6.1.3 Current and voltage ratio standard
6.1.3.1 Standard current and voltage transformer
The rated transformation ratio of the standard current and voltage transformer (including electronic standard voltage transformer) used for calibration shall be the same as that of the transformer to be tested, and the accuracy level shall be at least two levels higher than that of the transformer to be tested.
The actual error under the calibration environment conditions shall not exceed 1/5 of the basic error limit of the transformer to be tested.
The variation of the standard (the difference between the error values measured twice when the current and voltage rise and fall) shall not exceed 1/5 of its basic error limit.
The actual secondary load of the standard (including the differential loop load) shall not exceed the upper and lower load ranges specified.
If the error calibration value of the standard is required, the deviation between the actual secondary load of the standard (including the differential loop load) and the load specified in its calibration certificate shall not exceed 10%.
[Article Interpretation] The accuracy of power transformers operating on site is usually 02 level or below, so standard current and voltage transformers with an accuracy of 0.05 or 0.02 level should be used for on-site transformer calibration.
The secondary rated current of the standard current transformer used for calibrating current transformers is 1A and 5A, and the load capacity is 5VA.
The primary rated current can be distinguished according to the distribution network and the high-voltage power grid.
Usually, standard current transformers with a primary rated current of SA~500A can be used to calibrate distribution network current transformers, and standard current transformers with a primary rated current of 300A~3000A can be used to calibrate high-voltage and ultra-high-voltage power grid current transformers.
The equal ampere-turn method can be used to calibrate generator output current transformers.
Standard current transformers exceeding 5kA generally adopt a cascade structure, with the secondary rated current of the first stage being 50A, and then a 50A standard current transformer is cascaded to convert it to 1A or 5A output.
The standard voltage transformers used for calibrating voltage transformers are classified according to voltage levels.
Before the 1980s, my country could not manufacture high-accuracy voltage transformers with voltage levels above 110kV.
With the advancement of technology, my country is currently at the world's leading level in the manufacturing technology of ultra-high voltage and ultra-high voltage standard voltage transformers.
6.1.3.2 Capacitor voltage divider
Under the calibration environment conditions, the voltage coefficient of the capacitor voltage divider (the correlation between the voltage divider ratio and the voltage) should not be greater than 1/10 of the basic error limit of the voltage transformer being tested.
When using a capacitor voltage divider as a standard device to calibrate a voltage transformer, the substitution method circuit operation should be used.
The change in the voltage divider ratio of the capacitor voltage divider during the calibration process shall not be greater than 1/10 of the basic error limit of the transformer being tested.
[Article Interpretation] A high-voltage standard capacitor is connected in series with a low-voltage standard capacitor to form a standard capacitor voltage divider.
The voltage coefficient of the capacitor voltage divider is defined as the correlation between the voltage divider ratio and the voltage.
In fact, the electromagnetic voltage transformer also has a voltage coefficient, and the error at low voltage is not the same as the error at rated voltage.
Since the capacitance of the gas standard capacitor is mainly related to the geometric parameters of the electrode, the voltage coefficient of the gas capacitor with an ideal coaxial cylindrical electrode structure is zero in theory.
Therefore, as long as the design is reasonable and the manufacturing process meets the standards, the voltage coefficient of the high-voltage standard capacitor (the correlation between capacitance and voltage) can reach 10-order of magnitude.
The voltage of the low-voltage standard capacitor does not exceed 1000V, and the electric field force is much smaller than that of the high-voltage standard capacitor, so the voltage coefficient can be controlled to be smaller.
Therefore, the requirements of JJG 1021-2007 for the voltage coefficient of the capacitor voltage divider are easy to meet.
Although the voltage coefficient of the capacitor voltage divider is very small, the temperature and displacement stability of the capacitance are not very good.
During the transportation of the standard capacitor, the electrodes are vibrated and the geometric position will change.
Since the electrode spacing is only a few centimeters, even a micron-level change in the electrode spacing will cause a significant change in the capacitance.
On the other hand, the electrodes of the gas capacitor are made of metal materials, and the temperature coefficient of the capacitance is equivalent to the linear expansion coefficient of steel and copper, reaching 2x10-5/℃.
When used on site, the capacitance will change by 0.08% when the temperature changes by 40K, and this effect also needs to be considered.
In order to solve the problem of voltage divider ratio change due to unstable capacitance value, JJG1021-2007 stipulates the use of substitution method, that is, the capacitive voltage divider is only used as a measuring tool for voltage extrapolation, not as a measuring tool for proportional value traceability.
This ensures the reliability of the use of the capacitive voltage divider.
