Current Transformer
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 winding's providing different protection and instrumentation needs.
Specification of current transformers typically considers the following:
turns ratio - of the primary to secondary current (i.e. 1200/1)
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: 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'.
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 winding's (click for a larger version of the image)
