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Understanding Current Transformers – Part I Introduction

Current transformers are a commonly used device found in metering and protective relay applications for the electrical power industry.  The purpose of these devices is to scale large primary currents to a smaller, easy to measure, secondary current.  The device also isolates the measuring and recording instruments.  Current transformers (CT) together with voltage transformers (VT) or potential transformers (PT) are known as instrument transformers.

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Like any other transformer, a CT has a primary winding, a magnetic core, and a secondary winding.  Alternating current (AC) flowing in the primary winding produces a magnetic field in the core, which in turn induces a current in the secondary winding circuit. 

Figure 1: Basic CT design
N = Number of turns
I = Primary current

The objective of the CT design is to ensure that the primary and secondary circuits are efficiently coupled, so that the secondary current reflects an accurate relationship to that of the primary current.  The most basic design of a CT consists of a length of wire wrapped many times around a silicon steel ring passed over the circuit being measured.  The CT’s primary circuit consists of a “single turn” of conductor, with a secondary consisting of many hundreds of turns, Figure 1.  The primary winding may be a permanent part of the CT consisting of a heavy copper bar to carry current through the magnetic core, Figure 2.

Figure 2: CT with permanent primary

Window-type CTs are also very common. These CTs will have circuit cables run through the middle of an opening in the core to provide the single-turn primary winding, Figure 3.






Figure 3: Window type CT

The primary circuit is largely unaffected by the insertion of the CT.  The secondary current rating is typically standardized at either 1 or 5 amperes.  Therefore, if a CT is rated at 4000:5; the CT would provide an output current of 5 amperes when the primary is passing 4000 amperes. 

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Editor’s Note: This post was originally published October, 2011 and has been updated freshness, accuracy and comprehensiveness.