DC Interface Impedance Shaping for DC/AC Grid Forming Converters in Railway Applications

Among the different groups of stability issues that railway systems have to face are the electrical resonance instability phenomena. Within this group, one of the possible instability problems that appear is due to the constant power load (CPL) behavior of the traction and the auxiliary power system (APS) converters. This behavior interacts negatively with the variable impedance of the catenary and can cause catenary voltage fluctuations and compromise the stability of the railway line. To solve this problem, this article proposes the use of a structured H infinity synthesis to design an ac voltage controller for the APS converter. The algorithm models the dc input admittance of the converter addressing three objectives: catenary stability, tracking of the ac output voltage reference, and robustness against unmodeled uncertainties. Experimental results with different types of loads (resistive and/or induction machine loads) demonstrate its compliance with railway standards and the modeling of the specified admittance.