In a solid state transformer (SST), one of the most important roles is played by the dc-dc converter interfacing the high voltage port and the low voltage port while providing galvanic isolation. This paper proposes a quad-active-bridge (QAB) converter as the basic cell of a Modular Multilevel Converter (MMC) based solid state transformer for the integration of distributed energy resources (DER) and distributed energy storage systems (DESS) into the grid. This study comprises power transfer analysis, modeling and control of the QAB for this application. Furthermore, this paper shows an alternative method based on the star-mesh transformation to built the multiwinding tranformer equivalent circuit, that relates any two ports of the multiple-active-bridge (MAB) converter through a link inductance, which allows to describe power flow. Results are obtained by simulations in PLECS.

No abstract

Dynamic interactions between AC railway electrification systems and traction unit power converters can result in low frequency oscillation (LFO) of the contact-line voltage amplitude, which can lead to a power outage of the traction substation and the shutdown of train traffic. Several system parameters can influence the low frequency stability of the railway traction power system, including contact-line length and traction unit parameters such as transformer leakage inductance, DC-link capacitance, control bandwidths and synchronization systems. This paper focuses on the influence of these parameters on the LFO. The methodology is based on a frequency-domain analysis. Nyquist and Bode diagrams are used to determine the stability limit. The validation of the method is performed through the use of time-domain simulations.

No abstract

No abstract