Research On DC Bias Analysis And Suppression Method In Substation Of Transmission-end Power Grid

UHVDC transmission is superior to AC transmission of the same voltage level in terms of line cost,power loss,system stability,rapid regulation and non-capacitive charging current.As the projects are put into use one after another,when the UHVDC transmission system operates in the mono-pole ground return operation mode,the DC current in the power transformer near the DC grounding pole exceeds the safe value.The excitation current of the transformer is distorted,which will cause a series of hazards such as aggravated vibration and local overheating of the transformer.In this context,it is necessary to systematically research and manage the power transformers affected by DC bias in the power grid.The research contents of this thesis are as follows:This thesis firstly analyzes the reasons for the occurrence of DC bias in power transformers.Through the analysis of the AC-DC hybrid system of the transformer,the mechanism of DC bias is clarified,and the degree of distortion of the transformer excitation current and the content of various high-order harmonics under the condition of DC bias are theoretically analyzed;secondly,in order to verify that the autotransformer in the sending end power grid system will be affected by the DC bias,the model of the 330 kV and 750 kV autotransformers are established,and the DC bias simulation of these two autotransformers is carried out.The simulation results can provide a basis for DC bias management at the power grid system level.In order to solve the problem of DC bias in the sending end power grid,the complex network theory method is used to construct a complex power network topology model according to the structural parameters of the actual power grid in the sending end power grid,and the DC current flow at the grounding neutral point of the autotransformer in the power grid is calculated Judging the safety threshold,identifying the sites affected by the DC bias,taking control measures for the most affected substation sites,and reflecting the governance effect through the governance effect index;finally,thesis analyzes three DC bias suppression measures,and compares the device input based on the management effect during operation,the advantages and disadvantages of the three suppression measures are obtained,and the capacitor DC blocking measure is selected according to the simulation results.The simulation results show that the excitation current distortion degree of the autotransformer model is in line with the system analysis in the DC bias environment.In order to solve the problem of DC bias in the sending end power grid,the key sites are identified and governed according to the simulation results under the complex network theory.After the treatment is completed,the amount of DC current flowing into the neutral point of the autotransformer is below the safe value.The distribution of the current flowing through the neutral point of the station also tends to be uniform,and the value of the control effect index also decreases,which proves that the complex network theory has a good effect on the DC bias control of the sending end power grid.This method can be used DC bias for other power grids.