Current Transformers perform two critical roles in the power supply ecosystem. They are used as current measurement devices and as protection relays. However, there are different types of Current Transformers and several factors govern the appropriate choice of the Current Transformer.
The objective of this study is to understand the key considerations before choosing the right Current Transformer for a particular application.
If you are looking to read about the basic function of Current Transformers, compliance standards and applications, read our earlier blog here.
Current transformers are employed to measure power circuit currents using measuring instruments such as Ammeters, Kilowatt-hour meters and power factor meters. They work with a high degree of accuracy but within the rated current range. They must have a specified accuracy class as in IEC 60044-1. The secondary current is substantially proportional to the primary within a working range of about 5–120% of its primary rated current. The accuracy class required for various metering applications is listed below:
Two new accuracy classes have been developed by IEEE C57.13.6 to accommodate the shift towards electronic relays and meters from the traditional induction devices. Nowadays, manufacturers have been improving accuracy of instrument transformers to take advantage of the lower impedance of the devices.
If input current exceeds the rating, the metering CT will saturate thereby limiting the current level within the measuring instrument. Core materials for this type of CT typically have low saturation levels, such as nano-crystalline. They are generally available in wound or ring type.
KWM Series are a range of Measuring Current Transformers. These CTs measure current flowing through the primary conductor by converting them to measurable value.
KRM Series are a range of Measuring Current Transformers. This series of Ring type, also called window type, allows bus bars or cables to be passed through the CT and act as a Primary for the CT.
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