Research On Optimization Partition And Coupling Reduction Strategy Of AC/DC Hybrid Receiving System

In order to optimize the energy allocation and enhance the transmission capacity across regions,the ultra-high-voltage power transmission technologies have been widely used in China.Large-scale AC/DC hybrid grid pattern has been formed in the receiving grid.When the UHV network frame gradually comes into large scale,the operation state of the electromagnetic loop network with the original 500 kV power grid has formed,which brings many stability questions.Study on the electromagnetic loop decoupling of the system with multi-infeed HVDC systems is necessary.At the same time,with more HVDCs feeding in the grid,the interaction between multiple HVDCs and AC system is closer and more complex.When an AC fault occurs near the converter bus,the impact of AC fault and the interaction between multiple HVDCs overlay each other,which may cause cascading commutation failure,ultimately,leading to relatively severe power shock and voltage fluctuation.To improve the safety and stability of AC/DC hybrid system,it is of great importance to study on the solving methods of multi-HVDC commutation failure in fault scenarios.This thesis focuses on the problems of interregional electromagnetic loop network and commutation failure in multi-infeed HVDC system.The main jobs and results of the thesis are as follows.(1)Study on the risk analysis in AC/DC hybrid system is carried out.The forming reason and the harm of electromagnetic loop network such as excessive short circuit current and stability disruption are described.By summarizing the previous studies on the electromagnetic loop and multi-infeed HVDC system,the fundamental principles of the decoupling strategy at the AC/DC hybrid receiving system are proposed.Based on the analysis of the 6-pulse converter bridge,the development of commutation failure and its main influencing factors are studied.(2)A power grid partition optimization model with consideration of HVDC clustering optimization and electromagnetic loop decoupling is established.The multiple HVDCs are divided into different groups for cluster management.The potential relevance between HVDC systems on coordinated control relationship of voltage,the complementary characteristics of various types of energy transmitted by HVDC,the reactive power support for LCC-HVDC provided by VSC-HVDC and so on are quantified.Then,the correlation matrix of multi-infeed HVDC system is established.By applying the spectral clustering,HVDC clustering for optimizing HVDC management is solved.Based on the result of HVDC clusters,the optimization model of power grid partition is established,which aims to limit the short circuit current of the system and reduce the change of power flow after partition.NSGA-? is used as the solution method to deal with the problem of the expressions of objective and constraint functions are hard to derive.Finally,an actual planning grid is used as an example for the analysis of the proposed method.(3)The reactive power/voltage interaction among AC and multiple HVDC systems is described,and the development process of cascading commutation failure is analyzed in detail.To improve commutation failure questions in multi-infeed HVDC system caused by the close coupling relationship between HVDC,a strategy to reduce coupling by increasing the impedance of critical lines is proposed.The graph model of AC/DC hybrid system is established,and the critical paths with the relatively significant impact on the coupling relationship between each pair of HVDC are searched by applying Floyd algorithm.Then the importance of each line in the paths is measured by the power flow betweenness to avoid implementing the proposed strategy on important lines.2 models are built in PSCAD/EMTDC,and the effectiveness on the improvement of commutation failure of the proposed strategy is verified by the simulation results.