6.3.4.4 Comparison method circuit using electronic standard voltage transformer
The electronic standard voltage transformer consists of a gas-insulated capacitor voltage divider, an electronic potential follower and an electronic voltage divider, and is generally used only as a laboratory standard. When used on site, it is advisable to refer to the calibration method of the capacitor voltage divider before use.
[Article interpretation] Since the capacitance temperature and displacement stability of the capacitor voltage divider are not very good, it needs to be calibrated with a low-voltage voltage standard before use.
During calibration, the electronic voltage divider is adjusted to make the overall error of the electronic standard voltage transformer meet the requirements.
6.3.4.5 Inspection circuit using capacitor voltage divider
The circuit using the capacitor voltage divider as a standard to inspect the error of the voltage transformer.
The capacitor voltage divider is used for substitution measurement.
First, use the capacitor voltage divider to inspect a standard voltage transformer with the same transformation ratio as the transformer to be inspected, and adjust the voltage divider ratio of the capacitor voltage divider so that the calibration instrument indication is equal to the calibration value of the standard voltage transformer.
Then replace the standard voltage transformer with the voltage transformer to be inspected, and measure the error of the transformer to be inspected at the specified voltage percentage.
[Article Interpretation] Capacitive voltage transformers need to be tested with a series resonant boost device or a parallel resonant boost device.
The series resonant circuit consists of a boost transformer E, an iron core reactor L and a coupling capacitor C of the capacitive voltage transformer.
Electromagnetic voltage transformers are generally tested with a test transformer.
When conditions permit, the generator and transformer of the power plant can also be used to provide a boost power supply, but the generator and transformer are required to have good voltage regulation functions to ensure that they can stably provide 15%~120% of the test voltage.
6.3.4.6 Measurement of large current transformers by equal ampere-turns method
When the influence of the magnetic field of the primary return conductor on the error of the current transformer is not greater than 16 of the basic error limit, the equal ampere-turns method is allowed to measure the error of the current transformer.
[Article Interpretation] The operational variation of the equal ampere-turn test is mainly determined by the influence of the magnetic field of the primary return conductor.
Magnetic field calculations show that when the current is no more than 1000A and the distance between the return conductor and the center of the core is more than three times the radius of the core, the influence of the primary return conductor of the independent current transformer on the measurement result is generally no more than 1/6 of the basic error limit of the current transformer being tested.
For U-type primary conductor current transformers with a primary current greater than 1500A~3000A, a balanced winding is generally installed for compensation.
However, the equal ampere-turn method cannot reproduce the error of the current transformer equipped with a balanced winding, so it is not suitable for this type of U-type primary conductor current transformer.
If calibration is necessary, the distribution of magnetic flux in the transformer can be analyzed through magnetic shielding calculations.
If the variation in operation is determined to be within the allowable range in terms of structure, the calibration results of the equal ampere-turn method can be used.
