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# Explanation of the principle of current transformer -

by：Transmart     2023-01-03

1. Overview of voltage and current transformers Typical transformers use the principle of electromagnetic induction to convert high voltage into low voltage, or convert large current into small current, and provide suitable voltage or current for measuring devices, protection devices, and control devices Signal. The voltage transformer commonly used in the power system, its 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 types; and the current transformers commonly used in power systems, the primary side current is usually a few amperes to tens of thousands of amperes, and the standard secondary current is usually 5A, 1A, 0.5A, etc. 1. 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 W of the winding, and the magnetic flux Ф is: 2. The principle of the 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 ground voltage), 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 windings of a current transformer is inversely proportional to the number of turns of the primary and secondary windings. 3. The terminals of the transformer winding and the polarity voltage transformer winding are divided into the head end and the tail end. For a fully insulated voltage transformer, the ground voltage that the head end and the tail end of the primary winding can withstand is the same, while the semi-insulated The voltage transformer of the structure, the voltage that the end can withstand is generally only about a few kV. It is common to use A and X to represent the head end and tail end of the primary winding of the voltage transformer respectively, and use a, x or P1,
P2 indicates the first end or tail end of the secondary winding of the voltage transformer; L1 is commonly used for current transformers
, L2 represent the head end and tail end of the primary winding respectively, and K1, K2 or S1, S2 represent the head end or tail end of the secondary winding. The labels of different manufacturers may be different, and the subscript 1 is usually used to represent the head end. The subscript 2 indicates the tail end. When the direction of the induced potential of the terminals is the same, it is called the end of the same name; conversely, if the direct current of the same direction is passed through the end of the same name, the magnetic flux generated by them in the iron core is also in the same direction. The terminal with the same label as the head end or the same terminal and the direction of the induced potential are the same. The winding with 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 specified as depolarization. 4. The main difference in structure between voltage transformer and current transformer (1) Both voltage transformer and current transformer can have multiple secondary windings, but voltage transformer can share one iron core with multiple secondary windings, and the current The transformer must have an independent iron core for each secondary winding, and 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 number of turns of the secondary winding is small, and the wire is slightly thick; while the primary winding of the high-voltage current transformer used in the substation has only 1 to 2 turns, the wire is very thick, and the secondary winding has a small number of turns. There are many turns in the winding, 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. the
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