A Method Of Fault Line Selection For Small Current Single Phase Earth Fault Based On Improved S Transform

With the rapid development of economy,the power system is required to have better stability and higher reliability.Small current grounding system is widely used in power system under 66kV.Among them,more than 80%are single-phase ground fault.However,the current of single-phase grounding fault is small and has many the influence factors,therefore most of fault feature extraction of the line selection algorithm is not comprehensive,which exist the situation of false and leakage selection.So the accuracy of single-phase ground fault line selection of small current grounding system still needs to be improved.Based on the analysis of the zero-sequence current transient characteristics of the single-phase-to-ground fault in the small current grounding system,a new method of single-phase-to-ground fault line based on improved S-transform is proposed.Firstly,a small current grounding system simulation model is established using the simulation software MATLAB/Simulink.The simulation and analysis of the single-phase ground fault are carried out for the small current system of the three neutral point grounding modes,which is obtained in the different lines,different fault location,different fault initial angle and different transition resistance.Then,a low-current single-phase fault line selection algorithm based on hyperbolic S-transform(HST)is proposed,and the zero-sequence current of each feeder is analyzed using HST according to the zero-sequence current transient time.The results show that the algorithm has high accuracy and good applicability.Finally,a new method of single-phase-to-ground fault line selection based on fast S-transform(FST)is proposed.The possible fault lines are selected by the method according to the line energy value calculated from the one-dimensional modulus coefficient of each line fault signal FST,and then the fault lines is determined whether the line fault or bus failure according to the possible fault line and phase angle reference line FST after the main frequency point of the one-dimensional phase angle is greater than 30°.Comparing with the S transform and the improved algorithm,it can be found that the real-time performance of FST is better.Finally,a simulation experiment of FST algorithm with good real-time performance is carried out.Then,a small current grounding system fault simulation test circuit and fault signal acquisition circuit are built according to the "technical specification of single-phase grounding fault line selection device of small current grounding system",and the fault signal generated by the analog circuit is verified and analyze based on FST algorithm in MATABLE software.The simulation experiment and the fault simulation circuit experiment show that the method is suitable for three kinds of neutral grounding modes,and the line selection is accurate and real-time.