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2023-08-20

VILLESEC

19

The main function of current transformer is to induce small current from large wire (main line) according to a certain proportion for measurement and relay protection.Simply put, it is to pass the wire to be measured through the current transformer, and then the current transformer will induce a small current according to a certain proportion. We only need to measure the small current induced by the current transformer, and then multiply it by the corresponding multiple to know the large current.Principle of current transformer

The working principle of current transformer is similar to that of double winding transformer. It is also composed of primary coil and secondary coil. The coil connected to the power supply is called "primary coil" or "primary winding", and the coil connected to the load is called "secondary coil" or "secondary winding". The number of turns of the primary winding of current transformer is very small, generally only one turn. (for example, the bright red thick line in the figure above is the primary winding, and the dark red line is the output line of the secondary winding)When AC current is applied to the primary winding, an alternating magnetic field will be generated, which passes through the secondary winding under the constraint of the iron core. According to the principle of electromagnetic induction, the secondary winding will induce induced current. This current does not comply with transformer rulesWhere: U1 represents the primary winding voltage and U2 represents the secondary winding voltage; N1 represents the number of turns of primary winding and N2 represents the number of turns of secondary winding; I1 represents the primary winding current and I2 represents the secondary winding current.

According to the above formula, the ratio of primary current to secondary current of current transformer is related to the number of coil turns. The number of coil turns is fixed, so the transformation ratio of current transformer is also fixed, which is generally marked on the nameplate of current transformer.According to national specifications, the rated current of the secondary winding of the current transformer is specified as 5A or 1a, so the transformation ratio of the current transformer is also written as XX / 5 or XX / 1 (such as 500 / 5 or 500 / 1).For example, for a transformer with a transformation ratio of 500 / 5, where 500 represents the primary maximum current and 5 represents the secondary maximum current, the induced current of the current transformer is reduced by 500 5 = 100 times. For the same 500 / 1 current transformer, where 500 represents the primary maximum current and 1 represents the secondary maximum current, the induced current of the current transformer is reduced by 500 1 = 500 times.

Selection of current transformerAs mentioned above, the secondary rated current of current transformer is generally 5A or 1a. Generally, we give priority to the current transformer with secondary rated current of 5A; If the measuring instrument or relay line is far from the current transformer, we will choose 1A current transformer.General instruments have the highest accuracy at 60% of the range, so the full range value can be obtained by multiplying the calculated current by 1.3 times. For example, if the bus current (i.e. primary current) is 600A, the primary current should be 600 1.3800A In this way, it can be ensured that when the primary current is 600A, the instrument pointer is just at 60%, that is, two-thirds position.

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