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The design of protection systems associated with medium and low voltage networks has traditionally been based on the assumption of unidirectional power flows, making the use of time coordinated overcurrent relays an efficient and reliable way of protecting against network faults. The changes that these systems have undergone over the recent decades regarding distributed generation, along with the fact that many of these sources can be connected to form independent microgrids, have challenged this perspective. A number of alternative solutions have recently appeared in technical literature. Therefore, this paper aims to provide a comprehensive overview of the existing proposals for protection design in microgrids. Apart from describing the most relevant options presented to date and classifying them in specific groups, a comparative analysis is performed in which the most important benefits and drawbacks of each approach are presented. Finally, some conclusions and practical recommendations are derived from the analyzed references.

Abstract. Considering the importance that HVDC systems are expected to have in the future of power networks, this paper aims to provide a wide overview of the worldwide existing technologies and installations in the field of MTDC-HVDC systems. In particular, it analyses and describes both classic LCC-MTDC systems and present/future VSC-MTDC systems, the latter being based on multi-level converter technology. This way, it is expected that this paper will draw the attention of those researchers looking for a starting point, from where to be able to develop new analysis and developments that increase the applicability of future HVDC systems.

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Power quality and power disturbances have become an important increasing factor throughout electrical networks. Ferroresonance is one of these disturbances that can occur on distribution systems, causing quality and security problems. This paper analyzes the ferroresonance as a nonlinear resonance phenomenon of erratic nature and difficult prediction. The theoretical principles of this phenomenon and the particular symptoms that identify it will be thoroughly studied. Besides, due to its nonlinear behaviour, several ferroresonance analysis methods commonly used are presented. The theoretical principles are supported by several simulations based on the overvoltage behaviour of voltage transformers.