Grid Forming Inverter with Increased Short-Circuit Contribution to Address Inverter-Based Microgrid Protection Challenges
Authors: Maximiliano Ferrari, Leon M. Tolbert, And Emilio C. Piesciorovsky
Abstract:
One of the primary challenges of microgrid protection is the substantial difference in fault levels between grid-tied and islanded modes. In grid-tied mode, the bulk grid provides significant short-circuit current, while in islanded operation, the short-circuit magnitude is limited due to inverter-based resources, which restrict their current output to near nominal ratings. As a result, conventional distribution protection strategies based on overcurrent cannot provide reliable protection for microgrids in islanded mode. Fuses and overcurrent circuit breakers are especially impacted because of their inverse time-current characteristics. Currently, the lack of affordable protection solutions for microgrids in islanded mode often leads microgrid to implement unreliable overcurrent protection that leads to system shutdowns during electrical faults.
This article presents a grid-forming inverter prototype capable of delivering sufficient short-circuit current to support the use of cheap legacy overcurrent protection devices in inverter-based microgrids. Experimental results show that increasing fault current contributions from grid-forming inverters is achievable by overrating selected components and implementing control modifications to ensure stability during fault conditions. Specifically, the inverter’s power module must be overrated to handle higher current levels without exceeding safe operating limits. Additionally, control strategies must be adopted to maintain closed-loop stability when the grid-filter begins to saturate under high current stress. A cost analysis reveals overrating the silicon power module is unlikely to make a significant impact on the overall cost of the inverter, supporting the practicality of the proposed approach.
In the tested prototype, the inverter delivered more than three per unit (p.u.) of current during electrical faults, which was identified as the minimum required in the prototype test system to achieve protection coordination while maintaining grading margins between fuses and protective relays. The proposed modifications to the inverter provided sufficient fault current magnitude to enable fuse-relay and relay-to-relay overcurrent coordination in the microgrids operating in islanded mode. The novelties presented in this paper pave the way for the reliable use of conventional and cost-effective overcurrent protection in islanded microgrids.
