6.3.4.1 General requirements
According to the transformation ratio and accuracy level of the transformer to be tested, refer to Article 6.1.3 to select the standard instrument and use the test circuit recommended by this regulation to measure the error.
The measurement can start from the maximum percentage or the minimum percentage.
The calibration data of large current and high voltage transformers should be read after at least one full-scale rise and fall.
When large current transformers (rated primary current 3kA and above) are subsequently verified and inspected in use, with the approval of the superior metrology administrative department, it is allowed to merge the 100% and 120% rated primary current verification points into the actual maximum primary current point.
For voltage transformers with multiple secondary windings, except for the remaining windings, each secondary winding should be connected to the upper or lower load limit specified in Table 1.
Unless the user requires otherwise, current transformers and voltage transformers are only tested for the actual ratios used.
If the user plans to use another ratio during the calibration period, the user should request the calibration agency to increase the ratio to be tested before the calibration.
For transformers with calibration accuracy levels of 0.1 and 0.2, the ratio difference read is retained to 0.001%, and the phase difference is retained to 0.01'.
For transformers with calibration accuracy levels of 0.5 and 1, the ratio difference read is retained to 0.01%, and the phase difference is retained to 0.1'.
[Article interpretation] In GB 1207-2006 "Electromagnetic Voltage Transformer" and GB4703-2001 "Capacitive Voltage Transformer", the upper limit medium voltage of the transformer error test is specified as 120% of the rated voltage.
GB 311.1-2012 "Insulation Coordination Part: Definitions, Principles and Rules" also specifies the maximum voltage of power equipment.
Therefore, this regulation converts the highest voltage of power equipment to a percentage of the rated voltage, which are 115% and 110% respectively, as the upper limit test voltage for verification.
For medium voltage transformers, a higher or lower test voltage has little effect on the manufacturing cost of the equipment, but for ultra-high voltage and extra-high voltage transformers, increasing the rated voltage of the test equipment by 10 percentage points has a relatively large impact on the cost.
If it is not necessary, the increase in test expenses should be avoided.
The current transformers installed on site have been connected to the power grid and used at a certain current ratio.
Therefore, the regulations allow only the current ratio used to be verified. In general, other current ratios are not included in the verification content.
Considering that some users may change the current ratio of the current transformer during the verification cycle, in order to avoid re-verifying the newly enabled current ratio, users can make a request during the periodic verification and conduct verification in advance.
In order to meet the protection requirements, the rated primary current of large current transformers in substations and switch stations is greater than the actual operating current, usually only 30%~60% of the rated current.
Therefore, in theory, the error under the large current percentage has no substantial impact on the electric energy measurement.
On the other hand, the structure of current transformers makes them have stable errors.
If the first calibration is qualified, the subsequent calibration and use can be checked, and the metering performance can be controlled through the calibration points of 20% rated current and below and the maximum actual operating primary current point.
In order to ensure the legality of the calibration work, if the calibration personnel want to adopt the calibration method of combining 100% and 120% rated primary current calibration points into the actual maximum primary current point, they must apply to the superior metrology administrative department for approval in advance.
The calibration object of JIG 1021-2007 is working measuring instruments.
The calibrated transformers are not used for value transfer.
Only qualified and unqualified conclusions are required.
Therefore, data rounding is not necessary for power transformers.
In order to save the workload of calibration and improve efficiency, JJG1021-2007 does not require data rounding.
In order to obtain the same calibration conclusion without data rounding, it is necessary to ensure the accuracy of data reading.
JJG1021-2007 stipulates the following for data reading: For transformers with accuracy levels of 0.1 and 0.2, the ratio difference read during inspection shall be retained to 0.001%, and the phase difference shall be retained to 0.01'.
For transformers with accuracy levels of 05 and 1, the ratio difference read during inspection shall be retained to 0.01%, and the phase difference shall be retained to 0.1.
Since calibrators with digital display of measurement results are now widely used, their resolution can reach a ratio difference of 0.001% and a phase difference of 0.01', so it is not a problem to read the above accuracy.
The influence of the read data on the accuracy verification results can be analyzed using a 0.1-level transformer as an example: the basic error limits of a 0.1-level transformer are 0.1% and 5.
During the inspection, the ratio difference read is retained to 0.001%, the phase difference is retained to 0.01, and the recorded data is refined to 1/10 of a rounded unit.
The probability that the verification result falls on the critical point between qualified and unqualified errors is smaller than the original rounding method, and the possibility of misjudgment is also reduced.
In terms of workload, using one more significant digit is simpler than performing data rounding and reduces the possibility of error.
