Current Transformers: An Overview

CTs are used to reduce high-level currents to a smaller, more manageable level for use as inputs to protection relays and metering equipment. Current transformers are required in electrical systems to ensure the proper operation and control of equipment as well as to provide operational data and information.

This introductory note examines the construction and specification of current transformers.

Current transformers are classified into two types:

Current transformers operate on the same principles as voltage transformers. A magnetic core is wrapped in two (or more) windings. Current flowing through one winding [the primary] generates a magnetic field that drives current through the other winding [the secondary]. The current scaling is determined by the ratio of primary to secondary turns.

A current transformer’s physical construction can be as simple as one primary winding and one secondary winding on a core. Quite often, the construction is more complex, with multiple secondary windings providing varying levels of protection and instrumentation.

Current transformer specifications typically take the following factors into account:

burden – the normal load in VA that the CT can supply accuracy factors turns ratio – of the primary to secondary current (i.e. 1200/1) – the accuracy limits of physical configuration (both steady state and transient) – the number of primary or secondary windings, size, shape, and so on.

Current Transformer Specification

Accuracy of Current Transformers

The composite error is used to assess the accuracy of a current transformer. The difference between the ideal secondary RMS current and the actual secondary current is defined as this. It accounts for current errors, phase errors, and harmonic errors.

A current transformer designed for protection applications must be capable of handling a wide range of current. The ‘accuracy limit current’ is the current value up to which they will maintain accuracy. The ‘accuracy limit factor’ is the ratio of the accuracy limit current to the rated current.

CT Accuracy Class Measuring

Accuracy for measuring current transformers is achieved by assigning the CT an accuracy class. The standards define a maximum allowable current and phase displacement error for each class under various load conditions.

CT Accuracy Class Protection

Protection current transformers are classified as 5P or 10P. The current error, phase displacement error, and accuracy limit factor are defined for each of these.

Class ‘P’ current transformers are commonly used for overcurrent protection. An additional specification is required for more demanding applications. In this case, the maximum useful emf – defined as the excitation curve’s ‘knee-point’ – is frequently used ( the point at which a further 10 percent rise in emf, requires a 50 percent increase in excitation current).

In addition to the above, the following current transformer specifications are widely used:

P stands for general purpose, with accuracy defined by composite error and steady-state primary current.

TPS – low leakage, defined by secondary excitation and turns ratio error

Peak instantaneous error during specified transient duty is defined as TPX.

TPY – same as TPX, but remanent flux is limited to 10{8e0181f9b4047a7790fd20bbf2d43faff96926569f2820f3da0bb199a786bafe}

CT with large air gap TPZ breaker failure application

Ratings for Name Plates

A nameplate should be attached to all current transformers. An example of a typical nameplate for a current transformer with one primary and two secondary windings is shown in the image (click for a larger version of the image).

Sizing of Current Transformers

Transformer sizing and specification are critical for trouble-free operation of protection and instrumentation systems. A complete electrical note is available at:

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