Overcurrent Performance of Bushing-Type Current Transformers

Conventional methods used for defining and comparing current-transformer performances are commonly based on their ratio and phase-angle characteristics. For years, these characteristics have been continuously improved, particularly with respect to the requirements for metering service. However, from the standpoint of circuit protection, current-transformer application becomes more involved and as a result, increased attention has been given it. A better knowledge of transformer performance is necessitated by the requirements of new relays and protective schemes. Attention to the influence of the d-c component in short-circuit transients is especially necessary when using high-speed relays. Also, in differential connections, the necessity for equivalent matching of saturation characteristics is essential. These are illustrations of the types of problems which ratio and phase- angle characteristics alone do not solve. The analysis of open-circuit saturation curves has proved of immense value in the solution of these problems. This paper gives results of investigations on bushing-type transformers that indicate the proper choice of analysis procedure and the limitations of the various methods for determining performance under overcurrent operating conditions.