WG CF&E-CoronaGuideKuffel July2012

8/12/2019 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

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