frequency characteristics of leakage current waveforms

8/6/2019 Frequency Characteristics of Leakage Current Waveforms

1/5

IEEE Transactions on Dielectrics and Electrical Insulation Vol.8 No . 4, August 2001 705

Frequency Characteristics of LeakageCurrent Waveforms of an Artificially PollutedSuspension InsulatorT. Suda

Electrical Insulation DepartmentCentral Research Institute of Electric Power IndustryTokyo, Japan

ABSTRACTIn order to establish a method for monitoring contamination in insulators based on eakagecurrent waveforms and their frequency characteristics, the lea kage current waveforms and fre-quen cy characteristics of an artificially pollu ted 180 mm diameter cap and pin type insulatorwere investigated by th e wet contamina nt and the clean fog methods at fixed applied voltages.As a result, it was found th at leakage current waveforms become similar to the symmetricalwave when strong local arcs occur; hence, the in tensity of th e odd ord er of harmo nic compo-nents, e.g. 50, 150, and 250 Hz, s high. Furthermore, it was clarified that,th e transition ofthe leaka ge current waveforms, un til flashover occurs, is classified into six stages and that athreshold exists by which t he occurrence of flashover can be predicted.

1 INTRODUCTIONHE development of a reliable system for monitoring salt contami-T ation on insulato rs is strongly desired in order to take precautionsagainst possible accidents due to salt contamination. The most widelyused meth ods for pollution monitoring a re the equivalent salt deposit

density (ESDD), the surface conductance, the leakage current, air pollu-tion measurements, optical measurements, and the non-soluble depositdensity [l].The leakage current, which is driven by the source voltage and col-

lected at the ground ed end of the insulator, prov ides much useful infor-mation o ut of many para mete rs describing the state of a contaminatedinsulator. The leakage current surge counting, the highest leakage peakcurrent recording, an d charge measurements are three main m ethodsfor pollution monitoring [2,3]. In addi tion , the characteristics of leak-age current waveforms were investigated by applying Fourier seriesto clarify their influence on the test voltage in circuits [4 ] and severaldynamic models have been proposed to investigate ac source-insulatorinteraction during the critical arcing period of polluted insulators [ 5 ]and the feasibility of using th e dynamic arc modeling of polluted insu-lators, in order to characterize approaching flashover [6]. On the otherhand , the spectral analysis of the leakage current waveforms on con-taminated insulators under fog conditions was conducted by the autoregressive method [7]. The analysis of leakage current waveforms alsowas carried out for field-aged and polymer in sulators [8,9].

Performing a frequency analysis of leakage current waveforms witha spectrum analyzer in the we t contaminant flashover tests (equivalentfog method in [lo]), the author found that the third order harmonic

component, 150 Hz, increased when increasing the applied voltage,because of the wave distortion du e to local arcs [U] . It is expectedfrom these results that a novel monitoring system for salt contamina-tion could be developed from further investigations ab out frequencycomponents of leakage current waveforms as well as the magnitu de ofleakage currents.In this work, we investigated the characteristics of leakage currentwaveforms and their frequency characteristics of an artificially polluted180 mm diameter cap and pin type insulator from the viewpoint ofpollution severity monitoring, based on the frequency characteristicsof leakage current waveforms. Since power transmission voltages areconstant, we conducted experiments to examine the characteristics ofleakage current waveforms when constant voltages were applie d to anartificially polluted 180mm diameter cap and pin type insulator, usingthe wet contaminant method (equivalent fog method in [lo]) and theclean fog method (fog withstand method in [lo]or solid layer method in[12]). We describe here the characteristics of leakage curren t waveformsand their frequency characteristics to 400 kHz when strong local arcsoccur, as well as the distinct transition of the leakage curren t waveformsuntil flashover occurs. From the results, we show th e possibility thata threshold exists, because the probability of flashover increases whenthe magnitudes of leakage currents, prominent harmonic components,and their harmonic contents exceed specific levels.

2 EXPERIMENTThe experimental setup is shown in Figure 1. A cap and pin typeinsulator of 180 mm diameter, which is also shown, was used to clarify

1070-9878/1/ $3.000 001 IEEE

uthorized licensed use limited to: ULAKBIM UASL - MIDDLE EAST TECHNICAL UNIVERSITY. Downloaded on December 1, 2008 at 15:34 from IEEE Xplore. Restrictions apply.


2/5

706 Suda: Frequency Characteristicsof Leakage Current Waveforms

the funda mental characteristics of leakage curre nt waveforms of pol-luted insulators. Leakage current waveforms were detected with a cur-rent transformer (CT411, Pearson Electronics, Inc.; frequency range offlat response 1 Hz to 20 MHz), amplified by a dc am plifier (frequencyrange 0 to 5 kHz), and recorded by a da ta recorder (frequency range 0to 2.5 kHz). Frequency spectrum analysis was performed with a real-time signal analyzer having a Hanning window. When we examinedfrequency characteristics of leakage current waveforms < 400 kHz, andthe trans ition of leakage curre nt waveforms until flashover, we con-nected the output of the cu rrent transform er directly to the real-timesignal analyzer.

respectively. Figures 2(a), (b), and (c) correspond to the case of no stress,the case when local arcs occur, and the case with no local arc at the samevolta ge as that of case (b),respectively.

First, we examined frequency spectra < 3.125 kHz. In th e case of nostress (Figure 2(a) left), the frequency spectrum ha s a flat distributionwith no promin ent harmonic compone nt in general. In the absence oflocal arcs as shown in Figure 2(c) middle, the 50 Hz component hasthe highest inte nsity because of ohmic leakage current flow. The spec-trum at frequencies >550 Hz has a nearly flat distribution although theintensity of the o dd o rder of harmonic comp onents 6 550 Hz is highbecause of wave distortion. On the other hand, in the presence of lo-cal arcs as in Figure 2(b) middle, the o dd-order harmonic comp onents< 350 Hz become prominen t because leakage current waveform s aresimilar to symmetrical waves. However, the distribution is flat at fre-quencies2 1kHz.

In a symmetrical wave, we can write1 8 h $ cap & pi n type Insulator_________---- - f ( t+ T )= - f ( t ) (1)where f ( t ) s a funct ion of tim e t , epresenting the sym metrical wave,

Shield r m

Figure1. Experimental setup.The wet contamin ant method and clean fog method were adopted as

artificial contamination metho ds [lo , 121. The wet contaminant meth odis defined here as the method wherein a constant voltage is appliedwithin 2 to 3 min, immediately after an insulator is polluted with aspray containing contaminated suspension, and the voltage is main-tained u ntil a flashover occurs, or local arcs disappear. In the clean fogmethod, an insulator is left to dry naturally after it is polluted by spray-ing and by suspending in a vinyl-sheet-covered space of 1 5 0 ~ 5 0 ~ 1 7 0cm3 size. A constant ac voltage is applied immediately after the in-sulator is exposed to the artificial fog produced by a su personic-wavehumidifier and m aintained until either a flashover occurs, or for sixtyminutes. The contamin ated suspension was made from NaCl60 g/1 andTonoko (Japanese kaolin) 40 g/1(15 g/1 in the wet contam inant method)to simulate a severe degree of contam ination. The ESDD was measuredafter the experiments.

3 FREQUENCYCHARACTERISTICS OFLEAKAGE CURRENTWAVEFORMSIt is necessary to evaluate the frequency characteristics of back-

ground noise in order to clarify the frequency characteristic s of leakagecurrent waveforms < 400 kHz. These were obtained by comp aring thefollowing three frequency spectra: frequency spectrum of leakage cur-rent waveforms without an applied voltage (background noise), thatwithout local arcs at the same applied voltages as one with local arcs,and that w ith local arcs in the wet contamin ant method.

Figure 2showsleakage current waveforms, their frequency spectra6 3.125 kHz, and those


3/5

IEEE Transactions on Dielectrics and Electrical Insulation vol. 8 No. 4, August 2001 707

0.13 -20

50.0 3.125. I 8 0

-20

5-1800.0 kHz 3.125-20

5w 3.125-18000

54m.o-180OD

-20

5

-20

50.0 w 4W.0.I80

Figure 2. Comparison between leakage current waveforms and their frequency spectra in the case of no stress (a) , n the presence of local arcs (b), andin the absence of local arcs (c): (left) eakage current waveforms, (middle) frequency spectra < 3.125 khz, and (right) requency spectra < 400 kHz.

( just before flashover )

Figure3. Transition of leakage current waveforms until flashover occurs in the clean fog method.Stage 6: The peak value of leakage current waveforms becomes largeimmediately before flashover in the presence of intermittent, stronglocal arcs on both surfaces (arc elongation).Stage 7 Flashover occurs.The odd-order harmonic components from 50 to N 350 Hz in thefrequency spectra of leakage current waveforms increase in intensity

from stages 2 to 6 because the peak value of leakage current graduallybecomes large in time.

4.2 RELATIONSHIP BETWEENPROMINENT FREQUENCYCOMPONENTS ANDFLASHOVERIn this Section is presented the experimental result of the relation-ship between frequency spectra of leakage current and flashover ob-

tained after repeated artificial pollution experiments using the clean fogmethod. We conducted ten experiments as shown i n Table 1. Flashover

uthorized licensed use limited to: ULAKBIM UASL - MIDDLE EAST TECHNICAL UNIVERSITY. Downloaded on December 1, 2008 at 15:34 from IEEE Xplore. Restrictions apply.


4/5

708 Suda: Frequency Characteristics of Leakage Current Waveforms

V,kV10

Table 1. Applied voltage and ESDD.(FO/NFO) is also shown in the column. With/without flashover

FOINFOSDD ESDDupper bottommg/cm2 mg/cm2NF O 0.07 0.26

~

78910-

occurred five times.We measured and calculated the peak values of leakage currents,

the magnitudes of prominent harmonic components such as 50, 150,250 Hz, nd 350 Hz, nd the harm onic contents (ra tio of the magni-tude of prominent harmo nic components to that of the 50 Hz primarycomponent, O h ) of 150,250, and 350Hz.

The magnitudes of prom inent harmonic compon ents and the har-monic contents were defined a s follows: The leakage current I, of har-monic component i isI, = 1 0 K X ( 2 )

where K is the inverse of the dc amplifier magnification and V , s theoutput voltage of the curre nt transformer CT411 which ha s a relation of0.1 V/lA. The magnitude (dB) of harmon ic component i is

A , = 2 0 1 0 g I % = 2 0 ( 1 + 1 0 g K + l o g V , )where 20 log V is the reading of the real-time signal analyzer and thereference level is 1A .The ratio of the magnitude of component i to that of the main 50 Hzprimary component (Yo) is

(3)i = 50, 15 0, 250, 350

'4%: I t = 1020 (4)

AI-A5 0= 10 20,-I50 (5)i = 150, 250, 350

3w

250

12 1w -m I

Figure 4. Temporal variation of peak leakage current.

Figure 4shows the temporal variation of peak leakage current. (Thetest numbers appearing on Figures 4to 6 are the same as the numbersin Table 1.)The peak values of leakage currents gra dually become largewith time from the beginning of voltage application when flashoveroccurs; for example, they reach 0.25 A immediately before flashover intest 8. On the other han d, when flashover does not occur, most of themare small values,


5/5

IEEE Transactionson Dielectrics and Electrical Insulation Vol. 8 No. 4, August 2001 709over occurs and reaches a maximum immediately before flashover. Onthe other hand, w hen flashover does not occur, the magn itude of the50 Hz comp onent is small, 222,1906.1151 M. Toepler, h e r die physikalischen Grundgesetze de r in der Isolatorentechnikauftretencen elektrischen Gleiterscheinungen, Archiv fur Electrotechnik, Vol. 10,

pp. 157-185,1921,

199-208,1981.

*

957-963,1968.

Manuscript was received on 21June 1999, in finalform 23 April 2001.

frequency characteristics of leakage current waveforms

Documents