Post on 02-Dec-2014
1
Live line work on HV lines equipped with composite insulators:
state of the art
A. Pigini G. Pirovano G. Rocchetti
Consultant RSE MPS Italy Italy USA
1 Introduction
At present, non-ceramic insulators (NCI) also called composite insulators are widely used on overhead lines (OHL) all over the world with many advantages, e.g. in terms of weight and
performance under polluted conditions. An essential issue that limits an even wider application of NCIs is the difficulty to assess their condition in service before the application of live line working (LLW) techniques for the replacement or repair of the insulators or related hardware. Some utilities
already have good experience in inspecting the conditions of NCIs and in working on these components using LLW techniques while others need support in developing of such techniques.
Minimal required conditions for LLW are given in [1] [2]. IEC Standard 61472 [2] describes the method for the calculation of working distances in case of porcelain or glass cap-and-pin insulators, and defines the criteria to assess the maximum allowable number of damaged units depending on
the voltage class and the overvoltage level considered. Minimum requirements for composite insulators are not covered in the present Standard.
A proposal of revision of the abovementioned Standard, in order to include also composite insulators, has been made by the Italian National Committee. Following the proposal IEC 78 demanded to WG 11 and to an ad hoc PT the revision of the Standard. In the following the
available information derived from extensive experimental investigations performed is analyzed and discussed as a possible contribution to the foregoing revision.
In particular the minimal requirements in case of composites are evaluated with the same methodology used for glass and porcelain insulators and then the methodology to detect the composite insulator defects of interest for LLW is discussed.
2 Stresses of concern
LLW is performed in fair weather, far away from conditions that could lead to pollution flashover ,
thus only the dielectric performance under transient voltages is of concern. If lightning is probable, LLW is usually forbidden. Only long front overvoltages (LFO) are of concern, simulated in laboratory by Switching Impulses (SI).
The amplitude of the LFO can vary within a wide range depending on system design (Fig. 1). Furthermore the amplitude to be expected during LLW depends on the overvoltage control set up
during LLW (e.g. limitation of some operations, such as reclosure after a fault). Typical ranges of the statistical overvoltages U2 values assumed in the line design [3] are reported in Fig. 2 as a function of the system voltage (continuous lines), taking into account all the possible
operations which could occur in the system such as closing or reclosing operations. In practice the U2 value generally decreases when the system voltage increases, due to the specific solution
adopted to mitigate such overvoltages. Lower U2 values may also be assumed in the EHV range in case of compact line design. In practice most of the switching operations are not allowed during live line maintenance and thus
only overvoltages deriving from fault ignition and fault clearing are to be considered, having a much lower range, as shown in Fig. 2 (dotted lines). In particular for UHV it is feasible to limit such
vervoltages to 1.6-1.7 p.u.[4].
2
Fig. 1 Range of overvoltages depending on system design [1]
Fig. 2 U2 design values (range between continuous lines) and values expected during maintenance
(range between dotted lines) as a function of the system voltage
3 Minimum approach distances
During LLW a minimum approach distance (Da), also referred to as the safe approach distance, is to
be maintained, being the sum of the electrical distance Du and the ergonomic distance De. As from [2] Du is given by the following equation:
Du=2,17.(e
(U90/(1080.Kt) -1) +F (m) (1)
Where :
F is the floating object length in metres
U90 is the phase to earth (or phase to phase) statistical impulse withstand voltage in kV equal to Ks*U2 (with U2 statistical overvoltages, depending on the system and operation limitation
and Ks statistical factor)
Kt=ks.kg
.ka.kf
.Ki
Where ks is a statistical factor relating the withstand voltage to the 50% flashover voltage, kg the gap factor of the configuration on which work is performed, Ka a coefficient taking into account the altitude a.s.l. of the line installation place, kf a factor that takes into account the decrease of the
strength in presence of floating electrodes in the gap and Ki a factor that takes into account the presence of insulators in the gap.
1
1,5
2
2,5
3
3,5
200 400 600 800 1000 1200 1400
U2
(p.u
)
Um (kV)
3
As an example Fig. 3 a) and b) report the Du value evaluated for a “central window” configuration
kg=1,2 and a “lateral window” configuration with a gap factor kg=1,45, in the ideal case of absence of any floating electrode, at sea level and for various overvoltage values assumed during
maintenance.
a) b) Fig. 3: Du value as a function of system voltage and overvoltage value
4 Requirements to work close to insulator sets
A large amount of live working is devoted to the replacement of damaged insulator sets . Thus, it is important to know the extent of insulation damage that allows work on or near the insulation system
without risk of flashover. The residual electrical strength of a damaged insulator set can vary significantly depending upon the type of insulators, the size and location of the damage, and the
degree of damage.
4.1 Performance of cap and pin insulator strings with damaged insulators
The results of tests on string configurations with both glass and porcelain cap and pin insulators are summarized in Fig. 4. In particular the figure reports the residual flashover voltage as a function of
the amount Nd of failed units when grouped in the most critical place along the string. The flashover voltage is expressed in p.u. of the corresponding value for the sound string and Nd is normalized to the totale number of insulators in the string N t. The results are rather scattered, however the
following remark can be made:
The strength reduction is significantly larger with glass than with porcelain insulators. This
is due to the fact that pre-stressed toughened glass insulators always shatter completely, leaving a bare hub, while porcelain insulators may be damaged in different ways, so that the
strength depends very much on the portion of porcelain skirt that remains.
Obviously the larger the number of damaged units, the greater the strength reduction. But
even if all the insulators are damaged, an insulator string still maintains at least 20 % of its strength. The variation of the strength as a function of the type and number of damaged units can be assumed to be linear as a first approximation, and may be expressed by the following
equation
ki = 1− 0,8kd (Nd/Nt ) (2)
In the Standard Kd is assumed equal to 1 for glass insulators while is assumed equal to 0.75
for porcelain insulators to reflect the better performance.
0
2
4
6
8
10
12
14
0 500 1000
Du
(m)
Um (kV)
Kg 1,2
U2 1,8
U2 2,0
U2 2,2
0123456789
0 500 1000
Du
(m)
Um (kV)
Kg 1,45
U2 1,8
U2 2,0
U2 2,2
4
Fig. 4 Cap and pin insulator strings. SI tests positive polarity. Flashover voltage in p.u. as a function of the share Nd/Nt of failed units in the string ( g glass, p porcelain)
4.2 Performance of composite insulators partially damaged
Systematic positive SI tests were carried out to determine the dielectric performance of composite insulators in configurations simulating actual field conditions and different types of defects.
Insulator strings for 145 and 420 kV lines were considered (see the test configurations in Fig.5) [8],[9],[10],[11].
Fig. 5 Tested configurations in the RSE HV laboratory
Tests were performed simulating external defects by means of thin conductive foils or
semiconducting tape (defect Type A) having 5 mm width and variable length Ld applied along the insulator surface (total length L); for semiconductive defects type A the resistivity of the tape (in the
order of k .cm) was chosen to reproduce the values obtained from resistance measurements carried
out on fibre rod bar in presence of tracking phenomena on insulators removed from service. Internal artificial defects were also made applying on the insulator rod, prior to the application of the
housing, thin layers of varnish (defect type B) [6]. Examples of the results obtained by varying the position of the defect along the insulators are given in Fig. 6.
5
a) b) Fig. 6 Positive switching impulse strength U50 for 150 kV (Fig 6a) and 420 kV(Fig. 6 b)
configurations as a function of the conductive defect length and position
The Figure indicate that the minimum strength for a given defect length is obtained when the defect is close to the high voltage side, but not necessarily when it is just live side. The minimum results obtained for each defect length (minimum of the curves in Fig. 6) are given
in Fig. 7) and 8) as a function of the length of the insulator set damaged. In the Figures also the Uo value (50% flashover voltage of the minimum approach distance required
according to the Italian practice) is given to get a first idea of the allowable defect size.
a) b) Fig. 7 145 kV composite insulator configuration (a)with semi-conductive defect type A, b)with semi-conductive defect type B)
0
100
200
300
400
500
600
700
800
0,00 0,20 0,40 0,60 0,80 1,00
Fla
sh
over
vo
ltag
e
U50 [k
V]
Defect length (p.u.)
Conductive defect; live potential
Conductive defect; ground potential
Conductive defect; f loat ing potent ial
Semiconductive defect (type A); live potent ial
Semiconductive defect (type A); ground potential
Semiconductive defect (type A); floating potential
Semiconductive defect (type B); live potent ial
Reference configuration (gap-factor 1,1): Uo = 520 kV
Insulator with no defect
Uo = 520 kV
0
100
200
300
400
500
600
700
800
0,00 0,20 0,40 0,60 0,80 1,00
Fla
sh
over vo
ltag
e U
50 [
kV
]
Defect length (p.u.)
Semiconductive defect; live potential
Conduct ive defec t; ground potential
Semiconduct ive defect; f loating potential
Conduct ive defec t; f loating potential
Carbonisat ion along rod; live potential
Ax ial breakings of the rod; f loating potential
Reference condition (gap-factor 1,1): Uo = 520 kV
Insulator with no defect
Uo = 520 kV
6
Fig. 8 420 kV composite insulator configuration (semi conductive defects type A)
The 145 and 420 kV results are summarized in Fig. 9 where the flashover voltages are reported in p.u. of the flashver voltage of the sound unit.
Fig. 9. Dielectric strength (Cd in p.u.) under positive switching impulse for composite insulators as
a function of the length of the insulator set damaged (white and grey symbols: conductive type defects, black symbols: semiconductive defects).
The minimum dielectric strengths of the experimental data reported in the above figure can be expressed as:
Cd = 1 – Ld / L (3)
0
500
1000
1500
2000
0.00 0.20 0.40 0.60 0.80 1.00
Defect length (p.u.)
Fla
sh
over
vo
ltag
e U
50 [
kV
]
Conductive defect; live potential
Conductive defect; ground potential
Conductive defect; floating potential
Semiconductive defect, live potential
Semiconductive defect; ground potential
Reference configuration (gap-factor 1,1): Uo = 930 kV
Insulator with no defect
Uo = 930 kV
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0 0.2 0.4 0.6 0.8 1.0
Ld / L [p.u.]
Cd [
p.u
.]
7
The trend is similar to that of insulator strings with cap and pin units , with the exception that in this
case also the case of a conductive defects covering all the insulator length should be contemplated, so that the curve goes to zero for Ld=L
The above considerations allow the determination of the insulator string factor for composite insulators as:
ki = 1 –Kd (Ld / L) (4)
Conservatively in the present revision proposal of IEC 61472 kd value has been assumed equal to 1. However in analogy with the solution adopted in [2] for porcelain insulators and taking into account that actual defects are very likely of semi conductive type it could be possible to assume a
lower value for Kd (as the results in Fig. 6 and 7 seem to indicate), for or example Kd=0,75 as for porcelain. More adequate proposals for the value of Kd could be made on the base of extensive
measurements of the actual electrical resistance of the so called “semi conductive defects” (generally produced by tracking phenomena) of composite insulators and of switching impulse tests in presence of this type of defects.
4.3 Determination of safe conditions for LLW with damaged insulators
IEC 61472 Standard [2] prescribes that, when broken glass units affect the dielectric strength of an insulator string, LLW is permitted if the residual strength is higher than the maximum expected switching overvoltage.
The first step of the verification consists in the minimum electrical distance calculation that shall be guaranteed in presence of insulator damages, by means of the same relationship prescribed for the
electrical distance DU (equ. 1 together with equ. 2 and 3), 4) The second step of the verification consists in verifying that the obtained DU is less or equal to the string length. In this case, the safety conditions for LLW are guaranteed.
An example of evaluation for a typical 420 kV line is reported in Fig. 10 making reference for homogeneity also for cap and pin insulator strings to the length Ld corresponding to the number of
insulator damaged with respect to the total string length L. The following case is considered: LLW on the central phase equipped with I string (Kg=1,2), installation altitude a.s.l 1000 m, U2 assumed in line design 2.8 p.u. (consequent minimum clearance/string length L= 3,3 m), assumed U2 during
maintenance 2.0 p.u. For composites two assumptions were made in calculation Kd=1 (more conservative assumption, see Fig. 10a) and Kd=0,75 as for porcelain , see Fig. 10b).
Assuming Kd =1 the allowable defect size for composite insulators is around 25% while it becomes close to that for glass insulators (about 35%) for Kd=0,75 The maximum defect allowable for porcelain insulators is higher (about 45% of the insulator set). As previously indicated, in analogy
to the porcelain case it could be considered appropriate to select the coefficient kd=0,75 for composites as for porcelain.
Obviously in case of compact lines, with lower U2 assumed in design and consequently lower L values, the allowable defect size would be lower, as can be directly derived from Fig. 10, making LLW problematic. A clear example of the problem can be made with reference to UHV lines, which
due to the limitation assumed for the overvoltages in design, are to be considered as compact lines
8
a) b)
Fig. 10 Example of evaluation of the maximum allowable defect size for a 420 kV system. a) kd for composite =1, b) kd for composite =0,75 Dotted line: design value. Continuous lines: requirements during LLW depending on the defect size (in p.u. of the insulator set length)
An example of evaluation for a 1100 kV line is reported in Fig. 11. The following case is
considered: LLW on the central phase equipped with I string (kg=1,2), altitude a.s.l 1000 m, U2
assumed in line design 1.8 p.u. (consequent minimum clearance/string length L= 8,3 m), assumed U2 during maintenance 1.6 p.u, kd=0,75 for composites.
Fig. 11 Example of evaluation of the maximum allowable defect size for a 1100 kV system
Dotted line: design value. Continuous lines: requirements during LLW depending on the defect size (in p.u. of the insulator set length)
In this case only 15% of the insulator length with damage can be accepted for glass and composite and just a little more for porcelain. Since LLW is essential for UHV, the need of less compact
solutions may be considered in the final design phase.
5 Detectability of the composite insulator defects critical for LLW
Many types of possible NCI deteriorations in service do not affect the SI dielectric strength and thus are not of importance with respect to LLW feasibility. Examples of these defect types which
123456789
0 0,1 0,2 0,3 0,4 0,5 0,6
Du
(m
)
Ld/L
Um 420 kV ,U2 design 2,8p.u., U2 LLW 2 p.u., Kg 1,2
L composites glass porcelain
5
6
7
8
9
10
11
12
0 0,1 0,2 0,3
Du
(m)
Ld/L
Um 1100 kV ,U2 design 1,8 p.u., U2 LLW 1,6 p.u., Kg 1,2
L composites glass porcelain
9
are pure surface phenomena (external) and do not affect significantly the insulation strength
include Chalking, Colour Changes, Crazing, Grease Leakage, Surface Erosion, Minor Debonding, Local Abrasion, Alligatoring, Biological Growth. Even shed damages, e.g. punctures, splitting and
cutting, may be not such to cause a reduction of the strength, at least in dry condition, as typical for most of LLW conditions [8].
Only conductive or partially conductive defects may affect the SI strength. Tracking type defects
will always be potentially critical since they are conductive or partially conductive even in dry conditions. However the insulator can become partly conductive also following moisture ingress in
the core and at the interface. This occurs, as an example, when moisture penetrates toward the rod through the punctures/splitting of the sheath, thus increasing the conductivity of the rod or of part of it [8].
Prior to LLW, the insulation conditions are to be evaluated in order to detect any possible risk of flashover that may expose the maintenance personnel. Specific insulation check procedures are
applied for that scope. The aim of this activity is different from the assessment of the functional conditions of insulators (i.e. determination of all types of defects on insulators) and is focused on personnel safety and namely to detect the quite large defects non compatible with the LLW (see
examples in Fig.10 indicating that conductive type defects extending to about one third of the insulators are still acceptable during LLW ). This premise is very important for the assessment of
the feasibility and suitability of the available diagnostic methods. In the specific case of NCIs, in addition to visual inspection, three other diagnostic methods are generally suggested for the assessment of NCIs condition: infrared (IR)/ ultraviolet (UV) diagnostics and E-field
measurements. The efficiency of these methods is analyzed and compared based on laboratory investigation and field experience [8].
Visual inspection is a very useful tool for estimating the conditions of NCIs, as for ceramic ones. As reported in [8], the defects which are long enough to affect the feasibility of LLW are in most
cases visible on the outer insulator surface, as they show in terms of local sheath puncture or tracking. On the other end, insulators showing no visual evidence of severe defects are not expected
to have severe degradation of the dielectric strength. For this reason, many utilities conduct LLW with NCI after performing a thorough visual inspection of the insulator’s condition only. Preliminary visual inspection from ground level or from helicopters allows detection of only large
defects. The effectiveness of such technique is improved by the use of suitable binoculars or telescopes. Digital photo and video camera recordings allow to carry out both an immediate deeper
analysis and to compare subsequent observations. To be effective, visual inspection must be performed from different points of view, in order to cover the entire insulator. Visual inspection from ground may be improved by visual inspection from inside the tower. The latter must be carried
out by skilled personnel, trained and certified to climb the tower and to perform activity in the LLW zone.
A proper visual inspection may be in general carried out remaining inside the tower body. However a condition assessment based only on visual inspection could lead to very conservative decisions such as to avoid LLW any time that even small signs, which could be related to possible conductive
or semi-conductive defects, are evidenced, independently of their gravity. NCI inspection methods can be enhanced by using the various available diagnostic devices. These devices can help to
“quantify” the extent of damage. Infrared IR detection. Large conductive defects emit corona. Corona emission from conductive or
semi-conductive defects on the surface produces very limited temperature increase. Thus temperature measurements by means of infrared camera are not suitable to detect critical conductive
or semi-conductive defects creating corona on the insulator surface, as proven by the systematic laboratory investigations carried out in [6], [7], [8]: temperature variations measured in laboratory were always lower than a few Kelvin without a clear relation with the position or length of the
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defect, as shown in Fig. 12 where the difference in K between maximum temperature measured on
the insulator (hot spot temperature) and the ambient temperature is reported . Thus the method did not result as the most suitable to detect the most common defects detrimental for LLW.
The method to be effective needs a conductive/semi-conductive defect which develops for most of the insulator length thus causing a significant current flow and a consequent significant thermal effect (rather extreme but obviously very critical cases).
Fig. 12 Over temperature along the insulator as a function of the ratio defect length (Ld)/ insulator length L.
Electric Field (EF) Measurements EF longitudinal distribution along the NCI can be assessed by an EF probe handled by skilled LLW personnel. Defects that generate a distortion of the EF (i.e.
conductive or semi-conductive defects) can be detected by comparing the EF pattern obtained on the defective insulator with a reference fingerprint obtained on the sound insulator. To enhance the detection of defects the data can be normalized dividing the electric field value measured for each
shed by the corresponding value of the plot taken as reference (Fig.13). The maximum EF differences found between the corresponding values on defective and sound insulator (in p.u. of EF
of the sound insulator) are shown in Fig. 14 as a function of the defect length and position. It can be noticed that all critical defects (and even defects much lower than the critical level) may be identified by this method. In the attempt to derive general rules measurements were performed on
insulators having different defects generated during the manufacturing process (lack of primer, carbonization on the rod, breakings of the rod) or resulting from long duration aging test in different
environmental conditions. Electrical field deviations higher than the intrinsic sensitivity of the methodology were confirmed for all types of defects investigated [6]. Beyond being very sensitive, the power of the method relies on its capability of giving indications
about defect size and location and thus criticality [9] (Fig. 15). However the method is quite demanding in terms of time/cost and expertise need and thus is not
optimal to scan the entire OHL. On the contrary it may be very effective to ensure the safety of LLW on a specific insulator.
0
1
1
2
2
3
3
4
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Ld / L [p.u.]
Tm
ax
- T
am
b [
K]
Conductive LIVE POT. 145 kV Semi-Conductive LIVE POT. 145 kV
Conductive GROUND POT. 145 kV Semi-Conductive GROUND POT. 145 kV
Conductive FLOATING POT. 145 kV Semi-Conductive FLOATING POT. 145 kV
11
Fig. 13: Results of electric field measurements with live conductive defects of different lengths
Fig. 14 Conductive type defects. Maximum Electric Field deviation (p.u) as a function of the defect
length and position
Fig. 15 Normalized electric field graphs for live, ground and floating potential defects (conductive type)
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Shed n°
Ele
ctr
ic F
ield
Devia
tio
n[p
.u.]
Ld/L = 0.05 Ld/L = 0.07 Ld/L = 0.1
Ld/L = 0.13 NO DEFECT
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.1 0.2 0.3 0.4 0.5
Ld / L [p.u.]
MA
X E
lec
tric
Fie
ld D
ev
iati
on
[p
.u.]
LIVE GROUND FLOATING
Defects at live potential - 100 kV
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
2,2
2,4
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
n. Shed (1 earth side)
Ele
ctr
ic F
ield
(p
.u.)
C1
C2
C3
C6
Defects at live potential - 100 kV
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
2,2
2,4
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
n. Shed (1 earth side)
Ele
ctr
ic F
ield
(p
.u.)
C1
C2
C3
C6
Defects at ground potential - 100 kV
0,6
0,7
0,8
0,9
1,0
1,1
1,2
1,3
1,4
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
n. Shed (1 earth side)
Ele
ctr
ic F
ield
(p
.u.)
C4
C7
Defects at ground potential - 100 kV
0,6
0,7
0,8
0,9
1,0
1,1
1,2
1,3
1,4
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
n. Shed (1 earth side)
Ele
ctr
ic F
ield
(p
.u.)
C4
C7
Defects at floating potential - 100 kV
0,7
0,8
0,9
1,0
1,1
1,2
1,3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
n. Shed (1 earth side)
Ele
ctr
ic F
ield
(p
.u.)
C5
C8
cm
Live side C6 6
C1 9
C3 13,5
C2 16,5
earth side C7 11,5
C4 19
floating C8 15
C5 22,5
12
UV detection As previously mentioned large conductive defects often appear also on the surface (e.g. as tracking or sheath puncture) leading to heavy local field enhancement and possibly to
corona in air which can be nowadays detected in day light by special UV cameras. Corona emissions intensity may be evaluated by counting the number of pulses of light emission related to corona (named “blobs”).
The sensitivity of this method was investigated by simulating conductive defects (by metal wires) at live side, ground side and at floating potential (in the middle of the insulator), located on the
insulator surface [7]. The insulators were energized at the operating AC voltage and images were taken from about 20 m distance, to simulate service inspection. The defect length was progressively reduced in order to evaluate the limit of sensitivity of the method, defined in terms of the minimum
defect length characterized by a stable corona intensity value and a stable corona spot location. The results are summarized in Fig. 16 where the minimum detectable defects length (in p.u. of the
insulator length) is presented for different insulator ratings. The UV camera was in general capable to detect any conductive type defect longer than about 20-30%, which is the same order as the critical length for LLW for typical EHV line conditions.
Fig. 16: Minimum conductive defects lengths detectable by UV for different defect positions and for
insulators of different ratings
Analogous results were obtained by substituting the metallic wire with a semi-conductive tape with
resistivity in the order of k .cm, to reproduce typical tracking values.
The doubt remained about the capability of the method to detect defects located under the insulator surface, possibly leading to insulator flash-under. To study this aspect several 150 kV composite insulators were produced with a metallic wire purposely inserted by one manufacturer (MLP)
between the silicon housing and the fibreglass core during insulator manufacturing. Fig 17 reports the view of the insulators under tests in the RSE high voltage laboratory.
Fig. 17 Insulators installed in the RSE HV laboratory
0
5
10
15
20
25
30
35
40
100 150 200 250 300 350 400
Un [kV]
Ld
/ L
[%
]
floating potential defects
ground potential defects
live potential defects
13
The UV method was not able to detect inner type defects (which on the contrary were easily
detected by EM measurements). On some of the above mentioned insulators it was decided to remove small parts of housing, discovering the conductive end of the conductive defects (see Fig.
18).
Fig. 18- View of the insulator with removal of small housing part at the conductive defect tip
In this condition UV measurements allowed to detect exposed defects both at earth, live and floating potential defects (see Fig. 19), with results very similar to those obtained with defects
laying on the surface (cases of Fig.16). By comparison the same insulators were analysed by IR. Infrared measurements allowed to detect only some of the defects with a maximum variation of the temperature in the range of the centigrade, similarly to the case of fully conductive defects,
confirming the inadequacy of IR diagnostics for conductive or semiconductive defects.
Fig. 19 UV measurements on 145 kV insulators with inner type defects after removal of a small
part of the housing just to discover the tip of the defect.
It has to be pointed out that the condition of defects which fully develop inside the composite insulators are to be considered as a rare case, especially for new insulator vintages which assure a good adhesion between housing and core and a god sealing.
The evolution of defects from the inside to the outside is very common as proven by the experimental investigation reported in [8], [13] referring to 420 and 145 kV NCIs insulators with
intentional artificial defects, including core rod delamination, core rod to housing interface separation, core voids. In particular the experience reported in [13] refer to investigation carried out without and with moisture ingress [13]. The insulators were first tested in the laboratory and
afterwards energized for 2 years at the test station. An important observation after the two years energization was that the types of internal defects considered did not grow inside the insulators. Instead, original internal defects induced further deterioration on the surface. The results were
openings and punctures of the sheath and cracks as shown in Fig. 20 [8][13]. This behavior was more pronounced for larger defects, because of higher E-field concentration.
14
Fig. 20: Examples of development of originally internal defects into cracks or punctures of the sheath after two years ageing.
Preliminary investigations made with conductive defects have also shown that UV could permit to
get information about the defect location, as shown in Fig. 21 which reports the corona intensity as a function of the position of the defect extremity for different defect types (live, earth, floating). As an example Zone 1 identifies live defects while Zone 2 identifies ground defects [9].
Fig. 21 Example of defect identification graph for 220 kV composite insulators. Number of pulses
(blobs) per minute as a function of the defect position
6 Specific consideration for diagnostics on site
Environmental conditions can influence the measurements. Examples of recordings on site on insulators without defects at humidity level higher than 95% are shown in Figure 22. In this condition the UV emissions were not stable in terms of location and time along the insulator: the
number of blobs/min were also very variable. A similar behaviour was observed with IR, with maximum temperature deviation of about 5 K.The examples show that emissions of the same order
of those caused by defects can be observed also on insulators without defects in conditions of high humidity, especially for contaminated insulators.
0
5000
10000
15000
20000
25000
30000
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
co
ron
a e
mis
sio
n
[co
un
t/m
in]
1
1
2
2
3
4
Defect extremity position (sheds numbered starting
from live side)
15
Figure 22 – IR and UV detection at high humidity level method on insulator without defects.
7 Feasibility of LLW with Composite Insulators
Based on the recent level of development of diagnostic methods for NCIs it can be concluded that it is possible to identify the absence of critical defects of NCIs and thus to carry out safely LLW with
NCIs also. The following procedure is suggested just before LLW:
Visual inspection from the ground level or from a helicopter;
UV/IR corona inspection from ground level or from a helicopter;
Tower climbing and visual inspection from tower body
Finally, after having ascertained the absence of critical defects, and only if it is deemed
necessary, E-field distribution measurements may be performed
Based on the above approach Guidelines were set up and applied for LLW on NCI (fig. 23) [9].
Fig. 23 Example of LLW activity with composite insulators [9]
8 Conclusions
Generally speaking, principles and approaches for LLW on the OHL equipped with NCIs are the
same as those well-established for the strings of glass and porcelain cap-and-pin insulators. In
particular as for porcelain insulators, visual inspections, IR/UV inspections and E-field
measurements along the insulator can be applied.
Slow Front Overvoltage is the dimensioning stress for the evaluation of critical defects,
represented in laboratory by SI. Only conductive or semi-conductive type defects may decrease
the strength under SI. In analogy with the solution adopted by the standards for porcelain (to take
into account that non all the defects lead to the minimum strength) it is suggested to make
reference to the most likely case of semi-conductive type defects.
The critical defect size depends on the configuration and maximum Slow Front Overvoltage
value assumed during LLW. Examples indicate that conductive type defects extending to about
one third of the insulators are still acceptable during LLW for EHV configurations of usual
16
design. LLW can result more problematic for compact solutions and in the UHV range. The
LLW requirements are thus to be considered when analysing the benefits of compacting.
Usually large defects have an impact on the housing, independently of their origin. In fact even if
defects may originate internally, they would most likely induce some surface defects. Thus
visual inspection is always the first option. A number of guides for visual inspection of NCIs is
available and the work on their standardization is on the way within IEEE and CIGRE.
Visual inspection only can lead to rather conservative conclusions. Quantitative condition
assessment of NCIs can be performed by the use of UV/IR cameras. E-field measurements can a
be a complement to increase the level of information in specific cases.
Based on the available knowledge it is nowadays possible to safely carry out LLW and to set up
Guidelines for a safe LLW for OHL equipped by both ceramic insulators and NCIs.
9 References
[1] CIGRE WG 33.07 “Guidelines for insulation coordination in live working” Cigre Brochure 151 February 2000
[2] IEC 61472 2004 Live working - Minimum approach distances for a.c. systems in the voltage
range 72,5 kV to 800 kV - A method of calculation [3] CEI 11- 1 1999 Power installations exceeding 1 kV a.c.
[4] HuYi, Wang Li Nong, Shao Gui Wei, Liu Kai, Xu Ying, Liu Ting, Hu Jan Xun « Research of live working on UHV AC transmission line « IEC CIGRE UHV Symposium beijing 17-21-July 2007
[5] R.Bonzano, M. Ricca, E. Garbagnati, G. Marrone, A. Pigini “Experimental research on the behaviour of HV cap and pin insulator strings with failed units” ETEP Vol.1 1991
[6] M. de Nigris, F. Tavano, F. Zagliani R. Rendina “Diagnostic methods of non-ceramic insulators for HV lines” Cigre General Session 2000 paper 22-207
[7] A. Pigini, A. Colombo, M. de Nigris, "Diagnostics and Monitoring of Insulators for Power
System," CMD 2006 – International Conference on Condition Monitoring and Diagnosis, Changwon (South Korea)
[8] M. de Nigris, I. Gutman, A. Pigini “Live-Line Maintenance of AC Overhead Lines Equipped with Non Ceramic Insulators (NCI)” TD conference and Exhibition, New Orleans 2010 paper TDC.2010.5484679
[9[ G. De Donà, C. Milanello, A. Posati, R. Gallo, C. Valagussa, U. Leva, “Dielectric behaviour of damaged composite insulating strings. Minimum approach distances calculation and
individuation of the limit conditions for the safe live work”, ICOLIM 2008, Torun (POLAND)
[10] C. Valagussa, R. Brambilla, A. Colombo “ Diagnostics of composite insulators based on
electric field measurement (EFDM). Laboratory tests and computer aided simulation for the assessment of the sensitivity and the effectiveness of the method” ISH 2007
[11] C. Valagussa, N. Kuljaca, P. Mazza, G. Pirovano, G. De Donà, R. Rendina, “Minimum approach distance calculation for safe live works in presence of line composite insulators according to IEC 61472 criteria”, ISH 2009, Cape Town, 2009
[12] De Donà, G. Lanzavecchia, P. Guzzini, C. Valagussa, U. Leva, M. de Nigris, “Composite insulators diagnostic in Italian HV transmission system. Laboratory and on-site activities for
the definition of effective and safe criteria for the location of damages", ICOLIM, Torum, Poland, 2008.
[13] I. Gutman, K. Halsan, L. Wallin, T. Goodwin, G. Sakata, “Application of Helicopter-Based
IR Technology for Detection of Internal Defects in Composite Insulators”, World Congress on Insulators, Crete, 2009.