There are several types of current transformers, including protective, measurement, and metering current transformers. Protective CTs are used in power system protection schemes to detect and trip circuits during fault conditions. Measurement CTs are used in power monitoring applications, while metering CTs are used for billing and revenue metering.
A. According to construction:
Instrument transformer in which the secondary current, in normal conditions of use, is substantially proportional to the primary current and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections.
Current Transformer (Wound Type):
A current transformer having a primary winding of more than one full turn wound on the core (s).
Current Transformer (Bar Type):
A current transformer in which the primary winding consists of a bar of suitable size and material forming an integral part of the current transformer.
Current Transformer (Dry Type):
A current transformer which does not require the use of any liquid or semi-liquid material.
Current Transformer (Liquid Immersed):
A current transformer which requires the use of oil or other suitable liquid of suitable characteristics as insulating and/or cooling medium.
Hermetically Sealed Current Transformer:
A liquid immersed current transformer which is sealed and does not communicate with atmospheric air.
Current Transformer (Ring Type):
A current transformer which has an opening in the center to accommodate a primary conductor through it.
Multi-Core Current Transformers:
A current transformer having more than one secondary core and winding with a common primary winding.
Multi-Ratio Current Transformer:
A current transformer in which more than one ratio is obtainable by reconnection, or tappings, in primary or secondary windings.
B. According to application:
Metering CT's: verify that they are measuring the ratio error &phase displacement for metering CT's having accuracy class 0.2, 0.5&1.0.
> If it is 0.2 or 0.1 class then we have to take the values at 120%, 100%,
20%&5% of rated current with 100%, 25%of the rated burden.
> If it is 0.5, 1.0 class then we has to take the values at 120%, 20% of rated
current with 100%, 25%of the rated burden.
Protective CT's (class "p"):
verify that they are measuring the ratio error &phase displacement for protective CT's having accuracy class '5p' or '10p' at rated current with rated burden connected.
Protective CT's class 'ps':
turns ratio error of the special protection core can be measured directly by turns ratio meter. check that error is within limits or not.
Accuracy class:
It is the class assigned to the current transformer with the specified limits of ratio error and phase angle error.