2006 articolo cmd2006 isolatori mod def
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8/3/2019 2006 Articolo CMD2006 Isolatori MOD DEF
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Diagnostics and Monitoring of Insulators for Power System
A. Pigini* (Fellow IEEE), A. Colombo** and M. de Nigris (Senior Member IEEE)**
*CESI; Via Rubattino 54, Milan, I 20134 Italy
** CESI RICERCA; Via Rubattino 54, Milan, I 20134 Italy
Abstract - Tests under AC voltage and under
overvoltages were carried out on cap and pin insulator
strings and on composite insulator set in dry and clean
condition, simulating defects of different size and location
along the insulator length. On the basis of the results
obtained the severity of the various defects is derived,
allowing the assessment of the characteristics of the defects
still compatible with live line maintenance procedure and
with the required reliability of the line. As a consequence,indication about the minimum sensitivity required for
diagnostics is given.
The efficiency of the various proposed methods for the
diagnosis of insulators were analysed and compared by
systematic investigation in laboratory and in the field.
Index Terms- Cap and pin, Composites, Diagnostics,Insulators
I. I NTRODUCTION
Insulators are one of the most numerous components
in the electrical system and thus the assessment of their
conditions is essential to ensure the reliability of the line
and therefore the availability of the whole system. Theassessment of their reliability is of particular interest to
assure safety conditions in relation to live line
maintenance.
Insulators are characterised by a two-fold functions
that are to be guaranteed during the entire service life:
the mechanical and electrical function. This paper deals
mainly with the assessment of the electrical function.
The insulator dielectric performance may be
impaired by failures or damages of part of the insulator
set (e.g. failure of one or a few caps on cap and pin
insulators, degradation of the insulation on composite
insulators). The insulator performance may be also
affected by environmental conditions, especiallycontamination.
The verification of the adequacy of diagnostic
methods in respect to the electrical performance
requires:
- the assessment of the critical defect size, that is the
maximum defect size still compatible with the
acceptable system reliability and with live line
maintenance procedures
- the verification of the sensitivity of the diagnostic
methods with reference to the defects to be identified
Results of the research activity are reported in the
following. On the basis of the research carried out
guidelines for field inspection were set up and applied in
the field.
II. DETERMINATION OF THE MINIMUM REQUIRED
SENSITIVITY OF DIAGNOSTIC METHODS
The dielectric performance of insulator strings made
of cap and pin units depends on the number of damagedunits, on the degree of damage of the units, and on their
position along the string. The extent of the defect may
also be described by the length of the string with
damaged insulators with respect to the total string length.
The dielectric performances of composite insulators
can be seriously affected when the external or internal
surface of the housing is subjected to degradation
leading to the formation of conductive or semi-
conductive paths that reduce the withstand voltage of the
configuration.
The maximum allowable defective length of the
insulator string giving rise to a certain minimum
acceptable dielectric performance of the system isdefined as critical defect length. This type of defect
represents the absolute minimum that any diagnostic
method should be able to detect thus defining the
requirements for the diagnostic sensitivity. In the present
study the minimum acceptable dielectric performances
considered were those relevant to live line maintenance
operations: i.e. the critical defect length is considered as
that giving rise to a 50% probability flashover voltage
under positive switching impulse equal to that of the
reference configuration with a gap length equal to the
minimum approaching distance, as defined in the
Standards [1] for the system voltage considered.
Many data are available for cap and pin insulator ,
leading to the following relationship, interpolating the
lowest values of dielectric strength experienced, as a
function of the number of damaged insulators or length
of the insulator set damaged [2]
U50 = K * Cd * U50rp (1)
with:
- U50, U50rp: 50 % flashover voltage of the examined
configuration and of the reference rod plane one
under switching impulse;
- K: gap factor of the configuration;
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Cd = (1-0,8 Nd /Nt) = (1-0,8 ld /l ) (2) with
- Nd damaged units and Nt total units in the string;
- ld length of the damaged insulator set and lt total
insulator length.
An important observation is that the lowest strengthis obtained with grouped damaged insulators near the
line side, but not always at the line end. This fact
underlines the importance to detect damaged insulators
at line end, but also along the string (at “floating
potential”).
Less data are available for composite insulator sets.
Systematic tests were carried out in CESI Laboratories
to determine the dielectric performance of composite
insulators (non ceramic line insulators NCLI in the
following) in configurations simulating actual field
conditions and different types of defects. Insulators for
145 and 420 kV lines were considered. External defects
on the NCLI were simulated by means of thin
(conductive or semi conductive) layers having 5 mm
width and variable length applied along the insulator
surface; internal artificial defects were made applying on
the insulator rod, prior to the application of the housing,
thin layers of varnish having the required conductivity
and length. The results are given in Fig. 1 as a function
of the length of the insulator set damaged. In the same
Figure they are compared with data from (2):
Fig. 1. Dielectric strength (Cd in p.u.) under positive
switching impulse for composite insulators as a function of the
length of the insulator set damaged. Comparison with (2).
For 145 kV insulators all types of external defects
generated dielectric performance depending mainly on
the defect length having little dependence on defect
conductivity or position along the NCLI. For 420 kV
insulators the influence of the damage type and position
resulted more important. Only a few results obtained
with fully conductive layers are slightly lower than those
given by (2). Considering that the defects along the
insulators are to be expected not fully conductive in
service, (2) may be retained as a good interpolation of
the most critical condition for composite insulators also.
Comparing the dielectric strength of typical 145 and
420 kV configurations evaluated with (1), (2) with that
of the minimum approach distances from [1], the critical
defect length may be evaluated and results of about 30%
of the insulator length [3]. The detection of smaller
defects (15-20 % of the insulator length) may bedesirable, since if extended to a significant number of
insulator sets, can significantly affect the reliability of
the system, increasing the risk of failure in normal
operating conditions [4].
III. DIAGNOSTIC METHODS EVALUATION IN LABORATORY
The detection of defective insulators is quite easy for
glass insulator strings with shattered insulators, for
which an accurate visual inspection can be sufficient. It
is much more complicated for porcelain insulators and
especially for composite insulators [5]. Special attention
in the following will be paid to the composite insulator
case. Visual inspection is traditionally used also for
these insulators. The availability of powerful visual aids
and sophisticated image conditioning and interpretation
tools have increased the potential use of such diagnostic
techniques, especially for what concerns helicopter –
mounted inspections. However the environmental
conditions (daylight, sun reverberation, background
color, etc.) may introduce such high disturbances that
incipient defects can hardly be detected effectively.
The efficiency of three additional diagnostic methods
based on electric field measurement, on the
measurements of corona emission in the ultraviolet rangeand of temperature rise in the infrared range were
analysed and compared by systematic investigation in
laboratory.
A. Electric field monitoring
The Electric Field Distribution Measurement
(EFDM) diagnostic system for line insulators is based on
the assumption that any defect along the insulator is
likely to cause a distortion in the electrical field
distribution. Systematic laboratory tests were carried out
with power frequency voltage, to evaluate the EFDM
approach. The field probe was sled along the insulatorunder test by means of an insulating arm moved by
precision crane; the field distribution was determined
both on the onward and backward runs of the probe
along the insulator. Tests carried out on the same type of
insulators by different testing teams have proven that the
repeatability and reproducibility of the method is well
within 5%. To establish the method sensitivity similar
dielectric configurations as those for the determination
of the critical defect length were used. The
determination of the presence of a defect in a NCLI
tested according to this method was performed [3] by
comparing the pattern obtained on the defective insulator
with a reference fingerprint for the configuration
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considered, as shown in Figure 2. On the defective
insulator a tracking on the housing close to the live
terminal, affecting the first two sheds live side, was
simulated. It is evident from the figure that, although the
damage on the NCI is not very heavy (defect covering
7% of the insulator length), the change in the pattern isevident.
Fig.2. Comparison of measurements on insulator without
and with defects (shed 1: ground side, shed 15 line side) .
To enhance the detection of defects the data were
normalized dividing the electric field value measured for
each shed by the corresponding value of the plot taken as
reference (1 p.u), as shown in Fig.3. The result is
obtained on 145 kV NCLI with a conductive external
defect at live terminal: it can be noticed that a defect at
live terminal produces high electric field distortions.Lower distortion values are produced by conductive
defects at ground and floating potential (Fig. 4 and 5).
Semi-conductive defects produced similar results. From
analysis of the deviation pattern, as those in Fig. 3, 4 and
5, implemented by field calculations, indications on the
location of the defect can be obtained.
Fig. 3. Normalization of Fig. 2 data.
The maximum electric field deviation as a function
of the defect length for conductive defects in various
positions are reported in Figure 6. It can be noticed that
all critical defects and even defects much lower than the
critical value may be identified.
Beyond being very sensitive, the power of the
method is its capability of giving indications about
defect size an location and thus criticality.
Fig. 4. Normalized Electric Field for ground terminal defect
length.
Fig. 5. Normalized Electric Field for defects at floating
potential.
Fig. 6. Maximum Electric Field deviation (p.u.) as a
function of defect length and position.
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B- UVC method
The possibility of localizing initial corona activity
constitutes an interesting technical challenge, especially
in daylight conditions.
The diagnostic indicator considered is the emissiongenerated by the defects in the UVC range (i.e. with
wavelength in the range 240-280nm [6]) a bandwidth in
which the solar light is filtered by the atmosphere.
Corona emissions intensity is evaluated making
reference to the number of pulses of light emission
(named “blobs”). A counter gives a number proportional
to the quantity of “blobs” received by the sensor.
The first investigation aimed at checking the
sensitivity of the camera with respect to the standardized
procedures for corona inception evaluation: for this
purpose a standard-type 420 kV porcelain insulator
string was used.
Fig. 7. Sensitivity of UVC vs. Visual Corona.
The standard laboratory procedure (Standard Test)
for visual corona has evidenced the inception of corona
activity around the ground electrode of the protective
gap at 400 kV with extinction voltage at 385 kV. The
curve in Fig.7 indicate that the sensitivity of UVC, with
a gain of 160, is higher than that of the operator. An
equivalence between the two methods can be obtained
when reference is made to a “blob rate” of 500-800blobs/min. The experience suggests a possible use of the
methodology also for standard tests, thus removing the
very critical “human factor” of such tests.
Once established the sensitivity, measurements were
performed on composite insulators with artificial defects.
The corona emission level, in terms of blobs/min, is
reported in figure 8 as a function of the defect length for
defects live side. The data refer to tests at 100 kV. It is
evident that even very small conductive defects,
covering less than 4% of the total insulation, if at line
potential, may produce noticeable emissions.
Fig. 8. UVC sensitivity as a function of the defect length.
This means that this method, for this kind of defects,
is extremely sensitive, indicating the presence of defects
well below the critical one. However the relationship
between defect length and blobs/m is not alwaysmonotonous and the calibration of the method is not easy
to be assessed. Thus the method can give only
qualitative results and by itself can not furnish
indications about the defect criticality.
Furthermore the detection of the defect is possible
only if the defect makes corona at service voltage, which
is not the case for some defects at ground or floating
potential.
C. Infrared Thermography Measurement (ITM)
Thermal emission is associated with pre discharge
activity along the insulator and with significant current
flowing along the insulator, as for instance in presence
of tracking. With ITM the temperature distribution along
the insulator axis is measured by means of an infrared
camera, looking for hot spots, associated with possible
local defects. Laboratory tests were carried out to
evaluate the characteristics of the ITM diagnostic
approach. The thermo-camera used had a wavelength of
8-12 m, a field of view of 5 to 20 degrees (depending
on the lens used) and a resolution of 0.64 - 0.16 mrad
respectively. A sensitivity analysis was carried out to
check the effect on the accuracy of the emissivity
assumption and of the ambient temperature. Theinvestigation indicated that the measuring error can be
limited if the measurements are performed establishing
accurately the emissivity and avoiding atmospheric
conditions critical for the measurements.
Tests were carried out on insulators with simulated
defects For the examined insulators, an emissivity
coefficient of 0.95 was used .The results are reported in
Fig. 9. The vertical axis shows the difference in K
between the maximum temperature measured on the
insulator (hot spot temperature) and the insulator
temperature (Tamb).
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Fig. 12. ITM detection at high humidity level method on
insulator without defects.
Fig. 13. UVC detection at high humidity level method on
insulator without defects.
Examples of recordings on insulators without defects
at humidity level higher than 95% are shown in Figs. 12
and 13. The UVC emissions was not stable in terms of
location and time along the insulator: the number of
blobs/min were also very variable. A similar behavior
was observed with ITM, with maximum temperature
deviation of about 5 K.
The examples show that emissions of the same orderof those caused by defects can be observed also on
insulators without defects in conditions of high humidity,
possibly in presence of contamination.
Thus the campaign for defect detection are to be
carried out avoiding conditions of high humidity.
UVC and ITM may be used to have preliminary
indication about the presence of pollution, but the
analysis of the suitability of the methods to this purpose
is outside the scope of the present paper.
V. CONCLUSION
Live line maintenance requires the verification that
critical conditions are not present along the line, and in
particular that the dielectric withstand of the insulators
along the line is higher than that applicable to the
minimum approach distances. This implies that at least
defects affecting more than 30% of the insulator length
are to be identified.
Visual inspection remains of primary importance to
assess the insulator condition.
EFDM results the most sensitive method for
establishing the insulator condition. Unfortunately the
method can be applied only within live line maintenance
operations, and thus it finds an intrinsic limitation when
critical insulator conditions are expected.
UVC method is very sensible to defects causing
corona, being sometimes too sensitive, since some of the
defects detected can be far from critical.Some of the critical defects, e.g. long defects with
high resistivity at floating potential, may not cause
corona, but may produce heat. ITM is thus an important
method to complete the information on the insulator
condition in service.
The measurement are to be made in dry condition
and low humidity to optimize defects identification .
The investigation performed and the field experience
has allowed to set up guidelines for field inspection.
Some of the critical defects unfortunately cannot be
detected by any of the methods proposed (e.g. conditions
close brittle fracture), pointing out the need of additional
research.
R EFERENCES
[1] IEC 61472, “Live working - Minimum approachdistances for A.C. systems in the voltage range 72,5 kV to800 kV - A method of calculation”, July 2004.
[2] CIGRE WG 33.07 “Guidelines for insulationcoordination in live working”, CIGRE Brochure 151,2000.
[3] M.de Nigris, F. Tavano, F. Zagliani; R.Rendina,“Diagnostic methods of non ceramic insulators for H.V.lines”, CIGRE general Session 2000 paper22-207….
[4] G. Marrone, E. Garbagnati; C.Valagussa, D. Perin, M.
Ricca, R. Bonzano, “ Investigation on the dielectricstrength of damaged insulator strings of HV overheadlines during repair operations by live working”, CIGRE general session 1994, Paper 33-305.
[5] CIGRE WG 22.03 “Review of in service diagnostictesting of composite insulators”, ELECTRA N° 169, December 1996.
[6] P. Lindner “Inspection for corona and arcing with theDaycor camera”, 2005 World Congress and Exhibition oninsulators, arresters & bushings Hong Kong.
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