Protection Research Of Traction Transformer Insulation Under Lightning Intrusion

In recent years,with the rapid development of high speed railway,the operation safety relates closely to the traction power supply system for electrified railway.Traction substation occupies an important position in traction power supply system,the insulation safety of the electrical equipment was increasingly emphasized.Especially for the traction transformer,its insulation situation can directly influence the safety operation of traction substation.The research showed that after a lightning intrusion on traction substation,the over voltage amplitude can reach the highest value on transformer.Thus,the analysis should be pointed at the insulation conditions of the transformer under lightning intrusion.Taking the wide range frequency,high wave amplitude and long duration time of the lightning intrusion wave into consideration,one of the wave performance is resonance.This phenomenon can seriously threat the insulation safety,and the inhibitory effect is not ideal enough by the arrester protection.Thus,the transformer insulation effects caused by the resonance is analyzed.In addition,a paralleled transformer mental oxide varistor(MOV)method is raised to suppress lightning resonance over voltage in order to ensure the insulation safety.The main research contents are as follows:Firstly,an analysis is generated on traditional multi conductor transmission line model of transformer windings to point out that the difference for the turn length of real transformer winding was relatively large and the average turn lenth solving for traditional transmission line can lead to definite errors.In this case,an improved toroidal multi-conductor transmission line model is constructed under the polar coordinates with true length of the winding turn.The construction procedures and the winding terminal voltage are deducted.Secondly,the effects of the resonance on winding insulation under lightning intrusion wave are analyzed in detail.According to the toroidal transmission line model,the winding transfer function in frequency domain is deduced.The resonance situation is studied by analyzing the amplitude frequency characteristic curve and the insulation breakthrough points are located caused by the resonance.The results show that after the lightning intrusion,the main winding resonance frequency is around 0.8MHz and the 4th~8th windings of the first pie can be much easier to meet breakthrough.Thus,it should be paid more attention to lightning defence.Finally,the methods of resonance suppression and insulation protection under lightning intrusion is studied.The method of piecewise paralleled MOV is used to inhibit the resonance over voltage.The MOV is paralleled on the 10 th,20th,40 th and 60 th windings.A simulation analysis is generated on the MOV paralleled windings' amplitude frequency characteristic curve.The results show that the resonance over voltage deduce to half of the primary values after getting the MOV paralleled.Therefore,it can be proved that MOV contains favorable winding resonance protection performance.