Research On The Superimposed Effect Of Zero-sequence Electrical Quantities Of Large-scale Implementation Of Autotransformer Neutral Grounding Via Small Reactance

With the continued growth of the capacity of the units and substation capacity and loaddensity in the power grid, the grid structure is becoming more and more closely. It enablesincreasing levels of short-circuit current in the different voltage level of the power grid, andeven excessive. To suppress the short-circuit current in power grid,500kV transformersneutral grounding via small reactance is an effective measure. However, the zero-sequenceparameters of system will be changed. When unsymmetrical earth fault in transmission lineoccurs, the relay protection will be affected. Particularly the main transformer protection andtransmission line protection nearby, they may not act correctly.According to the Guangdong power grid concrete parameter, this paper builds a systemequivalent model in light of autotransformer in500kV substation. When the only oneautotransformer neutral is grounded via small reactance, the effects on power system zero-sequence parameters are analyzed. Meanwhile, a quantitative calculation is accomplishedbased on the relative variation of zero-sequence electric quantities. The following threeInfluencing factors，the unsymmetrical earth fault position variation and the small resistancesize variation and the transmission line length variation, are considered in the calculation.Furthermore, the impacts on protective relaying and coordination between the adjacent linesare analyzed according to curves drawn by Matlab. The zero-sequence current protection of500kV auto-transformer neutral should be re-set to meet sensitivity.On the basis of these studies, this paper puts forward an ring complex power systemequivalent model based on the500kV autotransformer, and uses the actual parameters of theGuangdong Power Grid500kV sites. With and without small reactance of the mainauto-transformer neutral in one substation, a relative variation quantitative calculation ofzero-sequence electric quantities is accomplished when the unsymmetrical earth failurelocation is different. Meanwhile, the superimposed effect of zero-sequence electrical quantities change is researched in different installation circumstances when theautotransformer neutral grounding via small reactance in multiple500kV substation.Furthermore, the effects on protective relaying are analyzed.The zero-sequence currentprotection of the main autotransformer neutral should be re-set to meet sensitivity. Thecorrectness of the theoretical analysis was verified by simulation of an actual large-scaleinterconnected power grid.