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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
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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
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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-
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8/16/2019 Analysis of Electrical Aging Effects of Insulating Tools for EHV Live Working
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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)
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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.
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1 0 - 1
Rcfcrcnccs
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bution live line maintenance , IEEE Trans.on
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J.
T.
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