## 1 Introduction  
The growth of distributed generation (DG) units within modern distribution networks (DNs) has opened up new possibilities in the diversification of energy supply, providing a better use of natural resources that contributes to sustainability. DG units are a promising alternative to meet the rising demand for energy, with a beneficial impact on electricity networks that enhances overall efficiency. Nonetheless, the inclusion of DG into traditional DNs leads to bi-directional power flows that alter the traditional load-generation dynamic interaction [1; 2; 3; 4]. This interaction causes the problem of protection coordination to be more complex and challenging than in traditional DNs. In current DNs, DG units are mainly related to solar and wind power resources; as a consequence, the traditional protection systems developed for DNs must contend with their inherent intermittency. Basically, DNs with the inclusion of DG units have become reconfigurable systems in which the level of current fault is variable; also, the inclusion of DG units converts load nodes into combined generation-load nodes, resulting in bi-directional power flows [5; 6; 7]. The inclusion of DG units has transformed the traditionally radial DNs into non-radial DNs with the advantage of making the system more flexible, but also with the drawback of the need for more complex protection coordination. Consequently, the coordination problem with the inclusion of DG that guarantees speed, selectivity and reliability has become one of the most challenging tasks when planning the operation of DNs [8]. Overcurrent relays (OCRs) are the preferred protection systems because of their performance, reliability, efficiency, simplicity, and facility of installation. However, their adequate integration and coordination in DNs when DG is incorporated is an emerging subject of research [1; 2; 9]. The choice and design of a protection coordination system are critical for the proper functioning and reliable operation of DNs [6]. Lately, researchers around the world have applied several optimization techniques to find a suitable protection coordination scheme that is characterized by minimum operation time and which, at the same time, guarantees reliability and safety [10; 11]. In [12], the authors presented an impact index to evaluate the coordination of OCRs when DG units are integrated in DNs. The coordination problem of directional OCRs is also discussed in [13] for DNs with the integration of significant DG. The authors of [14] proposed an approach for the coordination of OCRs that involves DNs under unexpected faults. In [15], a multi-objective optimization strategy for the coordination of OCRs in DNs with DG was developed. In [16], the authors propose an online coordination protection system for OCRs that is adapted under topological variations of DNs. This scheme uses intelligent communication devices to obtain real-time information to update relay settings, simulating operation under dynamically changing conditions, and also including loss of loads, generations and lines. In [17], the authors also provided a coordination protection scheme considering topological variations of DNs. Basically, a support vector machine combined with an artificial neural network was used for DN state recognition. Nonetheless, the protection systems that can be adapted under topological variations are usually costly and also feature a complicated implementation [18]. The authors in [19] proposed the concept of non-standard features for the coordination of OCRs in DNs which are not included in IEEE or in IEC standards. Technological developments have provided the platform with the latest generation of digital OCRs that permit the implementation of new protection schemes. The constant expansion of DNs is making the coordination of protections an increasingly complex problem. Non-standard features then emerge as an attractive alternative to enhance the safety and reliability of DNs.