Abstract-1 This paper presents a centralized protection strategy for medium voltage dc (MVDC) microgrids. The proposed strategy consists of a communication-assisted fault detection method with a centralized protection coordinator and a fault isolation technique that provides an economic, fast, and selective protection by using the minimum number of dc circuit breakers (DCCBs). The proposed method is also supported by a backup protection which is activated if communication fails. The paper also introduces a centralized self-healing strategy that guarantees successful operation of zones that are separated from the main grid after the operation of the protection devices. Furthermore, to provide a more reliable protection, thresholds of the protection devices are adapted according to the operational modes of the microgrid and the status of distributed generators (DGs). The effectiveness of the proposed protection strategy is validated through real-time simulation studies based on the hardware in the loop (HIL) approach.Index Terms-Adaptive protection, centralized protection, smart dc microgrids. I. INTRODUCTIONDue to the increasing penetration of DGs, especially in the form of renewable energy systems (RES), the concept of microgrids has been proposed as a method for DG integration into the electrical grids. Microgrid is a common concept in both ac and dc systems and is defined as a smallscale low or medium voltage grid consisting of loads and DGs. Such a system is capable of operating in both islanded and grid-connected modes [1]. Because of the advantages of the dc networks over the ac grids, and also because of the new developments in the technology of voltage source converters (VSCs), nowadays there is a major interest in dc grids in both research and industrial realms [2][3][4][5].At the present moment, protection is one of the most important challenges in the development of dc microgrids. Protection issues mainly arise due to the particular behavior of the fault current in VSC-based networks [6]. When a fault occurs in a dc grid, firstly, the dc-link capacitor is discharged causing the voltage of the main dc bus to drop precipitously. Then, the energy stored in the cable This work was supported in part by the Spanish Ministry of Economy and Competitiveness under Project ENE2013-48428-C2-2-R. The work of M. Monadi was supported by the Ministry of Science, Research, and Technology, Iran.M. Monadi is with Technical University of Catalonia (UPC) Barcelona, Spain and Shahid Chamran University of Ahvaz, Ahvaz, Iran (e-mail: meh_monadi@yahoo.com).C. Gavriluta is with the Grenoble Electrical Engineering Laboratory (G2ELab), France (email: catalin.gavriluta@g2elab.grenoble-inp.fr).A. Luna, J. I. Candela are with Technical University of Catalonia (UPC) Barcelona, Spain. (e-mails: luna@ee.upc.edu, candela@ee.upc.edu) P. Rodriguez is with Technical University of Catalonia (UPC) Barcelona, Spain and Abengoa research, Sevilla, Spain (e-mail: prodriguez@ee.upc.edu).inductance is also discharged through the freewheeling dio...

This paper proposes a hierarchical control architecture designed for an arbitrary high voltage multiterminal dc (MTDC) network. In the proposed architecture, the primary control of the MTDC system is decentralized and implemented using a generalized droop strategy. Design criteria for dimensioning the primary control parameters, including voltage limits, are offered by analyzing the transients appearing in the system. The proposed secondary control is centralized and regulates the operating point (OP) of the network so that optimal power flow (OPF) is achieved. Compared to previous works, this paper further elaborates, both analytically and through simulations, on the coordination between the primary and secondary control layers. This includes how local primary controllers have to be driven by the centralized controller in order to ensure a smooth transition to the optimal OP. IndexTerms-Droop control, hierarchical control, multiterminal dc (MTDC) systems, optimal power flow (OPF).

Multi-terminal dc networks based on voltage source converters (VSC) are the latest trend in dc-systems; the interest in the area is being fueled by the increased feasibility of these systems for the large scale integration of remote offshore wind resources. Despite the active research effort in the field, at the moment, issues related to the operation and control of these networks, as well as sizing, are still uncertain. This paper intends to make a contribution in this field by analyzing the sizing of droop control for VSC together with the output capacitors. Analytical formulas are developed for estimating the voltage peaks during transients, and then it is shown how these values can be used to dimension the dc-bus capacitor of each VSC. Further on, an improved droop control strategy that attenuates the voltage oscillations during transients is proposed. The proposed methods are validated on the dc-grid benchmark proposed by the CIGRE B4 working group. Starting from the structure of the network and the power rating of the converters at each terminal, the output capacitors and the primary control layer are designed together in order to ensure acceptable voltage transients.(C) 2014 Elsevier B.V. All rights reserved.Peer ReviewedPostprint (author’s final draft

As renewable sources increasingly replace existing conventional generation, the dynamics of the grid drastically changes, posing new challenges for transmission system operations, but also arising new opportunities as converter-based generation is highly controllable in faster timescales. This paper investigates grid stability under the massive integration of grid-forming converters. We utilize detailed converter and synchronous machine models and describe frequency behavior under different penetration levels. First, we show that the transition from 0% to 100% can be achieved from a frequency stability point of view. This is achieved by retuning power system stabilizers at high penetration values. Second, we explore the evolution of the nadir and RoCoF for each generator as a function of the amount of inverter-based generation in the grid. This work sheds some light on two major challenges in low and noinertia systems: defining novel performance metrics that better characterize grid behaviour, and adapting present paradigms in PSS design.

Multi-terminal dc (MTDC) networks are drawing a lot of interest lately in applications related to distributed generation, especially in those that also integrate energy storage (ES). A few approaches for controlling the operation of such systems have been proposed in the literature; however the existing structures can be significantly enhanced. This paper proposes an improved primary control layer, based on custom droop characteristics obtained by combining concepts of droop and dcbus signaling control. This approach is designed to be generic and takes into account the various operating states of the network. Five operating bands, similar to the operating states of the ac-grids, as well as various droop characteristics for different elements connected to the dc-network, are defined. For the energy storage, the state of charge (SoC) is taken into account at the primary control level and included in the droop characteristic, creating a two variables droop surface. The proposed control strategy is validated through simulation and experimental results obtained from a case study that involves a micro dc network composed of a PV generator, a lead-acid battery, and a connection point to the ac grid.