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This study is focused on the topical issue of increasing the energy efficiency in DC railway systems, in the context of global concerns for reducing the CO2 emissions by minimizing the energy consumption and energy loss. The main achievements in this complex issue are synthesized and discussed in a comprehensive review, emphasizing the implementation and application of the existing solutions on concrete case studies. Thus, all specific subtopics related to the energy efficiency are covered, starting with power quality conditioning and continuing with the recovery of braking energy, of which a large part is lost in the classic DC-traction substations. The solutions of onboard and wayside storage systems for the braking energy are discussed and compared, and practical examples are given. Then, the achievements in transforming the existing DC-traction substations in reversible substations with capabilities of power quality improvement are systematically reviewed by illustrating the main results of recent research on this topic. They include the equipment available on the market and solutions validated through implementations on experimental models. Through the results of this extensive review, useful reference and support are provided for the research and development focused on energy efficient traction systems.

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This paper is concerned with the possibilities of transformation the DC traction substations with uncontrolled rectifiers into active substation, being able to ensure both the braking energy recovery and active filtering function. There are a lot of approaches on the management of regenerative braking energy, most of them based on energy storage systems or the concept of reversible substation. Few references refer to the addition of complementary functions, such as harmonic filtering, and involve the adoption of specific structures without further argumentation. In this paper, the intention is to analyze and find argumentations on the possibilities of coupling a three-phase active power filter based on voltage source inverter topology in the power structure of a DC traction substation. The most used types of uncontrolled rectifiers, i.e. the three-phase bridge scheme and both 12-pulse parallel and series structures are taken into consideration. Based on correlation between the DCline and AC-line voltages, it is shown that the active power filter can be connected directly in the secondary of the existing traction transformer only in the case of 12-pulse series rectifier. A dedicated recovery transformer is required for the other two schemes of rectifiers and some considerations and results on the compensating power and correlation between the active filtering performance and the voltage in the recovery transformer primary are presented.

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The sound attenuation in a sonic composite without/with point defects is studied using a new method that combines the features of the cnoidal method and the genetic algorithm. Acoustic scatterers are composed by piezoceramic hollow spheres of functionally graded materials—the Reddy and cosine graded hollow spheres. This method enables to obtain the dispersion relation for defect modes, and the prediction of the evanescent nature of the modes inside the band-gaps.

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