This paper firstly analyses the fault equivalent circuit of DG (Distributed Generation) as a high-frequency harmonic source, and then deduces the fault characteristics of the electrical quantities at both ends on the basis of high-frequency harmonic injection, pointing out the weaknesses: easily affected by the load capacity of the T-connected load. Afterwards, this paper draws the equivalent circuit diagram of the positive sequence component after the fault and derives the difference expression of the virtual electrical quantity at the opposite end under internal or external faults at different locations, and then analyses the fault characteristics of positive sequence under the industrial frequency. On this basis, this paper proposes a double-terminal protection scheme based on the combination of high-frequency harmonic injection and virtual positive sequence differential principle. Finally, the protection scheme is verified on the MATLAB/SIMULINK platform. And the proposed protection method can identify faults within 30ms, with a capability of enduring transient resistance up to 200 Ω, noise immunity reaching 200 dB, and being unaffected by the size of T-connected load capacity or the fault location.