Metal oksit parafudrların modellenmesi

Abstract

Bu çalışmada, enerji iletim sistemlerindeki aşın gerilimlere karşı koruma aygıtlarından en gelişmişi olan metal oksit parafiıdrlar incelenmiştir. Yapılan literatür taramalarında, son yıllarda geliştirilmiş olan metal oksit parafiıdrlara ait, bunların koruma karakteristiklerini belirleyecek olan V-I özeğrilerinin yeterince verilmediği görülmüş ve bu eksikliği giderebilmek için çeşitli benzetim çalışmaları yapılmıştır. Çalışmalar altı bölüm altında toplanmış olup, bölümler hakkında kısa bilgiler aşağıda verilmiştir. Birinci bölümde; elektrik enerji sistemlerinde korumaya neden gerek duyulduğu ve bu korumanın hangi koruma aygıtlanyla ne kadar sağlanabildiği kıyaslamah olarak anlatılmıştır. îkinci bölümde; metal oksit parafiıdrların geliştirilmesinin nedenleri, üstünlükleri, temel çalışma ilkesi ve seçilme kriterleri alışılagelmiş silisyum karbür parafiıdrlara kıyaslamah olarak verilmiştir. Çeşitli elektrik aygıtlarına karşı nerede ve nasıl bir parafiıdr seçilerek, nasıl daha iyi bir koruma sağlanabileceği anlatılmıştır. Tezin üçüncü bölümünde; elektrik enerji sistemlerinde kullanılan tüm donanımlarda olduğu gibi, parafiıdrlar için de fiziksel davranışlarını en doğru şekilde yansıtabilecek model, teorik olarak tasarımlanmaya çalışılmıştır. Bu bölümde statik ve dinamik parafiıdr modelleri, dalga şekilleri ve süreleri gözönüne alınarak incelenmiştir. Dördüncü bölümde; enerji sistemlerinin güvenli çalıştırılabilmesi ve uygun genişletmelerin yapılabilmesi için, gerekli analizleri yapmak amacıyla geUştirilmiş, İT.Ü.'den temin edilen EMTP (Electromagnetic Transients Program) yazılımı tamtılmış, genel çözümleme teknikleri anlatılmış ve özel olarak parafiıdr modellemesi hakkında bilgi verilmiştir. Beşinci bölümde ise baz alman laboratuvar deneyleri ile aynı (minimum hatalı) sonuçlan verecek statik ve dinamik parafiıdr model parametrelerinin saptanması amacıyla yapılan benzetimler anlatılmıştır. Belirlenen parametrelerle modellenen parafiıdrlar, bazı uygulamalarda kullanılarak, modeller karşılaştmlmıştır. Tezin sonuçlar ve öneriler kısmında yapılanlar ve katkılar özetlenerek konuyla ilgili yapılabilecek çalışmalar hakkında önerilerde bulunulmuştur.

Overvoltages in electrical supply networks result from the effects of lightning strikes and switching actions and can not be avoided. They endanger the electrical equipment because, due to economical reasons, a sufficient voltage withstanding capacity can not be designed. A more economical and safe network therefore calls for extensive protection against unacceptable overvoltage loads. This applies to high voltage as well as medium and low voltage networks. Overvaltage protection basically can be achieved in two ways:. Avoid lightning overvoltage at the point of origin, e.g. through shielding earth wires in front of the substation to intercept lightning.. Limit overvoltage near the electrical equipment, e.g. through surge arresters in the vicinity of the electrical equipment In high voltage networks, both methods of protection are usual. In medium voltage networks the earth wire protection is generally not very effective. Due to the small clearance between the earth wire and the line wires, a direct lightning strike will usually hit the line wires as well. In addition, induced overvoltage (indirect effects of ligtning strikes) on the line wires can not be avoided by the earth wires. The most effective protection against overvoltages in a medium voltage network is the use of surge arresters in the vicinity of the electrical equipment. In important stations, protection against lightning surges requires the establishment of a protective voltage level by means of shunt connected protective devices. Satisfactory performance is required of surge arresters since they are essential for the insulation coordination in electric power systems. In particular, the protective characteristics of surge arresters must be improved since reducing the insulation level for any apparatus raises economic efficiency. This requirement imposes the necessity of lowering the protective level of the surge arrester and improving operating duty performance. According to the International Electrotechnical Commission, "insulation coordination comprises the selection of the electric strength of equipment and vi its application in relation to the voltages which can appear on the system for which the equipment is intended, and taking into account the characteristics of available protective devices, so as to reduce to an economically and operationally acceptable level the probability that the resulting voltage stresses will cause damage to eqipment insulation or affect the continuity of service" IEC 71-1, 71-2 (1976). The insulation design of high voltage stations must be based on different principles from those applying to transmission lines. Firstly, stations generally contain transformers and other valuable equipment with nonselfrestoring insulation which must be guarded most carefully against internal breakdowns. Secondly, since they have vital functions to fulfil in the power system, even the risk of flashover in the air, with the accompanying disturbance to normal operation, must be kept to a minimum. With nonselfrestoring insulation the risk of insulation failure should at all times be avoided. This is normally achieved by placing an overvoltage protective device, usually a surge arrester, in the vicinity of the equipment to be protected. If the insulation were subjected only to the normal operating voltage which varies within quite narrow limits, there would be no problem. In reality, the insulation has to withstand a variety of overvoltages with a large range of shapes, magnitudes and durations. These various parameters of overvoltages affect the ability of insulation to withstand them. The problem is therefore to ascertain the magnitudes, shapes, frequency and durations of overvoltages, and the changes they undergo when travelling from the point of origin to the equipment to determine the voltage withstand characteristics, in respect to these overvoltages, of varius types of insulation in use to adopt the insulation strength to the stresses. 1" =10 kA Un=4p.u. /2TTJn 10* IA] Fig. 1. Semi-logarithmic plot of current-voltage characteristics of MO and SiC resistors for U=4 kV. vii The operating voltage of surge arresters is directly related to insulated protection for each apparatus and it should be as low as possible. Metal oxide surge arrester is constructed by a series connection of zinc oxide elements having a highly nonlinear resistance. This is shown in figure 1 as compared with that of the conventional surge arrester. The development of zinc oxide element having a highly nonlinear characteristic has enabled to make a surge arrester without series gaps. Eliminating the series gaps, the metal oxide surge arrester is an ideal surge arrester provided with the following features.. Very small time delay in responding overvoltages.. No abrupt transient such as that occurs at the time of sparkover in a conventional arrester.. Negligible power follow current after a surge operation. A simple equipment circuit which is seen in the figure 2 is proposed to express the dynamic characteristics of zinc oxide element. Using this dynamic model, the effect of lightning strokes is analyzed by a computer under various conditions. The way of modelling the electrical properties of the arrester is described. The models are based on theoretical investigations on the individual characteristics affecting the behaviour of the arrester. The computer simulation is verified at medium voltage arresters. v(I).L.& Fig. 2. Dynamic characteristic of MO A and the equivalent circuit (dynamic model). The electrical properties of zinc oxide element, particularly, its transient response of V-I characteristic investigated in this thesis. The mathematical model proposed in this thesis is proved to be useful to calculate the discharge voltage upon the surge current of arbitrary waveform. The mathematical model is modified so that it can be applied in the simulation of surge vm propagation phenomena. Using this method, the overvoltages due to lightning strokes are studied in various conditions. Static and dynamic modelling of metal oxide surge arresters have developed in this thesis. A model is described which will give an appropriate voltage response for a current surge which has a time -to- crest anywhere in the range of 0,5 jis to 45 us. In static model a simple nonlinear V-I characteristic which is derived from achieved data with appropriate times to crest would be adequate in the absence of a frequency dependent model. Data on the characteristics of metal oxide surge arresters have been achieved by combining EMTP samples and lab test data of metal oxide surge arrester discharge voltage and current which have been referenced. Analysis of this achieved data suggested that switching surge studies could be performed by representing metal oxide surge arresters only with their V-I characteristics which is calculated by i=P(u/uref)