A Comprehensive Directional Overcurrent Protection Scheme for LVDC Microgrids Using Harmonic Voltage Injection: Analysis and Case Studies

Lowvoltage DC microgrids represent a highly adaptable approach to bringing renewable power generation closer to consumers, however their large, bidirectional fault currents and quick settling times complicate traditional protection methods. Therefore, an effective DC protection scheme must incorporate sufficient fault current limiting as well as an alternative method to coordinate protection relay tripping decisions. This article proposes a harmonic-based directional overcurrent protection scheme that limits fault currents through inductive fault current limiters and uses injectedAC voltages to coordinate protection relays. The injected harmonic quantities are measured by each relay and thus optimal protection coordination (OPC) can be performed. The OPC is completed through time-domain simulations and a non-linear constrained optimization program to determine relay settings. A 9-bus test system was used to evaluate the overall performance of the proposed protection scheme. The results demonstrate the scheme’s ability to quickly control and isolate faults of varying resistances. Significant improvements in relay operation times are made by exploring objective functions of varying parameter complexity. The proposed scheme offers a reliable and practical solution to the present challenges in DC microgrids.