Current transformers (CTs) are a primary function of electrical installations in many industries. A current transformer is a transformer used to measure alternating current or AC by scaling it down to a level suitable for metering and/or protective relays. The use of current transformers is ideal when the insulation of a metering or protection instrument cannot withstand line current without breakdown.
Current transformers and potential transformers (PTs) are collectively referred to as "instrument transformers". This is because they are designed to convert high currents/voltages to levels suitable for measuring and protecting equipment. However, there are some differences between these two transformer types.
CT is used to convert high current to lower current while PT converts high voltage to low voltage. In a CT, the primary winding has much fewer turns relative to the secondary winding, whereas in a voltage transformer the opposite is true.
Functionally, this means that the secondary current is proportional to the primary current in the CT and vice versa in the PT. CTs are standard rated at 1A or 5A, while PTs are standard rated at 120V secondary for up to 24kV primary and 115V for secondary over 24kV.
CTs are also connected in series with the equipment, while PTs are connected in parallel. These arrangements allow current to pass through the windings in the CT, while voltage appears on the windings of the PT.
Current transformers are essential for isolating electrical equipment from the more heavily used currents in AC transmission lines. They work by reducing supply current to levels that are safe for metering and protection equipment.
Primary applications for CTs include substations, commercial facilities, and industrial power distribution equipment. CTs are similar to potential (voltage) transformers but differ in construction and operation.
Choosing the right current transformer ensures its measurement accuracy. This requires ensuring that the CTs are rated to match the intended operation - to minimize unnecessary costs, energy losses, and equipment failures.
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