Impact Analysis And Risk Suppression Of DC Magnetic Bias Induced By UHV DC Transmission

Along with the rapid development of China’s economy,it is necessary to use UHV DC transmission to send surplus electricity from the western region to the more economically developed eastern and central provinces.When the UHV DC transmission in the commissioning,fault conditions will take the earth circuit mode of operation,then thousands of amperes of DC current are injected into the earth,and the earth surface potential difference appears.The grounded transformers and transmission lines provide the current paths for the potential difference on the earth surface,and the DC current flows into the transformer windings,thus causing DC bias phenomena such as transformer vibration,noise,heat,and harmonics in the excitation current,which affect the normal operation of the transformer and threaten the safe operation of the power system.Therefore,it is necessary to study the DC bias characteristics of transformers and their suppression measures.Firstly,the changes of each harmonic component of the transformer excitation current under different values of bias flux are simulated by Simplorer assembly in ANSYS.Different DC bias management standards and suppression measures are compared,then the DC bias management standards and suppression measures adopted in this paper are determined.Secondly,the formulas for calculating the earth potential under the earth circuit operation mode of DC transmission are analyzed,then a simplified formula for calculating the earth surface potential is derived by using the linear filtering method,and the calculation error of the formula is less than 1%,which proved the accuracy of the formula.Based on this,a simplified evaluation scheme of DC bias risk of power system considering the direct coupling relationship of field and circuit is proposed,and the effectiveness of the scheme is verified by evaluating the DC bias risk of Hebei South Network,the evaluation results show that the calculated errors of DC currents of transmission lines are less than 0.1A.Finally,based on the parameters of a power grid in Northwest China,a power system model with DC bias magnetization and considering direct field-circuit coupling is established and equivalently analyzed,then it is combined with a multi-objective discrete particle swarm optimization algorithm to optimize the configuration of DC bias magnetization suppression measures.The optimized results show that using the multi-objective discrete particle swarm algorithm can get better management results than the " treatment after exceeding the standard" solution.