A critical review of AC Microgrid protection issues and available solutions

One of the main technical issues in the practical implementation of a Microgrid is the design of the proper protection scheme. The scheme must be capable to meet the basic protection requirements of selectivity, sensitivity and reliability not only in grid-connected mode but also in islanded mode of operation. Since the introduction of Microgrid concept, many researchers have introduced various new protection schemes to be incorporated in medium and low voltage Microgrids. Most of the researchers agree that the conventional protective devices usually based on overcurrent principle are inadequate to provide complete protection for Microgrids during both grid-connected and islanded mode of operation, hence new protection techniques based on other principles must be developed. The new protection schemes should be not only adaptable according to operational modes of Microgrid but also sensitive enough to detect and clear the lowest possible fault currents within Microgrid quickly ensuring minimum supply disruption to consumers. This paper briefly reviews protection issues in AC Microgrids and presents state of the art protection schemes for AC Microgrids developed and proposed so far. It also gives a critical analysis of each proposed method and categorizes the protection issues and schemes on the basis of operational modes of AC Microgrid. At the end, it has been concluded that more effort is still needed to overcome the limitations of proposed protection schemes and to improve reliability of communication system for adaptive protection schemes or provide alternate means to cope with communication failures. Moreover, protection against cyber-attacks is crucial for safe and secure operation of future Microgrids.