Components of Protective Schemes
Current Transformers (CTs)
In order to obtain currents which can be used in control circuits and that are proportional to the system (primary) currents, current transformers are used. Often the primary conductor itself, for example a busbar, forms a single primary turn (bar primary). Whereas instrument current transformers have to remain accurate only up to slight over-currents, protection current transformers must retain proportionality up to at least 20 times normal full load. The nominal secondary current rating of current transformers is now usually 1 A, but 5 A has been used in the past.
A major problem can exist when two current transformers are used which should retain identical characteristics up to the highest fault current, for example in pilot wire schemes. Because of saturation in the silicon steel used and the possible existence of a direct component in the fault current, the exact matching of such current transformers is difficult.
The problems associated with current transformers have resulted in the development of devices called linear couplers, which serve the same purpose but, having air cores, remain linear at the highest currents. These are also known as Rogowski coils and are particularly suited to digital protection schemes.