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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)