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.

This article describes the circulating current phenomenon that takes place in bipolar, meshed DC distribution grids. A difference in resistance between the conductor changes the distribution of the current on the conductors. This provokes a net current to flow, which is in turn measured by the residual current protection scheme, used to detect and isolate ground faults in the system, even though there is no fault in the system. Due to the circulating tendency of this net current, the operation of the protection scheme can be prevented in such a case by the addition of a clever communication system.

Inside the meshed LVDC distribution grids the power flow is predominantly limited by the line impedances. In order to achieve economical and flexible operation of the meshed LVDC distribution grids, it is required to control the power flow. For that end, the line impedances need to be adjustable. The line impedance can be controlled by a DC-DC transformer, however, it needs to be rated for the full grid power. In order to reduce the installation costs, a partially rated device is desirable. Therefore, a partially rated power flow control converter (PFCC) is proposed. This paper presents the PFCC performance estimate and demonstrates the PFCC functionality. The PFCC is an economical solution for increasing the controllability of the power flow in the meshed LVDC distribution grids. However, due to high step down ratio inside the PFCC achieving efficient performance is challenging. Furthermore, due to unavoidable MOSFET paralleling the part count rises. The proposed PFCC consists of two cascaded converters, therefore the control range and stability depends, besides else, on the impedance interaction between the two stages.

Connecting large dc distribution grids on the same voltage level can allow superior utilization of infrastructure. Selective ground fault protection is necessary and can be done using low impedance grounding. Distributed energy resources would allow for higher resilience of the grid if islanding operation of dc microgrids is allowed. For safety, grounding is essential in these islanded microgrids so individual grounding points are needed. Multiple low impedance grounding points would cause dc ground currents that lead to corrosion. This paper introduces capacitive grounding which is high impedance in steady-state effectively eliminating ground currents but is low impedance for fault transients and thus can allow for selective ground fault protection. The sensitivity of the capacitor sizing for two grounding points is shown. Further an initial filter for ground fault discrimination is analyzed.

Nodal pricing or LMP is one option to deal with congestion in the distribution grids that are expected due to the emergence of electric vehicles. When optimal power flow (OPF) with losses is used to determine optimal dispatch and locational marginal prices (LMP) of a power systems, the prices include marginal losses. Marginal losses are double the amount of the real physical losses. This leads to over collection and higher prices for costumers far from generators. In this paper current pricing is proposed as a method to get nodal prices without marginal losses. DC grids are used for simpler exact modeling with an OPF formulation in terms of voltage and current instead of power. The nodal prices without marginal losses are derived by linearizing the quadratic OPF problem by only fixing the voltage, instead of using taylor approximation. An example illustrates the difference of nodal prices with marginal losses and current pricing.

This paper presents a transient analysis of various configurations of DC distribution systems. A combination of theoretical and practical methods were used to investigate the behavior of the system's variables after the occurrence of faults and the protection operation. A number of networks of varying complexity were built with detailed cable models using the EMTP/ATP software package in order to simulate a multitude of faults and assess each network's response at various positions and most crucially, the effectiveness of the protection scheme in reducing the impact of the transients after the fault. The travel speed of the transients was also investigated. At the same time, a set of measurements was conducted on a DC street lighting network to study the transient behavior of a real system. Another model was built to resemble the measured system, in order to compare between the simulation and measurement results. Finally, the evaluation of the measurement and simulation results leads to conclusions that will contribute to the creation of better, safer and more versatile DC distribution grids.

The emergence of distributed energy resources can lead to congestions in distribution grids. DC distribution grids are becoming more relevant as more sources and loads connected to the low voltage grid use dc. Bipolar dc distribution grids with asymmetric loading can experience partial congestion resulting in a nodal price difference between the two polarities. In order to take into account this price difference, this paper presents an exact optimal power flow (OPF) model, formulated in terms of voltage and current. In the case of bipolar dc distribution grids, the common ground plane assumption cannot be used as current can flow in the neutral conductors as well. Moreover, loads and sources can be connected between any two nodes in the network. The proposed OPF problem formulation includes bilinear cost functions, therefore the locational marginal prices (LMP) are solved in a second step.

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.