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# Explain the principle of current transformer

by：Transmart     2022-05-05

1. Overview of voltage and current transformers

A typical transformer uses the principle of electromagnetic induction to convert high voltage into low voltage, or convert large current into small current, to provide suitable voltage or current signals for measuring devices, protection devices, and control devices. Voltage transformers commonly used in power systems, the primary side voltage is related to the system voltage, usually hundreds of volts to hundreds of kilovolts, and the standard secondary voltage is usually 100V and 100V/ two; while the current transformers commonly used in power systems, The primary side current is usually several amperes to tens of thousands of amperes, and the standard secondary current is usually 5A, 1A, 0.5A, etc.u200bu200b

1. Principle of voltage transformer The principle of voltage transformer is similar to that of transformer, as shown in Figure 1.1. The primary winding (high voltage winding) and the secondary winding (low voltage winding) are wound on the same iron core, and the magnetic flux in the iron core is Ф. According to the law of electromagnetic induction, the relationship between the voltage U of the winding and the voltage frequency f, the number of turns of the winding W, and the magnetic flux Ф is:

2. The principle of current transformer It is also similar to the transformer in principle, as shown in Figure 1.2. The main difference with the voltage transformer is: under normal working conditions, the voltage drop on the primary and secondary windings is very small (note that it does not refer to the voltage to ground), which is equivalent to a transformer in a short-circuit state, so the magnetic flux in the iron core Ф It is also very small. At this time, the magnetic potential F (Fu003dIW) of the primary and secondary windings is equal in size and opposite in direction. That is, the current ratio between the primary and secondary current transformers is inversely proportional to the number of turns of the primary and secondary windings.u200bu200b

3. Terminals and polarity of transformer windings
The voltage transformer winding is divided into the head end and the tail end. For the fully insulated voltage transformer, the head end and the tail end of the primary winding can withstand the same ground voltage, while for the semi-insulated voltage transformer, the tail end can withstand the same ground voltage. The voltage that the terminal can withstand is generally only about a few kV. Commonly use A and X to represent the head and tail of the primary winding of the voltage transformer, respectively, use a, x or P1, P2 to represent the head or tail of the secondary winding of the voltage transformer; current transformers are commonly used L1, L2 represents the head end and tail end of the primary winding respectively, and the secondary winding uses K1, K2 or S1, S2 to represent the head end or the tail end. Mark 2 indicates the tail end.u200bu200b

When the induced potential direction of the terminals is the same, it is called the same-named end; on the other hand, if a DC current in the same direction is passed through the same-named end, the magnetic fluxes they generate in the iron core are also in the same direction. The terminal with the same label is the head end or the same end end and the induced potential direction is the same. The winding of this label is called minus polarity, and the voltage of the terminal is the result of the subtraction of the induced potential of the two windings. The correct designation in the transformer is for minus polarity.u200bu200b

4. The main difference between voltage transformer and current transformer in structure
(1) Both the voltage transformer and the current transformer can have multiple secondary windings, but the voltage transformer can share one iron core with multiple secondary windings, and the current transformer must be required for each secondary winding There are independent iron cores, and there are as many iron cores as there are secondary windings.u200bu200b
(2) The primary winding of the voltage transformer has many turns, the wire is very thin, the secondary winding has fewer turns, and the wire is slightly thicker; while the high-voltage current transformer used in the substation has only 1 to 2 turns of the primary winding, and the wire is very thick , the number of turns of the secondary winding is more, and the thickness of the wire is related to the rated value of the secondary current.u200bu200b
(3) When the voltage transformer is in normal operation, it is strictly forbidden to open the low-voltage terminal of the primary winding, and it is strictly forbidden to short-circuit the secondary winding; when the current transformer is in normal operation, it is strictly forbidden to open the secondary winding.u200bu200b
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