An AC line pilot protection scheme for AC/DC hybrid system based on composite mode power difference

•The protection scheme based on composite mode power difference is proposed.•The method can eliminate effect of commutation failure on protection.•The method can identify internal and external faults reliably and sensitively.•The method possesses strong immunity to fault resistance and is unaffected by fault type.•The method does not require real-time transmission of electrical information. This paper proposes a new AC line pilot protection scheme for AC/DC hybrid system on the basis of composite mode power difference, which can avoid the incorrect operation of AC line protection resulting from commutation failure. By analyzing the post-fault conducting states of inverter valves, the expressions of injection current from DC system in different operating states of inverter are derived. And combining the fault network of AC system, the models to calculate the 0/1/2-mode power injected from the backside systems of AC line are built. Then, the 0-mode power and 2-mode power are used to construct the composite mode power difference. Considering that the composite mode power varies with system topology, this paper forms the protection criterion according to the composite mode power difference which is related with the difference between system models of the backside systems in internal and external fault cases. Simulations based on RT-LAB platform confirm that, the presented method can correctly distinguish between different types of internal and external faults, and it is highly immune to the fault resistance and commutation failure.