N. Pamuk Statistička analiza učinka električne i mehaničke probojne čvrstoće na karakteristike izolacije kod transformatora snage visokog napona ISSN 1330-3651(Print), ISSN 1848-6339 (Online) UDC/UDK 621.315.61:621.316.926:621.314.222]:519.23 STATISTICAL ANALYSIS OF ELECTRICAL AND MECHANICAL BREAKDOWN STRESS FOR INSULATION PERFORMANCE IN HIGH VOLTAGE POWER TRANSFORMER Nihat Pamuk Original scientific paper In electrical power system, variety of solid, liquid and gaseous materials are used for insulation target to protect the incipient failure inside the high voltage power transformers. The insulation is practically ended when the insulation system has become fragile enough to flourish cracks under the electrical and mechanical stresses to which it is subjected. The electrical and mechanical stresses are induced by short circuit currents, thermal expansion and contraction of the conductors and vibration. The dielectric strength of insulation is not vitally diminished by brittleness alone, but electrical breakdown will rapidly follow the improvement of ensuing cracks. The purpose of this paper is to briefly cover the methods favourable for determining the insulating value of the insulation on high voltage power transformer windings and bushings. Many failures of insulation are caused by the entrance of moisture. Insulating materials used on high voltage power transformer windings have a high affinity for moisture from the surrounding atmosphere or oil. Effects of the stressed oil volume and stressing time on the breakdown stress of high voltage power transformer insulation were experimentally investigated. The factors influencing the effects and the results obtained based on them were statistically analysed and systematized by the Weibull distribution method. A Four-dimensional volume theory allowing good estimation of breakdown stress in terms of specific probable values has been established. Keywords: breakdown stress, high voltage insulation, power transformer, Weibull distribution method Statistička analiza učinka električne i mehaničke probojne čvrstoće na karakteristike izolacije kod transformatora snage visokog napona Izvorni znanstveni članka U elektroenergetskom se sistemu koristi niz krutih, tekućih i plinovitih materijala za izolaciju u svrhu zaštite od početnog proboja u transformatorima snage visokog napona. Izolacija praktički više ne postoji kad izolacijski sistem postane toliko krhak da se pojave napukline kod električnih i mehaničkih naprezanja kojima je izložen. Do električnih i mehaničkih naprezanja dolazi zbog struje kratkog spoja, termalne ekspanzije i kontrakcije vodiča te vibracije. Dijalektrička čvrstoća izolacije vitalno se ne smanjuje samom krhkošću, već će električni proboj uslijediti ubrzo nakon poboljšanja nastalih pukotina. Svrha je ovoga rada ukratko dati pregled metoda koje pomažu određivanju vrijednosti izolacije na namotima i uvodnicama transformatora visokog napona. Do mnogih kvarova izolacije dolazi zbog vlage. Izolacijski materijali na namotima transformatora visokog napona lako upijaju vlagu iz okolne atmosfere ili ulja. Eksperimentalno su istraživani učinci volumenskog napona ulja i vremena napona na probojnu čvrstoću izolacije transformatora visokog napona. Faktori koji djeluju na učinke i dobiveni rezultati statistički su analizirani i sistematizirani Weibull metodom distribucije. Postavljena je Četiri-dimenzijska teorija volumena koja omogućuje dobru procjenu napona kod električnog proboja u odnosu na moguće specifične vrijednosti. Ključne riječi: izolacija visokog napona, probojna čvrstoća, transformator snage, Weibull metoda distribucije 1 Introduction Nowadays, going on increasing in scale is requiring still higher voltage and power transformers; 500 kV, 250 MVA units have already been put to service. It is expected for electric power sources in future to be located much farther from service areas and to be much larger in capacity. In view of this situation, researches have already started to develop power transformers capable of bearing 1000 kV, a voltage presumed for next stage. On the other hand, the conditions of location for substations have become worse year by year. This places more severe requirements for transportation, which necessitates smaller size of equipment. It is required of the equipment to have much higher reliability because its failure might give a grave consequence to the community [1]. So a lot of electrical equipment production corporations have been positively engaged in theoretical analyses and fundamental studies on such problems as insulation, countermeasures against stray load loss caused by electromagnetic force, leakage magnetic flux and effective internal cooling of power transformers [2] and applied the results of these analyses and studies to actual equipment in order to grasp the problems observed as the result of the application and to inspect the performance of the objective equipment. An oil immersed power transformer has complex insulation structure composed of mineral oil and solid dielectrics [3 ÷ 5]. It is well known that, in such a power transformer, the partial discharge inception and dielectric breakdown will be predominantly affected by the breakdown strength of the insulating oil. Therefore, power transformer designers should have explicit figures regarding the stress exerted on the power transformer oil and the breakdown stress of the oil. Calculating techniques of stress exerted on the oil have made remarkable progress in recent years, but estimating techniques of breakdown stress seem to involve many problems to be solved. Breakdown stress of power transformer insulation cannot be easily estimated because of its likelihood of fluctuating and dispersing. Probably, these phenomena are caused by the presence of a measurable amount of impurities in the power transformer oil. Insulating oils used in power transformers contains 10 to 10.000 pieces of solid impurities per 100 ml [4 ÷ 7]. These impurities, considered to be factors causing discharges in the oil, thus demonstrate the probabilistic phenomena, such as the stressed oil volume effect or stressing time effect on the breakdown stress. I experimentally investigated these factors and effects on the breakdown of power transformer insulation, and systematized them on a statistical basis. The efforts have now led the work to the Tehnički vjesnik 21, 3(2014), 495-503 495