Tema da tese: Rail Power Conditioners Based on Modular Multilevel Converter in AC Railway Networks

Autor: Mohamed Tanta

Programa Doutoral: Programa Doutoral em Engenharia Eletrónica e de Computadores

Orientador: João Luiz Afonso; António Pina Martins

Data: 02/11/2020

Abstract: Railway systems have progressively been developed since James Watt presented a technique of converting steam power into a circular movement back in 1763. With the novelty of steam engines at that time and the increasing of railway networks, railway industry quickly became an economic catalyst throughout the world due to the advantages of passenger and freight transport. In 1879, Siemens & Halske company introduced the world’s first electric train in the city of Berlin, consisting of a locomotive and three wagons, and supplied via an insulated third rail with 150 V direct current (DC). From that time, the world has begun to recognize the important transition from steam power to electric power, and the potential of the electrified railway as a mode of mass transport.

Due to the plenty of fuel in the last century, Diesel trains were not only common, but they also dominated the railway sector for a few decades. Consequently, the development in the infrastructures of electric trains decelerated, and the path to having fully electrified trains was long enough. In this context, electric trains have introduced progressively, in which Diesel and electric power have been combined to create hybrid locomotives. However, and with the increased demand for transportation and the higher fuel prices in the last decades, electric trains can substantially offer lower operating costs and lower emissions compared with the Diesel‑powered trains.

Nowadays, most of the high-speed electric trains use alternating current (AC) power supply for their traction power systems, which provide better performance under long-distance power transmission than DC power supply. However, as the need for railway transportation increases due to more passengers and higher mobility requirements, more flexible and efficient traction systems are always needed.

In Europe, AC traction power systems are mainly classified according to the voltage and frequency parameters (15 kV, 16.7 Hz) or (1×25 kV or 2×25 kV, 50 Hz). In all cases, railway operators have an absolute interest to run the electrified trains with the lowest possible operation and maintenance costs. In this context, power quality improvement at the three‑phase power grid, associated with the AC electrified railway has drawn more attention in the last decades, especially after the evolution in the Power Electronics field. Subsequently, various solutions based on Power Electronics converters have been proposed to improve power quality in the electrified railway, e.g., the flexible AC transmission systems (FACTS).

The rail power conditioner (RPC) is one of the FACTS devices that can be used to improve power quality by compensating harmonic contents, reactive power and negative sequence components of currents generated by the railway system. Among the other possible multilevel power converters, the modular multilevel converter (MMC) is an attractive solution for medium-voltage applications due to harmonics reduction, lower switching losses, and higher flexibility, scalability and reliability. Therefore, the MMC has been enhanced to be combined with the FACTS family.

Taking into consideration the existing opportunities in the railway industry, not only in the development of the electric train itself, but also on the power quality improvement in the electrified railway, there is a strong investment in technological development for electrified railway systems. Therefore, this work presents a new topology of Power Electronics converter (RPC based on MMC) that compensates power quality problems associated with traction power systems, thus, reducing the operating costs of the electrified trains and increasing the power capacity of the electric traction grid.

The main innovations of the RPC based on MMC are the integration of the MMC topology to operate as a railway power quality conditioner, benefiting from the advantages of the MMC in the traction power supply system. In this context, the research work proposed and developed in this Ph.D. thesis aimed to design, develop and validate a reduced‑scale laboratory prototype of the RPC based on MMC, including all the necessary control algorithms and simulation models that are important to support the correct operation of the proposed system.

Under simulation conditions, this work developed control algorithms for different RPC topologies, (full‑bridge, half‑bridge, three-wire, etc.) for demonstrating the general capabilities of the RPC system, and also for two different transformers connections (V/V and Scott). The most favorable RPC based on MMC topology (based on half‑bridge MMC) was deeply and extensively simulated, namely employing predictive control approach. The experimental results obtained from a developed reduced-scale prototype confirm the validity of the presented control theory, as well as the power quality improvement capability of the proposed solution.

Keywords: Electrified Railway Systems, Flexible AC Transmission Systems (FACTS), Harmonic Distortion, Modular Multilevel Converter (MMC), Negative Sequence Component (NSC), Power Electronics, Electric Power Quality, Rail Power Conditioner (RPC), Traction Power System (TPS).