Research On Simulation Methods And Dynamic Performances Of AC/DC Power System

As HVDC systems have many advantages in long-distance bulk power transmission and non-synchronous network, in recent years they have been used widely in China. Long-distance HVDC transmission projects have become the investment and business focus in implementation of national strategy that is "west to east, north and south for each other, the national network". With the increasing proportion of HVDC transmission lines and capacities in the system, the research on simulation methods and dynamic performances of AC/DC power system becomes very important.In transient simulation analysis of AC/DC power system, there are electromechanical transient simulation and electromagnetic transient simulation. It needs select different methods for different purposes when the dynamic performances of AC/DC system are researched. This thesis firstly discusses the validity of electromechanical transient simulation and electromagnetic transient simulation. The differences between the two transient simulation methods on dynamic performance and commutation failure of DC system and transient stability of AC system are revealed by an example simulation. Then, the methods of researching dynamic performance of DC system and transient stability of large-scale AC/DC system are determined.Electromagnetic transient simulation model should be used for researching dynamic performance of DC system. Conditioned by the scale and speed of simulation, it is not realistic to establish the complete electromagnetic transient simulation model of large-scale AC/DC system and simulate accurately dynamic performance of DC system. On this, a new equivalent method is presented in this paper, which focuses on how to reserve DC system and reduce AC system while maintaining dynamic performance of AC and DC system. The original system and the equivalent system are simulated respectively. The result shows that the equivalent system can preserve the main dynamic characteristics of the original system well. Based on dynamic equivalent in AC system, electromagnetic transient simulation models of AC/DC system are set up, by which the dynamic performances of HVDC are studied under some kinds of AC system faults and DC system faults. The dynamic performances of the DC system itself are analyzed under the faults of converter station AC buses. Commutation failure and the dynamic process of DC control system and the recovery process of DC system are accurately simulated. The results show that the faults at the inverter side AC system are more likely to cause commutation failure than that at the rectifier side AC system. DC system of the model can recover normal operation state under the action of HVDC control.Finally, taking NingDong DC transmission system as an example, the interaction between AC and DC systems and the transient stability are analyzed. The simulation results show that most of the faults at the inverter side AC system will lead to commutation failure, and commutation failure time is closely related to the distance from the fault line to the converter station. Commutation failure will affect the voltage stability of AC system. In addition, fault clearing time and reactive power reserves are also important factors. The active power shortfalls in AC system near inverter station are caused by DC block, which doesn't trigger transient frequency and voltage stability problem.