2006 articolo cmd2006 isolatori mod def

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;



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



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.



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).



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.

2006 articolo cmd2006 isolatori mod def

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