A current transformer is an instrument that converts a large current on the primary side into a small current on the secondary side based on the principle of electromagnetic induction. The current transformer is composed of a closed core and windings. Its primary winding has a few turns and is stringed in the line of the current that needs to be measured.
Therefore, it often has all the current of the line flowing, and the number of turns of the secondary winding is relatively large. It is connected in series with the measuring instrument and the protection circuit. When the current transformer is working, its secondary circuit is always closed, so the measurement The impedance of the series coil of the instrument and the protection circuit is very small, and the working state of the current transformer is close to a short circuit. The current transformer converts the large current on the primary side into a small current on the secondary side for measurement, and the secondary side cannot be opened.
Current transformer is one of the main electrical equipment in substations. It mainly provides secondary current for protection and measurement. Its primary side is connected to primary high voltage equipment, and the secondary side is connected to secondary equipment. It can not only protect measuring instruments and relays The secondary electrical equipment and high-voltage electrical equipment are effectively isolated to ensure the safety of workers. It can also standardize and miniaturize measuring instruments and relays. Small cross-section wires and cables can be used for long-distance measurement; when the high-voltage side occurs When the circuit is broken, the current transformer can also protect the current coil of the measuring instrument from the damage of the large current.
Current Transformer Application Analysis1. Measurement of AC current
Whether the equipment is running at the rated current value, setting a current measuring device is a necessary technical measure. According to relevant regulations, equipment above 40KW must be installed with an ammeter for monitoring.
The measurement of alternating current has direct measurement and expanded measurement through current transformer. Direct measurement is to connect an appropriate ammeter in series to the current loop. The circuits shown in the figure below are all connected through current transformers for measurement.
(A) Use a current transformer to measure the current in the three-phase balanced line. The current transformer is selected based on the range of the ammeter, and the range of the ammeter is based on the actual value of the load current, which should account for 2/3 of the total range of the ammeter to the full scale value. For example, if a device has a rated current of 100 amperes, an ammeter with a full scale value of 150 amperes is selected. At this time, the mutual inductance ratio of the current transformer should be 150/5A, and the ammeter with a ratio of 150/5A should be selected accordingly. When reading, read directly with the indicated value of the meter.
(B) Use two current transformers connected in an incomplete star shape to measure the three-phase current in a three-phase balanced or unbalanced line.
(C) Three current transformers are used to connect into a complete star connection to measure the three-phase current in a three-phase balanced or unbalanced line.
2. Measurement of AC power
In order to expand the range of AC watt-hour meters, the most commonly used method in factories is to expand the range by using current transformers.
(1) The first figure is a measurement method using a single-phase watt-hour meter and a current transformer. It can be used to measure three-phase balance or single-phase electricity metering.
(2) The second figure is a measurement method using a two-element three-phase three-wire active watt-hour meter and two current transformers. It can be used to measure electricity metering in three-phase balanced or unbalanced lines.
(3) The third figure is a measurement method using a three-phase four-wire active watt-hour meter and three current transformers. It can measure the electricity meter in three-phase balanced or unbalanced lines.
3. Protection in the electric drive line
The overload protection in high-power motors is often due to the large current, and the corresponding thermal relay cannot be purchased. In this case, the method of installing a current transformer is generally used to solve it. The essence is to convert a large current into a small current and use a thermal relay within 5 A to meet the requirements of overload protection.
(1) Picture 1: The current transformer is used for high-power auto-coupling step-down start and overload protection in the motor.
(2) Picture 2: The current transformer is used for the start of high-power frequency-sensitive rheostat and the overload protection in the motor.
(3) Figure 3: Current transformer is used for high-power star-angle step-down starting, overload protection in the motor.
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