Frequency Characteristics of Leakage Current Waveforms of a String of Suspension Insulators

7/27/2019 Frequency Characteristics of Leakage Current Waveforms of a String of Suspension Insulators

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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 1, JANUARY 2005 481

Frequency Characteristics of Leakage CurrentWaveforms of a String of Suspension Insulators

Tomotaka Suda, Senior Member, IEEE

AbstractIn orderto establisha method for monitoringwhetherflashover occurs or not in a string of insulators based on leakagecurrent waveforms and theirfrequency characteristics, the leakagecurrent waveforms and the frequency characteristics of a stringof 120-kN suspension insulators were investigated with artificialcontamination tests and field tests. As a result, it was found thatleakage current waveforms become nearly the symmetrical wavewhen strong local arcs occur; hence, the intensity of the odd orderof harmonic components, e.g., 50, 150, and 250 Hz, is high. Fur-thermore, it was clarified that the transition of the leakage currentwaveforms until flashover occurs is classified into six stages andthat a threshold exists in the magnitude of peak leakage current

and prominent odd-order harmonic components by which the oc-currence of flashover can be predicted.

Index TermsFlashover, harmonic analysis, insulator contami-nation, leakage currents, signal analysis.

I. INTRODUCTION

THE SEVERITY of insulators such as flashover is mainly

due to salt contamination. The development of a reliable

system for monitoring salt contamination on insulators is

strongly desired in order to take precautions against possible

accidents due to salt contamination. The most widely used

methods for contamination monitoring are the equivalent salt

deposit density (ESDD), the surface conductance, the leakagecurrent, air pollution measurements, optical measurements, and

the nonsoluble deposit density [1].

The leakage current, which is driven by the source voltage and

collected at the grounded end of the insulator, provides much

useful information out of many parameters describing the state

of a contaminated insulator. The leakage current surge counting,

the highest leakage peak current recording, and charge measure-

ments are three main methods for contamination monitoring [2],

[3]. In addition, the characteristics of leakage current waveforms

were investigated by applying Fourier series to clarify their in-

fluence on the test voltage in circuits [4] and several dynamic

models have been proposed to investigate ac source-insulatorinteraction in contamination tests [5] and the feasibility of using

the dynamic arc modeling in order to characterize approaching

flashover [6]. Moreover, the spectral analysis of the leakage cur-

rent waveforms was conducted for polymer insulators [7], [8].

Performing a frequency analysis of leakage current wave-

forms with a spectrum analyzer in the wet contaminant flashover

Manuscript received November 19, 2003. Paper no. TPWRD-00345-2002.The author is with the Electric Power Engineering Research Laboratory, Cen-

tral Research Institute of Electric Power Industry, Kanagawa-ken 240-0196,Japan (e-mail: [email protected]).

Digital Object Identifier 10.1109/TPWRD.2004.837668

tests (equivalent fog method in [9]), the author found that the

third-order harmonic component, 150 Hz, increased when in-

creasing the applied voltage because of the wave distortion due

to local arcs [10]. It is expected from these results that a novel

monitoring system for the severity mainly due to salt contami-

nation could be developed from further investigations about fre-

quency components of leakage current waveforms as well as the

magnitude of leakage currents. Thus, a fundamental research

was conducted with regard to the characteristics of leakage cur-

rent waveforms and their frequency characteristics using single

contaminated 75-kN suspension insulator by the wet contami-nant method (equivalent fog method in [9]) and the clean fog

method (fog withstand method in [9] or solid layer method in

[11]) [12]. As a result, it was found that leakage current wave-

forms become similar to the symmetrical wave when strong

local arcs occur, hence, the intensity of the odd order of har-

monic components, e.g., 50, 150, and 250 Hz, is high. Further-

more, it was clarified that the transition of the leakage current

waveforms, until flashover occurs, is classified into six stages

and that a threshold exists by which the occurrence of flashover

can be predicted.

In this paper, the author presents the relationship between

frequency characteristics of leakage current waveforms and

flashover occurrence at a string of insulators (5 units of 120 kN

suspension insulators) on increasing the number of insulators.

II. ARTIFICIAL CONTAMINATION TEST

A. Experimental Setup

The experimental setup is shown in Fig. 1. The experiment

was conducted at a fog chamber (volume 7 7 7 m) in Yoko-

suka Research Laboratory of CRIEPI. A string of five units of

120-kN suspension insulators (diameter 254 ) were used to

clarify the fundamental characteristics of leakage current wave-

forms of contaminated insulators. The number of units of a

string and the applied voltage were 5 and 40 , respec-tively, at the 66-kV transmission line. The specification of the

ac source (capacity 50 kVA, impedance voltage 2.32%) is suffi-

cient to perform the artificial contamination tests [11]. The ex-

periment of two strings was performed simultaneously to im-

prove experimental efficiency. The Figure 1 or 2 preceded by

the underbar, for example test number 8_2, corresponds to each

string.

Leakage current waveforms were detected with a current

transformer (CT411 of Pearson Electronics, Inc.), amplified by

a dc amplifier, and recorded on a data recorder. Frequency spec-

trum analysis was performed with a real-time signal analyzer

having a Hanning window.

0885-8977/$20.00 2005 IEEE


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482 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 1, JANUARY 2005

Fig. 1. Experimental setup.

Insulators were contaminated by a spray containing contam-

inated suspension or by soaking in contaminated suspension

for ten seconds. Three kinds of contaminated suspension with

varying quantities of salt of 10 g/l, 50 g/l, and 200 g/l for the

constant quantity of tonoko of 40 g/l, were prepared in order to

simulate light, intermediate, and heavy degrees of contamina-

tion. Contaminated insulators were dried in a drying room and

moved to the fog chamber. Table I shows salt deposit densities of

contaminated insulators measured before the experiment. Test

numbers 1, 2, 5, and 6, test numbers 3 and 4, and test numbers

710 correspond to the light, intermediate, and heavy contami-

nation, respectively. Thus, a total of seven tests was conducted,

two of them for light contamination, one, for intermediate con-

tamination, and four, for heavy contamination, among which

flashover occurred in all the heavy contamination tests but did

not occur in any of the light or intermediate contamination tests.

The clean fog method was adopted as an artificial contami-

nation method [9], [11]. Applied voltage and artificial fog were

maintained until flashover occurred or for 60 min in the case of

no flashover after a constant 40 ac voltage was applied

and the artificial fog was generated.

B. Characteristics of Leakage Current Waveforms in the

Process of Flashover

Characteristics of leakage current waveforms in the process

of flashover at test number 10 of heavy contamination are de-

scribed. Flashover occurs through six stages in Fig. 2 as men-

tioned below.

Stage 1) Leakage current waveforms become sinusoidal at

the beginning of voltage application because of re-

sistive current flow. Thus, the 50-Hz fundamental

component becomes prominent.

Stage 2) The waveforms become triangular or sawtoothlike

when faint discharges occur. The odd-order har-

monic components become prominent because theyare symmetrical waves.

TABLE IEQUIVALENT SALT DEPOSIT DENSITIES

Stage 3) The tips of the triangular waveforms observed

during stage 2 become sharper and longer the mo-

ment the triangular waveforms become narrow inthe middle.

Stage 4) The tips become sharper.

Stage 5) The tips lengthen and intermittent waveforms

having large peak values add to the waveforms at

stage 4.

Stage 6) Groups of pulses having larger peak values occur

intermittently.

Stage 7) Flashover occurs.

The odd-order harmonic components from 50 to about 350

Hz in the frequency spectra of leakage current waveforms in-

crease in intensity from stages 2 to 6 because the peak value of

leakage current gradually becomes large in the meantime.

The transition of leakage current waveforms in the case of no

flashover is similar to that in the case of flashover up to stage

5 as described above. However, after stage 5, the intermittent

waveforms whose peak values are not larger than those in the

case offlashover do not appear frequently and their peak values

become smaller with time.

C. Relation Between Prominent Frequency Components and

Flashover Occurrence

1) Temporal Variations of Peak Leakage Currents: Fig. 3

shows the temporal variation of peak leakage current. Here,black symbols represent the cases of no flashover in both light

(four cases) and intermediate (two cases) contamination while

white symbols represent the cases offlashover in the heavy con-

tamination (four cases). In the case of no flashover (light and

intermediate contamination), the peak values of leakage cur-

rents gradually increase with time from the beginning of voltage

application, become maximum during about 20 min, and then

gradually decrease. The maximum peak value of leakage cur-

rent is at test number 3 of the intermediate contami-

nation. All other peak values are smaller than this. On the other

hand, in the case of flashover (heavy contamination), the peak

values of leakage currents gradually increase with time from the

beginning of voltage application. They increase slowly at first,then rapidly 46 min from the beginning, and finally become


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SUDA: FREQUENCY CHARACTERISTI CS OF LEAKAGE CURRENT WAVEFORMS OF A STRING OF SUSPENSION I NSULATORS 483

Fig. 2. Transition of leakage current waveform (artificial contamination test, heavy contamination).

Fig. 3. Temporal variation of peak leakage current.

maximum of about at test numbers 79 and at test

number 10 when flashover occurs.

2) Temporal Variations of the Magnitude of the ProminentHarmonic Components: As mentioned before, leakage current

waveforms become similar to the symmetrical wave when local

arcs occur; hence, the intensity of the odd order of harmonic

components, e.g., 50, 150, and 250 Hz etc., is high. Figure 4

shows temporal variations of the magnitude of the 150-Hz com-

ponent as a typical example. When flashover occurs, the magni-

tude of the 150-Hz component gradually increases from the be-

ginning to flashover occurrence while it tends to increase at thebeginning, become a maximum within 20 min, and then grad-

ually decrease when flashover does not occur as also shown in

the harmonic contents. This tendency can be seen for three other

kinds of prominent components. Furthermore, it is important

to point out from this figure that it is possible to distinguish

clearly the cases of flashover from those of no flashover from

the magnitudes of prominent frequency components. That is,

the magnitude of 150-Hz component exceeds in all

cases when flashover occurs while it is smaller than this level in

all cases when flashover does not occur. The similar levels can

be pointed out; e.g., for 50 Hz, for 250 Hz,

and for the 350-Hz component, respectively.

3) Temporal Variations of the Harmonic Contents: Next,temporal variations of the harmonic contents of the third, fifth,


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Fig. 4. Temporal variation of 150-Hz component.

Fig. 5. Temporal variation of 150-Hz harmonic content.

and, seventh order of harmonic components of leakage current

waveforms were examined. The harmonic content is defined as

the ratio of the magnitude of prominent harmonic components

to that of the 50-Hz fundamental component (%) [12]. Figure 5

shows the temporal variations of the 150-Hz harmonic content

at eachtest. It is noted from this figure that the 150-Hz harmonic

content tends to have maximum values within about 10 min

from the start of the test and decrease gradually after that in thecase of no flashover. On the other hand, when flashover occurs,

the 150-Hz harmonic content gradually increases from the

beginning of the test to flashover occurrence. This indicates that

the possibility of flashover occurrence becomes higher when

the 150-Hz harmonic content increases. However, flashover

does not always occur when the 150-Hz harmonic content

increases. Therefore, it is clarified that it is difficult to deter-

mine the threshold by which the occurrence offlashover can be

predicted as seen in the case of single insulator [12] because

the 150-Hz harmonic contents in the case of no flashover also

exceed almost the same level (50%60%) as seen in the case

of flashover. Moreover, whether flashover occurs or not cannot

be determined from the 250-Hz and 350-Hz harmonic contentsbecause they are nearly at the same level in the case of both

Fig. 6. Experimental setup.

flashover and no flashover, although the data are not shown in

this paper.

4) Correlation With the Results of the Single Suspension

Insulator [12]: We have already published the results of the

single suspension insulator [12].

In this paper, we concluded the following.

1) Leakage current waveforms become similar to the sym-

metrical wave in the presence of strong local arcs on the

surface of an insulator that is heavily contaminated and

wetted sufficiently. Hence, the intensity of the odd order of

harmonic components, e.g., 50, 150, and 250 Hz is high.

2) In the spectral area , distinguishing features

are shown only at frequencies in the presence of

local arcs.

3) The transition of the leakage current waveforms until

flashover is classified into six stages.

4) It is pointed out that a threshold exists; i.e., the possibility

of flashover occurrence becomes higher when the mag-nitudes and harmonic contents of the prominent compo-

nents exceed a particular level.

We compare these results with the above results of the string

of the suspension insulators. The results of the string of the sus-

pension insulators are similar to the results of the items 1, 2,

and 3. However, for item 4, a threshold does not exist in the har-

monic contents of the prominent components of the string of the

suspension insulators. This reason is not clear at the present and

future work is needed in this study.

III. FIELD TEST

A. Experimental Setup

We began to conduct field tests in December 1998 and could

collect the field data on February 11, 1999 and March 7, 1999.

Figure 6 shows the experimental setup at the Yokosuka field

exposure site of CRIEPI. A string of five units of 120-kN sus-

pension insulators was used and applied voltage was 77

in order to obtain data near the flashover. Leakage current wave-

forms were measured with a monitoring system of leakage cur-

rent waveforms for insulators [13] (Fig. 7). A clip-on-type CT

was used as a sensor for leakage currents and leakage current

waveforms were measured via optical signal from analog signal.

The measurement range of current is andthe frequency response is 50 Hz1 kHz in this system.


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Fig. 7. Transition of leakage current waveforms on February 11, 1999.

B. Insulator Contamination Condition and Meteorological

Condition

The ESDD of the bottom surface of the insulator ranged from

0.1 to 0.2 on February 11, 1999, and was estimated

as heavy contamination. The ESDD of the bottom surface of

the insulator was estimated to be less than 0.1 , that is,

intermediate contamination, on March 7, 1999.

On February 11, 1999, it began to rain at about 10:25 and

continued to rain until about 19:00. The temperature was about

3 and relative humidity was over 90%. Wind direction wasnortheast and wind speed was about 4 m/s. Rain was heavy at

12:00 and from 14:00 to 16:00.

On March 7, 1999, it began to rain at about 12:50 and con-

tinued to rain until 3:00 the next day. The temperature was about

7 and relativehumidity was over 90%. A windof about 5 m/s

and in the northeast direction blew, and it rained constantly from

14:00 to 18:00.

C. Characteristics of the Leakage Current Waveforms

Characteristics of the leakage current waveforms on February

11 when flashover was imminent are described. Leakage currentwaveforms varied through six stages, as follows.

Stage 1) Sinusoidal and ohmic leakage currents flow because

no discharge occurs due to the slightly wet sur-

face condition at the beginning of rainfall; hence,

the 50-Hz fundamental component becomes promi-

nent.

Stage 2) Faint discharges occur and leakage current wave-

forms become sawtoothlike.

Stage 3) The tips of the sawtoothlike waveforms observed at

stage 2 become longer intermittently.

Stage 4) The intermittent extension which began at stage 3

becomes larger.Stage 5) The pulse groups having larger peak values occur

intermittently.

Stage 6) The peak values of intermittent pulse groups be-

come higher when flashover is imminent.

The odd-order harmonic components up to 250 Hz become

prominent because the waveforms are symmetrical waves stages

2 through 6.

After stage 6, the peak values of the leakage current become

smaller and smaller because the discharges decrease gradually

and salt accumulated on the surface of the insulators is washed

out due to the continuous rain.

The transition of the leakage current waveforms on March 7

when no flashover occurred is similar to that mentioned above.However, it is different in that stage 2 and stage 3 occur simul-


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Fig. 8. Temporal variation of peak leakage current.

taneously and that the pulse groups having the large peak values

at stage 6 do not appear.

D. Characteristics of the Prominent Frequency Components of

Leakage Current Waveforms

1) Temporal Variations of Peak Leakage Currents: Figure

8 shows the temporal variations of peak leakage currents on

both February 11, 1999 and March 7, 1999. This figure shows

the temporal variations from 10:28 February 11 and those from

13:00 March 7, almost the same time as when it began to rain. It

is clear from this figure that peak leakage currents are high from

1 h 20 min to 2 h and 40 min after the beginning of the rain onFebruary 11. The maximum peak leakage current was .

The value of about is maintained for 3 h and 10 min

from 1 h and 20 min to 4 h and 20 min after the beginning of

the rain on March 7. The maximum peak leakage current is low,

. Thus, it is pointed out that the magnitude of the peak

leakage current differs greatly between the case where flashover

is imminent and the case where flashover does not occur. It can

be available as one method of contamination management for

insulators to fix a threshold level of the peak leakage current

beyond which the possibility of the occurrence of flashover be-

comes higher. This result corresponds to those of the artificial

contamination tests.2) Temporal Variations of the Magnitude of the Prominent

Harmonic Components: Figure 9 shows the temporal varia-

tions of the magnitude of the 150-Hz prominent harmonic com-

ponent on both February 11, 1999 and March 7, 1999. They

agree well with the temporal variations of the magnitude of the

peak leakage current shown in Fig. 8. It is clear from this figure

that it is possible to fix a threshold level beyond which the pos-

sibility of the occurrence of flashover becomes higher; that is,

for 50 Hz (not shown here), for 150 Hz,

for 250 Hz (not shown here), and for 350

Hz (not shown here). (The value of for 150 Hz equals

that obtained in the artificial contamination tests and shown in

Fig. 4.) These results correspond to those of the artificial con-tamination tests.

Fig. 9. Temporal variation of 150-Hz harmonic component.

Fig. 10. Temporal variation of 150-Hz harmonic content.

3) Temporal Variations of the Harmonic Con-

tents: Figure 10 shows the temporal variations of the

harmonic contents on both February 11, 1999 and March 7,

1999. It is clear from this figure that one cannot distinguish the

data for February 11 from those for March 7 with respect to

the harmonic contents because they are almost the same. This

result corresponds to those of the artificial contamination tests.

IV. CONCLUSIONS

Leakage current waveforms and their frequency characteris-

tics for a string of 120-kN suspension insulators were investi-

gated by means of artificial contamination tests and field expo-

sure tests in order to develop a monitoring and diagnosis system

for contaminated insulators.

The main results are as follows.

1) Leakage current waveforms become nearly the symmet-

rical wave when local arcs occur; hence, the intensity ofthe odd order of harmonic components is high.


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2) The thresholds by which the occurrence offlashover can

be predicted exist in the peak leakage currents and the

magnitude of the odd order of harmonic components.

Thus, these parameters show promise for use in moni-

toring systems.

3) However, the threshold by which the occurrence of

flashover can be predicted does not exist in the magnitudeof the prominent harmonic contents while it exists in the

single suspension insulator.

ACKNOWLEDGMENT

The author is grateful to Dr. T. Shindo, K. Takasu, Dr. K.

Izumi, and T. Takahashi of CRIEPI for providing important

suggestions and support.

REFERENCES

[1] Insulation Pollution Monitoring, CIGRE TF 33.04.03, ELECTRA no.152, Paris, France, 1994.

[2] A. G. Kanashiro and G. F. Burani, Leakage current monitoring of in-sulators exposed to marine and industrial pollution, in Conf. Rec. 1996

IEEE Int. Symp. Electrical Insulation, Montreal, QC, Canada, 1996, pp.271274.

[3] I. Ramirez-Vazquez and J. L. Fierro-Chavez, Criteria for the diagnosticof polluted ceramic insulators based on the leakage current monitoringtechnique, in 1999 Annu. Report Conf. Electrical Insulation and Di-electric Phenomena, Austin, TX, pp. 715718.

[4] M. P. Verma and W. Petrusch, Results of pollution tests on insulatorsin the 1 1 0 0 k V range and necessity of testing in the future, IEEETrans. Elect. Insulation, vol. EI-16, no. 3, pp. 199208, 1981.

[5] F. A. M. Rizk and D. H. Nguyen, AC source-insulator interaction inHV pollution tests, IEEE Trans. Power App. Syst., vol. PAS-103, no. 4,pp. 723732, 1984.

[6] G. G. Karady, F. Amarh, and R. Sundararajan, Dynamic modelingof AC insulator flashover characteristics, in Proc. 11th Int. Symp.

High-Voltage Engineering, London, U.K., 1999, Paper 467, pp.4.107.S254.110.S25.

[7] M. A. R. Fernando and S. M. Gubanski, Leakage current patterns oncontaminated polymeric surfaces, IEEE Trans. Elect. Insulation, vol. 6,no. 5, pp. 688694, Oct. 1999.

[8] A. H. El-Hag, S.Jayaram, and E. A. Cherney, Low frequency harmonic

components of leakage current as a diagnostic tool to study aging of sil-icone rubber insulators, in 2001 Annu. Report Conf. Electrical Insula-tion Dielectric Phenomena, pp. 597600.

[9] K. Naito, S. Kunieda, and Y. Hasegawa, DC contamination perfor-mance of station insulators, IEEE Trans. Elect. Insulation, vol. 23, no.6, pp. 3745, 1988.

[10] T. Suda, Frequency analysis for leakage current wave forms of pollutedinsulators, in Proc. 9th Int. Symp. HighVoltage Engineering, Graz, Aus-tria, 1995, pp. 3212-13.

[11] Artificial Pollution Test on High Voltage Insulators to be Used an A.C.Systems, IEC 507, 1991.

[12] T. Suda, Frequency characteristics of leakage current waveforms of anartificially polluted suspension insulator, IEEE Trans. Dielect. Elect.

Insul., vol. 8, no. 4, pp. 705709, Aug. 2001.[13] T. Suda and Y. Imano, View and Research Subjects on Monitoring Sys-

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Tomotaka Suda (M89SM92) was born inNagano Prefecture, Japan, on October 30, 1950. Hereceived the B.S. and M.S. degrees in electrical en-gineering from Tohoku University, Sendai, Japan in1973 and 1976, respectively, and the D.Eng. degree

in electrical engineering from Kyushu University,Fukuoka, Japan, in 1994.

In 1976, he joined the Central Research Institute,

Electric Power Industry, Tokyo, Japan, where hiswork focuses on the study of ion-flow electrificationphenomena on HVdc transmission lines, insulator

contamination, and lightning phenomena.Dr. Suda is a member of the Institute of Electrical Engineers of Japan and the

Society of Atmospheric Electricity of Japan.

Frequency Characteristics of Leakage Current Waveforms of a String of Suspension Insulators

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