Large power transformers tend to be used together with the Double Microcomputer Differential Protection. In the process of main transformer substitution operations, the differential protection current loop of the main transformer is switched to the bypass CT (2246 CT), with the main transformer attached to the outer CT (2203 CT). The switching operation of the main transformer used to be switched to the main transformer bushing CT, with the main transformer attached to the outer CT, leading to long exit time of the main transformer differential protection, the differential flow of the protection differential during operations, complicated operations and high risk. The present paper presents a new method in which the bypass 2246 circuit is accessed to the differential circuit in its open state, and the differential circuit of the 2203 circuit is lifted in its open state, so that the exit time of the main transformer differential protection is reduced, with the involvement of the two sets of differential protection in the operating process, ensuring the safety of operators and improving the protection reliability.%大型电力变压器均采用双套微机差动保护装置，在主变代路过程中，主变差动保护的电流回路须由主变外附CT（2203CT）切换至旁路CT（2246CT）．以往的倒闸操作沿用主变外附CT切换至主变套管CT的方法，该方法主变差动保护退出时间长，操作过程中差动保护将产生差流，操作繁琐，危险性高．作者提出了一种新的方法，即，将旁路2246的电流回路在开路状态接入差回路，将2203的电流回路在开路状态解除差回路，其主变差动保护退出时间短，倒闸操作过程中两套差动保护始终投入，保证了操作人员的人身安全，提高了差动保护的可靠性．
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