0885-8977 (c) 2019 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.2019.2929755, IEEE Transactions on Power Delivery TPWRD-01378-2018 1 Abstract— The increasing proliferation of distributed generators (DGs) demands a robust and comprehensive design of the communication-based adaptive protection scheme which must be able to deal with not only the known changes in the DGs- distribution system but also be able to handle the unknown changes due to variations in the operating mode, network configuration, and number, type, and size of DGs. On one side, with the increasing installations of DGs, the deployment of the communication system in protection is increasing to monitor the system status and select the relays’ settings accordingly. However, on the other side, if information flow doesn’t happen due to the presence of pre-fault failure of the relay-agents and/or communication links, a protection scheme may fail to provide the suitable protection. This paper proposes a centralized adaptive protection scheme to deal with such failures and varying operating conditions. It provides optimal relays settings for varying operating conditions of the DGs-distribution system with the aim of providing fast, comprehensive, and self-adaptive protection. In order to achieve these objectives, two algorithms have been developed: the first algorithm determines optimal hybrid pick-up settings of the relays, and the second algorithm determines the adaptive hierarchies of the relays. With the inclusion of these two algorithms, the relay coordination problem has been formalized linearly. The performance of the proposed scheme has been tested and validated for various known and unknown events with different types of short circuit faults of different fault impedances on the 37-bus IEEE test distribution system in a MATLAB simulation and coding environment. The results show that the proposed method not only provides the optimal relay's coordination in varying situations but also effectively reduces the protection latency due to pre-fault failures of the relays and communication links, and low level of fault currents. Index Terms—Relays Settings, relays coordination, communication failures, relays hierarchies, adaptive protection. I. NOMENCLATURE CPC: Center of Protection and Control RCF: Relay’s or its communication links failure Non-RCF : When a relay works properly without any RCF. RHs: Relays Hierarchies HPMS: Hybrid Pick-up Multiplier Setting PS : Pickup Signal R er : Represents an end-relay situated at an end (‘u’ or ‘d’) of a feeder. For ‘u’ and ‘d’ end of a z th zone, R er is represented by R zu and R zd respectively. th er_a nr R , th er_a far R : Represents the near and far adjacent relays located at the a th adjacent zone of a R er . h R T : Operating time of a R relay with h th hierarchy number TR_sum : Total sum of relays’ operating times II. INTRODUCTION DAPTIVE protection schemes are one of the promising approaches to protect the changing modern distribution system which is expanding faster with varying interconnections of the DGs. In the presence of DGs, the conventional schemes fail to provide selective protection because of exposure to the fault current of different characteristics with a wide range of fault current levels [1]. To protect a system from different types of fault-conditions, protection schemes based on only one electrical parameter of the feeder-line is not sufficient [2]. For example, schemes based on only overcurrent [2]-[8], [9] can fail to work in islanding or low fault current situations, and schemes based on only negative sequence current [10], [11] are less sensitive to the symmetrical faults, while voltage-based scheme proposed in [12] is limited to protect only DG-zones from outside faults and fails to protect the network feeders. In [13]- [14], modules of different protection schemes have been proposed for dealing with the different types of faults in grid- connected and islanding operating modes with different network-configurations, while considering the fixed number of DGs. In the presence of DGs, to deal with the bidirectional flow pattern of the fault current, in most of the previous researches [5], [10], [13], and [14], the feeder protection has been achieved by using one end-relay approach in which a relay is located at only one end of a feeder. Whereas some researches [3], [7], [9], [15]-[17] reported two end-relays, each at an end of the feeder, to protect the feeder. Unlike the one end-relay based protection design, in the latter design, for primary-level protection, there is no need to disconnect the adjacent feeder along with the faulted feeder to prevent the fault current flow from another A Robust Protection Scheme Based on Hybrid Pick-Up and Optimal Hierarchy Selection of Relays in the Variable DGs-Distribution System Ekta Purwar, Student Member, IEEE, S. P. Singh, Senior Member, IEEE, and D. N. Vishwakarma, Senior Member, IEEE A Manuscript received 7 Nov, 2018 (Corresponding author: Ekta Purwar.) The authors are with the Indian Institute of Technology (BHU) Varanasi, Varanasi 221005, India (e-mail: purwar.ekta21@gmail.com; spsingh.eee@itbhu.ac.in; dnv.eee@itbhu.ac.in ).