6.1.4 Current and voltage load boxes
The current and voltage load boxes used for power transformer verification shall have clear markings on the panel where the wiring terminals are located, indicating the rated ambient temperature range, rated frequency, rated current or voltage, and rated power factor.
The current load box shall also indicate the value of the external wiring resistance.
The rated temperature range recommended by this regulation is: low temperature type -25℃~15℃, normal temperature type -5℃~35℃, high temperature type 15℃~55℃.
The rated ambient temperature range of the current and voltage load boxes used during verification shall be able to cover the actual ambient temperature range during verification.
In the specified ambient temperature range, the relative error of the active and reactive components of the current and voltage load boxes within the range of 80%~120% of the rated frequency and rated current and voltage shall not exceed plus or minus 6%, and the residual reactive component (applicable to load boxes with a power factor equal to 1) shall not exceed plus or minus 6% of the rated load.
Under other specified current and voltage percentages, the relative error of active and reactive components shall not exceed plus or minus 9%, and the residual reactive component (applicable to load boxes with a power factor equal to 1) shall not exceed plus or minus 9% of the rated load.
[Article Interpretation] For a long time, current and voltage load boxes have been used as part of the calibration equipment and have been subject to the calibration procedures for current and voltage transformers.
Since the test temperature for on-site calibration has been extended to -25℃~55℃, it is difficult for existing load boxes to achieve the required accuracy in such a wide temperature range.
The main reason is that the resistivity temperature coefficient of copper is relatively large, reaching 0.004/℃.
When the temperature changes from 15℃ to -25℃, the resistance changes by 12%.
To reduce the proportion of copper resistance in the load, the diameter of the copper wire must be increased.
Such a design is not economical for low volt-ampere loads.
Therefore, JG 1021-2007 adopts the method of verifying the accuracy of the load box under the calibration environment, so that low-temperature, normal-temperature or high-temperature load boxes can be used according to the actual environmental conditions during calibration.
Among them, the normal-temperature load box is the load box produced according to existing technical conditions.
In this way, the manufacturer can use existing materials and processes, and after simple debugging, it can produce low-temperature and high-temperature load boxes, which meets the requirements of economic rationality.
If the use environment of the load box for power transformer does not exceed (5~35)℃, it should be calibrated according to the load box of instrument transformer, using the error limits specified in JJG313-2010 and JJG314-2010.
Generally speaking, the load box for power transformer can be calibrated on site using a transformer calibrator, because the transformer calibrator can meet the requirements for an error of 6%.
