Protection of Islanded Microgrid Fed by Inverters

As microgrids evolve and proliferate, with sharp increase in renewable-fed distributed generation, islanded operation of microgrids that are fed entirely by inverter-interfaced renewables has become an imminent reality. Commercially available grid-forming inverters have made it technically possible to form a stable microgrid island with other inverters following in a coordinated controlled operation. For reliable operation of islands, inverters participating in such islands may need to stay connected during faulted conditions until the fault is isolated. Though ample literature exists on protection of microgrids, protection of islands entirely fed by inverters has not been addressed well. This paper develops a simplified, yet accurate model for an inverter-interfaced distributed energy resource, and uses it to explore operation and protection issues in islanded microgrids that arise due to the control and design of commercial inverters.