This article is structured to present an overview of a DC ship power system. The main DC grid configurations will be presented and a difference to the AC system configuration will be highlighted. Compared to the AC power system used on board ships, DC has some obvious benefits which will be explored in this paper. These benefits include: improvement of prime mover efficiency and reduction of fuel costs, weight and space savings, unity power factor operation of generators, lower transmission losses, faster and simpler parallel connection of generators and simpler implementation of energy storage. Finally, some of the challenges introduced with the DC technology will also be explored. These include: high short-circuit currents, DC protection concept and expensive and possibly non-profitable energy storage system solutions.
The optimization of overcurrent relays’ operation is a topic associated with protection coordination of distribution networks. Usually, this refers to medium-voltage networks, since they are protected by numerical relay devices, as opposed to low-voltage networks, where utility operators allocate fuses. Correct setting of relays and optimal coordination is becoming a serious challenge to Distribution Network Operators around the world, since their networks’ passive operation has been greatly altered in the past two decades. Distributed generation units, a growing liberalized electricity market and more stringent legislation for distribution network planning and operation by state regulatory bodies have all indirectly affected the evolving of protection philosophy for distribution networks. In this paper the traditional optimization problem of overcurrent relay operation will be addressed and critically examined from both a theoretical and practical point of view. Optimization function, constraints and relay parameters will all be observed and compared with solutions used in distribution networks, and their modifications and improvements will be proposed and elaborated in detail.
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