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# Explanation of current transformer principle

2021-12-10

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, so as to provide appropriate voltage or current signals for measuring devices, protection devices and control devices. For voltage transformers commonly used in power system, 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 / respectively; For the current transformer commonly used in power system, 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. 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 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, winding voltage U and voltage frequency f, winding turns W and magnetic flux Ф The relationship is:

2. Principle of current transformer

It is also similar to the transformer in principle, as shown in Figure 1.2. The main difference from the voltage transformer is that 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 short-circuit state, so the magnetic flux in the iron core is very low Ф It is also very small. At this time, the magnetic potential f (F = IW) of the primary and secondary windings are equal and in the opposite direction.

That is, the current ratio between the primary and secondary windings of the current transformer is inversely proportional to the number of turns of the primary and secondary windings.

3. Terminal and polarity of transformer winding

Voltage transformer winding is divided into head end and tail end. For fully insulated voltage transformer, the head end and tail end of primary winding can bear the same voltage to ground, while for voltage transformer with semi insulated structure, the tail end can bear only about a few kV. Commonly, a and X are used to represent the head end and tail end of the primary winding of voltage transformer respectively, and a, X or P1 and P2 are used to represent the head end or tail end of the secondary winding of voltage transformer; For current transformers, L1 and L2 are commonly used to represent the head end and tail end of primary winding respectively, while K1, K2 or S1 and S2 are used to represent the head end or tail end of secondary winding. The labels of different manufacturers may be different. Generally, subscript 1 is used to represent the head end and subscript 2 is used to represent the tail end.

When the induced potential direction of the terminal is consistent, it is called the same name terminal; Conversely, if DC current in the same direction is applied at the same name end, the magnetic flux generated in the iron core is also in the same direction. The label is the same as the terminal at the head end or tail end, and the direction of induced potential is the same. The winding with this label is called depolarization, and the voltage of the terminal is the result of subtracting the induced potential of the two windings. In the transformer, the correct label is specified as minus polarity.

4. Main structural differences between voltage transformer and current transformer

(1) Both voltage transformer and current transformer can have multiple secondary windings, but voltage transformer can share an iron core with multiple secondary windings. Current transformer must have independent iron core for each secondary winding. There are as many iron cores as there are secondary windings.

(2) The primary winding of voltage transformer has many turns, the conductor is very thin, the secondary winding has few turns, and the conductor is slightly thick; The primary winding of the high-voltage current transformer used in the substation is only 1 to 2 turns, the conductor is very thick, the number of turns of the secondary winding is large, and the thickness of the conductor is related to the rated value of the secondary current.

(3) When the voltage transformer is in normal operation, it is strictly prohibited to open the low-voltage terminal of the primary winding and short circuit the secondary winding; When the current transformer operates normally, it is strictly prohibited to open the secondary winding.

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