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.@en

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 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

The emergence of distributed energy resources can lead to congestion 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 if a respective market model is applied. In order to take into account this price difference, this paper presents an optimal power flow (OPF) model formulated in terms of voltage and current. In the case of bipolar dc distribution grids, the single line approximation is no longer valid because 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 exact OPF formulation includes bilinear equations. The locational marginal prices (LMP) are derived by linearizing the problem at the optimal solution. Example cases show the various phenomena that can appear under asymmetric loading, such as pole-to-pole connections combined with pole-to-neutral connections, parallel sources, meshed grids and their effect on the LMP.@en

Low voltage dc distribution grids face issues associated with arc faults, aggravated by the absence of current zero crossing. The focus of this paper is to comprehensively develop a method of series arc fault detection at the load side power electronics, based on the electrode dependent initial voltage drop occurring at the arc initiation. The proposed arc detection algorithm is described along with the structure and time constants of the designed bandpass filter. The operational boundaries of the arc detection algorithm are defined for copper electrodes depending on the set threshold voltage and the system parameters, like grid inductance, resistance, and the load capacitance. Further, the detection time and the zone of guaranteed positive detection are depicted. These are validated through test simulations on the state space model of the system. Finally, experimental validation of the proposed scheme is carried out, wherein, a series arc is generated in the dc circuit and the programmed micro-controller provides a real time signal upon detecting the arcing event. The results on variation in detection time with set threshold voltage are also presented experimentally.@en

AbstractDue to an increasing number of power generation units and load devices operating with direct current (DC) at distribution level, there is a potential benefit of leading efforts toward building a DC distribution system. However, the implementation of DC distribution systems faces important challenges, including the market inertia of AC systems and standardization. Many of the benefits that are attributed to DC can only be realized if a complete DC system is developed, and not if only a few components are replaced. This paper presents the concept of a universal DC distribution system, as envisioned by the authors. The universal DC distribution system could be implemented in various use cases, but could also completely replace AC distribution grids. The paper covers the possibilities of having DC nanogrids inside buildings, DC microgrids in neighborhoods, and the connection to AC and DC medium voltage grids. Furthermore, considerations regarding flexibility, electricity market design, control, and protection are presented.@en

The traditional ac power system is challenged by emerging distributed renewable energy sources and an increase in installed load capacity, e.g., electric vehicles. Most of these new resources use inherently dc as do more and more appliances. This poses the question, if they should still be connected on ac in the low voltage grid, which was chosen a century ago, because at that time dc could not be easily transformed to higher voltage levels. In this dissertation, steps are set towards a universal dc distribution system that has the capability of replacing current low voltage ac grids. Standardization is very important at this voltage level due to the high number of connected devices. Therefore, an analysis of the future power system requirements is made and a modular architecture is proposed that consists of connected nano- and microgrids. The dc distribution grid could be meshed and these nano- and microgrids could be connected without a converter separating them, which has significant implications on the overall design of the system. Modular bipolar voltage levels can increase the efficiency of the system, but complicate its operation as well. The exact optimal power flow is formulated for bipolar dc distribution grids with asymmetric loading. It can be used to manage congestions that could affect only one pole. Congestions in distribution grids are likely to increase, due to the increase in installed capacity. They are also more severe in dc grids due to the use of power electronic converters that have very hard limits in comparison with ac transformers. A general method to calculate locational marginal prices between any two nodes in the dc grid is formulated. The optimal power flow formulation is extended to multiple periods in order to include storage operation. Protection is one of the main challenges in creating large dc grids, as short-circuit currents can be very high and there is no current zero crossing as in ac. A low short circuit current protection philosophy is formulated to deal with this. It also addresses the challenge of very low fault current contribution in case of islanded operation. Solid-state circuit breakers are proposed as the main protection devices for dc distribution grids in combination with fast fault detection and clearance. The challenges regarding fault discrimination and selectivity are addressed. Additionally a classification of protection zones in dc distribution grids based on risk is proposed. Experimental measurements of a developed prototype using current derivative tripping are shown. Finally, dc ready devices, that can operate on dc as well as ac, are introduced as a means of simplifying the transition towards dc distribution grids. The losses in the rectification components, when operated on dc instead of ac, are analyzed and it is found that the rectification components of wide input range devices do not need to be enhanced. @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 paper proposes a method to mitigate the problems related to series arcing in low voltage dc (LVdc) micro-grids by performing voltage drop detection using the power electronic device at the load-side. Specifically, the selectivity of the designed series arc detection algorithm is shown for two constant power loads connected in parallel at the point of common coupling with a stable voltage dc micro-grid during accidental unplugging. The influence of the variations in the grid parameters on the load capacitor voltage characteristics and the response of the designed detection algorithm is discussed, thereby, offering insight into the maximum voltage drop and rise time with different circuit configurations due to series arc initiation.@en

This paper presents a breaker arrangement concept, the Multi-Line Breaker (MLB), for the protection of multi-terminal high voltage dc (MTdc) networks. Based on the design of a hybrid breaker, the MLB is an economically attractive solution for the protection of multiple dc lines in nodal connection using a single main breaker path. By using commutation units, the MLB directs the fault current through the main breaker in a unidirectional way, irrespective of the fault location. Hence, this study presents the design requirements for the MLB, regarding both hardware and control, and evaluates its operation within a grid. For this reason, a four-terminal half-bridge MMC-based MTdc grid in radial configuration was used and pole-to-ground dc fault conditions were investigated. The dc fault response of the grid with one MLB at the central node is compared to the respective response of the grid when one hybrid breaker is employed at each dc line. The simulations show that the MLB is feasible and that the overall MTdc grid fault response for the two protection systems is very similar. As a result, the design advantages of the MLB make it a promising solution for the dc fault isolation in MTdc grids.@en

This paper presents a breaker arrangement concept, the Multi-Line Breaker (MLB), for the protection of multi-terminal high voltage dc (MTdc) networks. Based on the design of a hybrid breaker, the MLB is an economically attractive solution for the protection of multiple dc lines in nodal connection using a single main breaker path. By using commutation units, the MLB directs the fault current through the main breaker in a unidirectional way, irrespective of the fault location. Hence, this study presents the design requirements for the MLB, regarding both hardware and control, and evaluates its operation within a grid. For this reason, a four-terminal half-bridge MMC-based MTdc grid in radial configuration was used and pole-to-ground dc fault conditions were investigated. The dc fault response of the grid with one MLB at the central node is compared to the respective response of the grid when one hybrid breaker is employed at each dc line. The simulations show that the MLB is feasible and that the overall MTdc grid fault response for the two protection systems is very similar. As a result, the design advantages of the MLB make it a promising solution for the dc fault isolation in MTdc grids.@en