Voltage Unbalance Mitigation Strategies for Single-Phase Railway Systems

The electrification programme initialized by Banedanmark aims at electrifying the railway lines of the Danish territory, starting from the section between Esbjerg and Lunderskov. Electric trains present several advantages over the former diesel locomotives in terms of energy efficiency, speed and environmental impact.

One of the main concerns of this programme is addressed to the power quality of the transmission grid. The electric high-speed railway line built in Lunderskov is a single-phase type, connected across two phases of the transmission grid. During the acceleration of the trains, a significant but brief power demand occurs, leading to the generation of positive and negative sequence currents flowing from the grid to the railway lines.

While positive sequence currents are normally found in operative power systems, negative sequence currents cause voltage and power unbalance, threatening the stability of the components attached to the grid. The work presented in this thesis is finalized to the design of three different solutions for the mitigation of the caused voltage unbalance, through the implementation of a control strategy for flywheels energy storage and Negative Sequence Compensators (NSCOMs).

The initial part of this work is focused on the study of the concerned railway system. The measurements of the traction current undertaken at the substation of Andst show a high content of low-frequency harmonics. Moreover, consistent transients have been detected, typically generated when the trains leave the neutral zone of the railway line.

Such results are taken into account when designing the solutions for the voltage unbalance mitigation, as they can highly affect the performance of the components involved. The study then moves to the calculation of the positive and negative sequence currents absorbed by the railway system and to the estimation of the caused voltage unbalance.

Being the traction load a single-phase type, an equal amount of positive and negative sequence currents is absorbed by the railway lines. From the negative sequence currents and the single-phase traction power, it is possible to find a direct relationship with the caused voltage unbalance, setting the foundations of the solutions investigated in this work.

The first solution consists in the partial compensation of the negative sequence currents absorbed by the railway system through the use of a NSCOM. Similarly to the Static Synchronous Compensator (STATCOM), the NSCOM is a three-phase AC/DC converter connected across the lines of the transmission grid, in the proximity of the interested railway system.

The second solution, instead, makes use of a flywheel energy storage device for the implementation of the load management strategy called power-peak shaving. The flywheel is connected across the terminals of the railway system through a back-to-back converter with the objective of curtailing the peaks of traction power absorbed from the transmission grid.

The simulation results provided in this work prove that both solutions are able to maintain the voltage unbalance of the grid within the levels admitted by the transmission system operator. On the one hand, the NSCOM represents the cheapest solution, since is makes use of only one AC/DC converter. On the other hand, the flywheel energy storage has the additional benefit of compensating both the positive and negative sequence currents absorbed by the railway system, decreasing the overall power consumption and the costs related to it in the long term.

On the light of the obtained results, a third hybrid solution has been investigated to merge the benefits of the NSCOM and the flywheel. The designed hybrid system consists in a flywheel connected across the the grid lines through a three-phase back-to-back converter. The main advantage of this system is the ability of compensating the positive and negative sequence currents independently, allowing the installation of a smaller flywheel while providing the same voltage unbalance attenuation.

It has been demonstrated that in the case of a single railway line attached to the grid, the investment costs of the hybrid solution are more competitive than those of the flywheel solution when the ratio between the compensated positive and negative sequence current is below 85%. The thesis is closed with the experimental validation of the NSCOM solution.

The experiments are based on Power Hardware-In-the-Loop (PHIL) simulations finalized at emulating the response of the grid to the generated negative sequence currents. Particular attention is paid to the different interface algorithms and integration methods used for the resolution of differential equations.

The obtained outcomes show that the Ideal Transformer Method based on the Root-Matching technique can provide accurate results, while maintaining the simulated system stable with a relatively low computational burden. This test confirms the ability of the NSCOM to compensate the negative sequence current and decrease the voltage unbalance of the grid.