Accurate current transformer (CT) polarity and protection direction is to ensure the safe and reliable relay operation of the foundation, large transformer unit protection generally use the main protection and backup protection integration, duplex configuration mode, need to access the CT current loop more, And part of the CT current loop for protection within the main protection, backup protection and abnormal operation protect the common use, CT polarity must also meet the main protection, backup protection, abnormal operation protection requirements. Due to the protection of large transmission unit and the complexity of its secondary circuit, resulting in CT polarity and protection of the wrong direction caused by the protection of the group to prevent malpractice or refusal to happen when things have seriously affected the safety and stability of the grid run. According to the "hexagonal graph theory" of detecting CT polarity and protection direction with traditional artificial load, the criterion and function of CT polarity and automatic detection of protection direction are constructed in the protection device of the transformer group, and the protection group Safety and reliability, with high engineering practical value.