Study On Fault Analysis Method Of AC System Interconnected With HVDC System

With the development of modern power system and extensive application of HVDC (High Voltage Direct Current) technology, a large number of converter stations are introduced into the power system. Fault characteristics of the AC-DC (Alternative Current-Direct Current) system will be changed by DC power electronic devices, which are different from traditional AC system. Fault behaviors of the AC system interconnected with HVDC system cannot be studied by conventional fault algorithms. The effects of the interaction of AC and DC systems on the AC fault features are often ignored for the sake of analytical simplicity. On the basis of summing up the experience of the related research work at home and abroad, the operating characteristics of the HVDC system are studied when faults occur in the AC system. Combined with the requirements of steady-state fault calculation, a mathematical model for the HVDC system which is suitable for analyzing faults in the AC system is proposed in this paper. And the fault calculation methods for the AC system interconnected with the HVDC system are also developed. The accuracy and effectiveness of the proposed models and calculation methods are verified by simulations.The detailed research work is shown as following:(1) On the basis of extensive research on domestic and international related research papers, the characteristics and application scope of some mature HVDC models are analyzed and summarized, which are used in power flow calculation, stability and harmonic analysis. These models provide conditions and useful references for the establishment of the HVDC system fault models in this paper.(2) A three-phase switching functions model for converters operating under asymmetric conditions is proposed, based on the switching functions theory. The harmonic characteristics of converters are studied for asymmetric faults in the AC system, and the important harmonic characteristics have been taken into account. Then, fault models of converters in the sequence coordinates and the phase coordinates are established respectively, with considering some of the non-characteristic harmonics with larger influence. The validity of the proposed models is verified by simulations. (3) The functions of the main controllers for a general HVDC system and the cooperation among the controllers of the rectifier station and the inverter station are analyzed deeply. The steady-state response performances of HVDC control system during the AC fault period are discussed, and the feasible pattern changes of the control system are studied. The control models have been simplified, considering the complex transient process of the control system and the requirements of fault calculation. The control equations of control models are put forward according to the typical operation modes and control features in case of AC faults.(4) The Equivalent Models of Related Components in the HVDC System are built for analyzing faults in the AC system.These models are taken into the entire model of the HVDC system, together with the proposed converter models and control system models. Interfaced the equivalent model of the HVDC system to the AC network model, an equivalent circuit for fault calculation of the AC-DC hybrid systems is proposed. Combined with the proposed equivalent circuit, the sequence components method and the phase components method have been applied in the fault calculation of the AC system including the HVDC system. According to the description of the interface of the AC system and the HVDC system, iterative calculations are required between the AC system and the HVDC system. Detailed calculation procedures are given in this paper. The accuracy and effectiveness of the two proposed methods have been proved.The results above show that the establishment of the fault calculation methods for the AC system including the HVDC system provides valid calculation methods for setting calculations of protection relays and selections of electrical equipments. The methods are effective, simple and easy to understand. The research work would have important theoretical and practical significance.