### Literature Review  
Protection coordination of OCRs in a distribution network is severely affected by the presence of DGs [9]. According to different DG penetration levels, DG sites, and fault types and locations in the DN, determining the optimal setting values of pick-up current, TPS multiplier, and inverse-time curve coefficients of overcurrent relay is very necessary to ensure the adaptability and reliability of a DN protection system. Many coordination methods of overcurrent relays can be classified by trial-and-error methods [26], topological analysis methods [27], and optimization methods [28; 29; 30]. The latter is commonly used in recent years because the operating time of OC relays can be optimized against various coordination constraints as well as relay characteristic curves. The optimal coordination of overcurrent relays consists of an objective function (OF), parameters, constraints and a selected optimization method to solve the defined problem. A literature survey for the objective functions with parameters and constraints, type of OC relays, and optimization algorithms has been shown as the following. * Objective function and setting parameters: In [29; 30; 31; 32; 33; 34; 35; 36], objective functions are based on the overall operation time of primary and back-up overcurrent relays, where time-dial setting (TDS) multipliers are considered as the optimized parameters. In [37; 38; 39; 40], the optimized parameters of OFs are pick-up currents or both the pick-up currents and TDS multipliers. In [51; 52], a multi-objective function (MOF) with the overall operation time of relays and protection coordination constraints has been developed. The optimization parameters of the MOF are TDS multipliers, pick-up currents and coefficients of time-current characteristic curves. In [52], the authors have explained why adaptive coordination schemes are paid more attention to an overcurrent protection system of the DG-integrated DN when compared to others such as: immediate disconnection of DGs under a fault event; capacity limitation of installed DGs; protection system improvement by using more circuit breakers for sectionalization, distance relays, or directional OC relays (DOCRs); the use of fault current limiters to preserve the original settings of OC relays; and the use of fault ride through (FRT) strategies of inverter-based DGs. Based on the references [15; 19; 42; 43], an adaptive protection coordination (APC) scheme must first update the data from the latest change of DG-integrated distribution network, e.g., the operation status of DGs, the opened/closed status of circuit breakers (CBs), and then calculate power flows and perform the fault analysis to obtain the input data for optimal coordination algorithms. The APC scheme could require a data center to send/receive the data to/from digital relays before or after the sudden changes/disturbances of the DG-penetrated DN. A supervisory control and data acquisition (SCADA) system is needed to perform this adaptive coordination scheme. Moreover, digital relays can be remotely controlled through communication channels. According to the above references, the combination of digital relays, the SCADA system, and an appropriate optimization algorithm will be necessary for the adaptive coordination of OCRs in the DG-integrated DN. The optimal settings of both TDS multipliers and protection-curve coefficients for the inverse-time OC function could be easily implemented in a digital relay. Furthermore, several previous studies have not considered both the reliability of fault-current calculation results and the proper selection of tripping and pick-up thresholds of OCRs in the DN for the optimization of the objective function. Last but not least, it is needed to consider the operation characteristics of DG units, e.g., plug-and-play, peer-to-peer characteristics [53; 54; 55], for parameter settings of the adaptable relay coordination system in the DG-contained DN.