Electric rail transit systems are large consumers of energy. In trains with regenerative braking capability, a fraction of the energy used to power a train is regenerated during braking. This regenerated energy, if not properly captured, is typically dumped in the form of heat to avoid overvoltage. Finding a way to recuperate regenerative braking energy can result in economic as well as technical merits. In this comprehensive paper, the various methods and technologies that were proposed for regenerative energy recuperation have been analyzed, investigated and compared. These technologies include: train timetable optimization, energy storage systems (onboard and wayside), and reversible substations.Index Terms-Onboard energy storage, regenerative braking, reversible substation, wayside energy storage.

This paper is aimed at proposing a new hybrid method for the islanding detection of Distributed Generation (DG) units. Hybrid method operation is based on the combination of an active and a passive method, for which, Optimized Sandia Frequency Shift (SFS) method is used as the selected active method, and Rate of Change of Frequency relay (ROCOF) is used as the passive method. In order to demonstrate the effectiveness of the proposed technique on islanding detection, several simulation studies based on IEEE 1547 and UL1741 anti-islanding test requirements are carried out on a system equipped with the proposed hybrid method. The evaluation of simulation results reveals that the control system, based on the proposed hybrid algorithm, meets the DG islanding protection requirements efficiently. In other words, not only it holds all the benefits of both SFS and ROCOF, but also it removes their drawbacks by providing smaller Non Detection Zone, improved system's power quality and higher speed of response, and provides some other advantages. Moreover, it will be demonstrated that the proposed hybrid method is capable of accurately operating under multiple DG units, load switching in the grid connected mode, as well as different load quality factor conditions. Index Terms-Inverter-based distributed generation (DG), islanding detection technique, ROCOF relay, SFS method, hybrid method, negligible Non Detection Zone (NDZ), power quality improvement of system, multi DG system.

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