RELIABILITY OF THE IEC (1995) STANDARD FOR DETERMINING BREAKDOWN VOLTAGE OF ELECTROINSULATING OILS

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1 THE PUBLISHING HOUSE PROCEEDINGS OF THE ROMANIAN ACADEMY, Seres A, OF THE ROMANIAN ACADEMY Volume 8, Number 3/2007, pp RELIABILITY OF THE IEC 6056 (995) STANDARD FOR DETERMINING BREAKDOWN VOLTAGE OF ELECTROINSULATING OILS Mroslav PEŠIĆ*, Zora JEREMIĆ**, Predrag OSMOKROVIĆ*** a, Zora LAZARAVIĆ*** *Electrc power dustry of Serba, E.A. "Jugostok", Zoraa Đđća 46, 8000 Nš, Serba ad Moteegro **Electrc power dustry of Serba, E.A. "Cetar", Ulca Slobode 7, Kragujevac, Serba ad Moteegro ***Faculty of Electrcal Egeerg, Uversty of Belgrade, Bulevar Kralja Aleksadra 73, P.O.Box 3554, 000 Belgrade, Serba & Moteegro ****Faculty of Electrcal Egeerg, Uversty of Belgrade, Bulevar Kralja Aleksadra 73, P.O.Box 3554, 000 Belgrade, Serba & Moteegro, a E-mal: opredrag@verat.et Ths vestgato has aalyzed the recommeded test method for determg the breakdow voltage of electrosulatg ols accordg to the IEC 6056 (995) Stadard. Isuffcet accuracy of the Stadard has bee poted out, whch may lead to a large measuremet ucertaty, wth a allowable relablty. Specal cosderato s gve to the sze of the statstcal sample. By meas of Chauveet's crtero, U-test (Wlcoxo's rak-sum test) ad Studet's dstrbuto, adequacy of the recommeded sample, as well as the method of processg the obtaed results, have bee exhbted. Recommedatos for mprovg the Stadard are gve based o these cosderatos, regardg: measuremet procedure, statstcal sample sze ad statstcal treatmet of measuremet results. Key words: IEC, stadards, sulator testg, ol sulato, statstcs, error aalyss.. INTRODUCTION Breakdow voltage of electrosulatg ol s a dcator of ts capablty to wthstad the electrc feld wthout breakdow. It s a stochastc quatty, determed by stochastc result aalyss of a umber of repeated, mutually depedet (o-correlated) measuremets. Procedure set by the IEC 6056 (995) Stadard [,2] s used for determg the breakdow voltage of electrosulatg ols. Accordg to the IEC 6056 (995) Stadard, the procedure for testg delectrc hardess requres the use of a test voltage obtaed by usg a step-up trasformer suppled from a a.c. (48 Hz to 62 Hz) voltage source. After applyg such a voltage to the electrodes, the Stadard further gves the followg structos: "The frst applcato of voltage s started approxmately 5 m after completo of fllg ad checkg that o ar bubbles are vsble the electrode gap.... Uformly crease voltage from zero at the rate of 2,0 kv s - ± 0,2 kv s - utl breakdow occurs.... Carry out sx breakdows o the same cell fllg allowg a pause of at least 2 m after each breakdow before re-applcato of voltage. Check that o gas bubbles are preset wth the electrode gap...." [] Remarks o the IEC 6056 (995) Stadard are:. Although t s well kow that breakdow voltage of electrosulatg ol depeds substatally o ts humdty (water cotet), absolute humdty of a room where measuremets are performed s ot defed, or cted from aother stadard. 2. The requred umber of breakdows (sx) s suffcet for a thorough statstcal aalyss. The am of ths vestgato s to revse statstcal sample sze adequacy for determg the breakdow voltage of electrosulatg ol, as well as to provde recommedato for mprovg the Stadard regard to stated remarks.

2 Mroslav PEŠIĆ, Zora JEREMIĆ, Predrag OSMOKROVIĆ, Zora LAZARAVIĆ 2 2. STATISTICAL SAMPLE ANALYSIS Expadg the statstcal sample (to more the 6 measured values) would provde a more relable value of breakdow voltage. How bg the statstcal sample should be depeds upo a compromse betwee the umber of tests requred by the laws of mathematcal statstcs for obtag a relable mea value, tme spet performg the tests, ad rreversble chages ol resultg from breakdows, sce ol s ot etrely a self healg sulator (whch s cotrary to the assumpto of o-correlated statstcal sample elemets) [2]. I order to determe the optmal umber of tests, the followg expermet was performed: fresh, uused, meral ol was acqured. Breakdow voltage of ths ol was measured accordg to the IEC 6056 (995) Stadard (wth regard to everythg except the umber of tests), for two days. I order to mmze the fluece of humdty ad varous atmospherc codtos, measuremets were performed stable clmatc codtos durg two successve days. 20 tests were performed per day. Graphs Fg. show the values of crtcal electrc feld E c (correspodg to the values of breakdow voltage) versus the test umber for both days. It s clearly vsble from Fg. that, for both days, frst few measured values of breakdow voltage are cosderably lower the others, ad that breakdow voltage rses wth the test umber. Moreover, secod-day results do ot cotuously follow frst-day results. Therefore, t was't possble to form a sgle statstcal sample based o the tests performed o those two days, ad they were treated as two depedet samples. Values obtaed by tests performed o day oe ad day two were aalyzed as follows:. Statstcal samples wth 6, 8, 0, 2, 5, 20 ad 24 results were formed (separately for each day). 2. Separate mea values of the crtcal electrc feld (E c mea ), ts lmtg values (E c m ad E c max ) ad Type A measuremet ucertates u A (stadard devatos) [3,4] were determed for each sample, applyg Studet's dstrbuto. 3. The procedure was repeated after the use of Chauveet's crtero of spurous result elmato [Appedx]. Fg. Crtcal electrc feld versus the test umber (both days). Fg. 2 Crtcal electrc feld versus sample sze (frst-day tests).

3 3 Relablty of the IEC 6056 (995) stadard for determg breakdow voltage of electrosulatg ols Fg. 3 Crtcal electrc feld versus sample sze (secod-day tests). Fgs. 2 ad 3 show the crtcal electrc feld versus sample sze (umber of tests), for frst- ad secod-day tests, respectvely. Fg. 4 shows Type A measuremet ucertates u A versus sample sze correspodg to Fgs. 2 ad 3. Graph Fg. 5 shows the crtcal electrc feld versus sample sze after the applcato of Chauveet's crtero, for frst-day tests aloe. Type A measuremet ucertates for both days versus sample sze, wth Chauveet's crtero appled, are gve Fg. 6. I Fg. 7 Type A measuremet ucertaty versus sample sze s show, both wth ad wthout the applcato of Chauveet's crtero, for frst-day tests oly. Fg. 8 shows the mea value of crtcal electrc feld E c mea, obtaed from frst-day tests, versus sample sze, wth ad wthout applyg Chauveet's crtero. Based o the results preseted here t ca be deduced that a sample of 6 tests s suffcet for obtag breakdow voltage wth satsfactory Type A measuremet ucertaty. It s also otceable that measuremet ucertaty for samples of 5 tests (performed o each day) s much lower that the oe for samples of 2 tests, whle further expaso of sample sze (20 ad 24 tests) decreases the measuremet ucertaty oly slghtly. Based o ths aalyss t ca be cocluded that mmal sample sze for obtag suffcetly accurate measuremet ucertaty s 5 tests, wth the applcato of Chauveet's crtero. I order to verfy whether durg 5 tests rreversble chages ol occur, these samples are dvded to sub-samples, each wth 5 measuremets. U-test (Wlcoxo's rak-sum test) [5,6] s appled to the obtaed set of sub-samples order to establsh whether they belog to the same basc set. The result was postve both cases, wth ad wthout the applcato of Chauveet's crtero. The same test was egatve whe appled to 20 ad 24-test samples. I vew of these U-test outcomes, t s cocluded that rreversble ol chages do ot occur durg 5 tests, ad that all tests wth the sample belog to the same radom varable. Fg. 4 Type A measuremet ucertaty versus sample sze (both days). Fg. 5 Crtcal electrc feld versus test umber after the applcato of Chauveet's crtero (frst-day tests).

4 Mroslav PEŠIĆ, Zora JEREMIĆ, Predrag OSMOKROVIĆ, Zora LAZARAVIĆ 4 Fg. 6 Type A measuremet ucertaty versus sample sze wth Chauveet's crtero appled (both days). Fg. 7 Type A measuremet ucertaty versus sample sze, wth ad wthout Chauveet's crtero appled (frst-day tests). Fg. 8 Mea value of Crtcal electrc feld versus sample sze, wth ad wthout Chauveet's crtero appled (frst-day tests). 3. CONCLUSIONS It ca be ferred that the ICE 6056 (995) Stadard could be mproved toward reducto of combed measuremet ucertaty by:. Defg temperature ad humdty codtos of the room where tests are performed. 2. Codtog the ol sample wth 0 to 5 breakdows before testg. 3. Expadg statstcal sample to 5 tests. 4. Applyg Chauveet's crtero to obtaed results, ad oly the determg breakdow voltage as the mea value. Modfcato would reduce Type B measuremet ucertaty, ad modfcatos 2, 3, ad 4 Type A measuremet ucertaty.

5 5 Relablty of the IEC 6056 (995) stadard for determg breakdow voltage of electrosulatg ols APPENDIX Chauveet's crtero. It s checked whether the populato or sample dstrbuto s ormal, or t s asserted to be so. 2. Group parameter s determed as t t q = F ( ) = F - e dt = = t 2 π For a set of results x, x 2,..., x mea value ad stadard devato are determed as x = = 4. Chauveet's parameters q are determed as x ad = = ( x - x ) - 2 q x - x =,..., x - x q =,..., q 5. The set of Chauveet's parameters s sorted ad compared to the group parameter. If the largest of the parameters q s greater tha q, the correspodg result x s dscarded from the populato or sample as beg spurous, ad the procedure s repeated from step 2. = x - x REFERENCES. IEC 56 Iteratoal Stadard, Isulatg lquds - Determato of the breakdow voltage at power frequecy - Test method, Secod edto, Norther Techology & Testg (NTT) Techcal Bullets, Delectrc Breakdow Voltage, , NTT. Gude to the Expresso of Ucertaty Measuremet, Iteratoal Orgazato for Stadardzato, ROWLEY A., Evaluatg Ucertaty for Laboratores, Alle Rowley Assocates, West Ayto, HUSCHILD W., MOSCH W., Statstcal Techques for Hgh-Voltage Egeerg, IEEE Power Seres 3, P.OSMOKROVIĆ, B. LONČAR, S. STANKOVIĆ, The New Method of Determg Characterstcs of Elemets for Overvoltage Protecto of Low-Voltage System, IEEE Tras. o Istrumetato ad Measuremet, Vol. 55, No., 2006, pp Receved August 24, 2007