When the magnetic flux density of the current transformer core is 1.5 times the rated current and rated load, the error should not be greater than 1.5 times the error limit under the rated current and rated load.
[Article Interpretation] The selection of magnetic saturation margin is related to the instrument safety factor of the current transformer, because the current transformer for measurement has instrument safety factor requirements. When the primary current reaches the specified instrument safety current (rated primary current multiplied by the instrument safety factor), the error of the current transformer should not be greater than 10%, so as to protect the secondary instrument from damage caused by large current shock.
According to normal design, the instrument safety factor of the current transformer using cold-rolled silicon steel sheet core is about 10; the instrument safety factor of the current transformer using iron-nickel alloy core is about 3, but iron-nickel alloy is expensive and is only used in precision current transformers.
Therefore, the instrument safety factor of current transformer products in the past is not a problem that needs to be discussed.
In recent years, new and inexpensive soft magnetic materials, such as amorphous and ultra-fine crystal materials, have been widely used, and their magnetic permeability is several times higher than that of silicon steel sheets.
Higher magnetic flux density can be used to achieve the same accuracy.
In order to save core material and reduce volume, designers often take a relatively high rated magnetic flux density, which results in the instrument safety factor being reduced to less than 3.
On-site verification has found that the magnetic saturation point of some current transformer cores made of amorphous and ultra-microcrystalline materials is very close to the 120% point of the rated current, and a slight asymmetry of the magnetic circuit will exceed the tolerance.
Considering that during the error inspection and use of the current transformer, when the distance between the primary return conductor and the core is close to the geometric dimensions of the transformer body, the magnetic field generated by the large current conductor may cause the core magnetic flux density to have an asymmetry of 1/6.
If both parts of the asymmetric core are not saturated, the error of the current transformer will not be significantly affected.
However, if one side of the core is saturated, the error of the current transformer will change significantly, making the error of the transformer unqualified.
Therefore, it is necessary to ensure that the core has a certain magnetic saturation margin during design.
Since the maximum operating current of the current transformer is usually 1.2 times the rated current, it is more appropriate to increase it by 1.2 times on this basis and take 1.5 times.
