Download - WG CF&E-CoronaGuideKuffel July2012
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Dr. John Kuffel, FIEEEKinectrics Inc.
Development of IEEE Guide for Conducting Corona
Tests on Insulator Assemblies and Hardware for
Overhead Transmission Lines and Substations
July 2012IEEE PES General Meeting
San Diego
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POSITIVE CORONA VERSUS
NEGATIVE CORONA
Negative corona usually occurs at a lower voltage Discharges appear as a visible spherical bluish glow and an extending
conical column.
Hissing sound
Existing conical column reaches out with little branching
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As voltage is increased
negative-corona spots transform to positive glow at the positive halfcycle of the applied voltage and then turn into positive streamerswith increased voltage.
Positive corona discharges
white bright discharges
many filamentary branches extending from originproduce a crackling or snapping sound
significant RIV
POSITIVE CORONA
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(+) ive
(-) ive
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CORONA TESTING
Why worry?
Corona causes power losses, radio noise, audible
noise and damage to non-ceramic insulators
Corona Performance Requirement
Insulator assemblies and hardware must be free of
corona when operating under design conditions
How is this ensuredThrough laboratory testing
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TRADITIONAL CORONA TESTING
single phase mock-up of the conductor/hardware/insulator assembly
at a given height above the ground with 110% to 120% of the rated
line-to-ground operating voltage.
does not appear in any standards, but is used as a generally
accepted test method.
Acceptance Criterion
Mock up assembly is free of corona under the specified test voltage
Shortcoming
This test method can give erroneous results as corona inception is afunction of electric field not applied voltage.
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GUIDANCE
IEEE task force jointly sponsored by the insulator and corona and
field effects WGs formed
SCOPE
Prepare a guide for the performance of corona tests on insulator
assemblies and hardware for line and station applications
PROGRESS
PAR 1829 approved in 2011
Work ongoing for several years
Currently in final stages of review prior to ballot process
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CONTENTS OF GUIDE
1. Overview
The purpose of this guide is to establish uniform procedures for the
testing of transmission line and station bus hardware in high voltage
laboratories. A uniform procedure is a prerequisite to assure that:
hardware is free from visible corona and does not add any
appreciable radio interference (RI) Insulators are not installed in conditions where corona activity will
accelerate end of life
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2. DEFINITIONS corona: A luminous discharge due to ionization of the air
surrounding an electrode caused by a voltage gradient exceeding
a certain critical value.
corona inception gradient: The gradient on that part of anelectrode surface at which continuous corona first occurs as the
applied voltage is increased.
corona inception voltage: The voltage applied to the electrode to
produce the corona inception gradient.
corona extinction gradient: The gradient on that part of anelectrode surface at which continuous corona last persists as the
applied voltage is gradually decreased.
corona extinction voltage: The voltage applied to the electrode to
produce the corona extinction gradient.
radio influence voltage (RIV): The radio frequency voltageappearing on conductors of electrical equipment or circuits, as
measured using a radio noise meter as a two-terminal voltmeter in
accordance with specified methods (generally termed conducted
measurements).
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3. TEST ARRANGEMENTS COVERED
Lines
Insulator assemblies Tension
Suspension
Hardware
Dampers, compression sleeves, monitoring devices,etc.
Stations
Station bus hardware
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4. TEST PROCEDURE
Method based on utilizing the mid-span operating
conductor surface electric f ield as a basis for tests
Available from analytical calculations
Determination of acceptance conductor surface test
voltage gradient
Lab test conductor surface gradient established by
a) Calculation2 and 3 dimensional field calculations to determine test
setup gradient
b) Calibration
Use of calibrating spheres to determine the test setup
gradient
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DETERMINATION OF ACCEPTANCE TEST
VOLTAGE GRADIENT (contd)
Altitude Correction and Surface Ageing
1. Altitude Correction: Relative Air Density() corona onset is a function of both electric field intensity and relative
air density
voltage must be adjusted to account for the difference in RADbetween the test laboratory and the proposed line location.
corona onset voltage for the same test setup decreases as
decreases (altitude increases)
hardware in the lower elevation laboratories must be tested at a
higher voltage to account for the increased altitude.
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ALTITUDE OR AIR DENSITY CORRECTIONTo compensate for the difference in altitude between the
testing laboratory and the altitude for which the hardware
will be applied, one of following three formulas can beused:
V/Vo= o/, altitude correction term developed by Peek
V/Vo= (o/)2/3, altitude correction term developed by
PetersonV/Vo= (o/)1/2, altitude correction term developed at
Leadville, CO.
where
V is the voltage to be determined
Vois the specified acceptance voltage is the relative air density for the altitude at which
the hardware will be used.
ois the relative air density for the altitude of the
testing laboratory
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ALTITUDE OR AIR DENSITY CORRECTION
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
0 500 1000 1500 2000 2500 3000 3500 4000
ALTITUDE ABOVE SEA LEVEL, METERS
RELATIVE
AIR
DENSITY,P.U.
Relative air density versus altitude, based upon standard atmospheric tables
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ALTITUDE OR AIR DENSITY CORRECTION
1
1.05
1.1
1.15
1.2
1.25
1.3
1.35
1.4
1.45
1.5
0 500 1000 1500 2000 2500 3000 3500 4000
ALTITUDE ABOVE SEA LEVEL, METERS
VOLTAGE
INCREASE,P.U.
0.954 0.908 0.863 0.821 0.78 0.7410.704 0.668
RELATIVE AIR DENSITY. P.U.
PEEKPETERSON
LEADVILLE
Voltage increase required for testing hardware in HV laboratories
to account for higher altitudes
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DETERMINATION OF ACCEPTANCE TEST
VOLTAGE GRADIENT (contd)
2. Surface Ageing
Long term exposure to the environment causes
roughening of the surface of hardware
Surface roughening causes lower corona inceptionand extinction voltage levels
Corona inception and extinction voltage levels can
drop by up to 20% or 30 % for hardware that has
been exposed to environmental ageing for 20 plusyears.
Voltage should be adjusted to take this into account
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ACCEPTANCE GRADIENT
Test gradient for acceptance usually set between 1.1 and 1.3times max operating gradient
Agreement between utility and supplier
Depends on
Altitude (air density)
Environment (ageing of hardware over service life)
Assemblies must be shown as positive corona free when tested
at conductor or sub-conductor surface gradient defined as the
acceptance gradient
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StepsCalculate mid-span sub-conductor voltage gradient
Set required acceptance test gradient based on altitude
and surface ageing corrections
Select the appropriate size corona calibrating sphere
and calibrate the sphere on appropriate dia conductor
Use corona calibrating sphere to establish required test
voltage on full scale conductor setup
Build full scale single phase assembly mock-up in the
lab
Perform visual corona and RIV test
VISUAL CORONA TEST PROCEDURE
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CORONA CALIBRATING
SPHERES
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CORONA CALIBRATING
SPHERE APPLICATIONUsed to establish electric field at conductor surface through calibration
Individual spheres are assigned inception gradients based on a known geometry
and a given conductor size.
concentric cylinder geometry
EV
rR
r
=
ln
where:V = voltage applied to the conductor
R = radius of the concentric cylinder
r = radius of the conductor
single conductor above ground plane
r
hr
VE
2ln
=
where:V = voltage applied to the conductor
h = height of conductor above ground plane
r = radius of the conductor
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SETUP FOR GRADIENT ASSIGNMENT TO
CALIBRATING SPHERE
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CORONA CALIBRATING SPHERE
GRADIENT ASSIGNMENT
Range of sphere size vs conductor diameter is given inreference materials
Steps
sphere installed on conductor voltage increased
at positive corona inception, E at conductor surface is
calculated based on the applied voltage and geometry
EV
rh
r
=
ln2
EV
rR
r
=
ln
OR
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Height
above
ground
(ft)
Positive Corona Inception Voltage
(kV) Surface
Gradient
(kV/cm)
1 2 3 4 5 Average
2.0 92.7 91.6 91.6 91.2 91.7 91.6 15.49
3.0 101.5 100.5 100.0 100.0 100.0 100.5 15.48
TYPICAL CALIBRATING SPHERE
GRADIENT ASSIGNMENT RESULTS
3mm sphere on 26.8 mm bus
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CONDUCTOR ASSEMBLY CALIBRATION
Smooth metal tubes used as conductors or bundle sub-
conductors.Conductor assembly is suspended above a conducting
ground plane (usually the floor of a hvl)
Calibrating sphere is installed in the same location as which
the insulator assembly of hardware will be positioned for
testing.
Procedure
1. Raise voltage to above the corona inception level of the
calibrating sphere to condition the sphere2. Lower voltage to 30% below the corona extinction level
CONDUCTOR OR BUNDLE CONDUCTOR
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3. Raise and lower the voltage five times per steps 1and 2 and note the voltage at positive coronainception and extinction on the calibrating sphere
4. Average the five inception/extinction readings todetermine the positive corona inception/extinction
voltage of the calibrating sphere5. Previous calibration of the calibrating sphere on the
same diameter conductor shows positive coronainception gradient of X kV/cm.
6. Utilizing this pre-determined calibration data for thesphere, the relation between applied voltage andconductor surface gradient of X kV/cm isestablished.
CONDUCTOR OR BUNDLE CONDUCTOR
ASSEMBLY CALIBRATION contd
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DETERMINATION OF REQUIRED
TEST VOLTAGE
= The required applied test voltage
= The voltage gradient at which the test assembly must be free of
positive corona
= The positive corona inception voltage gradient for the conductormounted calibrating sphere (in this case X kV/cm)
= The applied voltage at which corona inception occurred during the
bundle calibration.
c
c
s
r VE
EV =
rV
sE
cE
cV
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Remove calibration sphere and install insulator assembly or
hardware to be tested
Install ground planes as required (ie. to simulate tower arms,
poles, etc.)
Raise the applied voltage to above the level at which positive
corona appears on the assemblies under test, and then lower thevoltage until corona extinction is observed.
Each reported corona extinction and inception voltage is based
on the average values of corona extinction and inception voltages
determined from a series of 5 voltage excursions.
If the voltage at corona extinction exceeds the required test
voltage, then the assembly is considered to have passed the test.
If corona extinction voltage is below the required test voltage,
then the assembly is considered to have failed the test.
TEST PROCEDURE
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Visible corona (or lack thereof) can be detected by
long exposure still photographs
light amplification systems (image intensifiers)
UV cameras
Corona cameras
IDENTIFICATION AND RECORDING OF
VISIBLE CORONA
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TESTING 765 kV POLYMER V-STRING
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TESTING 230 kV BUS CONNECTOR
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TESTING 345-kV HORIZONTAL VEE
INSULATOR ASSEMBLY
Corona inception Corona extinction
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Questions