## 3 Challenges and Issues in Hybrid AC/DC Microgrid
### Communication Challenges  
Communication is a big challenge in order to achieve a reliable and efficient system because communication significantly affects its design, such as topology, operational mode, coordination control of hybrid grid system, its protection schemes, control requirements of the system, and power management. Many different types of communication have been adopted to fulfill reliable and efficient operations, such as wireless and optical fiber. The hybrid microgrid is generally classified into two categories the communication-based system and the other communication less system. For control and power management, the communication-based system provides information regarding voltage, current, and frequency to determine the operation of a distributed system. Communication is most important for the protection of hybrid systems as well as power control [18]. The design of a communication system between the interlinked converter of a hybrid AC-DC microgrid system is a crucial step. Effective communication between different components to control and monitor in real-time is a big challenge. The development of hybrid microgrids as an integral part of a smart grid system requires intelligent coordination among controllers of the system, protection of the system, and the most important communication. Communication regarding coordination is the most important aspect of communication challenges. In the future grid composed of intelligent electronics devices with high complexity, the design of reliable communication and the standardization of the system holds a higher priority [19]. For the improved performance of control and protection coordination control, attention must be paid to develop proper infrastructure for communication between components to coordinate and perform an action. The different types of applicable standards are IEC 61850 standard, ANSI C12.22 standard, IEC 61499 standard, and IEC 61850 standard. Another important aspect is the communication regarding the control of the system. In hybrid microgrids, the system protection in both AC and DC has some issues such as self-healing of system, grounding, and no zero-crossing current in DC side which operates the circuit breaker. Therefore, to integrate a control system with protection, well-established communication infrastructure is required [20; 21]. Regulation is critical regarding the facilitation of microgrid structures. The practical implementation of hybrid microgrids needs to consider the regulatory issues and key drivers. Regulation is the combination of standards and guidance about distributed energy resources interconnections and integration with the main utility grid. In the case of hybrid AC/DC microgrid, the regulation needs to be further studied as the current regulatory process for AC network is already complex enough due to a combination of existing rules and standards that are not microgrid friendly. Therefore, there is a need for a clear regulatory framework that may include operations, interconnection, and different operating modes; all of these are reviewed in the following sections. The most common challenge for the interconnection of microgrids with the main grid is the connection cost for private microgrid inverters [22]. Moreover, the islanded operation of hybrid AC/DC microgrids poses a conflict with present rules and requires further research. Secondly, the installation of bidirectional energy meters to record energy transfer is another challenge [23]. Reference [24] provides a comparative assessment of regulatory challenges faced by different microgrid models, namely, distribution system operator (DSO) model, hybrid model, and third-party model.