The on-load tap-changer of UHVDC converter transformer has been basically monopolized by foreign enterprises due to its long-term carrying full load operated square wave curent

large switching current d

i

/d

t

difficult arc-quenching and frequent action. Firstly

as for such issues as ‘Black Box’ of traditional on-load tap-changer without any direct and effective electrical monitoring and protection and converter body damage due to fault which affects the utilizatino rate of UHV DC project

a kind of split-step on-load tap-changer topology is proposed to realize the split layout of the core components of converter-based on-load tap-changer and the step-by-step closed-loop control protection

and the electrical stress in engineering application is analyzed. Then

such key components as switching-over switch

tap selector and control protection device are developed. Finally

the switching test system based on

LC

resonance method is set up

and the switching test

capacity test

temperature rise test and insulation test are carried out for the developed split-step on-load tap-changer. The test results show that the developed split-step on-load tap-changer has a rated voltage of 4 kV and a maximum carrying current of 1 600 A. The switching-over switch is of extern

al and oil-less design

the inside is integrated with electrical measurement system and the switching process can achieve step-by-step operation control and protection

avoid fault aggravation and meet the engineering application requirements of ±800 kV/5 000 A high-voltage direct current converter transformer.