Analysis of Electrical Aging Effects of Insulating Tools for EHV Live Working

8/16/2019 Analysis of Electrical Aging Effects of Insulating Tools for EHV Live Working

1/6

10-1

Paper for ESMO 3,

USA,

Scptcmber 12 7, 1993

Analysis

of

Electrical Aging Effects of Insulating To ols

for

EHV Live Working

Zhihai Tim

China Live Working Center

Northeast Electric Power Research Institute

Shenyang

110006

P. R China

Abstract

The recent exploded accidents of rigid insula-

ting tools for EHV live workings were

introduced in the paper and long- term electri-

cal aging experiments of the 500 kV insulating

tools were

carried out.

Based on these results, the

electrical

aging

breakdown model

of

ong insulating

tools under high electric fields was researched and

the new concepts of erective insulation length and

safe

use

time

of

insula ting tools for live w orking

were

also

proposed

Keywords:

Extra High Voltage, Live W ork-

ing Rigid Insulating Tools, Electrical Aging, EUec-

tive Insulation Length, Safe Use Time

1

Introduction

The live workings on energized lines and high

voltage apparatus are very important because the

continuous supply of electric power and safe

eco

nomic operations of electric power systems

can

ef-

fectively

be

achieved in this way'

4.

The most

prominent characteristics

of

ive workings was that

they were performed on

or near

ive lines and appa-

ratus

so

that it was essential requirements that

the safety of linemen, norm al o perati ons of electric

power systems and the accomplishments of

live-working tasks must

be

ensured at the same

time. Live workings have widely been applied and

attracted more and more attention New technolo-

gies

such as robotic and helicopter maintenance

have alreadjl been adopted in live workings[5- 1.

EHV live

workings

have

already been applied

for more than ten years in China and various

kinds of live-workings tools have been developed

In order to develop live workings much better,

hina

Live Working C enter (CLWC) was founded

in

1988

and located in Shenyang(N0rtheast

Electric Power Research Institute,

P. R .

China),

which was in charge of live workin gs of China.

Insulating tools, such

as

various

poles,

insulating ladders and

ropes

etc, are the major and

common-use tools

of

live workings, whose per-

formances

and

reliabilities have direct con nections

with the safety of live workings. Although insula-

tion aging phenomena

of

high voltage apparatus

and rubber insulating gloves for live working have

been

st&ed -

Iq

little attention was paid to aging

erects and long- term pr opert ies of insula ting tools

for EHV live workingsp-

. I -

'?

In the recent years, several exploded accidents

of the EHV rigid insulating tools had happened

when they were used on the EHV transmission

lines for live workings. Fig.

1

to Fig.

3

have shown

the exploded parts of the EHV insulating tools

which had passed the tests according to the nation-

al standards. The internal carbonized paths in the

insulating tube of the 5OOkV insulating tools was

clear ly seen in Fig. 2.Several breakdown puncture

spots were also found on the 330kV insulating

tools alter the explosion The accumulative use

times of the insulating tools for live workings were

about U ours as shown in Table

1.

0-7803-1340-2/93/$3.00 Co p y r i g h t 1993 I

3 9 3


2/6

10-1

3. Analysis

of thc

expcrimtal

results

3.1

Expcrimcntal results of electrical tests

ThC aarptana test in Table 3 was frst

p e r -

formed and the leakage currents or

six

MO

kV

insulating poles were measured in the range of

60

lSO(pA)

which were much less than 1

(mA).

N o flashover or breakdown

he

tools

happened during the tes t The electrical aging test

of

the tools were then done for

100

hours under

the

actions

of the

maximum

operation voltage of

5ookV line. Surfacc discharges

started

from the

H. V. metal parts of the tools could clearly

bc seen

at nights which resulted in the

surface

insu-

lation aging as the nonuniform black marks ap

peared on

the

H.V. portion

of the

tools Another

typical

discharge w s

the parachute

discharge be-

cause it liked a parachute and scemed to be dcvel-

oped fiom thc interior to

the

outside

of

the tools,

which could

always

be

secn

at nights. It was noted

that the positions of the parachute discharges on

the poles were gradually developed

and

shifted

from the

H. V.

parts of the tools down and the

shifted distances s of the parachute discharges

along the tools were in the range

of

0.6 -

.3m

for diITerent kinds

of

tools as shown in Fig

S.

Fs 5 Relabom

dagd

tires

T dthe k d s and Ihc

s h i f ~ d i s t a n m s d p a r a c h u l e d i s ~

Although six tools underwent the long- term

elec-

trical aging tests without failure, the insulating

aging phenomena of the tools

became

more and

more serious. Then the tools were also tested

at S8OkV

for

minutes according to the

requirement of

the

preventative test [' Ihree of

six poles

were

fed

in

this

test ( 2 breakdowns

and 1 flashover). This indicated that the failure

percentage of t he tools was 50% and theelectrical

aging effects were prominent which decreased the

reliabilities of the insulating tools.

Thc local breakdown

marks

whose diameters

were

4mm and9

espcdivdy wen

round

on

two insulating tools broken down after the electri-

cal tests Tbe distances betwm local punctures

and earthed ends wcre

275

and

3 1 0

One break-

down pole was cut oIT for chccking internal insula-

tion marks and several intemal

carbonized

paths

developed along the insulating tool wcre

discovered The total length of

9

carbonized paths

was reached

534mm

and the longest path was over

28(hnm, which could

be

s n in Fig 6.

The

Fq. ll-cmkrnalcahmkdparhinchcira~

tulx

d 9 3 3 k V

iraulating

tod

carbonized paths were obviously the marks

of elm-

trical treeing in solid composite insulating mate-

rial, which were conductive

or

semiconductive sub-

stan-

so

that they led to decrease of insulation

length of the tool.

This

clearly indicated that

the

electrical aging eK s of EHV insulating tools

under the long-term actions of high electric Gelds

were rather serious

and

harmful which

r e d d

the electricd strengths of the insulating tools.

3.2

Influence of moisture

The

relative humidity of experimental environ-

ment during the aging test varied in 30 - 94

and the total time that the relative humidity

greater than 90% was over 10 hours.

S u r h x

discharges under high humidity were observed

and the

discharge

lengths

sometimes reached

1 m No flashoxr or breakdown of insulating

tools cccured even under such serious onditions

of high humidity and long-term voltage d u -

rancc

This

demonstrated

th t

moisture wasom

important factor of accelcrating'electrical

rather than

the

main cause of leading to flash-

over

or

breakdown d nsulating poles.

3.3 Electrical ging breakdown modcl of EHV

insulating tools

Based on t k

above

results,

it was

considered

3 9 4


3/6

that clectrical aging includcd surface and internal

insulation aging eKwts

under high

electric fields

was the main

cause

which lcd

to

the decreases ol

electrical strengths and lifes of

EHV

insulating

tools.

The electrical

aging

breakdown model

of ttc

insulating

tools

was proposed

in

Fig

7,

which

Idinsulatingtods

I

qqiiai vdtags?

Fig.

7

The dectdal

aging

kakdown model

d EHV

b r a y dcscnibed the aging breakdown processes

of

insulating tools. It must

be

stressed that the distri-

butions of

surface

and intemal electric fields along

the long insulating tools were certainly nonuniform

which surelyhad

siwcant

inlluences

on

the electri-

c l aging

crreCts

of

the

tools. The

4-conductor

bundle

was

used in 5

kV transmission lines and

thc maximum

electrical

M d strength Em was

around

the

surface of the bundle conductor which

could

be

calculated in

the

following equation

('?

irsulating

lods

where

%-Maximum

electric feld strength

dSOOkV transmission line

&-Maximum

opcration phase

voltage (line-to-ground voltage)

of

ss0

500KV

line,

U,

=

(kV-1

r- Radius

of single conductor of

5 kV

D -

Size of bundle conductor

of

5

kV

line, r= l.M(cm)

line, D=4ycm)

line to ground cm)

h- Height of bundle conductor of 500

kV

A t the condition of h =3 0( m) =3000(cm),

L= 2

v/cm) was the computed results accord-

ing to the

equation 1).

If there

were

some

tips or

edges on

the HV metal

parts

of EHV insulating

tools,

thc local

electric

filds

at thcsc tips

or

edges

of

the HV

metal

parts

would

bc much

higher.

Such high

clcctric

Ge lds

would initiate

s u r h x

discharges along EHV insulating tools which led

to surface insulation aging

of

the

tools. When intcr-

nal

voids or

cracks

existed

in

the composite

insulating materials of the insulating

tools

d e r

the

actions of

power

frequency high voltages,

internal partial discharges

(IPD)

would probably

be

initiated in voids or cracks in solid insulating

composites under

high

electric Gelds.

The

harmful

erectsd PD would surely

lead

to internal insulati-

on deterioration

Surface

and intemal insulation

ag-

ing

phenomena gradually decrtased the insulation

lengths and electrical strengths of the insulating

tools, which

also

reduced the lifes of the tools and

fially resulted

in

failures of the tools. The ex-

ploded accidents of the

EHV

insulating tools and

the experimental results above could satisfactorily

be explained by this model, which was also

demonstrated by the results of the internal

carbonitedlayers, breakdown punctures andsurface

insulation aging

eU-

of the insulating tools.

4 Suggestiors

4.1

EUective insulation length

of

insulating tool

The

minimum insulation length of insulating

tool for live working was i i dkd

as

the insulation

length

which

had enough

safety

margin to

prcrmt

flafhover

of

the

insulating tool.

In bet the

insbla-

3 9 5


4/6

10 1

tion lengths of he insulating tools were usually

considered as geometric sizes of the insulating

tools after subtractingthe sites of

metal

parts on

the

tools. Strictly

spcaking it may be

suitable to

new insulating

tools

instead of old insulating tools

bocause

the innuenas

of electrical aging

of

i nsul -

ating materialshave to be taken into acu)unt

The

total length of the internal carbonized layers found

in

the

insulating tools were longer than MO ,

which corresponded

to the

reduction of insulation

lengths

of

the tools. However, it was diflicult to

measure

the

accurate lcngths of internal electrical

trees in

the

insulating tools. In spite of this, the

new conapt of

ellective

insulation length of

insulating tools for live working must be proposed

on the

basis

of the above defiition and thc insula-

tion aging duc to surface and internal

par-

tial discharges must be considered so

that

the

e l k -

tive insulation length

Le

of insulating tool

was

de-

fined as

the net insulation

length

of insulating tool

alter subtracting the sizes of all conductive parts

along the axa direction of the insulating tool,

such as metal parts, surface discharge lengths and

internal

carbonizedpaths

etc.

Thus Le can be

cal-

culated in equation 2):

where L- EBdve insulation length of insulating

L-Total length of insulating tool

L,-Sum of sizes of all metal parts on

L,-ss~m of lengths of all internal carbo-

tool

the insulating tool

( i = 1 2, -a ,

n,)

nized

paths

in

the

insulating tool

L-Sum

of surface discharge lengths or

surface

insulation aging lengths d

the

insulating tool (k=

1,2,-

.-,n,)

ci=1,2*.-, q

L

and Li

in

equation 2)wtre the oral v lues

for insulating tools,

but

Lj and wen variables

for the tools.In general, the higher applied voltage

was and the longer the use times of

the

insulating

tools, the more serious the electrical aging ere ts

were.

Ths

L

and

L,

were

increased,

which decreased the efTeedive insulation lengths

or

the tools

so

that the

electrical

strengths

and

the

lifes

of t k insulating tools were also

rcduccd It

must

be

stressed that

the

etTective insulation

length of insulating tool proposed in this paper

was variable which reflected the dynamic process

of decrease of insulation length of

the

tool with

the

increase

of

live working time or development of

electrical

trees, although

the

insulation length of

insulating tool was considered as a Gxed length

without variation in t k pis t It was very impor-

tant to understand tbcsc as thc euects had direct

conncdions with the reliabiditics

of

insulating tools

and

the safety of live working When

the

ellcctive

insulation length of the insulating tool decreased

and

reachcd below the

critical

point, flashover

or

breakdown of the insulating

tool

would

occur and

result in catastrophic failure.

4 2

We use time of insulating tool

One

important question is how long the

insulating tool

can be

uscd

safely

or what the

life

expectancy

of

the insulating tool

is.

Unfortunately,

no quantitative results or criteria about

this

problem

was

available till now. The exploded

accidents

of EHV

rigid insulating

tools

and

the experimental results have clear ly demon-

strated that the Me expectancy or safe use times

of

various insulating tools for live working must

be

de-

termined in order to ensure the safety of live work-

ing and avoid accidents.

This

problem should

be

considered from the design steps of the insulating

tools, which also depended on the mandacturing

methods, materials and tool tests etc

T h e lie

pre-

dictions and non-destructive

tests

of

insulating

use

times of

the

tools

so

that

the

dangerous tools

with diferent defects

can be

abandoned prior

to

failure.

The

safe

use time

Ts of

insulating tool for

live working was suggested in this paper and

determid in equation

(3):

tools arc also ncces~ary or determining the safe

Ts=K

Te

(3)

3 9 6


5/6

whcrc

Ts-Sak

use time

or

insulating tool

Tc- Lire expactancy of insulating tool

K-Life factor of insulating tool

The life factor K should

be

varied for diUerent

insulating tools

as

shown in Fig. 8 and Table

4.

0

250 MO750

lmu(kv)

F i g 8

RdatiOn d r y s t a n

vdta+pUand lilc Iador K

d h d tools

The higher the system voltages, the more

serious

the insulation aging efects of insulating tools under

high electric fields. With

increase

of system

voltages,

the

requirements

on

thequality and prop

ertics of insulating tools must

be

more stringent

so

Tde4

ugpestcd

life

faders

K

dins

icds

that the safety margins of the tools should

be

greater at higher system voltages. That was

why the life factors K of the insulating tools slight-

ly decreased with increase of system voltages.The

prediction of life expectancy, determination of safe

use

times and non-destructive tests of insulating

tools will be researched in the future.

conclusiors

Some conclusions according to the results in

this paper can

be

summarized

as

follows:

1)

Electrical

aging phenomena of EHV rigid

insulating tools under

high

electric fields were quite

serious, which decreased the eUective insulation

lengths and electrical strengths of the insulating

tools so that it was detrimental to the safety of

live working.

(2) The

internal carbonized paths found in the

5OOKV exploded insulating pole and the pole bro-

ken

down &er

electrical

aging test could

sat isktor ily be explained by the eU of

el&ricd treeing

in so l i d

composite insulating mate-

rials, which rcflcctcd thc dcgradation proccsscs of

thc internal insulation or

EHV

insulating tools

under strong electric Ti l .

(3) Moisture was an important factor of

aaxlerating insulating aging, but electrical aging ef-

fects of EHV insulating tools were still serious

even under

dry

conditions.

( 4 ) Electrical aging breakdown model of EHV

rigid insulating tools was studied. It was consid-

ered that the internal and surface insulation

aging

e l b s fmally led to flashover or breakdown of

the

tools and explosions of the hollow insulating tools

when the live workings were performing on the

real transmission lines.

5 ) Thc new conapts

of

the clrcctivc insulation

length and safe

use

times of insulating tools were

proposed in this paper. It must be

stressed

tb t the

eKective insulation lengths of the tools were varia-

bles which reflected the dynamic proasses of insula-

tion aging

so

that the safe use times of the

insulating tools must

be

determined in order to

ensure the safety of live working.

( 6 ) I t was suggested that electrical aging ta t s of

samples of insulating tools

be

included in type test

and sampling t a t of the tools to assure the

long- term properties

and

reliabilities of insulating

tools. Researches on life predictions and

non-destructive tests of live working tools should

be carried out in the future.

Acknowledgements

The author

is

greatly indebted to Mr. Qingnai

Wang Director of China Live Working Center, for

his encouragement

and

help in the work

Thanks

are

due to the supports of

Mr

Naiqian Jiq

Mr.

Hongren Sun, Mr. Tongsi Sun, Mr.Juntao

Xian

and the colleaguesof the author in performing

the experiments. The author would

also

like

to express

his

gratitude to Ms. ao and

Ms. Fenghua Zhang for their assistance in typing

the paper.

3 9 7


6/6

1 0 - 1

Rcfcrcnccs

1 G. &la et al. ‘Insulation coordination

in

distri-

bution live line maintenance , IEEE Trans.on

Power Delivery, Vol.

3,

No. 4, pp 1922- 1927,

1988.

2

J.

T.

Tyner, ‘Sdety

in

live

-

ine work is focus

of

ESMO

-9

Transmission& Dirtribution, pp.

22 - 9, November 1990.

Geld parameters during live- line maintenance’,

IEEE Trans. on Power Delivery, Vol. 6,

No.

3,

pp 1187- 1195, 1991.

3 Yan et

al.,

‘Calculation and measurement of

4 G. Gela et al., IEEE Committee Report

‘Bibliography of literature for live- ine maintc-

nancc and

d a t e d topics’, IEEE Trans.

on Power Delivery, Vol. 7, No. 3 pp.

1552-1562, 1992

5.

J.

M. Van Name et al.,

‘Robotic maintenance

of

overhead transmission lines”, CIGRE, Pro-

ceedings

of

the

32nd

Session,

V.

1,

1988, p.

6 M. Tsuchihashi et al., ‘Manipulator system for

construction overhead distribution lines”, IEEE

Trans.

on

Power Delivery, Vol. 4, No. 3, pp.

1904- 1909, 1989.

7. S N. Burchette, ‘Helicopter maintenance on

energized EHV transmission

lines’,

Transmis-

sion

&

Distribution, November 1989, pp.

58

- 59.

8 G.

Katsuta et

al.,

’Development

of

a method

of partial

&charge

detection in extra-high

voltage

cross

linked polyethylene insulated

cable lines”, IEEE Trans. on Power Delivery,

Vol. 7, NO. 3, pp. 1068

-

079, 1992

9 R Bartnikas et al. Engineering Dielectrics,

Vol.HA: Electrical Properties of Solid Insulating

Materials,

ASTM,

USA, 1983.

10.P.

Qgan

et

al.,

‘Models for insulation aging

under electrical and thermal multistress”,

IEEE Trans. on Electrical Insulation, Vol. 25.

No.

5, p.

923, 1990.

11. IEC Standard Publication 855 (1985):

sulating foam-iilld tubes

and

solid rods

for live working

22-12/1-11.

2 IEC Standard Publication 832 (1988):

‘Insulating polcs and universal tool attach-

ments for live working ”

13. National Standard of

China, GB13398-92,

‘Insulating poles for live working ( I n

Chinese), 1992

Beijing, 1986.

14. N. Fanget al., Live Working (In

Chinese),

3 9 8

Analysis of Electrical Aging Effects of Insulating Tools for EHV Live Working

Documents

Transcript of Analysis of Electrical Aging Effects of Insulating Tools for EHV Live Working