Line Side Protection of Substation at Powerhouse

There is no single "normal practice." What is used for # of CTs and # of cores depends on the protection schemes used and the protection and design philosophy of the engineer(s) doing the design, and also any codes or design criteria which may be mandated by regulatory bodies that have oversight of the transmission system.

Sometimes it is accepted that different redundant protection systems can use different cores of the same CTs, but other people or design requirements call for them to be on different CTs. Some people allow metering or SCADA circuits to be on the same CTs, even the same CT secondary, as protection circuits, while others require them to be separate.