ABSTRACT
The selection of instrument transformers, current and voltage, must be carried out with care if the protection, instrumentation, and measuring systems are to function correctly. Most modern digital relays have been made more able to function correctly in the presence of current transformers (CT) saturation by using adaptive filtering to circumvent CT saturation. The compact design of modern metal-clad switchgear has limited space for the number and size of current transformers that are required to perform satisfactorily in a high-fault-current environment. A bushing CT consists of secondary windings on an annular magnetic core. A bar-type CT is of similar construction as a bushing type. These current transformers have a single concentrically placed primary conductor, sometimes permanently built into the CT and provided with the necessary primary insulation. High-voltage CTs for external use need physical height to provide the required phase-to-ground insulation while providing the same insulation level between the primary and secondary windings.
