Reasons affecting the current transformer error

- Jul 31, 2020-

1. Effect of return conductor

In an ideal situation: the wires of the secondary winding of the transformer are evenly distributed on the ring-shaped iron core,-when the secondary winding is evenly distributed or the primary winding is an infinitely long conductor and passes through the center of the iron core, the magnetic flux of each section of the iron core The density is equal. In fact, there is a return conductor in the primary winding structure of the transformer itself (except for inverted and penetrating transformers). For example, the U-shaped primary winding used for high-voltage transformers consists of two leads, and the secondary winding is sheathed on one lead and the other. The bow|wire and loop (bent) conductor constitute the return conductor.

When analyzing the influence of the return conductor, a simple return conductor parallel to the transformer secondary winding conductor is used as an example. Due to the magnetic flux generated by the current in the return conductor, the magnetic flux density of the core passing through the core close to the return conductor increases, while the magnetic flux density of the core away from the return conductor decreases, making the magnetic flux density of the core section Wait. Because the magnetic flux path generated by this external magnetic field mostly passes through the air, the core magnetic circuit is only a very small section, so its magnetic induction is proportional to the magnetic permeability of the air; and the magnetic field lines are only linked to some secondary windings. In this case, it is the magnetic flux leakage that increases the secondary leakage resistance. However, in the case of large overcurrent, the core magnetic flux density near the return conductor will increase rapidly and the error will increase rapidly, especially for the protection current transformer, it may cause the compliance error to exceed the limit.

2. Dispersion of iron core excitation characteristics

In the case of rated ampere-turns, secondary load, core cross-sectional area, and conductors are the same, due to the different core excitation characteristics, the measured error results are also different, and the difference is very different. The excitation current is the direct cause of the error. The smaller the excitation current, the smaller the current error; the larger the excitation current, the larger the current error.