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# Explain the principle of current transformer

by：Transmart     2022-08-04

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 for measuring devices, protection devices and control devices Signal. 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/
There are two kinds; and 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.​​

1. The principle of voltage transformer

The principle of a voltage transformer is similar to that of a 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 in principle to a transformer, 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 (F=IW) 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.

3. Terminals and polarity of transformer windings

The voltage transformer winding is divided into a head end and a tail end. For a fully insulated voltage transformer, the head and tail ends of the primary winding can withstand the same ground voltage, while for a semi-insulated voltage transformer, the tail end can withstand The voltage is generally only about a few kV. Commonly used A and X are used to represent the head and tail of the primary winding of the voltage transformer, respectively, and a, x or P1,
P2 represents the head or tail of the secondary winding of the voltage transformer; L1 is commonly used for current transformers
, L2 respectively represent the head and tail of the primary winding, and K1, K2 or S1, S2 represent the head or tail of the secondary winding. Different manufacturers may have different labels. Usually, the subscript 1 is used to indicate the head. The subscript 2 indicates the tail. 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.​​

4. The main difference between the voltage transformer and the 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 have an independent secondary winding for each Iron core, there are as many iron cores as there are secondary windings.​​

(2) The primary winding of the voltage transformer has a lot of 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, the wire is very thick, and the secondary winding is slightly thicker. The number of winding turns is large, and the thickness of the wire is related to the rated value of the secondary current.​​

(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.
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