Research Of Impacts On The Grid Operation By Harmonic And Negative Sequence Current Of The Traction Load

With the advantages of its high-speed, heavy loads, little pollution, energyconservation, electrified railway has been developed rapidly in China, bringingenormous economic benefits to the national economy. Electrified railways, withoutenergy in themselves, need to obtain power frequency energy from the grid. Becausethey are asymmetric, nonlinear loads, when exchanging power with the network side,electrified railways send harmonic and negative sequence currents to the grid. Withthe power of electric locomotives increasing, it will inevitably affect the safeoperation of power system.This paper has concentrated on the impact of harmonic and negative sequencecurrents on the operation of grid. Its major works and accomplishments are shown asfollow.Based on the measured data of three110kV traction substations, analysis of theharmonic and negative sequence characteristics of the locomotives under differenttraction working conditions was carried out. As a result, it turns out that the tractionworking condition is the primary factor to determine its harmonic and negativesequence level, and the traction working condition of locomotives is a directreflection of the power exchange between locomotives and power system. On thebasis of previous analysis, using measurement-modeling method, this paper hasestablished a model with the three-phase fundamental positive active power andreactive power exchanged between locomotives and the system on the network sideacting as excitation, and the real and imaginary part of harmonic and negativesequence currents as the responses respectively. Considering the fact thatnetwork-sided voltage fluctuates in a certain range, polynomial model can be selectedto describe the harmonic and negative sequence characteristics.Based on the field data parameter identification and the comprehensiveclassification large number of measured samples and their identification results.Thetraction load different operation mode and different traction conditions and differentload level of the harmonics and negative sequence current source model recommendedparameters are identified. Then, the harmonic and negative sequence source modelestablished in this article are validated to reflect the essential harmonic and negativesequence characteristics of the electrified railway traction loads from these three indicators, i.e. the maximum relative error, the correlation coefficient, and the averagedeviation.Based on the analysis of the user-defining model(UDM) functions and features ofthe power system analysis software package(PSASP),the interface between establishedmodels and harmonic simulation and analysis in PSASP was realized. The correctnessof interface was validated through illustrating the realization of UDM interface andconducting simulation based on EPRI-36system. At last, the paper has introduced thecalculation flow of harmonic and negative sequence current in PSASP.Backed by some regional grid in Hunan, massive simulations of variousoperation modes and different load levels and traction working conditions wereconducted. Consequently, systematic and quantitative analysis of the impact thatnegative sequence and harmonic currents generated by traction loads have ongenerator, transformer, motor, line and relay protection was carried out with relevantanalytical results. Meanwhile, a series of countermeasures to handle harmonic andnegative sequence currents were proposed.