What is the use of current transformers

Ct is an instrument that converts large current on the primary side into small current on the secondary side according to the principle of electromagnetic induction.The ct is composed of a closed core and winding.Its primary side winding has a small number of turns and is strung in the line of the current to be measured.

So it often have all the current flows through the line, the secondary winding circle number is more, concatenated in measuring instrument and protection circuit, current transformer at work, its secondary side circuit is always closed, so the measuring instrument and protection circuit in series with the coil impedance is very small, the working state of the current transformer is close to a short circuit.The ct is used to measure the large current on the primary side into the small current on the secondary side.

What is the use of current transformers

Current transformer is one of the main electric equipment of substation, mainly provide protection, measuring the secondary current, one side is connected to a high voltage equipment, secondary side is connected with the secondary equipment, it can not only make a measuring instrument and relay protection secondary electric equipment and high voltage electrical apparatus effective isolation, guarantee the safety of the staff, still can make measuring instrument and relay standardization and miniaturization, and small section of wire and cable used for distance measurement;The ct can also protect the current coil of the measuring instrument from the damage of large current when the high-voltage side is broken.

Application analysis of current transformer

1. Measurement of ac current

It is necessary to set up the current measuring device if the equipment is running at the rated current value.Relevant regulations above 40KW equipment, must be installed ammeter monitoring.

The measurement of ac current includes direct measurement and expanded measurement by current transformer.Direct measurement is to connect the appropriate ammeter in series to the current loop.The circuit shown in the figure below is all measured by current transformer.

(a) use a current transformer to measure the current in the three-phase balance circuit.The current transformer is based on the ammeter range, while the ammeter range is based on the actual value of the load current, which should account for 2/3 of the total ammeter range to the full value.For example, if a device is rated at 100 amperes, choose an ammeter with a full degree value of 150 amperes.At this time, the mutual inductance ratio of the current transformer shall be 150/5 ampere, and the ammeter shall be 150/5 ampere.When reading, read directly with the indicated value of the meter.

(b) two current transformers, which are connected into an incomplete star shape, are used to measure the three-phase current in the three-phase balance or unbalanced circuit.

(c) three current transformers, connected into a complete star connection, are used to measure the three-phase current in the three-phase balance or unbalanced circuit.

2. Measurement of alternating current

In order to enlarge the measuring range of ac watt-hour meter, current transformer is used to enlarge the measuring range.

(1) the first figure is the measurement method using single-phase watt-hour meter and current transformer.Can be used to measure three - phase balance or single - phase electricity measurement.

(2) the second figure is a measurement method using a two-element three-phase three-wire active watt-hour meter and two current transformers.Can be used to measure the three - phase balance or unbalanced lines in the electricity measurement.

(3) the third figure is the measurement method using three-phase four-wire active watt-hour meter and three current transformers.It can be used to measure electricity in three-phase balance or unbalanced circuit.

3. Protection in electric drag line

The overload protection in high-power motors is often unable to purchase corresponding thermal relays due to the large current. In such cases, the solution is generally to install current transformers.Its essence is to transform large current into small current with 5 ampere thermal relay enough to meet the requirements of overload protection.

(1) FIG. 1: the current transformer is used for high-power auto-coupling step-down start and overload protection in the motor.

(2) FIG. 2: current transformer is used for starting high-power frequency-sensitive rheostat and overload protection in motor.

(3) FIG. 3: the current transformer is used for high-power star-angle step-down start and overload protection in the motor.

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