Abstract: The increasing penetration rate of inverter interfaced distributed generator （IIDG） in microgrids has led to unclear fault characteristics,complex power flow,and difficulty in protection setting.Traditional protection schemes require the use of highspeed communication systems to analyze the electrical quantities of different nodes and extract fault information.A harmonic current injection based active protection scheme for microgrids is proposed,which utilizes IIDG to actively inject harmonics during faults,achieving the transmission of fault detection information without communication,and solving the difficulty in protection setting caused by bidirectional power flow,transition resistance and changes in operating mode.By controlling the superposition of harmonics in the current inner loop,IIDG actively injects harmonic current into the microgrid.Based on the calculation results of the equivalent impedance change of the IIDG port,the amplitude of the injected harmonics is modulated.By adjusting the amplitude and frequency of harmonics,the fault information transmission at different nodes is achieved.Based on the characteristics of significant differences in harmonic components at different frequencies within and outside the region,a protection criterion based on the ratio of harmonic current of IIDG is constructed,and an open/close logic is proposed based on the direction of harmonic flow.A practical engineering microgrid model is established in PSCAD/EMTDC,and the effectiveness,selectivity,and rapidity of the scheme under different fault conditions are verified through simulation.