Study On Fault Interaction Analysis And Calculation Model Of AC/DC System And Its Application

In recent years, with more and more HVDC projects put into operation, a large-scale national interconnected AC/DC power system has been formed in our country. Apart from those existing multi-infeed AC system, to the provinces which energy are relatively concentrated such as Sichuan and Ningxia has formed a multi-send HVDC system. As to the North China Power Grid and the Northeast power grid, central China Power Grid and the northwest power grid are both interconnected by the back-to-back HVDC. To such a huge interconnected AC/DC power systems, due to the existence of DC system, it will have difference in the AC power grid fault when compared with the traditional AC system. So it is necessary to take control characteristics of DC system and the interaction of AC/DC power system into account in fault analysis. Therefore, the study of fault analysis for interconnected AC/DC power systems while according to its’ characteristics of different forms can provide important theoretical and practical value for the protection analysis of power systems and the safe and stable operation of large scale ac/dc hybrid system.A dc system equivalent model which considering the dc control characteristic is established in this paper firstly. The model reflects the relationship between each order current which injected by dc system and each order converter bus voltage, which determined by dc system. It provides the interface model for MSDC system and BTB HVDC ac power network fault analysis calculation. Based on the simplified model, combined with the topology structure of multi-send power network and the fault boundary conditions of asymmetric fault, a multi-send HVDC system fault calculation method is proposed. Based on CIGRE HVDC benchmark model, a three-send HVDC system is built to testify the proposed method fully. The results are compared with those obtained by simulation using PSCAD/EMTDC software. It is shown that the proposed method is of simplicity, rapid convergence and more accuracy..As to the BTB HVDC, based on the switch equivalent model that takes DC control system characteristics into account, the harmonic interaction and the fault effect mechanism between the connected AC systems, a fault calculation model is built for BTB HVDC.Combined the model with the topology structure of both connected power networks and the fault boundary conditions of asymmetric fault,an fault analysis method for power system with BTB HVDC is proposed.The method is used to the fault analysis and calculation of Lingbao BTB project.The results are compared with those obtained by simulation using PSCAD/EMTDC software. It is shown that the proposed method is accurate and effective.The methods described above for the interconnected AC/DC power system provide the quantitative analysis of fault and protection setting and cooperation.