Research On DC Magnetic Bias Of Transformer Netural Point And Its Suppression In Power Grid

In the AC-DC hybrid transmission network,the pole operation is unbalanced or the DC transmission is operated on a unipolar earth.The DC surface potential ACTS on the substation grid,and the DC current flows through the neutral point of the transformer,causing the main transformer to be deflected by DC.As a result,noise,vibration and harmonics occur in the transformer,which has a huge impact on the whole AC system and the operation of the transformer.The theoretical analysis can be used to understand the DC transmission system as DC transmission conductors in the ground during the return operation of mono-polar earth.At this point,the current of the system is the current of the current,which changes the surface potential near the converter station.At this point,the ground transformer will form the DC component(neutral point),and the transformer will generate DC bias.At present,China is actively implementing the policy of power transmission from west to east,and domestic power grid will inevitably make more frequent use of HVDC transmission technology.At the same time,the DC magnetic biasing problem is further highlighted due to the lack of relevant vertical separation devices.In this paper,the influence mechanism of dc bias on transformer is analyzed.In view of the variation of transformer loss,noise and vibration,many adverse effects of dc bias on transformer and power system are discussed.Then taking 110 kV transformer as an example,the finite element simulation software was used to establish the transformer 3d simulation model.The variation of excitation current and harmonic component of the transformer under different dc bias conditions is analyzed by taking single-phase transformer as an example.Then,the variation of leakage magnetic field in these areas and the stray loss caused by leakage magnetic field are analyzed for the metal structure of transformer.It is concluded that the leakage magnetic field increases significantly with the increase of dc bias,and the stray loss caused by leakage magnetic field becomes more and more serious in the metal structure.In view of the above abnormal operating conditions,this paper analyzes the dc bias magnetic suppression method of the transformer,mainly including: the neutral point injection reverse current method of the transformer;Capacitance straightening method for neutral point;Small resistance current limiting method and direct current compensation method are added to the neutral point.The operating principles and advantages and disadvantages of these methods are compared and analyzed,and the guiding Suggestions for the anti-dc magnetic bias method of transformer are proposed.