Analysis, simulation and operation of advanced converter-based electrical railway power supply systems for high-speed lines

Resumen

Railway is a key player in the construction of more efficient and environmentally friendly transport systems. Within the different railway modalities, high-speed lines seem to be attracting special attention due to their velocity, reliability, safety and comfort. At present, these lines are typically electrified by means of conventional transformer-based substations that must be connected to utility grids with high short circuit capacity because of their high energy consumption and their single-phase nature. This fact involves a significant increase in the cost of the electrical installation that seriously limits their advancement. In this situation, researchers are proposing new electrical power supply systems based on modern electronic power converters capable of minimizing the principal drawbacks of conventional transformer-based configurations. One of the most promising ones are the so-called advanced systems that combines the use of reversible and totally controllable converter substations with a continuous catenary scheme. These systems enable not only to balance the load, and thus reducing the short circuit capacity of the utility grid needed, but also to perform an optimal control of the power flows within the railway grid. This latter aspect involves a real qualitative leap in railway electrification that allows to align them with the current electrical power system transformation towards smart grids. The development of simulation tools becomes therefore a crucial factor for the progress of these new electrical power supply configurations. Railway simulation is a complex task due to the singular characteristics presented in these systems that has been aggravated with the new difficulties brought by the advanced schemes. In this context, the thesis proposes a new simulation tool capable of determining with great detail the operation of both conventional transformer-based and advanced converter-based configurations. Using a decoupled model, the tool is divided into two parts. The first part is devoted to the simulation of the traffic system. It determines the consumption and position of the trains over the time considering the topographic profile of the line, the kinematic limitations of the sections and the dynamic characteristics of the motor and the brake. The second part is devoted to the electrical system simulation. It determines the voltage and current distributions of the conductors, as well as the power flows of both railway grid and electrical grid. To this end, a multiphase power flow based on a modified nodal analysis has been implemented, that includes a mathematical representation capable of integrating the AC and the DC equations. Finally, the power flow has been solved using the complete version of the Newton-Raphson method and a new initialization procedure has been proposed. The next topic addressed in the thesis has been the study the operation of the advanced systems. In contrast to conventional transformer-based ones, the operation of the advanced systems is determined by the type of control implemented. In this respect, the thesis has developed a control strategy based on the droop approach and valid for both alternating current and direct current. This strategy distributes the electrical power according to the distance between the trains and the substations, but it is capable of increasing or decreasing the degree of cooperation between traction substations depending on the conditions of the utility grid without the need of communication between their elements. In the case of alternating current, it is necessary a communication system that synchronizes the angles of the converters. Finally, the effectiveness of the control strategy has been validated by the simulation tool previously described and compared with the operation of the conventional transformer-based systems for a simple electrical scheme. Finally, the simulation of a complete high-speed line has been carried out, specifically the Spanish line connecting the cities of Madrid and Valencia. This analysis has made it possible firstly to verify the robustness of the simulation tool and the effectiveness of the control system for larger and more complex systems. Secondly, it has allowed to analyze the feasibility and potential of advanced converter-based systems over conventional transformer-based systems for the electrification of high-speed lines.