Raylı ulaşım sistemlerinde tahrik amaçlı katı hal transformatörü kullanımı
The use of solid state transformer for traction in railway systems
Yazar: UĞUR EMRE DOĞRU
Danışman: PROF. DR. ÖZCAN KALENDERLİ ; PROF. DR. MUSTAFA BAĞRIYANIK ; DR. ÖĞR. ÜYESİ OKTAY ARIKAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Raylı Sistemler Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering ; Ulaşım = Transportation
Dizin: Elektrik motorları = Electric motors ; Güç elektroniği devreleri = Power electronic circuits ; Güç transformatörü = Power transformer ; Tahrik sistemleri = Propulsion systems

Özet:
Demiryolları ve trenler, buharlı makinenin icadından bu yana, yüklerin ve yolcuların çok uzak mesafelere güvenli ve etkili bir biçimde taşınması için her zaman önemli bir taşımacılık sistemi olmuştur. Demiryollarına olan ihtiyaç, şehirler ve ülkeler büyüdükçe arttı ve modern dünyada hem diğer araçlara karşı sağladığı enerji verimliliği hem de sağladığı erişilebilirlik ile cazip bir taşımacılık yöntemi haline geldi. Dünyada giderek artan nüfusun etkili ve hızlı bir biçimde taşınabilmesi demiryollarının ülkeler içerisindeki ağırlıklarının artmasıyla gerçekleşmektedir. Aynı zamanda sahip olduğu düşük karbon iziyle, enerji ve iklim krizi yaşayan dünyada demiryollarını önemli kılmıştır. Avrupa Birliği'nin, birlik içerisinde gerçekleştirdiği ortak trafik yönetim sistemi (ERTMS), Çin'in oluşturduğu İpek Yolu projesi ve benzeri projeler demiryolu taşımacılığının modern dünyadaki rolünü göstermektedir.
Demiryollarında kullanılan tahrik sistemlerinin gelişimi yaşanan endüstriyel devrimlerle paralel bir şekilde ilerlemiştir. Elektriğin kullanımıyla başlayan ikinci endüstriyel devrimle birlikte trenler de elektrik motorları kullanmaya başlamışlardır. Dizel ve buharlı motorların düşük verim yoğunluğuna karşı, elektrik motorları iyi bir seçenek olarak ortaya çıkmıştır. Buna karşılık, elektriğin iletilmesi ve üretilmesi bu tipte trenler için problem oluşturmuştur. İlk olarak dizel-elektrik hibrit çözümü denenirken, teknolojinin gelişmesiyle birlikte elektrikli trenler hayatımıza girmişlerdir. Trenlerde elektrik enerjisinin motorlara aktarılmasında en önemli rolü tahrik sistemleri almaktadır. Alternatif akım ve doğru akım çözümleri güç elektroniği teknolojilerinin gelişmesiyle birlikte daha da verimlileşmiş ve yayılmıştır.
Demiryollarında kullanılan mevcut tahrik sistemlerinin yerine, katı hal transformatörleri bir alternatif olarak gelişmiştir. Güç elektroniği teknolojisinin gelişimiyle birlikte 50/60 Hz dağıtım transformatörlerinin yerine yüksek frekanslı işaret kullanan DC/DC çeviricilere katı hal transformatörleri denmektedir. Katı hal transformatörlerinde şebekeden gelen işaret doğrultulur daha sonra bir DC/AC çevirici vasıtasıyla yüksek frekanslı bir AC işarete dönüştürülür. Yüksek frekans tahrik transformatörü vasıtasıyla ikincil tarafa geçen işaret bir AC/DC çevirici ile tekrar doğrultularak, motor sürücü devresine verilir. Kullanılan motorun tipine göre motor sürücü devresi de değişiklik gösterebilmektedir. Katı hal transformatörlerinin en büyük özelliği 50/60 Hz transformatörlerin aksine daha az yer kaplamasıdır. Bu da tahrik sistemini üstünde taşıyan lokomotifler için hacim kazancı sağlayarak daha fazla yolcu ya da yük alınmasına olanak tanır. Ayrıca lokomotifin hafiflemesiyle, trenin atıl kütlesinden kaynaklanan kayıplar azalmaktadır. İkinci bir avantajı ise sahip olduğu modüler yapıdır. Modüler yapısı ile dağıtıcı merkezlere istenilen yapıda takılarak, istenilen güç değerlerinin istenildiği şekilde sağlanması mümkün olmaktadır.
Bu çalışmada, ilk olarak mevcut tahrik yöntemleri incelenmiştir. Daha sonra katı hal transformatörleri tanıtılmıştır. Son adımda ise mevcutta bulunan bir tramvayın özellikleri alınarak devre benzetim yöntemiyle bir katı hal transformatör devresi kurulmuştur. Devrenin içerisinde benzetim sonuçlarında katı hal transformatörü tramvay vagonunun ihtiyaçlarına göre gereken hız, güç ve gerilim değerlerinin karşılayıp karşılamadığı kontrol edilmiştir. Aynı zamanda bu katı hal transformatörünün içerisinde kullanılan rezonans tankının değişen frekansla birlikte çıkış gerilimine olan etkileri de incelenmiştir. Yapılan çalışmanın sonucunda kurulan katı hal transformatörünün tramvayın sahip olduğu değerlere uyumlu bir şekilde çalıştığını, tramvayı temsil eden motorların şaft hızlarına bakıldığında uyumlu bir tekerlek hızı elde edildiği gözlemlenmiştir.

Abstract:
Since the invention of steam engine, railways and trains has always been a fast, efficient and reliable way to transport passengers and freight. The need for a better and extensive railways has continuously increased as the cities and countries developed and became crowded. As the world population and interdependence between countries increase railways became an essential part in transportation. At the same time, railways become advantageous as they have lower carbon footprint when compared with any other land, sea or air routes since the world challenges the climate change and energy insufficiency. European Railway Traffic Management System (ERTMS) project of European Union and modern Silkroad project of China prove the importance of railways as they will being used for connection of cultures and economies.
The development of traction systems of trains is always in parallel with industrial revolutions. As the second industrial revolution greets the electricity into world, trains also had started to be electrified. The use of electric motors has gradually increased.  The high power density of electric motors make them a viable solution against diesel and steam engines. The process started with diesel-electric motors and evolved into fully electrified trains. The increased role of electricity made the electrified traction systems a core design criteria for trains.  The alternative current (AC) and direct current (DC) solutions in traction systems has gradually gained importance because of the increased usage of electrified trains. In addition, the development of power electronics technology made traction systems more and more efficient.
The electrified traction systems of trains started in late 20s. The Mexican Railway had electrified its rail route as the steam engined trains had gradually being abandoned. Another electrification had been made by Brazilian government for Sorocabana railway as the usage of coal and wood became expensive in trains. Earlier systems like examples above generally uses DC systems as mercury arc rectifiers are present at the traction system of train. These two works made by J. B. Cox and Durval Muylaert respectively, mark the early start of electrified traction system for railways. For long-distance freight transportation, the cost is really high as the railway needs energy distribution centers to be constructed. As an alternative solution presented by Bharat Bhargava, the low frequency (16,67 Hz) AC traction systems solves the construction costs problem because as the frequency decreases the distance between distribution centers may become greater. This kind of solution is specifically good for freight trains in USA. Another study by Bhargava categorizes the usage of electrified traction systems according the need for energy. According to the study, the low voltage DC systems are being used when the energy need is low like trams and light rail trains. The voltage level increases as the transportation load increases. AC systems are being used for long distance freight and passenger trains.
The solid state transformer solution for traction systems has appeared as an alternative to traditional AC or DC traction systems. Development speed of power electronics technology rapidly increased with Silicon based semiconductor technology. In early years, semiconductors were being used in series and parallel connections because they were not meeting the needed power level. The rated powers of MOSFETs, diodes and thyristors increased year by year thus the circuits used for high power demand became more reliable. In addition, with the switch control technology, power electronic circuits became efficient and available. Solid state transformers are DC/DC converters with high frequency transformer inside. The general working principle of solid state transformers are formed by four steps. The electric energy coming from grid is rectified. Then the rectified signal is fed to a DC/AC inverter which converter transforms the signal into a high frequency signal at kHz grade. Then by a high frequency transformer, the energy is passed to secondary side. At secondary side there is an AC/DC rectifier which it makes the energy available for motor drive utility. The motor driver unit changes according to motor used in train. One of advantages of solid state transformers is the volume gain. Since the frequency at transformer is significantly increased, the dimensions of transformers are smaller when compared with 50 or 60 Hz bulk transformers. Gain on volume, makes trains available for more passengers or freight. In addition, their modular structure is another advantage. Solid state transformers can work independently as modules thus in case of a need for power demand increase, more modules can be added into distribution centers, if the grid is available.
The use of solid state transformers are not limited to traction systems. It can be also used in distribution systems since it can meet high power and high voltage demands. It can be used at wind, solar and tidal power plants for grid implementation, to charge and discharge the fuel cells connected to grid, VAr compansation and power and harmonics filtering. It can be used at car fast charge station since it give advantage with size and efficiency. Smart grid implementation is also another future area. The main withdraws are cost and reliability since it complicates whole energy transformation process with complex power electronic circuits and high frequency transformers but it will continue to gain momentum as electric traction systems become popular.
In this study, current traction systems was researched then solid state transformers was investigated. Stages of solid state transformers was taken into account one by one. After then a solid state transformer model was proposed. Solid state transformer in this study, was proposed for a tram named Silkworm which is currently used at Bursa railway network. The proposed system includes three main stages; rectifier, solid state transformer and motor driver unit. At rectifier stage, a 12 pulse controlled rectifier was used for rectification process. At solid state transformer stage, two full bridge converter with resonant tank was proposed. The resonant frequency of the circuit is 60 kHz and switching frequency is 56 kHz. To observe whether solid state transformer works or not, a motor driver unit was proposed. three phase full bridge inverter feeds two permanent magnet brushless DC motors. To control the speed of these motors, a PI controller was used. The reference speed of motor shaft was given by a step input. The proposed system was simulated with circuit simulator program PSIM. Then output values of circuit such as shaft speed, train wheel speed, output voltage and power of solid state transformer were investigated.
In conclusion, it was observed that the proposed solid state transformer is a viable solution for traction system of selected tram. Motors used in tram have 2 x 100 kW rated power and the maximum speed is 70 km/h. In simulation it was seen that the proposed solid state power, can feed the motor driver unit with 185 kW, 92,5 kW for each. The train wheel speed is 65 km/h which it is also suitable for tram. Since an LLC resonant converter was used in solid state transformer, the output voltage response against changing switching frequency also investigated. The efficiency of the whole system is 93% which is suitable with solid state transformers currently in the market.