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Current transformer

A current transformer is a device for measuring a current flowing through a power system and inputting the measured current to a protective relay system. Electrical power distribution systems may require the use of a variety of circuit condition monitoring devices to facilitate the detection and location of system malfunctions. Current transformers and current sensors are well known in the field of electronic circuit breakers, providing the general function of powering the electronics within the circuit breaker trip unit and sensing the circuit current within the protected circuit. Ground fault circuit breakers for alternating current distribution circuits are commonly used to protect people against dangerous shocks due to line-to-ground current flow through someone's body. Ground fault circuit breakers must be able to detect current flow between line conductors and ground at current levels. Upon detection of such a ground fault current, the contacts of the circuit breaker are opened to deenergize the circuit. Current transformers are an integral part of ground fault circuit breakers. Current transformer assemblies are often positioned between the line side of a trip unit of a circuit breaker and the load side in order to monitor the current therebetween. Current transformers in electrical substations measure the system currents at predetermined measuring points of the switchgear with a certain measurement inaccuracy. The measuring points are typically located at all incoming and outgoing lines and possibly also within the system, e.g. for the busbar protection. The current measurement signals are used for protective functions, for monitoring the substation, for calculating performance data for operating purposes or for consumption billing and for the representation on a display. The output of the current transformer provides a representation of the current flowing through the assembly that is being monitored. Associated monitoring and control instrumentation in combination with the current transformer may provide critical system functions such as overload protection and power usage monitoring.

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