Considering the impact of the second harmonic on DC side under asymmetric conditions

a DC system model for AC system of fault conditions is proposed. It can be easily embedded into the traditional AC system fault calculation process. The model is solved through utilizing the secant unified iterative method

which is a super linear convergence algorithm. The DC system can be regarded as the positive and negative sequence current injected into the AC bus. Therefore

the node admittance matrix and Norton equivalent susceptance of the AC network is calculated only once for interfacing with the original AC system fault calculation program. The CIGRE-Benchmark system and a three-infeed DC system are established in PSCAD. The results of simulation of various faults applied show that the proposed model is efficient and the secant unified iterative method is reliable in convergence. This work is supported by National Natural Science Foundation of China (No. 51277068)

Program for New Century Excellent Talents in University (No. NCET-12-0846) and Program of Introducing Talents of Discipline to Universities (“111” Program) (No. B08013).