Influence Of DC Bias On Operation Characteristics Of HVDC System And Differentiation Of Bias Current

China has a vast territory,but the distribution of population and energy is extremely uneven.The distribution of load centers and primary energy bases is oppositely distributed.HVDC transmission has become an inevitable trend of development because of its many advantages.With the vigorous development and construction of China's HVDC transmission project,the phenomenon of DC biasing in converter transformers is increasing.When this problem occurs,it will lead to problems such as increased harmonics,increased noise,increased vibration,and increased loss,which seriously threaten the safe and stable operation of China's power grid.At the same time,the serious accident of bipolar blocking caused by DC biased differential current in the Southern Power Grid is in urgent need of in-depth study to avoid such accidents from happening again.In this context,the DC biasing of the converter transformer is deeply studied based on both theory and simulation.First,the reasons for the DC bias are studied.The mechanism of DC bias magnetism is analyzed,and the formula for calculating the DC bias current is given by the formula.The distortion of the excitation current and the content of each harmonic in the DC bias of the converter transformer are analyzed theoretically..Secondly,in the finite element simulation software MAGNET,the three-dimensional calculation model with the same size as the product-level converter transformer of Xi'an Xidian Transformer Co.,Ltd.is established.The simulatio n analysis of the converter transformer in rated and different magnitude DC bias current conditions is carried out.The magnetic field distribution characteristics of the flow transformer are analyzed,and the distribution of leakage flux in the fuel tank,windings,clamps and pull plates and the distribution of eddy current loss of the transformer under DC bias are analyzed.Then,based on an actual ±800kV UHVDC transmission project,a system-level simulation model corresponding to the parameters is built in the PSCAD/EMTDC simulation software.The relationship between the blocking fault and the DC bias current is studied in this model.At the same time,based on the simulation model,a DC bias simulation platform based on system level is established.By ad justing the resistance values of different sizes,the variation law of the DC bias current of the converter transformer under different operating modes can be simulated.During the unipolar operation of the system,by changing the transmission power on t he DC line,the mapping relationship between the transmission power on the DC line and the DC bias current of the neutral point of the converter transformer is established.At the same time,the converter transformer adopts the UMEC model with core segment ation characteristics.The magnitude of the excitation current and the characteristics of each harmonic,the current and voltage of the primary secondary winding and the harmonics of different secondary DC bias currents entering the converter transformer Characteristics and reactive power consumption characteristics have been studied.Finally,according to the principle of transformer magnetomotive force balance,the DC bias current difference of YY and YD converter transformers in the high-voltage DC transmission project of China Southern Power Grid is theoretically analyzed.The advantages and disadvantages of the traditional methods for suppressing DC bias current are explained.At the same time,in order to suppress the differential current,a topology of DC transmission system using YYD/YD converter transformer is proposed and theoretically analyzed.At the same time,based on the PSCAD/EMTDC simulation software,the corresponding simulation model is established,which illustrates the correctness of the proposed method.