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This paper presents a protection framework for low voltage dc grids, which segments these grids into zones and tiers according to their fault current potential and provided protection. Furthermore, the technology and applications of different protection devices are examined. It is demonstrated that the utilization of fast fault interruption and fault limiting inductors are vital for the protection of low voltage dc grids. Moreover, a design of a solid-state circuit breaker is presented, and this devices is experimentally verified. The experimental results showed that the total time for the detection and interruption of faults can be lower than $1 \mu \mathrm{s}$ with solid-state protection devices.

This paper presents a breaker arrangement concept, the multiline breaker (MLB), for the protection of multiterminal 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 multilevel modular converters-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.

This paper presents the design of an energy management system (EMS) capable of forecasting photovoltaic (PV) power production and optimizing power flows between PV system, grid, and battery electric vehicles (BEVs) at the workplace. The aim is to minimize charging cost while reducing energy demand from the grid by increasing PV self-consumption and consequently increasing sustainability of the BEV fleet. The developed EMS consists of two components: An autoregressive integrated moving average model to predict PV power production and a mixed-integer linear programming framework that optimally allocates power to minimize charging cost. The results show that the developed EMS is able to reduce charging cost significantly, while increasing PV self-consumption and reducing energy consumption from the grid. Furthermore, during a case study analogous to one repeatedly considered in the literature, i.e., dynamic purchase tariff and dynamic feed-in tariff, the EMS reduces charging cost by 118.44 $\%$ and 427.45 $\%$ in case of one and two charging points, respectively, when compared to an uncontrolled charging policy.

Due 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.

This paper discusses a qualitative comparison between Two and Three-Level Voltage Source Converter (VSC) topologies for battery energy storage applications. Three-Level Neutral Point Clamped (NPC) and T-Type circuit topologies are benchmarked versus the state-of-art Two-Level VSC in terms of efficiency and power density considering a 100 kW system. Analytical equations for determining the power losses in the semiconductor modules are given, and the procedure for designing the output LCL filter and the DC-link capacitors is described. The analysis, based on off-the-shelf circuit components, shows that the Three-Level topologies perform better than the Two-Level one in both considered metrics, mainly due to their lower switching losses that allow operating at higher switching frequency without significantly degrading the system efficiency, and, at the same time, increasing the system power density. Additionally, it is found that the T-Type topology shows better performances than the NPC topology at full and high partial loads, being then more suitable for applications that require most of the operation at maximum power.

Parallel ac-dc reconfigurable link technology can find interesting applications in medium voltage power distribution. A given system can operate in different configurations while maintaining equivalent capacity during (n-1) contingencies. It is proved that within the defined operating boundaries, a parallel ac-dc configuration has higher efficiency as compared to pure ac or pure dc power delivery. Using sensitivity analysis, the variations in these efficiency boundaries with power demand, power factor, grid voltages, link lengths, conductor areas and converter efficiency is described. It is shown that parallel ac-dc system can have smaller payback time as compared to a purely dc power transmission for the same capacity due to lower investment cost in converter station and superior efficiency. As compared to a purely ac system, the payback of a refurbished parallel acdc configuration can be less than 5 years for a 10 km, 10 kV distribution link within the specified assumptions and operating conditions., Comment: \c{opyright} 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, including reprinting/republishing this material for advertising or promotional purposes, collecting new collected works for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works

Flexible dc interlinks can be used for compact and efficient active power redirection by creating a meshed structure in existing radial distribution network. It is shown that with optimal location and sizing of a back-to-back voltage sourced converter based point-to-point dc link connecting two ac buses of the test 33-bus radial distribution system, 10-15% network loss minimization can be achieved. This possibility can be advantageous in efficiently redirecting active power flows in urban and semi -urban areas because intermittent distributed grid resources (such as photovoltaics and wind farms) and energy intensive loads (such as ultra fast charging stations for electric vehicles) are resulting in local pockets of excess generation and demand. The impact of distributed generation and high powered loads on placement of dc interlink is numerically quantified to suggest that both capacity and efficiency enhancement can be achieved in the distribution grid. It is envisioned that this method of integrating dc power transmission can be the first realistic step towards realizing a multi-terminal dc medium voltage distribution evolving out of the existing infrastructure.

Since the voltages and currents in dc grids do not have a natural zero-crossing, the protection of these grids is more challenging than the protection of conventional ac grids. Literature presents several unit and non-unit protection schemes that rely on communication, or knowledge about the system&rsquo, s topology and parameters in order to achieve selective protection in these grids. However, communication complicates fast fault detection and interruption, and a system&rsquo, s parameters are subject to uncertainty and change. This paper demonstrates that, in low voltage dc grids, faults propagate fast through the grid and interrupted inductive currents commutate to non-faulted sections of the grid, which both can cause circuit breakers in non-faulted sections to trip. A decentralized plug-and-play protection scheme is proposed that ensures selectivity via an augmented solid-state circuit breaker topology and by utilizing the proposed time-current characteristic. It is experimentally shown that the proposed scheme provides secure and selective fault interruption for radial and meshed low voltage dc grids under various conditions.

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.

Inductive power transfer (IPT) is gaining popularity across a wide range of battery charging applications like biomedical, consumer electronics and electric vehicle (EV) charging. One of the major challenges in designing IPT charge pads is determining the optimal physical sizes of the magnetic couplers resulting in efficient power transfer and low cost of materials. In EV applications, it is especially difficult due to the variation in nominal air gap, required power levels associated with different vehicle classes, and charging locations that may be encountered. This paper aims to determine the relationship between optimal coupler sizes and the nominal air gap of an IPT system. Finite element analysis (FEA) is used to model the electromagnetic behavior of the magnetic couplers. A multi-objective optimization framework is built to reveal the Pareto fronts which show the trade-offs between the power transfer efficiencies and the coupler power densities at different air gaps. This method is applied on polarized double-D (DD) couplers for a 5 kW IPT system at different air gaps. Analyzing the power densities of the Pareto Optimal designs an approximate relation between optimal pad sizes and the air gap is derived. Results show that there is an exponential relationship between the optimal coupler sizes and the nominal air gap.

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.

This paper provides an empirical study on series arc behavior in low voltage dc microgrids. The response of an R-L-C dc microgrid abstraction towards series arcs is studied experimentally for varying grid inductance, dc voltages, load capacitances and load currents. In order to account for the stochastic nature of arcs, experiments are repeated multiple times under similar conditions to gain statistical significance. Thereby, insight on percentage occurrence and burn time of initiated series arcs is provided. Load side voltage response is studied to gain insight on the expected peak drop and fall time. This empirical evidence was judged to be a necessary requirement in developing a novel series arc extinguishing method from load side power electronic devices.

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.

This paper considers a three-phase grid-connected photovoltaic (GCPV) system, which comprises a 120kW PV farm connected to the grid through a step-up transformer. GCPV is considered as a distributed generation (DG) serving a variable local load (LL) connected to the low voltage side of the transformer. A power conditioning unit (PCU) consisting of a two-stage energy conversion device is used to (i) maximize the extracted PV farm output power, and (ii) control power exchange with the grid. The PCU regulates the LL voltage level by means of reactive power compensation (RPC). The proposed system is equipped with an intelligent energy harvesting method based on particle swarm optimization-subgradient algorithm (PSO-SG). It is shown that the system operates efficiently using the proposed PCU. Validation of the model has been carried out by using MATLAB/Simulink.

It is anticipated that with the thrust towards use of clean energy resources such as electric vehicles, future distribution grids will face a steep increase in power demand, forcing the utility operators to invest in enhancing the power delivering capacity of the grid infrastructure. It is identified that the critical 5–20 km medium voltage (MV) underground ac distribution cable link, responsible for bulk power delivery to the inner urban city substation, can benefit the most with capacity and efficiency enhancement, if the existing infrastructure is reused and operated under dc. Quantification of the same is offered in this paper by incorporating all influencing factors like voltage regulation, dc voltage rating enhancement, capacitive leakage currents, skin and magnetic proximity effect, thermal proximity effect and load power factor. Results are presented for three different ac and dc system topologies for varying cable lengths and conductor cross-sections. The computed system efficiency is enhanced with use of modular multilevel converters that have lower losses due to lower switching frequency. A justified expectation of 50–60% capacity gains is proved along with a generalized insight on its variations that can be extrapolated for different network parameters and configurations. Conditions for achieving payback time of 5 years or lower due to energy savings are identified, while the socio-economic benefits of avoiding digging and installing new cable infrastructure are highlighted. The technical implications of refurbishing cables designed for ac to operate under dc conditions is discussed in terms of imposed electric fields, thermal profile and lifetime. A novel opportunity of temperature dependent dynamic dc voltage rating to achieve additional capacity and efficiency gains is presented.

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.

Refurbishing existing ac distribution cables to operate under dc conditions can offer several advantages in terms of capacity enhancement, efficiency and flexibility in power and voltage control, among others. In this paper, technical aspects such as insulation ageing, capacity and efficiency enhancement are explored. A novel idea of dynamic cable voltage rating based on the temperature dependent electric field is developed. The proposed algorithm can be incorporated in the dc link converters to obtain additional efficiency and capacity gains. Finally, challenges in the presented concept are highlighted.

This paper derives the mathematical expressions for computation of the load impedance magnitude, such that rated cable current is imposed at the sending end, with sending end voltage as the reference phasor. Thereby, the variation in maximum power transferred to the receiving end of a medium voltage cable link is described for varying link length, conductor cross-sectional area and load power factor. The percentage error in transmitted power computation due to simplification by neglecting the cable capacitance is quantified. The merit of the developed theory for future use is highlighted.

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.

Unplugging of energized loads in dc microgrids can cause safety concerns to users in residential areas. Sustained series arcs are more relevant in dc systems than in ac due to absence of current zero crossing. In this paper, the series arcs are detected from the load side, wherein, the proposed algorithm monitors the drop in load capacitor voltage associated with the intial electrode dependent series arc voltage. The original contribution of this work includes the experimental analysis of response of the arc detection algorithm with different discrete bandpass filter configurations and time constants. Experiments are conducted by generating series arcs for a stable 100V supply and noisy 600V supply with 300 Hz noise. The design requirements like robustness to low frequency grid voltage fluctuations, high frequency switching noise, detection time, and the threshold detection voltage are achieved and the most favourable design choices are highlighted.

The paper provides a comparison of four PV-battery architectures with dc and ac backbones, in terms of autarky, energy efficiency, battery size and reduction of annual electricity cost. The comparison is conducted based on the residential load and irradiation data from the Netherlands. The effect of different PV generation is also analyzed by comparing the results with irradiation data from Costa Rica. The results show that the ac coupled architecture gives the best performance.

We present a novel approach to physically integrate a PV module, dc/dc converter, dc/ac microinverter and a battery pack. The main advantages and challenges to realize this integration are presented. Since the thermal management is one main challenge to tackle, a 1D thermal model is developed, and the respective simulation results are obtained.

Corrections were made to author information for the paper, “Energy management system with PV power forecast to optimally charge EVs at the workplace,” (van Der Meer, D., et al), IEEE Trans. Ind. Informat., vol. 14, no. 1, pp. 311–320, Jan. 2018.

This paper focuses on the most common PV-storage architectures that are designed for residential applications and that incorporate storage devices like batteries, hydrogen systems, supercapacitors, and flywheels. The main motivations and a comparison of the advantages and disadvantages of the architectures are presented. Moreover, some common approaches to performing intelligent power management are introduced.

EV Charging is expected to be a significant load on the grid in the future. But they can also be a part of the solution to grid violations. As the amount of renewable energy generation increases, there is a large amount of power flowing in the reverse direction in the distribution network, which is undesirable. To reduce this, EVs can charge at strategically located “Grid Management Parking Lots”. Here the EV charging power level varies in accordance with the amount of capacity available in the grid. This is a win-win situation for both parties, the grid operator and the EV owner, as the grid operator reduces the amount of violations in the grid and the EV owner has his/her vehicle charged sooner.

Micro-grid for villages using renewable energy is becoming a trend for developing countries like in Africa. Commonly, ac microgrids are used for the simple reason of ac household equipments and protection abilities. DC microgrid is a better choice in terms of efficiency, control simplicity and converter prices. However, whether it is an AC or DC, the demand-supply management is the most basic problem in developing countries which causes more outage due to overloads and/or equipment failures. This paper proposes, a weakly coupled dc grid, a systematic approach to building electrical grid with the consideration of social behavior, future power system considerations and social participation in decision making.

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.

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.

Due to the absence of current zero crossing, dc arcs do not extinguish as easily as ac ones. It is essential to detect and eliminate series arc faults in dc microgrids in order to ensure safety, particularly while unplugging loads. Several detection and extinction methods using various means are known in literature. In this paper a novel arc detection method is proposed. It detects the load side input voltage drop due to the initial electrode specific minimum arc voltage. Only the local input voltage has to be measured and selectivity is given. The arc can be extinguished by shutting down the load's power electronics converter. The proposed method is elaborated through simulation of arc behaviour for constant resistor and constant power loads with input capacitors. First experimental results correspond with the theoretical analysis.

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.