Current Transformer (CT) – Construction and Working Principle

Class T and Class C Current Transformers
ANSI/IEEE standards classify CTs into two types:

1. 
   Class T Current Transformer
   
2. 
   Class C Current Transformer
   

Typically, a class T CT is a wound type CT with one or more primary turns wound on a core. It is associated with high leakage flux in the core. Because of this, the only way to determine it’s performance is by test. In other words, standardized performance curves cannot be used with these types of CTs.
For class C CT, letter designation ‘C’ indicates that the leakage flux is negligible. Class C CTs are the more accurate bar type CTs. In such CTs, the leakage flux from the core is kept very small. For such CTs, the performance can be evaluated from the standard exciting curves. Also, the ratio error is maintained within ±10% for standard operating conditions.
Construction of Current Transformer
As we discussed above, there are three types of constructions used for the indoor current transformers which are,

1. 
   Wound Type CT
   
2. 
   Toroidal (Window) Type CT
   
3. 
   Bar Type CT
   

Wound Type Current Transformer – The transformers primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. The magnitude of the secondary current is dependent on the turn’s ratio of the transformer. 
Toroidal (Window) Type Current Transformer – These do not contain a primary winding. Instead, the line that carries the current flowing in the network is threaded through a window or hole in the toroidal transformer. Some current transformers have a “split core” which allows it to be opened, installed, and closed, without disconnecting the circuit to which they are attached.

Wound Type CT Toroidal (Window) Type CT Bar Type CT

Bar-type Current Transformer – This type of current transformer uses the actual cable or bus-bar of the main circuit as the primary winding, which is equivalent to a single turn. They are fully insulated from the high operating voltage of the system and are usually bolted to the current-carrying device.