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The increasing amount of distributed energy resources requires significant changes to today's power systems. Most of the distributed resources are dc inherently or have a dc link. Therefore, connecting them with a dc distribution system seems beneficial. This paper presents the opportunities and challenges of dc distribution systems, starting with the requirements of future power systems. It will be looked at how dc can fulfill them with meshed grid architectures, increased system availability and new market models. New protection strategies for large dc distribution systems and their open research questions are discussed.978-1-4799-9880-7/15/$31.00 ©2015 IEEE

Meshed dc distribution grids have advantages over radial connected ones. Challenges arise when this is done on the same voltage level and the full potential power might not be usable in certain configurations. This paper shows how the power flow in meshed dc grids can be increased by limiting the currents at congested lines. This can be done in a decentralized way with only local current measurements. Several realization possibilities for current limiting devices are introduced and discussed. Dynamic simulations are performed.978-1-4799-9880-7/15/$31.00 ©2015 IEEE

This paper describes the behavior of voltage weak dc microgrids. These are dc microgrids with a relatively small system capacitance. The large amounts of stored energy in the passive component of a network has a considerable effect on the size of the fault currents, control and reliability. The use of a complex control approach to stabilize voltage weak microgrids is a possibility that requires further attention. In this paper, however, a simple way to stabilize such a system is proposed by limiting the rate of change of the power electronic interfaces. The small signal analysis of a three node system is analyzed to see the effect of system capacitance, inductance, resistance and PI and Droop values on the control of the system. The small signal analysis is implemented to estimate the rate of change of loads and the effect of step load changes on the system. In the modeling, a combination two types of loads, Constant power loads, and resistive loads, is used to see the effect of on the system stability. The source and converters are modeled as droop controlled current sources in parallel with capacitors.

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