Current Transformer (CT) – Construction and Working Principle

Types of Current Transformer
On the basis of their applications in the field, current transformers can be broadly classified into two types,

1. 
   Indoor current transformers
   
2. 
   Outdoor current transformers
   

Indoor Current Transformers
Current transformers designed for mounting inside metal cubicles are known as Indoor Current Transformers.
Depending upon the method of insulation, these can further be classified as:

• 
  Tape insulated
  
• 
  Cast resin (epoxy, polyurethane or polycrete)
  

In terms of constructional aspects, Indoor Current Transformers can be further classified into the following types:

1. 
   Bar Type CT: The CTs having a bar of suitable size and material used as primary winding are known as bar-type CT s’. The bar may be of rectangular or circular cross-section.
   
2. 
   Slot/ Window/ Ring Type CT: CTs having an opening in the center to accommodate a primary conductor through it is known as ‘ring-type’ (or ’slot/ window type’) CT.
   
3. 
   Wound Type CT: A CT having a primary winding of more than one full turn wound on the core is known as wound type CT. The connecting primary terminals may be similar to those of a bar type CT or rectangular pads can be provided for this purpose.
   

Outdoor Current Transformer
These current transformers are designed for outdoor application. They use transformer oil or any other suitable liquid for insulation and cooling. A liquid-immersed CT which is sealed and does not communicate with the atmosphere is known as a hermetically sealed CT.
Outdoor oil-filled CTs are further classified as

1. 
   live tank type CT
   
2. 
   dead tank type CT
   

Most of the outdoor current transformers are high voltage current transformers. Based on the application they are further classified into :

1. 
   Measurement Current Transformer
   
2. 
   Protection Current Transformer