CT Secondary Connections: A Practical Guide for Secondary Connections of Current Transformers in Substation

What is the significance of the connection Diagram of a 33 kV Outdoor 3 Core CT on the Name Plate ?

The above Diagram is of 33 kV 3 Core CT, which is given in the Nameplate of the CT.
1) Generally P2 is connected Bus Side and P1 towards the Line. Whichever is the case, it is generally maintained same for all the CT’s in a Substation.
2) In the Secondary Terminal the Prefix represents the Core Number and the suffix represents the Terminal Number.
So in 1S1, prefix 1 shows that it is 1st Core and suffix 1 represents it is the first Terminal.
Likewise, in 1S2, 1 represents the 1st Core and 2 represents the second Terminal of the core 1.
3) If we connect Full winding i.e. 1S1 and 1S3 we get the full ratio i.e. 400/1 and if we connect Half Winding i.e 1S1 and 1S2, we get half the Ratio i.e 200/1.
4) With the help of Class, we can identify the Core like say PS Class is used for Protection Relay, while 0.2S Class Core is used for Metering.

Why are CT secondary terminals Star Connected ?

Answer:
The Secondary Winding of R, Y and B Phase CT’s are mostly Star connected and then connected to the Relay/Meter. Current Transformer Secondary Connection is very important for proper functioning of the Meters and relays.

Please find below, some of the advantages of a Star Connected CT:
1) CT Earthing:
CT Secondary Earthing, which is very important for Safety, is possible with Star Connection and not with Delta Connection.
2) Metering:
In Star Connection, Line current is equal to phase current and so the current flowing through the Meter will reflect the Line Current of the System.
As the Meter measures exact CT Secondary Current, there is no conversion required.
Let say if we use a Delta Connection for CT Secondary,  it will reflect the phase current of the system. So we have to calculate the phase current to match the line current.
The phase current is equal to root three times of line current. 
Let’s say if the Line current is 100 A, the Phase Current will be 173 Amps approximately.
3) Protection:
Since  all the Protections are directly set with Line current, it is very easy to do the relay Settings  for a Star connected CT.