Current transformers sample the line current and convert it into safety and measurable values for the normalized standards of instruments, metering devices and other metering and control devices.
Nominal values of the current transformers are defined as the ratio between the primary and the secondary current.
They are classified according to the main isolation used: Wound Primary type, Bar Primary type, Toroidal type and for Terminal blocks type.
These current transformers can be used for two different purposes: Measurement or Protection. The correct choice of the CT is essential in order to avoid faults and degradation that would lead into economic losses or even into dangerous situations.
Both Measurement and Protection current transformers have to provide a secondary current that is proportional to the one of the primary.
THE MAIN PURPOSE OF MEASUREMENT TRANSFORMERS is to measure currents without the activation of corrective responses when abnormal values arise. Within the range of the nominal current good accuracy is needed while out of the threshold of the nominal range that accuracy is unnecessary. For that reason, the measurement transformers have a very low saturation factor and a high security factor to avoid overloads in the metering devices.
PROTECTION TRANSFORMERS are designed to give a warning or correction advice when abnormal values are measured. With high values of fault current, a high saturation factor is required in order to maintain a good accuracy. The secondary current, proportional to the primary, could reach very high values and should always be supported by protection devices.
The Power Transformers have a special winding which allows them to have a high voltage primary and a low voltage secondary. They have a very low nominal power and their unique purpose is to give a voltage sample of the power system to measure it with the incorporated instruments. Being their principal objective the voltage sampling, they must be specially accurate so that they do not to distort the real values. The selection of the transformer will be conditioned by the accuracy required in its readings.