Study Of Mechanism And Elimination Measure Of Ferroresonance In Ungrounded System

In ungrounded power distribution network, the ferroresonance causes over-voltage andover-current due to saturation of electromagnetic voltage transformer (PT), which seriouslythreats the safety and stable operation of power system. In recent years, ferroresonance wasresearched a lot, and plenty of elimination measures were proposed by Domestic and Foreignscholars, but it still frequently occurs because every measure has its limitation, because of thecomplexity and volatility of power system. So, it is particularly important to further study themechanism and suppression of the ferroresonance in order to improve the security andstability of power system.In this thesis, the mechanism of ferroresonance was analyzed，the behavior andcharacteristics of high-frequency, fundamental frequency and sub-frequency resonance wereillustrated, and the necessary conditions for ferroresonance were summarized. In order toaccurately simulate calculations of the ferroresonance, a PT model that parallels withnonlinear inductance and nonlinear resistance was adopted, fully taking into account the ironloss and the nonlinear. Asimulation model of10kV ungrounded power system was built withthe PT model in ATP-EMTP. Matlab program was written to convert the valid values ofvoltage, current, iron losses power measured when the transformer has no-load to the i characteristic of inductance and u i characteristic of nonlinear resistor with point by pointmethod.In this thesis, the effect that excitation way, the linearity of excitation characteristics andiron loss of the PT, three-phase asymmetry to resonance were studied by stimulation with theabove simulation model. The results show that, single-phase ground fault can easily exciteslow-frequency resonance, and the excitation degree is closely related to the moment of thefault disappears; the different stage of three-phase closing can easily excites high-frequencyresonance; and the iron losses of PT should not be ignored in study; the better of thetransformer linearity, the smaller of the probability of resonant, but resonance can not be eliminated virtually; resonance occurs easier when there is an asymmetry of three PTexcitation characteristics or unbalance of three relatively capacitance. The two measures ofaccessing resistance and fourth PT to the PT primary winding neutral point were studied indepth, its principle and influence to relay were analyzed, the resistance and capacity werecalculated, and determined the resistance, capacity, linearity of the fourth PT, and the effectwas proved by simulation. The results show that, the resistance should be greater than50kΩ,when the linear resistance was used, and the relay need to re-tune; when4PT was used, thep of the linearity ratio of the fourth PT with three phase PT can be simulation calculated withthe method in this thesis; the relay can be calculated by the formula deduced in this paperwhen different p.The amplitude of voltage, current and the resonance characteristic bysimulation under different conditions were detailed in paper, which expect to provide areference to fault diagnosis, the choice of PT model and resonance elimination measures.