Analysis Of The Main Transformer Neutral Point Of 220kV Substation Through Small Reactance Grounding Operation

In the 220kV system,the neutral point of substation is accustomed to adopt partial grounding operation mode.When a ground fault occurs,the ungrounded main transformer neutral point will generate overvoltage,the main transformer winding will withstand excessive short-circuit current,and even the single-phase short-circuit current is greater than three phase short circuit current.Although it can be protected by the main transformer neutral point gap parallel lightning arrester.However,the gap discharge is easily affected by environmental factors,the gap dispersion is large,the distance setting is difficult,and the gap is prone to "reject action" or "misoperation".And there is a possibility of imbalance between the gap and the lightning arrester.Practice shows that the main transformer neutral point can effectively solve the above problems through small reactance grounding.Therefore,this paper has earried out a more in-depth and eomprehensive study on the problems related to the small reactance grounding of the main transformer neutral point in the substation.Firstly the zero-sequence equivalent reactance change of the transformer is analyzed when the neutral point is direct grounding or small reactance grounding,and the value of small reactance is recommended to maintain the zero-sequence reactance of the substation,moreover,the effect of grounding mode's change on zero sequence current and neutral point overvoltage is also analyzed.Then,the theoretical analysis and calculation of the short-circuit current of the fault point and the neutral point overvoltage of the ungrounded main transformer are carried out under the operation mode of the neutral point directly grounding and small reactance grounding for a 220kV substation when the 110kV busbar occuring single-phase short circuit.And the PSCAD/EMTDC software is used to model and simulate.The analysis results show that the consequences of the two analytical methods are very consistent,which verifies the correctness of the model.After analysis and comparison,an operation mode that can reduce the short-circuit current to a large extent is recommended.Then,under the recommended operation mode,a more in-depth and comprehensive analysis of various types of fault overvoltage and short-circuit current in the 220kV side of the main transformer under partial grounding and different resistance of small reactance grounding operation modes are carried out,and the best small reactance value is recommended.The neutral point overvoltage protection and insulation coordination and the selection of small reactance parameters of small reactance grounding are proposed through the simulation results.Then the principle of parameter determination of traditional gap parallel arrester protection mode and various new neutral point protection methods including advantages and disadvantages are introduced.And the current neutral point gap protection setting principle and existing problems and improvement suggestions are analyzed.Finally,the practical application of the main transformer neutral point through the reactor grounding is introduced.In this paper,the influence of neutral point overvoltage,short-circuit current and relay protection setting are analyzed after the main transformer neutral point of 220kV substation is changed from partial grounding to small reactance grounding.The conclusions obtained ean provide reference for the promotion and application of the small reactance grounding operation mode in the 110/220kV substation,which has strong engineering application value.