Study On Dc Transmission And Ac Grid Transformer Dc Bias

In our country, the disproportion of supply and demand of power resources promotes the development of long-distance, high-capacity transmission line. Therefore, HVDC plays an increasingly important role in China’s power grid construction. The DC current generated by the HVDC system flows into the ground point of transformers and other equipments in the system. The resulting de flux will lead to a large number of harmonics. The reactive power consumption of transformer increases, voltage of system is severely degraded and relay protection is falsely operated. Severe magnetic saturation will make the closed magnetic flux in normal situation leave the core, which results in the structures overheating, insulation ageing.In addition, the duration time and crest value of de current also affect the safe operation of transformer.Repeated damage to various parts of transformer will bring on damage accumulation, which results in eventual destruction. The paper makes the UHVDC fire electricity in ShanXi province as the background. The generation of de bias current effect of de magnetic bias and measures to restrain the current will be studied in this paper.Based on the grounding analysis software CDEGS, the paper builds AC-DC interconnected model under the HVDC single-pole mode and studies the ground surface potential distribution around de ground electrode. Meanwhile, the effect of soil parameters on the ground surface potential is analyzed in this paper. With discussing the phenomenon of diffluence when DC current flows into AC power grid caused by ground potential difference, the author to the DC component of transformer neutrals in different substations and draws the DC current distribution graph in AC power grid.The impact HVDC ground electrode current on transformer and waveform characteristic of exciting current are analyzed exhaustively in the paper. The system simulation model of transformers under DC magnetic bias is established by Matlab/Simulink. By studying the flux of transformer with different structure and the exciting current waveform, the author obtains the limits DC bias current that transformer can endure, which provides a basis for research of transformer DC bias restraining measures. The results can provide a reference for the choice of de grounding site and a theoretical basis for the protection of DC bias current in the substations around the DC ground electrode.