Selection of current transformers and wire sizing in substations

While retrofitting protections and meters in substations, the older equipment get replaced with newer ones with the rest of the installation such as Current Transformer (CT) and lead remaining undisturbed. Utilities have standardized on their older current transformers and want to maintain the same current transformer specifications throughout their system even in their newer installation. An extension of the above approach is that even the cable leads and panel wiring sizes are over-specified resulting in totally unjustified cost of installations as well as difficulty in terminating such wires in the modern panels and equipment. On the other end of the spectrum, in rare cases, the existing CT sizes and lead sizes of electromechanical relays may not be adequate for the latest protective relays which might demand a different current transformer performance. This paper systematically lists and analyses such requirements for various modern protective relays.