A common mistake and its harm in CT secondary connection of transformer differential protection were analyzed, the correct connection mode was explaned, and how to use the hexagon vector diagram of checking was introduced.

• 摘要分析了一种常见的变压器纵差保护电流互感器错误接线及其危害，说明了正确接线方式，并介绍了如何通过作六角图来检查纵差保护二次接线的正误。