Current transformers: how to specify them

byWriter •January 19, 2023

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Current transformers: how to specify them

Introduction

Current transformers: how to specify them

Electrical power management requires implementation of data processing units able to monitor networks or equipment and, as applicable, to initiate the appropriate actions. Data sent by current transformers are processed by protection, control and monitoring units that send signals to operate switchgear and/or information to a supervisory unit or to a central control room.

The task, consisting of identifying and rating the current transformers and associating them with the protection and/or metering units, has always given rise to problems, both for electrical engineers (oversizing the characteristics) and for the manufacturer (random feasibility, excessive sizing, high costs).

This document does not cover the technical demonstrations amply referred to in the literature (see Cahiers Techniques no. 164 and 170). Its purpose is to remind users of a few simple rules enabling the best possible definition of the secondary characteristics of a current transformer (CT) according to the protections and applications concerned.

Practically, it offers constructive assistance for technicians who have reached a dead end:

c either because they do not possess the necessary information,
c or because the results of their design have led to current transformers that cannot be manufactured by the potential suppliers.

Distribution Automation Handbook