Distributed Generation has created the need for new types of smart dc grids developments. The multiple generation sources, bi-directional power flow, power flow time co-ordination and management bring significant benefits as well as challenges for current ac electrical grids and emerging dc microgrids. In particular, the effect of voltage weak distribution systems on protection concepts and approaches needs to be understood, investigated, and accounted for. This paper describes principles of protection of voltage weak dc grid concepts. New alternative protection designs are proposed and their benefits and challenges are discussed. Likewise, special circuit configurations are proposed to improve fault discrimination. Fault-induced voltage transients are used to detect and isolate faulty part of the dc distribution grid.@en

This paper reviews the currently available dc micro-grids and dc grids. Currently, commonly existing and proposed dc microgrids are designed to provide sufficeint energy during system disturbance as in ac grid and, are mainly voltage stiff systems. It also compares the voltage stiff systems to a low energy system, which represents voltage weak dc system, from systems point of view. First, the definition of voltage weak dc systems is provided from a system perspective. Comparisons of both dc systems are reviewed on stability complexity, fault detection and isolation strategies and devices, and power management approaches. Various challenges and opportunities of voltage weak dc systems are briefly addressed. And also several architectures and topologies for voltage weak dc distribution systems are reviewed. Finally, this paper identifies a comparative advantage of such systems from a control, protection and system operation standpoints.@en

This paper describes the behavior of voltage weak DC distribution systems. These systems have relatively small system capacitance. The size of system capacitance, which stores energy, has a considerable effect on the value of fault currents, control complexity, and system reliability. A number of potential definitions of voltage weak DC distribution systems are proposed. These definitions address the main characteristics of voltage weak systems. A small signal model of a general voltage weak DC distribution system is developed in order to observe sufficient conditions for system stability. This is achieved by analyzing the dominant poles. The source converters are modeled as droop-controlled current sources in parallel with their respective terminal capacitors. As constant power loads have incremental negative impedances, which affect the system stability, especially, in voltage weak system, ideal constant power loads with their terminal capacitors are included in the small signal model. A three-node voltage weak DC distribution grid is analyzed, as a case study, by implementing the developed small signal model. The effects on system stability by the values of system capacitance, cable inductance, and cable resistance are investigated using dominant pole placement. Likewise, the influence of proportional-integral regulators and droop coefficients of source converters on the stability of the system is examined. Finally, the three node DC distribution grid is developed in MATLAB/Simulink in order to demonstrate the influence of small capacitance on system stability. Moreover, effect of the rate of change of constant power loads on the system stability is simulated. These results are further compared and verified on a voltage weak DC experimental test bench with a 350 VDC bus voltage.@en

DC microgrids can connect directly dc renewable energy sources with increasing amount of dc loads. In this paper it is looked for possible architectures for integrating PV panels into dc microgrids, by means of microconverter strings. Three topologies are considered, featuring only buck microconverters and only boost microconverters, since they promise higher efficiency due to fewer semiconductors in the current path. The topologies under exam are tested with a perturb and observe MPPT, in the cases of abrupt local shading and uneven shading over a solar panels' array. Among the three, the best topology by response time and control ease is found.@en

The traditional ac power system is challenged by the increased amount of distributed energy resources. Enormous changes and investments are necessary in order to achieve an ac smart grid capable of coping with the challenges introduced. In dc, solutions seem to be more straightforward since most of the distributed energy sources and most of the loads connected to the low voltage grid operate with dc. For this reason, dc distribution systems should be considered as an alternative with significant potential. Nevertheless, nowadays research is mainly focused on local dc nano- and microgrids. This paper introduces relevant aspects related to dc distribution networks. A wide field of opportunities and challenges are briefly exposed.@en

This study evaluates the dynamics of DC microgrids (DCMG) with the integration of voltage dependent demand response (VDDR). Previous work evaluates price prediction models for grid operators to estimate local price as a demand side management (DSM) technique. VDDR has been proposed to implement DSM without the need of a dedicated communication link. The dynamic response of a DCMG with VDDR has been evaluated in a situation with limited source power output. This has been done with two different VDDR controller logics: dynamic and fixed hysteresis control. Furthermore, the effect of current rate of change control is presented. The results provide further understanding on the dynamic behaviour of DCMGs, which can be used to create a guideline for the design of source and load control systems.@en

This paper introduces a new power system that can be implemented on all levels by using the bottom-up approach of system design. It is based on orthogonal power flow at different frequencies. The power exchange between different elements of the grid as well as the additional features are based on implementing power electronics (PE) interfaces and applying multiple frequencies (zero- DC is also a valid frequency). Thus, the micro-grids are decoupled into multiple systems which can then be controlled independently, paving the way for a more flexible and stable general system. By implementing multiple frequencies (MF) additional features can be achieved that would otherwise not be possible with conventional AC, newly- emerging DC systems or any of their hybrid forms. In this paper, the concept of orthogonal power transfer, which is the basis for this system will be presented, and the multifrequency system will be defined and compared to existing solutions. Finally, this novel concept will be demonstrated on a simulation incorporating renewable energy sources, energy storage and a constant power load.@en