0885-8977 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TPWRD.2015.2509784, IEEE Transactions on Power Delivery > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract—The constantly increasing presence of distributed generation (DG) in modern distribution systems induces grid configuration alterations, affecting thus the short-circuit levels and fault current paths. To address all arising protection challenges, adaptive protection is being implemented. This paper presents an innovative hardware-in-the-loop (HIL) adaptive protection scheme (APS), which incorporates real time simulation, multifunction protection, centralized control, and optimal calculation of protection settings. The proposed adaptive scheme is based, firstly, on the determination of optimal relay setting groups, and then on their online self-adjustment, providing a continuously tuned protection scheme to the variable system operating modes. The efficacy of the proposed solution is demonstrated through two distribution test networks with embedded DG. Index Terms—Adaptive protection scheme, directional overcurrent relays, distributed generation, hardware-in-the-loop simulation, nonlinear programming, protection blinding, sympathetic tripping. I. INTRODUCTION OWADAYS, distributed generation (DG), based on renewable energy sources and natural gas, offers on-site power generation close to consumption points, leading to better efficiency, lower emissions, and economic gain for end-customers. These benefits have motivated governments worldwide to push towards integration of DGs at the distribution level, causing therefore a paradigm shift in power delivery systems. However, the widespread penetration of distributed energy resources in distribution grids has a significant impact on the existing protection schemes. Indeed, a DG unit introduces an additional fault current source, which increases the total short-circuit level, while altering the magnitude and direction of fault currents sensed by protective devices. Hence, the advent of DGs has posed new threats to distribution protection schemes, such as blinding of feeder protection, sympathetic tripping, failed This work was supported in part by the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement number 308755–the SuSTAINABLE project, funded under the EC call "ENERGY.2012.7.1.1". The authors are with the School of Electrical and Computer Engineering, National Technical University of Athens, Iroon Polytechniou 9, 15780, Athens, Greece (e-mail: vpapaspi@power.ece.ntua.gr; gkorres@cs.ntua.gr; vkleft@mail.ntua.gr; nh@power.ece.ntua.gr). reclosing, and recloser-fuse miscoordination [1]–[7], compelling engineers to reconsider long-established practices. Recently, several protection solutions for the previous complications have been suggested. The predominant protection strategy concerns the so-called adaptive protection, which is actually an automatic procedure that monitors and amends protection relay settings, in order to make them better attuned to prevailing system operating conditions [8]. Characteristic applications of adaptive protection systems to distribution utilities and microgrids can be found in [9]–[13]. Directional overcurrent relays (DOCRs) are used as the first line of defense in modern distribution systems, where ring and multi-source configurations are adopted. Currently, the DOCR coordination process is performed by optimization methods, aiming at the determination of optimal time dial (TD) and pickup current settings. Various approaches have been proposed in literature for optimal DOCR coordination. Nonlinear programming (NLP) approach solved by heuristic methods [14]–[16] and expert solvers [17]–[20] has dominated, with both time dial and pickup current settings being considered as continuous decision variables. This assumption meets the specifications of modern numerical relays, since they provide accuracy of many decimal places for their setting parameters. The contribution of this work is manifold. A thorough examination of DG impacts on conventional distribution protection is presented. Then, a hardware-in-the-loop (HIL) infrastructure is introduced, providing a complete testbed for adaptive protection schemes which address the previous problems. Furthermore, the integration of state-of-the-art optimization techniques in the adaptive procedure is analyzed. This novel concept is based not merely on the flexibility of automatic relay setting group adjustment, but also on the determination of optimal relay setting values, leading to great enhancement of adaptive protection systems developed so far. The rest of the paper is organized as follows. In Section II, protection blinding and sympathetic tripping issues are investigated. Section III firstly describes the architecture of adaptive protection schemes, and then proposes a new concept combining adaptive protection and optimization techniques. Test results from the application of the proposed scheme to different DG-penetrated distribution systems are discussed in Section IV. Finally, Section V highlights the conclusions of this work. Hardware-In-the-Loop Design and Optimal Setting of Adaptive Protection Schemes for Distribution Systems with Distributed Generation Vasileios A. Papaspiliotopoulos, Student Member, IEEE, George N. Korres, Senior Member, IEEE, Vasilis A. Kleftakis, and Nikos D. Hatziargyriou, Fellow, IEEE N