The fault characteristics of the inverter-based distributed generator(IBDG)differ from those of traditional resources

which makes it difficult for traditional current relay to clear fault fast and reliably. Also affected is the action performance of the existing transmission line protection principle directly used in distribution network with IBDG. By drawing on current differential protection for reference

the amplitudes of the defined positive sequence differential impedance for the internal and external fault are analyzed

and a novel positive sequence differential impedance protection principle is proposed to account for the impedance amplitude difference. To further improve the performance

an operation criterion with braking characteristic is constructed to overcome the adverse effects of load branch. An auxiliary criterion based on the amplitude of positive sequence impedance and current is formulated for internal three-phase short circuit faults without transition resistance. Compared with the traditional current differential protection

the proposed protection strategy has no need for synchronized measurement. The feasibility of protection scheme is verified by simulation results. This work is supported by National High Technology Research and Development Program of China(863 Program)(No. 2012AA050213)and Shandong Province Key Plan of Research and Development(No. 2015GGX101011).