -------- - - - -

-----

---- - - - -

-

-------------- - - - - - - -

----

--- +

++++

+

+

++ + +

+

++++++

++

+ ++--

------

-----

- - - - ---

--

------

----

-- - - - -

--

---- -

+ + +

++

+++

+

+++

+ +

+

++

( a )( b )

( c ) ( d )

( e ) ( f )

I

dba

ct

c

e gf

UI cor

Icorona es capacitiva, por desplazamiento de la carga espacial.

Main Types of DC Transmission Main Types of DC Transmission LinesLines

• Unipolar Lines

• Bipolar Lines

Metallic return

( Optional)

AC System AC System

AC SystemAC System

Physical Description of Unipolar Physical Description of Unipolar CoronaCorona• Unipolar ions created near the conductor drift

towards the ground, filling the entire space

Physical Description of Bipolar Physical Description of Bipolar CoronaCorona• Ions of both polarities fill the space, creating

two unipolar regions and a bipolar region

PositiveNegativeRegionRegion

BipolarRegion

Generation of RIGeneration of RI Corona current pulse

trains are injected into conductors

The high-frequency current components propagate along the conductors and produce RI near the transmission line.

time

T

T

c+

DC Positive

DC Negative

AC

time

time

c-

c+T

Pulsos aleatoriamente

Pulsos menores

Ambos tipos

Corona & Gap Discharge Current Corona & Gap Discharge Current Pulse CharacteristicsPulse Characteristics

• Both positive and negative corona, as well as gap discharge, current pulses have a fast-rising front (1 a 50 ns) and a slowly decaying tail (50 a 200 ns) as shown

cr

cr0.1 I

0.9 IcrI

Tr Time

Cur

rent

dT

Corona Current Pulse CharacteristicsCorona Current Pulse Characteristics

Type of Pulse Amplitude(mA)

Rise-time(ns)

Duration(ns)

RepetitionRate

(pulses/s)

Positive Corona

Negative Corona

Gap Discharge

10 – 50

1 – 10

500 - 2000

50

10

1

250

100

5

103 – 5.103

104 - 105

102 – 5.103

Corona Current Pulse CharacteristicsCorona Current Pulse Characteristics• Frequency Spectra

of Corona and Gap Discharge Pulses

0.01 0.1 1.0 10.0 100.0 1000.050

110

100

90

80

70

60

Frequency, MHz.

F (

)

, d

B

PositiveGap

Negative

RI Characteristics of AC LinesRI Characteristics of AC Lines• RI from transmission lines is generally defined in terms of

three characteristics:1. Frequency Spectrum

0

10

20

30

40

50

60

70

0.2 0.3 0.5 1 2 3 5 10 20 300.1

Frequency, MHz

RI,

dB

( µ

V/ m

) Q

P

RI Characteristics of AC LinesRI Characteristics of AC Lines2. Lateral Profile (proporcional a 1/D)

0

10

20

30

40

50

60

70

075 50 25 25 50 75 100100- - - -

RI,

dB

( µ

V/ m

) Q

P

Distance, m

RI Characteristics of AC LinesRI Characteristics of AC Lines3. Statistical Distribution

40 6020 25 30 35 45 50 55 65

0.51.02.0

5.0

10.0

20.030.040.050.060.070.0

80.0

90.0

95.0

98.099.0

99.899.9

Per

cent

age

Tim

e A

bove

Abs

ciss

a

RI, dB ( µ V /m ) QP

RI Characteristics of DC LinesRI Characteristics of DC LinesLateral Profile

0

10

20

30

40

50

60

70

075 50 25 25 50 75 100100- - - -

RI,

dB

( µ

V/ m

) Q

P

Distance, m

El positivo contribuye mucho más a la radiointerferencia.

Producida por las descargas tipo streamer.

RI Characteristics of DC LinesRI Characteristics of DC LinesStatistical Distribution

40 6020 25 30 35 45 50 55 65

0.51.02.0

5.0

10.0

20.030.040.050.060.070.0

80.0

90.0

95.0

98.099.0

99.899.9

Per

c en t

a ge

Ti m

e A

b ov e

Ab s

c iss

a

RI, dB ( µ V /m ) QP

Audible Noise Generation and PropagationAudible Noise Generation and Propagation

Generated Corona Acoustic Pulse

• AN Propagation

- 2

- 1

0

1

2

400 10 20 30 50 60 70 80

Time, µs.

Pre

ssur

e, P

a.

r

P

O

2

xx

R

1-

AN Characteristics of AC LinesAN Characteristics of AC Lines• Audible noise from AC lines is described, similar

to RI, in terms of frequency spectrum (figure below), lateral profile and statistical distribution

6331 125 250 500 1000 2000 8000 160004000

30

40

50

60

Sou

nd P

ress

ure

Lev

el, d

B a

bove

20

µ P

A

Frequency, Hz

Corona-generated Hum NoiseCorona-generated Hum Noise• Oscillatory movement of the ionic space charge

creates hum noise at twice power frequency; Figure shows lateral profile of hum noise

70

60

50

40

0 10 20 30 40 50

Lateral Distance From Center Phase, m.

Sou

nd P

ress

ure

Lev

el, d

B a

bove

20

µ P

A

AN Characteristics of DC LinesAN Characteristics of DC Lines• Lateral profile & Statistical distribution are

similar to those for RI;Frequency spectrum is given below

6331 125 250 500 1000 2000 8000 160004000

30

40

50

60

Sou

nd P

ress

ure

Lev

el, d

B a

bove

20

µ P

A

Frequency, Hz

DC Electric Field & Space Charge DC Electric Field & Space Charge ProfilesProfiles

E

(kV

/ m

)

20

30

10

40

60

70

80

90

100

-10 0 10 20 30 40 500

5

10

15

20

25

Computed Electric Field

j (

nA /m

)

- +

Distance from Centre of Line, m

NegativePole

Positive Pole

Measured ElectricField

ComputedCurrent Density

MeasuredCurrent Density

g

50

g

2

Corona Effects Design CriteriaCorona Effects Design Criteria• Corona Loss

- Economic Choice of Conductor Bundle

Conductor Diameter, d

dm

dc1

dc2

Tota

l Cos

t

B

A

por pérdidas

por radiointerferencia

Corona Effects Design Criteria (Corona Effects Design Criteria (at 1 MHzat 1 MHz))

• Radio Interference

USA

RI from power systems is governed by the FCC Rules

Canada

Design Limits

Nominal Phase-to-

Phase Voltage(kV)

Interference Field Strength

(dB above 1 μV/m)

Below 7070 – 200

200 – 300300 – 400400 – 600Above 600

434953566063

Corona Effects Design CriteriaCorona Effects Design Criteria• Audible NoiseUSA The Environmental Protection Agency (EPA)

published guidelines for AN in general. However, each state is responsible to legislate

noise regulations and these regulations may vary widely from state to state.

The EPA document recommends that the day-night average sound level, Ldn, be limited to 55 dB(A) outdoors and 45 dB(A) indoors.

DC Fields & Ions Design CriteriaDC Fields & Ions Design Criteria

• Design criteria for electric fields and ion currents under DC lines are established on the basis of human perception studies

• Based on such studies, the following design limits have been proposed:

E = 25 kV/m (en ca 10 kV/m)

j = 100 nA/m2 (corriente iónica)