Research On DC Power Flow Controller In Multiterminal Flexible HVDC System

With the increasing consumption of traditional fossil energy, the problem of ecological environmental and energy security is increasingly prominent. The strategic adjustment of energy structure has become the extremely urgent task in our country, even the other countries. China has been rising up the development of renewable energy to an important position. The multi-terminal flexiable DC system has the advantages of in favour of renewable energy absorption, reduce power transformation and reduce the effects of intermittent and volatility of renewable energy, and will become the main development trend of the future power grid. Due to the particularity of its structure, the MTDC has the disadvantage of cannot control the power flow flexibly, so we need DC power flow controller to achieve reasonable distribution of the power flow, the same as AC system.This thesis introduce the research status of DPFC first, the three basic forms of DPFC(series variable resistors, series DC/DC Converter, series variable voltage source) is analyzed and compared. Through the analysis of the node type, station level control and system level control of the MTDC system, this thesis proposes the two different category of power flow control. And the second power flow control category is the main research contents. Then the thesis analyzes the power flow of the MTDC system with DPFC, and points out the main function of the DPFC according to the analysis of the above. Mean while, this thesis also introduces the configuration of DPFC in the system briefly. Next,we introduced the thyristor anti-parallel type DPFC and four-quadrant chopper type DPFC, include the topology and control strategy of the two different kinds of DPFC, and validates it by simulation use PSCAD/EMTDC. Based on the analysis of series voltage source type DPFC, the thesis summarizes two methods of site selection of the DPFC:site selection based on sensitivity analysis and site selection based on the operating space of power flow. Through the appropriate site selection of the DPFC, it can effectively prevent the line overload in MTDC system and achieve reasonable distribution of power flow.In the last section, based on the traditional series voltage source type DPFC and the theory of power balance, the thesis propose the power balance type DPFC. By analysis of simulation test result, it has good performance in power flow control, low power rating and do not exchange energy with the AC system.