This report examines the protection problems that must be dealt with to successfully operate a microgrid when the utility is experiencing abnormal conditions. There are two distinct sets of problems to solve. The first is how to determine when an islanded microgrid should be formed in the face of the array of abnormal conditions that the utility can experience. The second is how to provide segments of the microgrid with sufficient coordinated fault protection while operating as an island separate from the utility.
As used in this discussion, the term microgrid refers to conventional distribution systems with distributed resources (DR) added. This is not to imply that the simple addition of DR to a distribution system creates a microgrid. In a microgrid the DR(s) has sufficient capacity to carry all, or most, of the load connected to that portion of the distribution system that houses the DR. In addition, a microgrid can operate as an electrical island in times of disturbance to the main utility system. Thus, there will be a well-defined interconnection point where the microgrid can be disconnected from the bulk of the electric utility system if so desired.