A kind of mechanical three-pole-gang-operation SF6 circuit breaker with hydraulic mechanism was developed based on present 252 kV self-energized single-pole-operation SF6 circuit breaker with spring mechanism

and a suitable hydraulic mechanism and its transmission structure were selected through calculation to make it match with the no-load characteristic of single-pole-operation circuit breaker for simplifying the test.The emphases were given to the equations for calculating the related physical quantities in opening process of the circuit breaker

and the programming and computation of no-load characteristic of the three-pole-gang-operation circuit breaker based on the equations.The characteristic curves of several different hydraulic mechanisms were calculated and compared with the referenced mechanical characteristic curves.It is shown that the calculated results for selecting the hydraulic mechanism and its transmission structure meet the requirement of simplifying the test.This study may facilitate the product design.