Research On Magnetizing Inrush Current Characteristics And Identification Method Of Converter Transformer

HVDC transmission technology and its engineering have developed rapidly.As a key equipment of HVDC transmission system,converter transformer is essential for safe and stable operation.Converter transformers differ from conventional power transformers in terms of wiring and structural characteristics,including DC bias during normal operation and magnetizing inrush current during no-load closing and their effects on current differential protection.To this end,this paper has carried out research work on the inrush current characteristics of the converter transformer and its identification method.The ultra-high-voltage and high-voltage large-capacity converter transformer is generally composed of a star-angle transformer and a star-connected transformer in parallel,and provides an alternating-current voltage with a phase angle difference of30° for the converter valve,and accordingly realizes 12-pulse AC-DC conversion.This paper describes the generation mechanism and influence of DC biasing of converter transformers,as well as the characteristics and classification of the inrush current of converter transformers.It is analyzed that the excitation inrush current of the converter transformer may have a serious impact on the safe and reliable operation of the current differential protection under different operating conditions.For the converter transformers operating in parallel,this paper deeply studies the intrinsic relationship between the angular circulation of the star-angle transformer and the zero-sequence current of the star-side neutral point during the no-load closing of the star-angle transformer.The composite circulating current and composite zero sequence current of the converter transformer group formed by the star angle transformer and the star transformer are defined.On this basis,the relation expressions of the commutating transformer composite circulating current and the composite zero-sequence current under different conditions of no-load closing inrush current and internal grounding fault are derived.A new method for identifying the inrush current of the converter transformer based on the waveform correlation coefficient and the current differential protection scheme are proposed.Theoretical analysis and simulation verification show that the new method can effectively avoid the misoperation of the converter transformer differential protection during the inrush current and the refusal of the airdrop in the fault,effectively improve the operational safety of the converter transformer and ensure the reliable operation of the AC and DC system.