Research Of Traction Transformer Lightning Protection Considering Wave Process

With the rapid development of electrified railway power system, the requirements of its security performance have been being gradually improved. As the most important facility in substations, it is extremely important guarantee the traction transformers’ insulation safety and reliable operation. Lightning voltage has relatively high amplitude, large gradient and wide frequency range, when invading the traction transformers, it can create the wave process on windings. Once the transformer is inappropriately designed, the phenomenon of insulation breakdown would appear which can seriously threaten the insulation safety of windings. This thesis combines the time and frequency domain analysis to finally obtain the windings’ vulnerable spots after the lightning invasion on transformers aiming at indicating a comprehensive analysis on traction transformers’ insulation influences caused by the lightning originated wave process. The main research contents are as follows.Firstly, with the transient analysis on the whole traction substation, the electrical equipment such as transformer is equated with the form of entrance capacitor. Then the electro magnetic transients program(EMTP) model is constructed, chooses the proper lightning strike point, the amplitude and gradient of overvoltage formed on transformers is calculated when entrance tower is hit in traction substation as well as analyzes the protective effect on traction transformers when just installing lightning arresters. Then results showed that lightning arresters failed to decrease the transformers’ gradient from lightning wave invasion and insure the insulation safety inside the windings. Thus, the influences of winding wave after the invasion of steep wave must be considered.Secondly, the thesis has analyzed the situation of extremely heterogeneous potential distribution on windings caused by the invasion of steep wave. Through building multiconductor transmission line transient model of traction transformer in time domain, the voltage iteration formula was deduced based on Finite Difference Time Domain(FDTD). At the same time, with applying vector matching method, the problem of FDTD failed to achieve frequency dependent parameters was solved. In this case, the FDTD voltage iteration formula with considering the frequency dependent parameters can be obtained to calculate the potential distribution inside the windings any time at any point.Finally, the thesis discussed the situation of high frequency resonance overvoltage formed on windings caused by invasion of steep wave. In order to locate the exact turn where resonance appeared inside the windings, a port model was built in frequency domain to calculate the voltage transfer function of each turn. Through spectrum analysis on the model,the resonance frequency and the point where resonance most easily happened can be found.The results showed that when lightning invasion happened on traction transformers, withconsidering overvoltage and resonance, the 6thto 8thturns of the first area were the points where insulation safety was mostly threatened. So it is strongly recommended to specially conduct the insulation protection in these areas. This thesis compared the results obtained in both time and frequency domain at last, and illustrated the methods applied in the thesis are reliable as well as the results.