Yüksek darbe gerilimlerinin sayısal kaydı ve bilgisayarda değerlendirmesi

Abstract

Yüksek gerilim deney çalışmaları dijital kaydedici olarak dijital osiloskopların kullanımı ile bilgisayarlardan doğrudan yararlanma olanağına kavuşmuştur. Dijital kaydedicilerin en önemli özelliği analog kaydedicilerin (hafızalı analog osiloskop) aksine kaydedilen işaretleri bilgisayara aktarmaya olanak sağlaması ve bu işaretlerin bilgisayarda depolanabilir olmasıdır. Darbe şekli parametreleri hafızalı bir analog osiloskop ekranından çekilen fotoğraf yardımı ile uzun uğraşılardan sonra hesaplanabilirken, ölçme devresinde bir dijital kaydedicinin kullanılması kaydedilen işaretin bilgisayar ortamına aktarılmasına ve bir yazılım yardımı ile otomatik olarak elde edilmesine olanak vermiştir. Deney sırasmda kaydedilen darbe gerilimi işaretinin analizinden darbe gerilimi üzerindeki varsa bozucu etkilerin ne zaman başladığı, frekansı ve genliği gibi konular hakkında da bilgi elde edinilebilir. Bozucu etkiler hakkındaki bu gibi bilgiler ölçme devresinde yapılması gereken düzeltmeler için de bir ipucu olmaktadır. Bu çalışmanın amacı darbe gerilimi parametrelerini otomatik olarak hesaplayabilen bir yazılımla desteklenmiş dijital ölçme sisteminin kurulmasıdır. Bu amaçla darbe gerilimi üretecinin ölçme devresine veri transferine izin veren bir dijital osiloskop bağlanmıştır. Analog darbe işareti osiloskoba kompanze edilmiş bir zayıflatıcı ile on kat zayıflatılarak uygulanmıştır. Osiloskobun veri transferine izin vermesi verinin bilgisayarda kaydedilmesi ve işlenmesi olanağı vermiştir. Bu tezde geliştirilen bilgisayar programı ile osiloskoptan alman bilgiler değerlendirilerek bilgisayar ekranına darbe şekli çizdirilmekte ve darbe geriliminin tepe değeri, cephe ve sırt yan değer süreleri gibi önemli parametreleri hesaplandıktan sonra ekranda yazdınlmaktadır. Geliştirilen program gerçek deney devresinde farklı darbe şekilleri ile denenmiş ve sonuçlar ölçme devresindeki diğer ölçme aletleri ile karşılaştınlmıştır. Sonuçların diğer ölçme olanakları ile elde edilen sonuçlarla uyumlu olduğu görülmüştür. Kurulan deney düzeneği ve geliştirilen bilgisayar programı ile bilgisayar destekli darbe gerilimi deneyleri yapma olanağının bulunması ile çalışma hedefine ulaşmıştır. Ulaşılan bu hedef bundan sonrasında bilgisayar programının daha da geliştirilmesi, yeni özellikler katılması (veri işleme, tanıma, analiz, rapor gibi) ve deney otomasyonuna gidilmesi gibi yeni ufuklar açmıştır.As the custom barriers disappear in the world trading, the essential key to success seems to be the improvement of the equipment quality. So in the last years a lot has been done in order to improve the quality control systems and consequently the tests involved. By the other hand, the technology development has been giving us the possibility to use new measuring equipment in the tests. Their characteristics allows the improvement of the test efficiency and sometimes, the possibilities offered are so wide that we can even think in changing the tests procedures in order to improve further. This panorama can also be found in the high voltage field. In the last decades, a lot of effort has been done in order to improve the quality of the equipment and due to their dimensions and cost, to optimize the design and the manufacturing. An advent of digital recorders has a profound impact on high voltage test and measuring technique. The effectiveness of quality control of power apparatus can be enhanced by signal processing and computer assisted analysis of digitally recorded high voltage test impulses. Valuable hints as to the origin and location of partial discharges in high voltage insulation are provided by statistical analysis and pattern identification of digitally recorded partial discharges. The digital instrumentation has enabled development of monitoring devices, which can track the long term performance of high voltage apparatus insulation in service. These aims claim for good high voltage laboratories, having test systems and procedures with a high level of performance. One of most complicated tests the importance of which has being more and more accepted, is the high voltage impulse test. These tests simulate the stress to which the high voltage equipment is submitted by an over voltage due to a lightning stroke or to the switching of a high voltage circuit component. So, the impulses used in the tests have very high amplitudes and extremely high steepness. This causes many problems of electromagnetic interference during the tests, and the final result is, most of the times, the presence of disturbances in the waveforms. During several decades the evaluation of the test results was made by the hand of a skilled operator based in an analog record (photo of the oscilloscope display) of waveform applied. The development of the digital recording equipment, together with the improvement of shielding techniques allowed the attainment of very fast digitizers with performance well suited to be used in the high voltage laboratories. That was a time of changing in the high voltage measuring system philosophy and together with the need of improvement of the test quality, gave the high voltage test experts the opportunity to think on the evaluation of the performance of their test systems and procedures and also the adequacy of the international standards to the new philosophy and needs. So, in the last years, many actions have been taken in order to improve the performance of the high voltage laboratories. These actions comprise intercomparison tests among laboratories belonging to different countries, the revision of the international standards and the creation of new standards ruling the new equipment and procedures. However, digital instruments can not be introduced to a high voltage test bay or substation without an appropriate protection against the electromagnetic interference. At present, commercially available modern analog to digital converters can sample the signal at 100 MHz rate and offer 1000:1 resolution, and they have already replaced analog impulse oscilloscopes in many high voltage test stations. The new instruments can detect minor differences between wave forms, which could have been masked by the trace width of the cathode ray tube. Digital recording systems contributes to the analysis of the wave shape during tests. The noise can be found by the analysis which can also give information about the reasons of the disturbances to the test operator. The operator then can modify the measuring device to avoid these disturbances. The digital philosophy presented by the new measuring systems, gave place to the need for software adapted to the analysis and processing of the high voltage impulses. Due to the wide possibilities of disturbance on the impulse waveforms the development of such a software is not an easy task. As there is any international standard ruling the processing techniques and the analysis criteria, in the case of digital analysis and processing, different vi laboratories have being developing their own software, with quite different options in what concerns the analysis and processing. One software intercomparison showed that, whenever there is disturbance in the impulses to evaluate, the final results are greatly influenced by the performance of the software. So, the overall quality of a high voltage measuring system depends also on the performance of the software. Being aware of this, the experts have being studying ways to improve the performance of the software, and also developing an international standard for the software. This thesis presents such a software which is written in Microsoft Visual Basic 3.0. The software yet can calculate the peak value, the front time (Tj) and time to half value (T2) of an impulse. The software is calculating time to half value (T2) for chopped impulses as the chopping time. In the tests the impulse voltage is produced by a single stage impulse generator and measured by a digital oscilloscope. EEEE 488 interface board in the oscilloscope contributes to processing of the digitized data. The connection is established from the oscilloscope's 25-pin port to the computer's 9-pin comm. port. Processing the data, sent by the oscilloscope, the graphic can be drawn just as the same as seen on the oscilloscope's display. Figure. 1.a and b show the impulse generator and remote control table and measuring system used in the experiments. Figure.2 shows the obtained impulse waveforms in different environments. For this purpose a computer with 386 processor and 4 Mbytes RAM could be enough. Recorded data have the size of approximately 2,5 kbytes in the computer. For the accuracy of the calculations made by the software, the trigger input to oscilloscope is very important. In this study 8 m coaxial cable is used for delaying the trig. For the chopped impulses the trigger input is greatly influenced by the breakdown of the air gap. The oscilloscope could take this discharge as a trig signal. To avoid taking discharge signals as trigger input the place of the antenna is changed and placed away from the discharge area. An advent of digital recorders has a profound impact on high voltage test and measuring technique. The effectiveness of quality control of power apparatus can be enhanced by signal processing and computer assisted analysis of digitally recorded high voltage test impulses. Valuable hints as to the origin and location of partial discharges in high voltage insulation are provided by statistical analysis and pattern identification of digitally recorded partial discharges. The digital instrumentation has enabled development of vii (a) (b) Figure. 1. a. Impulse generator. b. Control desk and measuring system. (1) Control desk, (2) Digital impulse voltmeter, (3) Digital oscilloscope, (4) Computer. vin (a) Dosya "Tepe Degeı ve Qluslugu An" 1 3.75 KV 26 nukııuaniye Cephe Suı esi [|ısj rpıfl Ya» Peğer SûresiH>1~.300G.31 ~&aat ve Tdiıh" Z2:13 5/8/1 99B (b) ıx (c) Figure.2.a. Impulse voltage photograph taken from the digitaloscilloscope screen b. View of the main frame of the computer program c. Impulse voltage photograph taken from the computer screen monitoring devices, which can track the long term performance of high voltage apparatus insulation in service.