A fault analysis model for AC system interconnected by HVDC system

Fault behaviors of the AC system interconnected by HVDC system cannot be studied by conventional fault algorithms. The effects of interaction of the hybrid systems on the AC fault features are often ignored for the sake of analytical simplicity. In this paper, a mathematical model for HVDC system which is suitable for analyzing faults in the AC system is presented. First, for analysis of HVDC converter under the asymmetric AC system, an equivalent model of the converter based on the switching function method and sequence components method is put forward. Because it has taken the important harmonic characteristics into account, it can improve the calculation accuracy. Meanwhile, it has a convenient interface with the AC system and a simple calculation. Then, the response performances of HVDC control system during the AC fault period are discussed and its model is established. Finally, a general algorithm is developed to incorporate the proposed HVDC model into the conventional AC short-circuit calculation algorithm. The effectiveness of the proposed HVDC model and the algorithm has been verified by using simulation studies and by comparing the results with those obtained from the EMTDC simulation.