Based on function:
Current transformers used in metering and indicating circuits are often referred to as measuring CTs. They have a lower saturation point. In the event of a fault, the iron core saturates and the secondary current does not damage the measurement equipment connected to it.
The current transformer used in conjunction with the protection device is called a protection CT. The purpose is to detect fault currents in the system and transmit a signal to the relay. Since it operates at a higher current value than its rated value, its core has a high saturation point.
Strip current transformer:
This type of current transformer uses the actual cable or busbar of the main circuit as the primary winding, equivalent to a single turn. They are fully insulated from high operating voltages.
Wound Current Transformer:
The transformer's primary winding is in series with the conductor that carries the measurement current flowing in the circuit.
Ring/Window Current Transformer:
These do not contain primary windings. Instead, the lines that carry the current flowing in the network go through windows or holes in the toroidal transformer. Some current transformers have a "split core" that can be opened, installed, and closed without breaking the circuit to which they are connected.