Study On Impedance Frequency Characteristics Of Traction Substation

There are more and more nonlinear loads in power system, leads to the harmonic content increasing and the harmonic pollution is increasingly serious, has severely threatens the safety of electric power system and electric equipments. The analysis of system impedance frequency characteristics has very important significance for the harmonic governance and the resonance analysis of parallel capacitor group, accurate assessment of the system harmonic impedance can improve power quality and guarantee the safe operation of the capacitor group. In this paper, the main research object is the system harmonic impedance measurement methods and harmonic amplification mechanism of parallel capacitor group.Existing harmonic impedance methods were studied, including harmonic current injection method, regression method, fluctuation method and switch element method. Contrast the advantages and disadvantages of each method and considering the actual situation, regression method and capacitor switching method is selected for further analysis, because they are easy to implement and low cost.In order to overcome the interference of abnormal data in samples when use regression method to estimate harmonic impedance, a new method was proposed, which based on median parameter equivalent weight regression method. Build simulation model in Matlab, some conclusions can be obtained from the simulation results, when excluding outliers interference, the result of improved algorithm is close to the existing methods, but in the abnormal, the improved algorithm is more close to the theoretical value. Actual measurement data is used for further analysis,the results of improved algorithm and fluction method is consistent and smaller fluctuations in the results of the improved algorithm, the practicality of algorithm was proved. The superiority of the proposed algorithm was validated through theoretical research, simulation analysis and experimental data caculate.Capacitor switching harmonic impedance measurement method was studied, and field test was conducted on the traction substation.We can found that this method greatly influenced by external but has fast calculation speed through theoretical analysis. Some conclusions can be obtained through simulation analysis, when there is interference, if the total interference rate is less than one harmonic ration then the harmonic impedance calculation results could be believed, otherwise the result is not credible. Calculated by the measured data, can found that there must be large enough harmonic sources, namely the harmonic power relative system capacity is large enough, the harmonic impedance can be measured accurately. According to the measured datas of traction substation, capacitor switching method and improved regression method was used to calculate, the harmonic impedance results especially odd results were very close, the practicability and feasibility of the method is validated.Power system harmonic resonance mechanism are analyzed, emphatically analyzes the substation harmonic amplification of parallel capacitor group. Parallel capacitor group branch harmonic amplification and harmonic resonance analysis criterion was put forward:①each harmonic current exceeds bid;②analysis the result of theory resonance points;③ matching relations of system harmonic impedance calculation results and capacitor harmonic impedance.