Effect of Connection Schemes of 500kV SF_6 Current Transformer on Protection Operation

During an internal fault of 500kV sulphur hexafluoride(SF6) current transformer(CT),its top shell with high voltage discharges to earth through insulator support,capacitive screen or supportive glass fiber reinforced plastic column.The secondary currents of CT are different because of the different primary connections,and the operation of relay protection is affected.Therefore,the operation of line protection and bus protection under parallel connection and series connection on primary side during switch unit CT fault on bus side is discussed with matters needing attention indicated.During CT shell grounding fault,line protection operates reliably and bus protection doesn't operate under parallel connection;both line protection and bus protection may not operate under series connection.