4. The influence of high-voltage leakage current on the error of current transformer
When high-voltage current transformers are calibrated in the laboratory, the method of measuring the error under low voltage is generally adopted.
For current transformers with equipotential capacitor screen design, the measurement error under low voltage is not significantly different from the measurement error under high voltage in terms of results, because the current flowing from the high-voltage primary conductor to the secondary winding on the low-voltage side is blocked by the capacitor screen and flows into the ground terminal, and will not enter the secondary winding.
However, the situation is different for current transformers without capacitor screens.
At this time, the inter-pole current flowing from the high-voltage primary conductor will enter the low-voltage secondary winding through the main insulation, and superimpose with the secondary current obtained by electromagnetic induction conversion to become the measured current, thereby generating additional errors.
The influence of high-voltage leakage current of the current transformer is defined as the ratio of the current flowing from the high-voltage conductor to the secondary winding to 5% of the rated secondary current.
5. The influence of equal ampere-turn busbar on the error of current transformer
The current conductor can generate a magnetic field on the adjacent current transformer core.
The highest accuracy level of current transformer is only 0.1.
The rate of change of magnetic permeability of most ferromagnetic materials under the operating magnetic flux density (0.01T~1.5T) is not large, and a slight change in magnetic flux density will not have a substantial impact on the error.
Therefore, as long as the external magnetic field does not significantly disturb the magnetic field inside the core, for example, the change of magnetic flux on both sides of the core magnetic circuit is only 10%, the error of the transformer can be considered to be basically unchanged.
However, if the external magnetic field causes the difference in magnetic flux on both sides of the core magnetic circuit to exceed 30%, the error change will be obvious.
In particular, if the magnetic flux on one side increases to close to the saturation magnetic flux, the error will be out of control and even cause the transformer winding to overheat and be damaged.
There are two situations in which the current conductor is affected.
One is that the through-core busbar deviates from the core axis, and the other is that the return conductor is too close to the transformer core.
The uneven magnetic field generated by the equal ampere-turn busbar can be equivalent to the eccentric busbar.
When the primary current increases to the point where the magnetization curve of the core enters the nonlinear region, the magnetic field generated by the eccentric primary busbar in the core is unevenly distributed along the circumference, making the synthetic magnetic field unevenly distributed along the circumference of the core.
The research results show that when the primary current of the current transformer is not greater than 4000A, the error is basically not affected by the eccentricity of the busbar.
6. The influence of the primary conductor magnetic field on the voltage transformer error
The core of the voltage transformer will be affected by the magnetic field of the current conductor installed nearby.
Among them, the primary conductor magnetic field of the current transformer in the combined transformer has a significant effect on the voltage transformer error and needs to be controlled.
During the test, the tested voltage transformer is connected to 1/4 of the rated secondary load, and the terminals on the primary side of the transformer are connected according to the operating state.
7. The influence of external electric field on the error of capacitive voltage transformer
In addition to the primary high-voltage conductor during operation, the capacitive voltage transformer installed on site will also form a space capacitance with the adjacent non-charged metal frame and power equipment, thereby flowing stray capacitance current in the coupling capacitor.
The test shows that even if the same model and specification of capacitor voltage transformers are used in a group of three phases, different error values will be obtained during the inspection due to installation in different positions.
The reason is that the surrounding objects have different capacitive coupling with the three transformers, resulting in different interference.
8. The influence of frequency change on the error of capacitor voltage transformer
The voltage dividing capacitor and electromagnetic unit of the capacitor voltage transformer form a series resonant circuit.
The influence of frequency on the error of capacitor voltage transformer can be analyzed according to the method introduced in the interpretation of Article 4.2.
