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What is a current Transformer?
Current transformers are largely used in areas in which it is not possible to measure current directly. They are a special type of transformer with a defined grade of accuracy, which translates the primary current into a (typically) reduced, standardised secondary current, as well as galvanically separating primary and secondary circuits from each other. The physical saturation of the core material additionally guarantees protection of the secondary circuit from higher currents.
It is essentially possible to distinguish among single-phase current transformers and wound primary current transformers. The most common form of single-phase current transformer is the moulded case feed-through CT, which is mounted onto the current-carrying phase and consequently forms a transformer with primary winding (and secondary wingdings in unity with the transformation ratio).
Current Transformer Ratios - Primary / Secondary
The transformation ratio is the association between the primary rated current and the secondary rated current, and is calculated and labelled as an unsimplified fraction.
Most often, 5A secondary output current transformers are installed. For technical and moreover economic reasons, 1 A output current transformers are recommended with long measuring cable lengths (burden/losses with 1-A transformers is only 4 % in contrast to 5-A transformers)
VA Rating / Power Rating
Rated power defines the capacity of a CT to "drive" the secondary current within the error limitations through a load.
When selecting the CT power rating, it is essential to take into consideration the following parameters: Meter power consumption (in-series), cable length, wire cross-sectional area. The larger the cable size and smaller the cross-sectional area, the greater the losses through the supply, i.e. nominal power of the CT must be adequate for the installation
Types of Current Transformers - Split-Core or Solid-Core
Moulded Case: Phases are fed through the CT aperture (cable or Bus-bar) which forms the current transformer. Moulded-case CT's are the most cost-effective option and are primarily used in the new system installations
Split-Core Current Transformers: Split CT's are frequently used in retro-fit applications because of their ability to 'open', which allows for easy installations without interpreting the primary conductor.