Renewable energy represented by grid-forming inverter-interfaced distributed energy resources (DERs) are increasingly connected to microgrids that can operate independently and have various modes

resulting in traditional relay protection schemes that cannot meet the requirements of protection selectivity and speed. To address this issue

the fault characteristics of grid-forming DERs are analyzed. On this basis

the current

voltage and phase angle characteristics between them under the microgrids' grid-connected/islanded operation mode are analyzed. To tackle the problem that the traditional protection loses selectivity

the phase angle difference between the pre-fault voltage and the post-fault current is used to determine the fault direction

Additionally

an internal fault acceleration factor

combining the integrated phase angle difference and the measured impedance

is introduced to enhance the action equation of the inverse-time overcurrent protection. Furthermore

a low-voltage protection criterion based on comprehensive phase angle difference characteristics is established to address the challenge of insufficient fault current to trigger inverse-time overcurrent protection during two-phase short-circuit faults in the islanded state. This leads to a microgrid protection scheme that integrates the improved inverse-time overcurrent protection with low-voltage protection. The model is built by PSCAD. The theoretical analysis and simulation results show that the scheme can operate quickly and reliably in the grid-connected/ islanded operation state of the microgrid

which verifies the effectiveness of the proposed protection scheme.