The role of current transformer:
The main function of the current transformer is to induce a small current in a certain proportion from the large wire (main line) for measurement and relay protection.
Simply put, the wire to be measured is passed through the current transformer, and then the current transformer will induce a small current in 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 a current transformer is similar to that of a dual-winding transformer, and it is also composed of a primary coil and a secondary coil. The coil connected to the power supply is called the "primary coil" or "primary winding", and the coil connected to the load is called the "secondary winding" or "secondary winding". The primary winding of a current transformer has very few turns, generally only one turn. (For example, the bright red thick wire in the above picture is the primary winding, and the dark red is the output wire of the secondary winding)
When an alternating current is applied to the primary winding, an alternating magnetic field will be generated, and the alternating magnetic field will pass through the secondary winding under the constraint of the iron core. According to the principle of electromagnetic induction, the secondary winding will induce an induced current. This current conforms to the transformer rule: U1 represents the voltage of the primary winding, U2 represents the voltage of the secondary winding; N1 represents the number of turns of the primary winding, N2 represents the number of turns of the secondary winding; I1 represents the current of the primary winding, and I2 represents the current of the secondary winding.
According to the above formula, the ratio of the primary current and the secondary current of the current transformer is related to the number of turns of the coil. The number of turns of the coil is fixed, so the transformation ratio of the current transformer is also fixed, which is generally marked on the nameplate of the current transformer.
According to national regulations, the rated current of the secondary winding of the current transformer is uniformly 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, etc.). For example, for a transformer with a transformation ratio of 500/5, where 500 represents the maximum current of the first time and 5 represents the maximum current of the second time, the induced current of the current transformer is reduced by 500+5=100 times. For the same current transformer of 500/1, where 500 represents the maximum current of the first time, and 1 represents the maximum current of the second time, the induced current of the current transformer is reduced by 500+1=500 times.
Current transformer selection:
As we mentioned above, the secondary rated current of the current transformer is generally 5A or 1A. Under normal circumstances, we prefer to choose a current transformer with a secondary rated current of 5A; if the measuring instrument or relay line is far away from the current transformer, then we choose a 1A current transformer
Generally, the accuracy of the meter is the highest at 60% of the range, so the full-scale value can be obtained by multiplying the calculated current by 1.3 times. For example, the bus current (that is, the primary current) value is 600A, then the primary current should be 600x1.3≈800A. In this way, it can be ensured that when the primary current is 600A, the pointer of the meter is just at 60%, that is, the two-thirds position.
How to choose a current transformer for 600A current?
If the measuring instrument or the relay line is close to the current transformer (such as the same power distribution cabinet), we give priority to the 800/5 current transformer.
If the measuring instrument or the relay line is far away from the current transformer (for example, in two different power distribution cabinets), we give priority to the 800/1 current transformer.
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