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Tips to obtain better knowledge in current transformer

A current transformer (CT), simply stated, is a device for measuring current flowing through a power system and inputting the measured current to a protective relay system. Electrical power distribution systems require a variety of circuit condition monitoring devices to facilitate the detection and location of system malfunctions. Current transformers 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.

As you are aware, the key to all of the AC power transducers is the measurement of current. This is accomplished using a current transformer (CT), a device through which is threaded the wire whose current is to be measured. A current transformer is designed to produce either an alternating current or alternating voltage proportional to the current being measured.

Current transformers are meant to perform circuit control, measure current for power measurement and control, and perform roles for safety protection and current limiting. They can also cause circuit events to occur when the monitored current reaches a specified level.

Current transformers have many laid-down performance specifications such as primary current, secondary current, insulation voltage, accuracy, and burden. Primary current is the load of the current transformer. Secondary current is the range of current outputs. Insulation voltage represents the maximum insulation that current transformers provide when connected to a power source. Accuracy is the degree of certainty with which the measured current agrees with the ideal value. Burden is the maximum load that devices can support while operating within their accuracy ratings.

There are two basic types of current transformers: wound and toroidal. Wound current transformers consist of an integral primary winding that is inserted in series with the conductor that carries the measured current. Toroidal or donut-shaped current transformers do not contain a primary winding. Instead, the wire that carries the current is threaded through a window in the toroidal transformer.

Current transformers are commonly used in metering and protective relaying where they facilitate the safe measurement of large currents, often in the presence of high voltages. The current transformer safely isolates measurement and control circuitry from the high voltages available on the circuit being measured.

The proper application of current transformers involves the consideration of many factors such as: mechanical construction, type of insulation (dry or liquid), ratio in terms of primary and secondary currents or voltages, continuous thermal rating, short-time thermal and mechanical ratings, insulation class, impulse level, service conditions, accuracy, and connections.

There are several applications for current transformers. Some CTs are used to measure current in electronics equipment or motors. Others are used in street lighting. Current transformers with small footprints mount on printed circuit boards (PCBs) and are used to sense current overloads, detect ground faults, and isolate current feedback signals. Larger devices are used in many three-phase systems to measure current or voltage.

Many voltage transformers have adjusted ratios that produce the desired output voltage and compensate for losses. The turns-ratios or actual turns need not be the primary concern of any end-user. Only the voltage output and possibly regulation and other loss parameters may matter to him. With current transformers, the user must know the current ratio to use the transformer. The knowledge of amps in per amps out is the basis for use of the current transformer. Quite often, the end users provide the primary with a wire through the center of the transformer. They must know what secondary turns are to determine what their output current will be.