Physics of Corona and Gap
Discharges
AC and DC Transmission Line
Corona Effects
UV Inspection User’s Group Meeting
February 11-13, 2004
ORLANDO, Florida, USA
By
Dr. P. Sarma Maruvada
Notas en español agregadas por Ing. Ariel Lichtig
exclusivamente 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 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 Processes
• Ionization and excitation by electron impact
A: molécula neutra A*: excitada e inestable
A + 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 & ionization
A + hfp  A*
(photo-excitation)
A + hfp A+ + e
(photo-ionization,
con más
energía)
Basic Ionization Processes
• Electron attachment
A+e A
• Recombination
A+ + 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 Fields
• Field intensified ionization & electron avalanche
+ Ө: iones, más lentos
- : electrones libres
electric field
Discharge in Uniform Fields
•
Discharge development & breakdown
Los iones positivos golpean el cátodo y liberan más electrones. Si hay
suficiente campo, el proceso se autosostiene
Cathode
HV
A
i
Anode
Breakdown 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 conductorplane gap, only partial breakdown or corona
occurs (al disminuir el campo).
Modes 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
Appearance of Negative DC Modes
Para 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 Air
• Positive DC Corona
Modes:
- Onset Streamer (el
más importante)
- Positive Glow
- Breakdown Streamer
Modes of Corona in Air: Visual
Appearance of Positive DC Modes
Para 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: AC Modes
Escalas: 50 microA/div - 1 ms/div
Se observan varios modos de corona durante cada ciclo al cambiar
el campo continuamente en amplitud y polaridad
Glow
Breakdown
Glow
Gap 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 Air
General Mechanism
Z
1
Divisor
U
capacitivo
Z2
Ug
Gap
Gap Discharges in Air
Typical Current Pulse Produced
cr
C u rren t
I
Time
T
r
ns ó μs
T
d
Light 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
Energy Levels of the Nitrogen Molecule
Distintos tipos de
fotón según el
salto de energía
Light 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 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).
Photoabsorption
• 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
Photoabsorption
• Overall photoabsorption may be characterized
by I (intensidad de fotones):
I  I0 e   x
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 ,
-1
Air,
μ = 5 cm
A menor μ, se propaga mayor distancia
The presence of moisture in air reduces μ by
about 25%.
Radiation from a Corona Discharge
Radiation from Sun
Corona Onset Gradient (en kV pico/cm)
Ec


m E 0  1 




 rc 
K
• 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
Transmission Lines
For 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 Loss
-
-
-
-
-
-
-
-
++
+
-
-
-
-
-
-
-
- - ++
+ + ++
+ ++
+ +
- - -
(c)
+
I
+
+
a
+
+
cor
+
b
c
e
c
f
d
+
- -
+
+
+
+
+
+
+
(d)
+
- -+ - -- - - +
-- -+
+
+ + +
(e)
U
-
I
+
+ +
+ ++ + + +
+ + ++ + +
+ +
+
+ ++ + ++
+ + + +++
+ +
+
-
(b)
(a)
+
-
- - - - - - -- - - - -- - - - - - - - - - - - - - - - - - - - - -- - - -- - - - - -
(f)
Icorona es capacitiva, por
desplazamiento de la
carga espacial.
g
t
Main Types of DC Transmission
Lines
• Unipolar Lines
AC System
AC System
Metallic return
( Optional)
AC System
• Bipolar Lines
AC System
Physical Description of Unipolar
Corona
• Unipolar ions created near the conductor drift
towards the ground, filling the entire space
Physical Description of Bipolar
Corona
• Ions of both polarities fill the space, creating
two unipolar regions and a bipolar region
Bipolar
Region
Negative
Region
Positive
Region
Generation of RI
 Corona current pulse
trains are injected
into conductors
DC Positive
time
Pulsos aleatoriamente
DC Negative
Pulsos menores
time
T c-
AC
T c+
Ambos tipos
T c+
time
 The high-frequency
current components
propagate along the
conductors and
produce RI near the
transmission line.
Corona & Gap Discharge Current
Pulse Characteristics
I cr
0.9 I
cr
C u rren t
• Both positive and
negative corona,
as well as gap
discharge, current
pulses have a fastrising front (1 a 50
ns) and a slowly
decaying tail (50 a
200 ns) as shown
0.1 I
cr
Tr
Time
T
d
Corona Current Pulse Characteristics
Type of Pulse
Amplitude
(mA)
Rise-time
(ns)
Duration
(ns)
Repetition
Rate
(pulses/s)
Positive Corona
10 – 50
50
250
103 – 5.103
Negative Corona
1 – 10
10
100
104 - 105
Gap Discharge
500 - 2000
1
5
102 – 5.103
Corona Current Pulse Characteristics
• Frequency Spectra
of Corona and Gap
Discharge Pulses
Positive
Gap
110
100
F ( )
, dB
Negative
90
80
70
60
50
0.01
0.1
1.0
10.0
Frequency, MHz.
100.0
1000.0
RI Characteristics of AC Lines
• RI from transmission lines is generally defined in
terms of three characteristics:
1. Frequency Spectrum
70
R I, dB ( µ V / m ) Q P
60
50
40
30
20
10
0
0.1
0.2
0.3
0.5
1
2
Frequency, MHz
3
5
10
20
30
RI Characteristics of AC Lines
2. Lateral Profile (proporcional a 1/D)
70
R I, dB ( µ V / m ) Q P
60
50
40
30
20
10
0
- 100
- 75
- 50
-
25
0
Distance, m
25
50
75
100
RI Characteristics of AC Lines
3. Statistical Distribution
0.5
1.0
2.0
P ercen tage T im e A b ove A bscissa
5.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
95.0
98.0
99.0
99.8
99.9
20
25
30
35
40
45
RI, dB ( µ V /m ) QP
50
55
60
65
RI Characteristics of DC Lines
Lateral Profile
70
El positivo
contribuye mucho
más a la
radiointerferencia.
Producida por las
descargas tipo
streamer.
R I, dB ( µ V / m ) Q P
60
50
40
30
20
10
0
- 100
- 75
- 50
-
25
0
Distance, m
25
50
75
100
RI Characteristics of DC Lines
Statistical Distribution
0.5
1.0
2.0
P ercentage Tim e A bo ve A bscissa
5.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
95.0
98.0
99.0
99.8
99.9
20
25
30
35
40
45
RI, dB ( µ V /m ) QP
50
55
60
65
Audible Noise Generation and Propagation
P ressure, Pa.
2
Generated Corona
Acoustic Pulse
1
0
-1
-2
0
10
20
30
40
50
60
70
80
Time, µs.
x
x
- 1

2
O
R
r
P
• AN Propagation
AN Characteristics of AC Lines
S ou nd P ressure L evel, dB above 20 µ P A
• Audible noise from AC lines is described, similar
to RI, in terms of frequency spectrum (figure
below), lateral profile and statistical distribution
60
50
40
30
31
63
125
250
500
1000
Frequency, Hz
2000
4000
8000
16000
Corona-generated Hum Noise
S oun d P ressure L ev el, dB ab ove 20 µ PA
• 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
Lateral Distance From Center Phase, m.
40
50
AN Characteristics of DC Lines
S ou nd P ressure L evel, dB above 20 µ P A
• Lateral profile & Statistical distribution are
similar to those for RI;
Frequency spectrum is given below
60
50
40
30
31
63
125
250
500
1000
Frequency, Hz
2000
4000
8000
16000
DC Electric Field & Space Charge
Profiles
25
-
Negative
Pole
Positive Pole
+
Computed Electric Field
100
20
90
Eg
(kV / m )
2
j g (nA /m )
Measured Electric
Field
80
15
70
60
Computed
Current Density
50
10
40
30
Measured
Current Density
5
20
10
0
-10
0
10
20
30
Distance from Centre of Line, m
40
50
Corona Effects Design Criteria
• Corona Loss
- Economic Choice of Conductor Bundle
T otal C o st
B
A
d c1
dm
por pérdidas
d c2
Conductor Diameter, d
por radiointerferencia
Corona Effects Design Criteria (at 1 MHz)
• Radio Interference
USA
RI from power
systems is governed
by the FCC Rules
Canada
Design Limits
Nominal
Interference
Phase-toField Strength
Phase Voltage
(dB above
(kV)
1 μV/m)
Below 70
70 – 200
200 – 300
300 – 400
400 – 600
Above 600
43
49
53
56
60
63
Corona Effects Design Criteria
• Audible Noise
USA
 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 daynight average sound level, Ldn, be limited to
55 dB(A) outdoors and 45 dB(A) indoors.
DC 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)
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Clase 10 (´05)-Efecto corona