Research On Transformer DC Bias Caused By UHVDC In Suzhou Power Grid

UHVDCtransmission has the advantages of large capacity and long distance power transmission. The uneven distribution of energy and load impel China’s west to east power transmission project. Vigorously exploiting west power and using UHVDC to transmit power to coastal economically developed areas is imperative. However, when the UHVDC transmission system works in monopolar operation, DC flows through the neutral point and causes DC bias of transformer around UHVDC grounding polar,which will threaten the operation security of transformers. Therefore, it is important and necessary to research on transformer DC bias and it’s restraining. Some key problems are reseached in this paper, such as the distribution of earth surface potentials and DC grounding current, the mechanism oftransformer DC bias and its effect, transformer grounding via small resistance to limit DC bias current and so on.According to the structural parameters of Suzhou grid and Suzhou geological situation, grid topology model is established.Earth surface potentials distribution and DC grounding current around UHVDC grounding polar are calculated in this paper based on CDEGS.Proceeding from the theoretical model of the transformer, the excitation current and itsharmonic components of transoformer in the DC bias case are analyzed.This paperprovides test data of DC bias in several Suzhou substations in Suzhou,when the UHVDC transmission system of Jinpin-Sunan works in monopolar operation.Restraining method on transformer DC bias is studied in this paper.The mechanism of transformer grounding via small resistance to limit DC biascurrent isrecommended in Suzhou power grid. And the feasibility of this scheme in Mudu substation is studied.