The Calculation Of DC Transmission Capacity Of VSC-HVDC System Supply To The Passive Network And Its Improving Method

Along with the shortage of global energy and the increasing demand of electricity, the grid scale expands rapidly. People speed up the development of solar energy, wind power and renewable energy sources. The renewable energy has a dispersed nature, and the dismatch of the power geographical distribution and the power supply demand center, which requires the transmission technology can adapt to the characteristics of dispersion, intermittent, and long distance to the load center. While conventional HVDC transmission have many shortcoming, such as: the grid of capacity need largely enough and a large number of reactive power will be consumpted. Comparing with traditional transmission technology, VSC-HVDC technology has many advantages, such as it can control the active and reactive power rapidly and independently, meanwhile, it can improve the existing power quality and increase transmission capacity of the system, can supply power to passive network, easy to make the trend reverse, and so on. Therefore, VSC- HVDC transmission technology is very suitable for urban and isolated power supply, the grid connection for renewable energy power generation, the asynchronous interconnection of ac system, the power market trading and others fields. There are a broad commercial value and application prospects in the field of electric power system.As a complex control system, the primary objective of the VSC-HVDC system is to ensure the power system can work security and stability. Harmonics, fault and disturbance factors such as load variations will make VSC-HVDC system working unstable. Meanwhile VSC is a nonlinear electronic power converter with very high control precision. VSC-HVDC systems often take a fast and no error control to the AC bus voltage of the inverter, the ac voltage response speed of disturbance is in milliseconds or less in DC side, so that the load of the DC system(as viewed from the DC side of the inverter station) presents constant power characteristics. The constant power characteristics may destabilize the stable operation. Thus the stability study of the system impacted by the characteristic of constant power is critical.According to the external voltage/current characteristics of VSC-HVDC's rectifier and inverter stations, the DC equivalent circuit is established, then Through the small signal stability analysis to study system, and obtain the corresponding relationship between transmission capacity of VSC-HVDC supply to passive network and the DC voltage, DC capacitor and line parameters. At the same time through the design of a virtual resistor as a feed-forward control can greatly improve the transmission capacity of the system. The dynamic simulation is conducted by using PSCAD/EMTDC software. The simulation results demonstrate that small signal stability analysis strategy is effective and validity.