| In power systems, the waveforms of voltage and current are distorted due to nonlinear loads, and the quality of electricity will decline for the great deal of harmonics generated. With the increasing nonlinear loads, harmonic problems become more serious in power systems, even threat the safely and economical operation of power systems. Now it's time to find ways to prevent and eliminate these harmonics in the power systems.Firstly, this paper gives the main harmonic sources and their harms to power systems, and then lists some technique methods now used to handle harmonics.This paper takes TangYin substation as the study object. TangYin substation is an important substation which has two transformer of 120MVA, eight 220kV lines and three 110kV lines.TangQian line is a 110kV line specially for traction transformer substation.Because traction load is high-power nonlinear and asymmetry.it will produce much harmonic and negatvie sequence current,which have done harm to compensation capacitors of TangYin substation.In recent years, the Tang6C capacitor often interrupted and in 2001 July the Tang7C capacitor's discharge PT exploded.Because the harmonics in TangYin substation are primarily from electric traction loads, this paper gives the structures of several main locomotive models and general harmonics they produce.The harmonics produced by electric locomotive have a characteristic that their range and phase are both dispersive.This paper also analyzes the impedance-matching balance transformer's structure and harmonics injected into the power system at its primary side, and sums up the characteristics of the harmonics from electric traction loads in the TangYin traction substation.Due to the complexity of harmonic problems, it's necessary to use a method of theory analysis and field measurement to solve them. In this paper, the harmonic measurement and the harmonic data processing software developed by us are introduced. The way of receiving harmonic data from harmonic testing instrument is improved and this work greatly increases the sampling speed and the data handling efficiency. In this paper.In accord with the state standard about harmonic.the harmonic permission value of Tangqian line is calculated. Some measurement data are shown and analyzed,and some main conclusion is drawed that the 7th harmonic voltages of TangQian line is near to the permission value, total harmonic voltage distortion ratio has oversteped the permission value.the 3th harmonic, the 5th harmonic, the 7th harmonic and the 9th harmonic current from TangQian line all overstep the permission, the largest 5th harmonic current ratio in TangSC reaches 44%, the 7th harmonic and the 9th harmonic current in Tang7C and TangSC is large maybe because the harmonic current is magnified.This paper studies the model of each element in Tang Yin substation. The way of taking one nth out of capacitor's the first harmonic capacitive reactance as its nth harmonic impedance is verified whit the data from Tang7C.According to the approximately estimating method, the first harmonic impedance of 220kV system is calculated.On the basis of the models, this paper gives the harmonic equivalent circuit of substation and gives the B(n) function of the harmonic time n.When the capacitors has different combination, B(n)'s value will also change, with the inputting local measurement data, the paper studies the series and parallel resonance at the substation, and deduces that the reason the capacitor's fuse of Tang6C is frequently interrupted is that its capacitor current contains a large number of 5th harmonics, and that the reason the discharging PT of Tang7C capacitor being exploded is also that the 7th and 9th harmonic currents are amplified. As a result, this paper gives a corresponding harmonic prevent measure.Finally, according to the asymmetrical characteristics of electrical traction loads, this paper studies the negative sequence currents under different loads and makes a piece of advice for reducing the negative sequence currents produced... |
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