Research On The Connection Mode Selection Of Energy Base DC Transmission System

With the implementation of the "The Belt and Road Initiative" and the "The Global Energy Internet" strategy,the development of large-scale energy bases in various continents and the interconnection of power grids across the country has been promoted.The large scale,wide range and highly efficient optimal deployment of clean energy will be an important development direction of power gird.Because of its obvious advantage in large-capacity and long-distance transmission,DC transmission has become an important technical method to promote China’s power development.At present,some new energy bases are located far away from the load center.However,the small local power grids often have weak structure,whether using grid-connected with the DC transmission system to achieve the technical and economic requirements or not,it is an urgent issue to be solved in planning and designing.Accordingly,this paper studies the connection mode selection of energy base HVDC transmission system based on the planning perspective.The main content of this thesis is as follows:(1)The energy base DC transmission system model considering local power grid is established.This paper analyzes the characteristics and existing technical problems of grid-connected mode and islanded-mode of DC transmission system.The DC main circuit module,control system module and DC additional control module are established.Then the function of DC additional control is verified.The dynamic model of local power grid is established by using the dynamic equivalent method.The simulation model is established by BPA simulation software with reference to the actual parameters of one DC project.(2)The principle of mode selection based on power analysis and system spare capacity analysis is determined.Taking the grid-connected and island-connected mode as the control group,the mode selection method of energy base DC transmission system is proposed from the view of power flow,short circuit current and transient stability,the overall process and steps of the mode selection method are given;the power flow,short-circuit current,transient stability verification contents which are suitable for the selection of grid-connected DC transmission mode selection problem are designed.Considering the probability of failure and the severity of its impact on the stability of the sending end system,the order of transient safety and stability verification of the first stage and the second stage and standards are determined.According to the AC and DC characteristics reflect the sensitivity of the system stability,temporary stability test is given.Finally,the simulation model established in Chapter 2 is used as an example to verify the effectiveness and adaptability of the model selection method.(3)To solve the problem of mode conversion between grid-connected and island-connected operation mode,a grid connection mode which meets the requirements of grid-connected and island-connected operation mode is proposed and its wiring scheme is determined.Then,a mode selection method GICM mode selection is put forward,and a method of scheme modification including power flow adjustment,short circuit current restriction and transient stability control is designed.The validity of the GICM mode selection method is obtained in this paper.