Current Transformers (CTs) are used to convert high-level currents to a smaller more reasonable level for use as inputs to protection relays and metering equipment.  Within electrical systems, current transformers are essential to ensure the correct functioning and control of equipment and for providing operational data and information.

This introductory note looks at the construction of current transformers and their specification.

There are two broad categories of current transformer:

Measuring CTs – provide signals to meters and instruments.

Protection CTs – provide signals to protective relays to enable correct operation under steady state and transient conditions.

Current transformers work on a similar principal to normal voltage transformers.  Two (or more) winding are wound round a magnetic core.  Current flowing in one winding [the primary] creates a magnetic field which drives current in the other winding [the secondary].  The ratio of the primary turns to the secondary turns provides the current scaling.

Example: a 600:5 ratio CT, for every turn on the primary would have 120 turns on the secondary.  A primary current of 600 A would cause 5 A to flow in the secondary. 

The physical construction of a current transformer can be as simple as one primary winding and one secondary winding on a core.  Quite often the construction is more complex with several secondary windings providing different protection and instrumentation needs.

Specification of current transformers typically considers the following:

turns ratio – of the primary to secondary current

burden – the normal load in VA that the CT can supply

accuracy factors – the accuracy limits of (both steady state and transient) 

physical configuration – the number of primary or secondary windings, size, shape, etc.

Safety Note:If a CT secondary is not connected to any load, then it should be short circuited.  If the secondary of the CT was left open during operation, the potential of dangerous voltages would be induced at the secondary terminals. 

Current Transformer Accuracy

Accuracy of a current transformer is measured by the composite error.  This is defined as the difference between the ideal secondary RMS current and that of the actual secondary current.  It considers current errors, phase error and harmonic errors.

Current transformer intended for protection applications need to cover a wide range of current.  Then current value up to which they will maintain accuracy is the ‘accuracy limit current’.   The ratio of the accuracy limit current to the rated current is the ‘accuracy limit factor’.

Generally, current transformers are used to measure high currents; higher than 5A on the primary. The most important parameter in defining a current transformer is the Ratio between the primary and secondary.

There are two mayor groups of Current Transformers:

Protection current transformers.

Measurement current transformers.

They both have a primary current, a secondary current and a ratio and from these 3 parameters you can define important property’s related to accuracy for each class.

But the components of the two classes are different. For example, in a measuring class current transformer, the core material must have a high permeability so that the magnetizing current is low.

Measuring CT Accuracy Class

Measurement CT’s are often being used for billing of electrical power consumption and their accuracy is important because money is involved.

The accuracy of a measurement CT is given by it’s accuracy class that corresponds to the error% at rated current and at 1.2 times rated current.. The standard accuracy classes according IEC are class 0.2, 0.5, 1, 3 and 5. For classes 3 and 5, no angle error is specified. The classes 0.2S and 0.5S have their accuracy shifted toward the lower currents. This means that they have 5 measuring points instead of 4 (or 2 for class 3 & 5).

Protection CT Accuracy Class

 Protection Current Transformers are designed to measure the actual currents in power systems and to produce proportional currents in their secondary windings which are isolated from the main power circuit. These replica currents are used as inputs to protective relays which will automatically isolate part of a circuit in the event of an abnormal or fault condition therein, yet permit other parts of the powersystem to continue to operate.

Satisfactory operation of protective relays can depend on accurate representation of currents ranging from small leakage currents to very high overcurrent’s, requiring the protective current transformer to be linear, and therefore below magnetic saturation at values up to perhaps 30 times full load current .

This wide operating range means that protective current transformers require to be constructed with larger cross-sections resulting in heavier cores than equivalent current transformers used for measuring duties.

This is a very basic discussion of the operation, types, and construction of current transformers. This series on current transformers will continue with more in-depth discussions on the accuracy and standards assigned to current transformer.