The use of solid state transformer for traction in railway systems

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.