Alçak Gerilim Ark Modelleri Ve Arkın Isıl Analizi

Abstract

Bu tezin amacı yüksek ve orta gerilimde açma-kapama işlemi sırasında meydana gelen arkların modellenmesi ve bilgisayar simülasyonları ile arkın elektriksel ve ısıl analizinin yapılmasıdır. Güç sistemlerinde kısa devre arızalarının meydana gelmesi durumunda enerjinin sorunsuz bir biçimde kesilmesi ve gerekli korumanın sağlanması büyük önem taşımaktadır. Güç sistemlerinde kısa devre olaylarının büyük bir bölümü ark şeklinde meydana gelmektedir. Ayrıca kısa devre durumları veya normal şartlar altında kesici açma işlemini gerçekleştirdiğinde kontaklar arasında yine bir elektrik arkı oluşabilmektedir. Bu gibi durumlarda sistemden geçen akımın ve gerilim dalgasının biçiminde meydana gelen değişimin bilinmesi gerek koruma açısından, gerekse devre kesicilerinin tasarımı açısından büyük önem taşımaktadır. Elektrik arkı sabit bir direnç olarak modellenmekle birlikte, doğru bir analiz için daha detaylı modellere gerek bulunmaktadır. Bu amaç için, sabit direnç yerine karakteristiği zamanla değişkenlik gösteren dinamik ark modelleri kullanılmaktadır. Ancak bu gibi detaylı modellerin simülasyonunda zorluklar çekilmektedir. MATLAB-Simulink kullanılarak oluşturulan modeller yardımıyla arkın bu dinamik karaktersitiği kolaylıkla incelenebilmektedir. Yapılan çalışmada dört farklı ark modeli için simülasyon modelleri oluşturulmuştur. Oluşturulan bu modellerde bir periyot içinde farklı zamanlarda (sıfır geçişlerinde ve sıfır geçişleri dışında) ark oluşturulmuş ve davranışları incelenmiştir. Ayrıca bu dört modelin akım ve gerilim karakteristikleri birbirleri ile karşılaştırılmış ve modellerin farkları ortaya konulmuştur. Arkın ısıl etkilerini görebilmek adına üç farklı elektrot tipinden(düzlem-düzlem, çubuk düzlem ve küre düzlem) oluşturulan sistemlerde arkın elektrot yüzeyinde oluşturduğu zamana bağlı ısı geçişi ve sıcaklık dağılımları sonlu elemanlar destekli bir analiz programı olan COMSOL 3.4 ile simule edilmiştir. Ayrıca düzlem elektrot sistemi için sıcaklık dağılımı elle hesaplanıp benzetim verileri ile karşılaştırılmıştır. Elektrot açıklığının sıcaklık dağılımına etkisini görebilmek için simülasyonlar sırasıyla 1mm, 2mm ve 3mm elektrot açıklıkları için de yapılmıştır. Ark sütununun sıcaklığının ve arkın süresinin sıcaklık dağılışını nasıl değiştirdiği simülasyonlarla incelenmiştir.

Current interruption is the core technology in low voltage circuit breakers and it is characterized by the presence of an electrical arc. The electrical arc is a luminous, noisy and hot electrical discharge that leads to large scale of temperature. Also arc flash is a common issue for electrical insulation due to its hazardous effects on high voltage equipments. High voltage circuit breaker is the key element for power systems. Line faults lead flowing fault currents through the lines. When the current reaches critical level circuit breakers protect the system with interrupting the current. Arc occurs when the current is interrupted in the circuit-breaker. Electrode types affect the interrupting ability of the arc in the circuit-breaker. The main effect of the arc is causing large scale of temperature due to joule heating. This over-temperature causes heavily damage on system equipments especially the circuit-breakers. For this reason, arc flash tests play an important role on system safety. The purpose of this thesis is to investigate the dynamic models of the electric arcs occurs during the switcing operatrions in high voltage and thermal analysis of arc. In power systems, it is very important to switch the load and source without any problem and to provide a reliable protection in the case of short circuit faults. Most of the short circuit faults are in the form of electrical arcs. Also, electrical arcs occur between the contacts of the circuit breakers. Determination of current flowing in the system and changes in the voltage under these conditions is important for the protection process and for the design of the circuit breakers. Although an electrical arc can be modeled as a constant resistor, more accurate models are needed for a detailed analysis. To achieve this, dynamic arc models whose characteristics varying with time are used instead of constant resistance models. However, there are some difficulties in simulation of these models. By using MATLAB-Simulink programme the dynamic characteristics of arc can be easily analyze. Simulation models for four different arc models are composed in the study. In composed models, arcs are simulated in different instants in one period (current zero points and non current zero points), and their behaviour is investigated. Furthermore, voltage and current characteristics of these four models are compared with one another and differences between the models are set for the experiments. In order to observe the thermal effects of the arc, time dependent heat transfer which occur on electrode surface because of arc, and temperature distributions are simulated by COMSOL 3.4 which is a Finite Element Method based analysis software, on the systems that are composed of three different electrode types (plane-plane, rod-plane, sphere-sphere). In addition to that, temperature distribution in plane electrode type is calculated analytically and compared with simulation results. Besides simulations are performed for 1mm, 2mm and 3mm electrode gaps respectively, to see the effect of electrode gap on temperature distribution. It is analysed by simulations, how the temperature of the arc column and arc period is changing the temperature distribution. The effect of electrode types on arc flash testing are investigated experimentally. In experiments, three different electrode systems such as plane-plane, rod-plane and sphere-sphere electrodes are used to observe the electrical field effect on arc flash testing. 50 Hz AC voltage is applied to the electrodes in short air gap as 5 mm. In the test setup, voltage and current amplitudes are measured for all three electrode systems respectively. The measurement process is repeated for different gap spacing among 1-5 mm between the electrodes. Voltage and current characteristics with respect to the electrode type and air gap are obtained graphically by using the data acquired from experiments. The experimental results are compared with the theoretical results obtained from the formulation mentioned in IEEE 1584 Guide for Performing Arc-Flash Hazard Calculations. In conclusion, both of the experimental and theoretical results seem to satisfy each other. In this study, influence of using electrode shape on heat dissipation of the electrical arc is investigated experimentally and the test results are presented. Besides, the paper describes the laboratory work including formation of electrical arc and measurements of arc properties such those arc temperature, arc voltage and current. Different shapes of the electrodes such as sphere, plane, and rod electrodes are used for the experiment setup. Different voltage levels are applied to the electrodes to obtain arc. Temperature of the arc plasma column and the electrode surface are measured by infrared thermometer for each electrode shape and configuration to obtain the characteristics of both column temperature and surface temperature with respect to arcing voltage and current. Also the tests are repeated for the electrodes having different surface areas under fixed voltages to obtain relation between the electrode surface and the heat dissipation. After the experiments, heat analysis of the system is performed by using the finite element method. The system having different electrodes is simulated with program Finite Element Method Magnetics (FEMM). All the results are given graphically. Consecutively, from studies on heat dissipation of the arc it is found that area of electrode surface is dominant on this effect. This experimental study was intended to investigate the electrode type effect on arc flash for AC systems. According to experiments, it is clear that different electrode types have different arc characteristics. However, the nonlinearity of the arc resistance can be seen easily by the graphics. The tests show the biggest breakdown voltage occurs in between the plane-plane electrodes among the others. Unlikely the current characteristics for bigger gap spacings are different than the voltage characteristics. For bigger gap spacings, rod-plane system has lowest current values. The reason is that its capacitance is relatively small than the others and small capacitance yields big reactance. Due to the high level of the current for plane-plane system it is certain that the arcing energy of the plane-plane electrode system has higher energy values than the others.