1. Generally, the transformer can guarantee linearity with excellent accuracy within the range of 12%~90% of its rated current. Therefore, most of the time when selecting a transformer, it will be properly enlarged.
2. The meter head also has good accuracy within a certain range. The general choice is: it is ideal to keep the working current at about 30% to 75% of its rated current.
If the transformer ratio is too large, the current flowing through the meter head will be too small, and the characters will be slower.
If the current transformer ratio is too large or too small, it will have an adverse effect on the measurement.
The protective current transformer mainly cooperates with the relay protection device and provides a signal to the relay device to cut off the faulty circuit when the line occurs short-circuit overload and other faults, so as to protect the safety of the power system.
Non-sinusoidal wavelets cannot be analyzed by phasor diagrams, and the concept of compound error needs to be analyzed, which makes i0 increase rapidly, which is equivalent to part of i1 failing to be converted into i2, and i2 and i1 no longer change proportionally, thus increasing the TA error.
When a short-circuit fault occurs in the power system and the relay protection action occurs, the short-circuit current i is very large, generally several times the rated current, which increases the error and endangers the sensitivity and selectivity of the protection device.
When selecting the accuracy level of TA for protection, the accurate limit coefficient must be selected at the same time, such as 5P20 and 1200/5A, which means i≯20 times the rated i1, that is, when it is within the range of ≯20×1200=24kA, the compound error is ≯5%; select both directions, you can select the rated current ratio according to the given ilmax, secondary load value and 10% error curve, accurate limit coefficient; in steady-state operation, TA secondary load should meet the requirements of 10% error curve.
For example the rated current of the power distribution cabinet to supply a certain line is 1450A, then the current transformer of 1500A/5A should be selected, and the three-phase 5A watt-hour meter should be used, then the multiplier is 150.
The selection of current transformers should pay attention to the following points:
1. Accuracy grade: Generally, current transformers are divided into 1.0, 0.5, 0.2, 0.5S, 0.2S grades, which should be selected according to the requirements of sausages!
2. The inner aperture size of the current transformer: Different models have different sizes. The general current transformers are of the through-core type. The time to select the model should first see whether the busbar or cable can pass through!
3. Whether to choose the open type: Open type transformers are generally used in reconstruction projects, which means that the cables or busbars have been installed, and it is not convenient to re-install. At this time, it is more convenient to use open-type transformers!
Summary: The choice of magnification of the current transformer is not a concept of good or small. There is no such thing as a large or small magnification.
It mainly depends on the size of the primary side current and the size of the current measured or controlled on the secondary side. Usually, the nominal current of the secondary side of the current transformer is 5A. The main is to select the appropriate (close to the nominal current value of the transformer) nominal value of the primary side current of the current transformer according to the supply current of the AC power supply or the current of the electrical load.
