Current Transformer
A current transformer (CT) is a type of transformer that is used to reduce or multiply an alternating current (AC). It produces a current in its secondary which is proportional to the current in its primary. Current transformers, along with voltage or potential transformers, are instrument transformers. Instrument transformers scale the large values of voltage or current to small, standardized values that are easy to handle for measuring instruments and protective relays. The instrument transformers isolate measurement or protection circuits from the high voltage of the primary system. A current transformer provides a secondary current that is accurately proportional to the current flowing in its primary. The current transformer presents a negligible load to the primary circuit.
Construction
In CT manufacturing, the selection and manufacturing of the core will almost determine the final performance of the CT to be manufactured. Inter Win CT uses high grade grain oriented silicon steel. It is rolled by precision coil winder. The cores are annealed in vacuum furnace and varnished. These cores are graded for its induction properties for various applications. The core is insulated and secondary coil is wound by precision toroid winding machine. Multilayers of non-flammable PVC tape are then applied to provide a tough moisture resistance coating.
Specification
* Thermal current (Ith) & dynamic current (ldyn)
**lth is the highest primary current (effective value), the ldyn is the highest primary current (Peak Value) that the CT can support for 1 second without damage, owing to excessive overloads with secondary short circuits.
*** 1A secondary CT will usually have smaller size and weight, with lower cost. Higher accuracy and burden.
**lth is the highest primary current (effective value), the ldyn is the highest primary current (Peak Value) that the CT can support for 1 second without damage, owing to excessive overloads with secondary short circuits.
*** 1A secondary CT will usually have smaller size and weight, with lower cost. Higher accuracy and burden.
Selection of Current Transformer
CT is used to accurately measure high alternating primary current. This is done by converting the primary current into a proportional secondary current, usually 5A or 1A, as required by measurement instrument. Inter Win CT is manufactured to exact specification and individually calibrated.
When ordering the below information are needed:
1. Ratio – Rated Primary and Secondary current. e.g. 400/5A or 400/1A. We manufacture CT from 30A – 6,000A.
*Small ratio CT: As CT works on the principle of mutual inductance, CT such as 60/5A or less will be extremely difficult to manufacture to high degree of accuracy.
The use of high cost, high permeability steel for this application is unavoidable.
It is therefore recommended to use multiple turns for primary to reduce size, weight and cost. Its accuracy and rated burden can be greatly increased with multiple primary turns.
2. CT Burden – Usually measured as VA or Ohm. It is the external load (burden) imposed on the CT secondary output terminals. It is the sum of the resistance of the connection pilot cables and the meter load. The common burdens are 1.5VA, 2.5VA, 3.75VA, 5VA, 7.5VA …... etc.
Solid state electronics meter burden - usually about 1.0VA or less.
Moving magnet analog meter burden – depend on size and design, usually 0.75 – 2.5VA
Connection terminal lead cables burden – it depend on sectional area and the total length from CT to metering instrument. The chart below is for your easy quick reference.
When ordering the below information are needed:
1. Ratio – Rated Primary and Secondary current. e.g. 400/5A or 400/1A. We manufacture CT from 30A – 6,000A.
*Small ratio CT: As CT works on the principle of mutual inductance, CT such as 60/5A or less will be extremely difficult to manufacture to high degree of accuracy.
The use of high cost, high permeability steel for this application is unavoidable.
It is therefore recommended to use multiple turns for primary to reduce size, weight and cost. Its accuracy and rated burden can be greatly increased with multiple primary turns.
2. CT Burden – Usually measured as VA or Ohm. It is the external load (burden) imposed on the CT secondary output terminals. It is the sum of the resistance of the connection pilot cables and the meter load. The common burdens are 1.5VA, 2.5VA, 3.75VA, 5VA, 7.5VA …... etc.
Solid state electronics meter burden - usually about 1.0VA or less.
Moving magnet analog meter burden – depend on size and design, usually 0.75 – 2.5VA
Connection terminal lead cables burden – it depend on sectional area and the total length from CT to metering instrument. The chart below is for your easy quick reference.
