Solar Home Systems (SHS) have proven to be an effective means to tackle the global energy poverty that still affects around 1 billion people. However, present-day SHS (which are standalone systems with usually a purely dc architecture) have a limited power rating (usually up to 100 Wp). To enable higher power levels of electricity access in an economically viable way, energy sharing between these individual SHS through interconnectivity is a logical progression. The interconnectivity has to be implemented at a higher voltage level in order to reduce the conduction losses and cable costs. Existing control schemes do not take into account the multi-voltage dc microgrids. In this paper, the state of charge (SOC) balancing in such an interconnected SHS-based dc microgrid is addressed. In particular, the adaptive droop-based SOC control is extended for multiple voltage levels in a dc microgrid without any means of active communication. This is achieved through the creation of a voltage dead-band, SOC-based droop resistances, and the use of voltage ratios in dc-dc converters.

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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 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.

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DC distribution grids are an option for future smart grids in order to directly connect distributed energy resources like photovoltaics, storage, electric vehicles and loads that already use dc internally. Due to the increased installed power capacity, voltage deviations and line congestion are likely to occur. Exact optimal power flow with locational marginal prices (LMP) is a way of tackling this problem.In this paper a fully distributed solution for the exact optimal power flow problem in dc distribution grids is presented. It uses the consensus and innovations approach and includes line losses as well as line current limits and over- and under-voltage bounds. Zero marginal cost and linear cost functions for generators and loads are possible in addition to quadratic cost functions. A simulation example shows initial results on a 4-node test network.

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Flexible power flow control is one of the main challenges in the development of the meshed low voltage direct current distribution system. The most widely adopted approach to achieve flexible power control in the network is to use various solid-state based solutions which are rated for the peak power of the grid. In this paper, we propose a solution based on a converter which has several times smaller power rating than the grid power rating. Due to the multi-port nature of the proposed solution, it is well suited for the bipolar networks.

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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.

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In meshed low voltage DC distribution grids, one of the main challenges is achieving controllability of the power flow. The most common approach to achieve the full control over the power flow in the meshed DC grid is to deploy DC-DC converters rated for the full power rating of the grid. The drawbacks of this solution are costs and losses of such DC-DC converters. To reduce the costs and the losses from the system standpoint, we introduce a partially rated power flow control converter. This paper presents the control modes of the partially rated power flow control converter and experimentally demonstrates its functionality. The proposed power flow control converter consists of two cascaded converters. On one side the converter is rated for the maximum grid voltage but only a fraction of the grid current, and on the other side, it is rated for the maximum grid current but only a fraction of the grid voltage. The proposed converter can operate in three modes and demonstrates the power flow control capability.@en

The two main challenges of meshed low voltage DC grids today are the flexible control of power flow and the short-circuit protection. The conventional approach to deal with both problems is to incorporate galvanically isolated DC-DC converters with integrated short-circuit protection, which are rated for the full power rating of the grid. In this paper, we describe an approach based on the combination of a converter which has a partial power rating with respect to the grids power rating and a circuit breaker with full rating with respect to the grid. Based on the review of abnormal operating conditions of the power flow control converter and its requirements on protection. We propose a new protection strategy for the power flow control converter and evaluate the applicable circuit breaker technologies based on the review of the protection requirements.

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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.

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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

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

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.

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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.

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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.

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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.

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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.

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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.

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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.

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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 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