Yıldırım boşalmaları ve etkinlik bölgelerinin incelenmesi

Abstract

Bu çalışmada, yıldırım boşalmaları ve etkileri » teorik ve deneysel olarak incelenmiştir. Bu amaçla bulut ve yeryüzünün düzlem elektrotlarla, pilot boşalmanın pilot elektrot olarak adlandırılan çubuk elektrot ile modellendiği elektrot sisteminde, pozitif ve negatif kutbiyetteki darbe gerilimleri ile çeşitli deneyler yapılmıştır. Deneyler, 1,2/SO /js'lik standart darbe geriliminde, hem pozitif, hem de negatif kutbiyette yapılmıştır.. Böylece kutbiyetin % SO atlama gerilimi üzerine etkisini görebilme olanağı bulunmuştur. Deneylerde yıldırımın binalara etkilerini incelemek İCİn çeşitli yükseklikteki bina ve elektrot modelleri kullanılmıştır. Model elektrotların, pilot elektrodun düşey eksenine göre yatay uzaklıkları değiştirilerek her bir durum için % SO atlama gerilimleri saptanmıştır. Böylece belirli elektrot yükseklikleri için % SO atlama gerilimlerinin yatay uzaklığa göre değişimleri elde edilmiş ve sonuçlar karşılaştırılarak yıldırımın etkinlik bölgeleri, dolayısıyla yıldırıma karsı koruma bölgeleri incelenmiştir. Deneylerden sonra yapılan karşılaştırmalarda, aynı elektrot yüksekliği için negatif darbe geriliminde elde edilen % SO atlama gerilimlerinin, pozitif kutbiyette ölçülen % SO atlama gerilimlerinden çok daha büyük olduğu görülmüştür. Her iki kutbiyette bulunan % SO atlama gerilimlerinin oranı kullanılan her iki model için de yaklaşık 1,5 civarındadır. Ayrıca yapılan deneyler sonunda, negatif darbe geriliminde bina modelinin veya sivri uçlu elektrodun yatay açıklığının, çalışılan düzen için pozitif darbe gerilimindeki ne göre yaklaşık 3, S oranında daha fazla arttırılabildiği görülmüştür. Bu, pozitif kutbiyetli yıldırımın atlama olasılığının daha dar bir aralıkta daha büyük olması demek ti r. Aynı elektrot yüksekliği h ve aynı kutbiyetteki darbe geriliminde, sivri uç ile bina modeli için elde edilen sonuçlar karşılaştırıldığında» % SO atlama gerilimlerinin hemen hemen eşit olduğu belirlenmiştir.

Power systems are often subjected to over voltages that have their origin in atmospheric discharges in which ease they are called external or lightning overvoltages, or they are generated internally by connecting or discon necting the system» or due to the systems fault initia tion or extinction. The magnitude of the external or lightning over- voltages remains essentially independent of the system's design» whereas that of internal or switching overvolt ages increases with increasing the operating voltage of the system. Hence» with increasing the system's oper ating voltage a point is reached when the switching over- voltages become the dominant factor in designing the sys tem's insulation. Up to approximately 300 kV, the sys tem's insulation has to be designed to withstand primari ly lightning surges. Above that voltage» both lightning and switching surges have to be considered. For ultra h. v. systems» 76S kV and above switching overvoltages in combination with insulator contamination becomes the pre dominating factor in the insulation design. For the study of overvoltages occurring in power systems, a thor ough knowledge of surge propagation laws is needed. According to theories generally accepted» light ning is produced in an attempt by nature to maintain a dynamic balance between the positively charged ionosphere and the negatively charged earth. Over fair-weather are as there is a downward transfer of positive charges through the global air -earth current. This is then coun teracted by thunderstorms, during which positive charges are transferred upward in the form of lightning. During thunderstorms, positive and negative charges are separated by the movements of air currents forming ice crystals in the upper layer of a cloud and rain in the lower part. The cloud becomes negatively charged and has a larger of positive charge at its top. As the separation of charge proceeds in the cloud, the vi potential difference between the concentrations of charges increases and the vertical electric field along the cloud also increases. The total potential difference between the two main charge centers may vary from lOO to lOOO MV. Only a part of the total charge-several hundred coulombs- is released to earth by lightning; the rest is consumed in inter cloud discharges. The height of the thundercloud dipole above earth may reach S km in tropi cal regions. Physical manifestations of lightning have been noted in ancient times, but the understanding of light ning is relatively recent. Franklin carried out experi ments on lightning in 1744,-1730, but most of the knowl edge has been obtained over the last SO years. The real incentive to study lightning came when electrie transmis sion lines had to be protected against lightning. Fundamentally, lightning is a manifestation of a very large electric spark. Several theories have been advanced to explain aecummul ati on of electricity in clouds. In an aetive thunder cloud the larger particles usually possess negative charge and the smaller carriers are positive. Thus the base of a thunder cloud generally carries a negative eharge and the upper part is positive, with the whole being electrically neutral. There may be several eharge centers wihtin a single cloud. Typically the negative charge center may be located anywhere be tween SOO m and İOOOO m above ground. Lightning dis charge to earth is usually initiated at the fringe of a negative eharge center. To the eye a lightning discharge appears as a sin gle luminous discharge, although at times branches of variable intensity may be observed which terminate in mid-air, while the luminous main channel continue in a zig-zag path to earth. High-speed photographic technique studies reveal that most ligtning strokes are followed by repeat or multiple strokes which travel along the path established by the stroke. The latter ones are not usu ally branched and their path is brightly illuminated. The various development stages of a lightning stroke from cloud to earth is observed by the high-speed photography. The stroke is initiated in the region of the negative charge centre where the local field intensi ty approaches ionization field intensity 50M 200. 150 100 50 poaitiv» polarity -I 1 1 1 1 1 1 1 1 1- x (cm) Fig. 1. An example of lK50?S>=f in both polarities. The results obtained from the experiments were illustrated in form of the curves U<509D=f and compared with each other, where Uis SO % sparkover voltage and x is the horizontal distance between the axis of pilot electrode and the electrode in form of a building model. An example of compared curves is shown in Fig.l. During the tests, temperature, humidity and pres sure in the laboratory are measured. Thus, relative air density and humidity correction factor are taken into account. By applying the impulse voltages in both polari ties, the following results are obtained: 1) The SO % sparkover voltage increases with the horizon tal distance from the axis of pilot electrode in both polarities. 2> The value of the SO % sparkover voltage for negative polarity is higher than the value of the SO H break down voltage for positive polarity. 3> Sparkover ranges of the impulses which are the hori zontal distance between the axis of pilot electrode and the electrode in form of a building model are larger for the negative impulses than the positive impulses. 4> The heights of the buildings are very important for lightning strokes. Flashover probablity of lightning strokes of the tall buildings was observed to be higher than that of the short buildings. The lightning impulses that stroke to the walls of tall buildings were observed during the tests. Therefore, this phenomenon must be taken into account for the protection of the tall buildings against to damage of lightning.