Research On Transmission Technologies Of Intermittent Power Based On VSC-HVDC

As energy shortage and the pollution of the environment are more and more serious, the renewable energy, for example the wind energy and solar energy, development take seriously highly. Because wind generation is a clean, renewable and rich resources and has mature technologies, it is supported by national energy policy in China. At present wind farm connected to the AC grid has basically three methods including AC connection, traditional HVDC connection and VSC-HVDC connection. This paper focuses on the comparative analysis of three respects about the line loss, low voltage ride-through capability and transient stability, when the wind farm is connected to the AC grid through two different methods of AC and VSC-HVDC. At first, the paper establishes the doubly-fed induction electric generator mathematical model and its control system, which laid the foundation for further study of wind farm. Through programming calculation of specific examples, compared with wind farm connected by AC, wind farm connected by VSC-HVDC has higher active power loss and no reactive power loss. The wind farm connected to the nearest AC grid by two different methods of AC and VSC-HVDC were modeled in MATLAB/SIMULINK software, and then the paper analyzes the response characteristics of wind turbine based on doubly-fed induction generator at voltage dips and introduces the grid demand for LVRT of wind farm and the present research results of LVRT technologies of wind farm based on DFIG. Through the simulation results, yet for wind farm connected to AC grid by VSC-HVDC system, if VSC-HVDC system has chosen right dc-capacitors, the dc-capacitors can temporarily store energy during system fault and the store energy can be transmitted to the grid when the system can normally operate, which can ensure the LVRT standard of wind farm. Compared with AC connection, wind farm connected to the grid by VSC-HVDC is more favorable. Finally, the simulation analyses the effects of transient stability for system, when wind farm is connected to the grid by two different methods. Through the comparative analysis of the calculation of fault critical clearing time and specific examples, the simulation results can indicate that wind farm connected to the AC grid by VSC-HVDC system is more favorable to the stability of the system.