Transformatörlerde kalkanlı sargı uygulaması ve yansıyan gerilimlerin incelenmesi
Shielded winding application in transformers and transferred voltages
Yazar: AHMET KEREM KÖSEOĞLU
Danışman: DOÇ. DR. GÜVEN KÖMÜRGÖZ KIRIŞ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Elektrik Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2017
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering ; Enerji = Energy
Dizin: Aşırı gerilimler = Overvoltage ; Darbe gerilimleri = Impulse voltages ; Delinme gerilimleri = Breakdown voltages ; Güç transformatörü = Power transformer ; Yüksek gerilim = High voltage

Özet:
Şebekede üretimi en uzun süren ve en maliyetli ekipmanlardan birisi transformatördür. Transformatörün kapasitesi (gücü) ve gerilimi arttıkça; üretim süresi, maliyeti ve boyutları büyümektedir. Bu sebeple; transformatörlerin uzun ömürlü olması ve korunması, şebeke sağlığı açısından büyük önem taşımaktadır. 
Güç transformatörlerinin işletmede sıklıkla karşılaştığı arızalardan birisi de aşırı gerilimlerden dolayı arızalanan sargılardır. Bu aşırı gerilimlerin transformatörde oluşmasının en sık nedeni; transformatör ekipmanlarına ya da yüksek iletim hatlarına düşen yıldırım darbeleridir.
Oluşacak yıldırım darbelerin sargıda daha iyi dağılmasını sağlamak için, sargı kapasitelerinin uygun şekilde dağıtılması gerekmektedir. Bunu sağlamak için gerilim seviyelerine göre farklı sargı tipleri kullanılmaktadır. Fakat genel olarak; gerilim arttıkça işçilik artmaktadır.
Bu tezde önerilen sargı tipi olan kalkanlı sargı ve bu sargının getirdiği avantajlar incelenmiştir. Önerilen sargı tipine ait hesaplamalar aktarılmıştır.
Darbelerin etkileri sadece darbenin düştüğü sargıda değil, manyetik olarak birbirine bağlı olan devrede, diğer sargıda da gözlemlenmektedir. Bu gerilim etkilerine transfer edilen (yansıyan) gerilimler denmektedir. Tezde yansıyan darbelerin etkileri de incelenmiştir ve bağlı olduğu değişkenlere göre etkisi gözlemlenmiştir. Önerilen kalkanlı sargı çözümünün yansıyan gerilimlere etkisi de ayrıca irdelenmiştir.
Son olarak tezde önerilen kalkanlı sargıya ait deneysel çalışmalar yapılmış, yansıyan darbe geriliminin, en etkili parametre göz önünde bulundurularak düşürülmesi sağlanmıştır.
Türkiye'de artan güç transformatörü imal eden firma sayısı ile birlikte tez konusu daha çok önem kazanacak ve bu alanda geliştirilecek yenilikler ile transformatörlerin kalitesi ve şebeke ömrü arttırılacak, maliyetleri de azaltılacaktır.

Abstract:
Transformers are an essential element of power systems. Among other equipment; transformers are very costly and hard to manufacture (takes longer time). With increasing power and voltage levels; manufacturing time, cost and dimensions of transformer increases. For this reason, it is very important to make transformer life as long as possible and protect the equipment from hazardous effects in operation. 
One of the most common and dangerous damage to transformer windings in operation is overvoltages. Most common overvoltage situation is the lightning impulses effected to transformer terminals or transmission lines.
There are couple of protection method from overvoltage. Some of them are at outside of transformers; like surge arresters. Other method is inside the transformers; to absorb the voltage or increase the withstand ability. Both could be made with adjusting the equivalent circuit of transformer according to.
In order to distribute lightning impulse in transformer windings better; winding turn to turn and turn to earth capacitances must be distributed according to best option. To give best capacitance distribution, different winding types are used depending on voltage level of winding.
In this thesis, shielded winding is suggested to give best capacitance distribution for high voltage windings. Advantages on lightning distribution and labor cost is explained. Calculations depending on this winding is also explained with details.
Effects of impulses are not damaging only the winding which lightning stroke, it also damages the other windings, which are magnetically connected to primary winding via core. Not only magnetic effects, also capacitive effect for transferring the voltage from effected winding to other winding is essential. These transferring voltage situations are called as transferred surges (voltages). Transferred surges are also inspected in this thesis, dependencies to design parameters are measured with simulation. Shielded winding performance in transferred voltages are also simulated.
Firstly observation made on high voltage terminal neutral side and it's dependency with applied voltage to main terminal. In order to compare shielded winding with disc winding, lightning distribution through winding is essential.
After that, effect of earthing the low voltage side is inspected. Of course, on LV terminals, earthed voltage levels are 0kV and non-earthed simulation results were a lot higher but in order to compare low voltage winding completely, inside turns of low voltage winding (voltage levels) are important. It is observed that voltage induced in middle part of the winding and it could not be ignored.
In order to perform real tests on transformers without damaging the winding, relationship between applied voltage and transferred voltage is observed. If the equal circuit is same for two applied voltage, no change should occur. For this purpose; lightning impulse voltage raised systematically and transferred voltage is observed. 
It is proposed that there are two types of transferred voltages from high voltage winding to low voltage winding. First one is capacitive effect, which takes low voltage and high voltage winding together as capacitance and transfers voltage from one to another. Other is the magnetically transferred effect via core of transformer. Even the core is earthed, for very fast voltage changes on high voltage terminal, very low voltages will be induced on low voltage winding.
Direction of turns of low voltage winding is also simulated. Right wounded high voltage winding and right wounded low voltage winding simulation is compared with right wounded high voltage winding and left wounded low voltage winding. 
With a three-phase simulation; effect of connection symbol of transformer is observed. In order not to confuse the situation with winding direction, only same high voltage and low voltage winding direction connection groups are checked. It is observed that the effect is minimal.
After that; capacitance network of model is changed by switching the winding parameters. For this reason, multiple variance of distances are made on:
High voltage and low voltage winding distance is changed
Low voltage and core distance is changed
Number of turns per layer of low voltage winding is changed
Effect of winding copper dimension ins observed by changing the axial length of winding. Also some simulations made to see total dimension effect by changing only winding or only copper dimensions.
Number of layers in low voltage winding and effect to transferred voltage is observed
Changing the high voltage winding type effect is also observed.
Simulation results shows that for transferred voltage effect; most effective way is to add a shield between high voltage winding and low voltage winding. This will limit all capacitance-transferred voltages; only induced transferred voltages will affect the low voltage winding.
For a transferred voltage from a 1050kV high voltage winding to low voltage top terminal approximately 275kV; only 76.5kV of it is induced voltage, rest is capacitive effect. The result shows to focus on capacitive network, more than induced network.
Changing the distance between windings is an easy solution to change capacitive network. For this purpose; distance between high voltage and low voltage winding is changed from 38mm to 758mm step by step. Increasing the distance is positively affect on parallel capacitance so transferred voltage is reduced. But increasing distance to abnormal values is not reasonable since cost of transformer will increase too much. The logic behind increasing the distance is carrying capacitive effect further from winding. Changing the distance 20mm reduces the transferred voltage at low voltage top terminal approximately 8%.
In next simulation; distance between low voltage winding and core is changed. This is also an interference to capacitive network, but this time a negative effect is observed. Form 10mm to 160mm, 6 simulations performed. Change on transferred voltage is not huge but increasing distance between core and low voltage winding, increases the transferred voltage. Changing this distance 20mm increases the transferred voltage around 6%.
Number of turn at low voltage winding is one of the key parameter in design. First analysis made without changing the overall low voltage winding dimensions. Total height of winding is kept constants and only number of turn is changed by adjusting height of the copper inside winding. This result lead to very small change in transferred voltage, only radical change is observed after 1.5 times the actual turn is created.
Number of turn per layer effect is simulated. Without changing the copper dimensions, number of turn per layer at low voltage winding has very limited effect on transferred voltage. With keeping volt/turn ratio constant and changing the number of turn per layer and adjusting copper length accordingly (changing both HV and LV side number of turn), increasing the turn per layer decreases the transferred voltage. Adding 20 turns to low voltage side reduces transferred voltage at 10% level.
Last simulation made with changing capacitive network of high voltage winding. This made by converting high voltage winding to inter-shielded winding. This change leads both to lightning voltage distribution better on high voltage winding and also reduces the transferred voltage accordingly. Inter-shielded winding reduced the transferred voltage more than half value.
All simulations together; most effective way to reduce transferred voltage effect is to place an earthed shield between high voltage winding and low voltage winding. After that; turning high voltage winding to shielded winding is more effective than any other simulation. Increasing distance between high voltage winding and low voltage winding is also have positive effect but since it increases the cost a lot, it is not preferable. To make small changes in transferred voltage; increasing number of low voltage turns is also a solution.
In last chapter; experimental tests are performed on a manufactured transformer. Shielded winding distribution is tested and transferred voltages are observed. It shows the accuracy of simulations and advantages of shielded winding. 
With increasing number of power transformer manufacturers in Turkey, this thesis will improve overall quality and know-how of companies. Also it will reduce costs and increase lifetime of a transformer.