Practical Considerations of CT Performance

Current transformer (CT) has a critical role in protective and measurement devices used in both the industrial and the utility worlds. Selection of a CT type, class, and ratio for a utility distribution feeder is a function of several variables. Type of feeder, available fault duty, CT types, and values of connected burden can significantly influence the selection parameters. An optimal procedure for selection of the correct type and ratio can be challenging depending on the application. These challenges usually stem from a variety of reasons. Differences in the available standards, unfamiliarity of the user with some of the basic concepts, and complexities of the analyses for transient responses are among some of the major difficulties in need of further studies and clarification. In this paper, a brief discussion about the theory and the governing fundamentals of a CT is presented. A classical modeling of the CT identifying the essential parameters is introduced and the basic formulas relating the CT voltage, current, burden, and their relationships have been discussed. Practical procedures for measuring of the phase to phase and phase to ground burdens of the CTs connected to a typical distribution circuits have been shown and analyzed. ANSI and IEC standards and definitions regarding CT types, accuracy class, CT characteristics, and CT burdens have been introduced and compared. Concerns of CT saturation have been addressed and examples showing step by step calculations regarding simple analyses have been provided. Finally, dynamic characteristics of a CT for generalized saturation analyses during transient conditions have been introduced. Concluding remarks regarding practical approximations and guiding principles have been presented