Impact Of Faulty Inrush Current Of Converter Transformer On Differential Protection And Its Countermeasures

With the advancement of HVDC transmission technology and the rapid completion of HVDC transmission projects,China has gradually formed a large-scale AC/DC hybrid system.But the challenges that DC systems bring to traditional relay protection cannot be ignored.This article focuses on the differential protection of converter transformers in AC/DC hybrid systems,and selects the single-phase short-circuit grounding with the highest probability of occurrence on the grid side of the converter transformer as the fault condition in the engineering practice.Inrush current was discussed in depth,and the operation characteristics of converter transformer differential protection under the conditions of commutation failure and TA effects were studied.First of all,from the perspective of analyzing the DC conditions flowing into the converter transformer and combining the switching function theory,the current characteristics of the two cases of commutation failure after the ground fault of the inverter-side converter transformer grid side are not taken into account An analysis was performed to determine the saturation of the converter transformer.Based on this,the effect of this fault type on the differential protection operation characteristics is further analyzed.It is pointed out that the ground fault of the inverter-side converter transformer will not saturate the converter transformer,that is,no faulty inrush current will occur Its differential protection has little effect.Secondly,from the perspective of engineering practice,considering the influence of TA on the side of the inverter-side converter transformer and the external ground of the zone when taking into account the effects of TA on the differential protection operation characteristics.It is pointed out that when taking into account the impact of TA,faults in the area will cause the saturation of the TA on the grid side of the converter transformer,and the transmission characteristics will be seriously affected,leading to the risk of false blocking of the converter transformer differential protection.Finally,to account for the phenomenon of mis-blocking of the differential protection of the converter transformer after a fault in the grid side of the inverter-side converter transformer after taking into account the effects of TA,the secondary-side currents on both sides of the TA are based on the S-transformation and relative energy entropy theory.By comparison,it is determined whether the fault occurred in the region or outside the region.Based on the second harmonic braking principle,the latching logic of the converter transformer differential protection is improved to some extent.Simulation results verify the improved latching scheme.It can effectively identify the mis-blocking of the differential protection caused by the excessive second harmonic content,ensure the reliability of the differential protection,and ensure the safe operation of the AC/DC hybrid system.