Introduction to Current Transformers 

 

High Voltage SF6 Current Transformers
Image Source: Courtesy Siemens 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:

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

Safety: 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, then you would effectively have a transformer with one turn on the primary and many turns on the secondary.  Large and potentially dangerous voltages would be induced at the secondary terminals. 

Specification of Current Transformers

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 takes into account 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'.

Measuring CT Accuracy Class

Accuracy for measuring current transformers is achieved by allocating an accuracy class to the CT.  For each class, the standards define a maximum allowable current and phase displacement error for different load conditions.

Protection CT Accuracy Class

Protection current transformers are defined as either 5P or 10P.  For each of these the current error, phase displacement error and accuracy limit factor are defined

Class 'P' current transformers are generally used for overcurrent protection applications.  For more demanding applications, an additional specification is required.  In this instance, the maximum useful emf is often used - specified as the 'knee-point' of the excitation curve ( the point at which a further 10% rise in emf, requires a 50% increase in excitation current).

In addition to the above, other current transformer specifications are also in widespread use:

• P - general purpose with accuracy defined by composite error and steady-state primary current
• TPS - low leakage with performance defined by secondary excitation and turns ratio error
• TPX - defined by peak instantaneous error during specified transient duty
• TPY - as per TPX, but remanent flux limited to 10%
• TPZ - breaker failure application CT with large air gap

Name Plate Ratings

Typical Current
Transformer Nameplate
Image: Courtesy Schneider Electric All current transformers should have a nameplate attached.  The image shows an example of a typical nameplate for a current transformer with one primary and two secondary windings (click for a larger version of the image).

Sizing of Current Transformers

The correct sizing and specification of transformers are essential to ensure trouble-free operation of protection and instrumentation systems.  There is a full electrical note dedicated to this at:

How to Size Current Transformers