Geri Besleme Kontrollü Rezonans Transformatör Tasarımı

Abstract

Dünya geliştikçe elektrik enerjisine olan gereksinim hızla artmaktadır. Üretilen elektrik enerjisinin ekonomik nedenler ve doğal kaynakların sınırlı olması sebebiyle tüketim merkezlerine en az kayıpla ulaştırılması gerekmektedir. Bu durum elektrik enerjisinin üretiminde, iletiminde ve dağıtımında yüksek gerilimlerin kullanılmasını gerekli hale getirmektedir. Yüksek gerilim üretmek için birçok yöntem mevcuttur. Üretilecek yüksek gerilimin doğru ya da alternatif gerilim olmasına ve kullanım amacına göre en uygun yöntem seçilmelidir. Yüksek gerilime, elektrik iletim ve dağıtım şebekelerinin yanında yüksek gerilim altında çalışacak malzemeleri ve aygıtları denemek için de ihtiyaç duyulmaktadır. Bu çalışmada yüksek alternatif gerilim üretim yöntemleri ele alınmış, özellikle kapasitif karakterli yapıları deneme süreçlerinde yüksek alternatif gerilim üretmek amacıyla kullanılan, yirminci yüzyılın ikinci yarısında geliştirilen, rezonans transformatör ile yüksek gerilim üretimi üzerinde durulmuş ve şebeke gerilimi alçaltıcı bir trafodan geçirilerek beslenen bir seri rezonans devresi ile oluşturulan rezonans transformatörün, giriş gerilimini kalite faktörü kadar kuvvetlendiren bir uygulaması yapılmıştır. Değişken endüktans elde edilerek endüntans ayarlı bir rezonans transformatör tasarlanmıştır. Özellikle kapasitif özellikli yüksek gerilim elemanlarının denenmesi gibi endüstriyel amaçlarla kullanılan, endüktans ayarlı rezonans transformatörlerinde rezonans anını yakalamak için endüktans el ile değiştirilmekte, herhangi bir kontrol uygulanmamaktadır. Bu uygulamada ise rezonansa otomatik olarak gelen, geri beslemeli kontrollü rezonans transformatör tasarlanmıştır. Devreden akım geri beslemesi alınmış ve rezonans anında akım en büyük değerini alacağından, mikroişlemci içinde koşturulan maksimizasyon algoritması ile rezonans anı belirlenmiştir. Tasarlanan rezonans transformatörü endüktans ayarlı olduğundan değişken endüktans elde etmek üzere Henry mertebesinde endüktansa sahip bir bobin sarılmıştır. Endüktans, boş bir silindir etrafına bobin teli sarılarak elde edilmiştir. Tek kat sarımda elde edilen değerin yeterli büyüklükte olmaması nedeniyle katmanlar arasına gerilim ve ısı dayanımlı bant konulmak suretiyle çok katlı sarım yapılmıştır. Toplamda 16 kat olmak üzere 870 m uzunluğunda 0,4 mm2 kesit alanına sahip bobin teli sarılmıştır. Endüktans değişimi, içi boş silindirin içerisine manyetik geçirgenliği yüksek demir alaşımlı kütle sokularak sağlanmıştır. Bu kütle, dört kutuplu bir adım motor ile tahrik edilen, lineer kızak sistemi ile hareket ettirilmiştir. Akım maksimum değerini alana kadar demir kütle ileri ve geri yönde hareket ettirilerek endüktans değişimi sağlanmış ve rezonans noktası tespit edilmiştir. Adım motor, mikroişlemci yardımıyla sürülmüştür. Mikroişlemci dijital çıkışları, adım motorun herbir bobinine sıra ile enerji verecek şekilde programlanarak ileri hareket etmesi sağlanmıştır.

In today's developing world, the need for electrical energy is increasing rapidly. Due to our limited natural resources, electricity should have transferred with low losses to end users. To achieve this goal, high voltage must be used in distribution and transmission of electricity. Today, one of the most widely used sources of alternative high voltage is transformers. Also there are many methods for producing high voltage that can be change due to purposes of applications. High alternating voltages is required for high voltage laboratory experiments, alternative high-voltage experiments, and also for the majority of the circuits used in the production of direct and impulse voltages. In general, transformers used for this purpose have higher conversion rates and lower power than power transformers. These transformers are fed mainly on the network a regulator or synchronous generators. Besides distribution, transmission and experimental purposes, high voltage can be used in order to test the capacitive materials and equipments that work under high voltage. Transformers are also can be used as single ply or a cascade connected in High-voltage test systems. For production of high alternating voltage to the level of several hundred kV, single high voltage transformer is the most economical way. For the electricity more than 300 kV, transformers which are connected each other cascade is used instead. Cause, single transformer is not suitable in case of size and portability. Test transformers connected cascade, is used by the first time in 1915, W. Petersen, F. Dessauer and E. By Welter . Usually between two and four transformer connected each other. The most common usage is three-transformers performed the use of cascade connection. Basically, the high voltage side of the secondary windings connected in series cascade structure consisting of several transformers, low voltage side of the primary windings is fed through the its predecessor transformer excitation coil. The current that flows through the Low voltage and excitation coil of test transformers connected cascade, larger than high voltage winding. In the same way the output power is shared by multiples of the transformer. This study is focused on production method of high alternative voltage by resonant transformers. Resonant transformers which were invented in the second half of twentieth century strengthen the input voltage by the rate of Q (quality factor). This transformer works on the principle of resonance. Electrical resonance, the capacitive and inductive reactance in a circuit state of being equal to each other at a particular frequency. Basically, the current through the inductor creates a magnetic field through the feeding capacity, formed during the discharge current generates a magnetic field on the back of inductance and the resonant frequency of the circuit of capacitive and inductive reactances are synchronized to each other. In this way, in a sense the electrical system at resonance frequency can be considered as a pendulum. Reactive power flow due to capacitive and inductive elements in the system to oscillate between the each other. So, reactive power is cancelled at resonance. Thus, power drawn from the source at resonance, is equal to the power expended in the circuit resistors. DC is input to the transformer, which is converted to a high-frequency driving signal, essential for the resonant condition to occur in the device. The high-power resonant transformer is driven at relatively high frequencies, up to a hundred kilohertz, made possible by advances in solid state power transistors. This driving signal initiates the resonant effect in the primary and secondary coils of the transformer, converting the input to a high-voltage signal at a comparatively high frequency, present on the device's secondary coil. The resonant transformer effect is an old technology renewed by the infusion of modern semiconductor technology. A novelty of resonant transformers is that the high voltage developed is a consequence of the resonant effect, rather than winding ratio of the coils. In fact, the early use of these devices was for the generation and transmission of radio. Once a high voltage is developed in the transformer, the output is pulled from the secondary coil by an output coil. The output is then converted into DC and then to any other utilization power by a novel rectifier arrangement. A resonant transformer is one that operates at the resonant frequency of one or more of its coils. The resonant coil, usually the secondary, acts as an inductor, and is connected in series with a capacitor. If the primary coil is driven by a periodic source of alternating current, such as a square or sawtooth wave, each pulse of current helps to build up an oscillation in the secondary coil. Due to resonance, a very high voltage can develop across the secondary, until it is limited by some process such as electrical breakdown. These devices are therefore used to generate high alternating voltages. Tunable inductance and tunable frequency resonant transformers are available. Tunable inductance reactor modules for series resonant circuits are available between 250 and 1200 kV and test power between 1000 and 20000 kVA. Tunable frequency modules for series resonant circuits are available between 100 and 1600 kV and test power between 50 and 9000 kVA. The high voltage experiments which are made with variable frequency resonant transformer as the experiments with the series resonant tuned transformer, follow some sequences. First the of the circuit resonance state, than output voltage value is made desired test voltage, and try to perform a particular operation will follow. Unlike serial resonant transformers, adjustment of voltage is provided with frequency change. The inductance tuned resonant transformers, allow the production of a high voltage and make circuit resonance in constant frequency by means of variable inductance. There are two types of variable inductance resonant transformer according to position of the elements in circuit. One of them is series other one is parallel resonant transformer. Series resonant transformers, regulators, transformers, for the variable inductance, one high voltage reactor and test material that connected in series to reactor occurs. Regulator provides the ability to change the voltage to connected transformer. The next part of the high voltage winding of the transformer circuit generates the high voltage side. The transformer which connects regulator and resonance circuit, is provide the galvanic isolation between the two sides at the same time. Similar to the series resonant transformer, parallel resonant transformer is consisting of one regulator and transformer connected to regulator and test material. Unlike a serial resonance circuit, high voltage reactor and test material connected parallel to the circuit and each other. Because, parallel resonant transformer's output voltage is more constant than serial resonance, it is used especially in the tests which include big currents such as large generator windings, corona loss experiments. In addition, an advantage of placement in parallel, while the circuit is not connected to the load, the system can be operated at full power. This is very advantageous for the system elements, the calibration and measurement of partial discharges. An application of feedback controlled resonant transformer with serial resonance will be take place in last part of this study. Moving the iron back and forth with in the coil will change the inductance value with the help of a linear slide which consist of the stepper motor driven by microcontroller and gears. This circuit is supplied through mains voltage after a reduction transformer. Therefore, the 50 Hz must be the resonance frequency of the circuit. At the resonant frequency, capacity and inductance act as a short circuit than the current which flows through the circuit is limited by only resistor. Current flowing through a resistor which connected in series to circuit will take the highest value at resonance frequency. This current is given as feedback to the controller and controller drives the iron mass to the position that inductance reach the required value for resonance frequency. In the designed resonant transformer, circuit current is measured and given as a feedback to controller. As mentioned above, the current takes its maximum value at the moment of resonance. Increasing in resistance of circuit will result decrease the quality factor of resonant transformer. Thus, current measuring should be taken without increase the resistance of the circuit. To achieve this goal 1 Ohm resistor that is small enough to be neglected according to circuit resistance, is connected series to circuit. With help of this feedback voltage, controller is programmed to maximize the current flows through the circuit. Voltage wave forms are implemented in last part of this study and controller software is explained in attachments.