Off-circuit tap-changer is an important component of UHV transformer, and its independent research and development has made great progress in China. A prototype of off-circuit tap-changer for UHV transformer was developed，and it adopts single-phase cage structure, divided into A and B two layers layout. In this paper, the electrical performance, temperature rise performance and mechanical performance of key components of tap-changer are calculated and checked to verify the reliability of its design.
      In this paper, the finite element method is adopted to conduct two-dimensional and three-dimensional modeling of the off-circuit tap-changer, and the maximum field intensity of the contact head position is calculated, and the maximum electric field calculated was 2.043kV /mm. In the case of good contact between the moving contact and the connecting plate, the temperature rises to 11.2K at 1.2 times the rated current. The moving contact bracket adopts brass silver-plated material, and the friction torque generated during the contact action is 14± 2.5Nm, far less than the maximum torque that the bracket can bear 940Nm, and the safety margin is large enough.
      The calculation results show that the product has good current-carrying capacity, electrical and mechanical properties, the current-carrying capacity of the contact is not less than 1.2 times of the highest rated current, the insulation margin is at least 5% higher than the standard requirements, and the product design has high reliability.
 

Off-circuit Tap-changer, Transformer, UHV