Fault calculation of AC system interconnected by HVDC system

Conventional fault analysis algorithms may not be suitable for the AC/DC hybrid systems. To solve this problem, a mathematical model for HVDC (High Voltage Direct Current) system, which is suitable for analyzing faults in AC system, is presented in this paper. Firstly, based on the switching functions method and sequence components method, an equivalent model of HVDC converter under the asymmetric AC system is put forward. The unique advantage of the converter model lies in that it has taken harmonic characteristics into account and can improve the calculation accuracy significantly. Secondly, response performances of HVDC control system in cases of AC faults are analyzed and expressions for the control response characteristics are discussed under 3 modes. 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 algorithm has been verified by simulation studies and by comparing the results with those obtained from the EMTDC simulations.