【JJG313-2010】5.1.6 Current load box
When the current load box is at rated frequency 50Hz (60Hz), 20%~120% of the rated current, and the ambient temperature is 20℃±5℃, the relative error of the active and reactive components of the current load (calculated together with the specified secondary lead resistance) shall not exceed ±3%.
When cos=1, the residual reactive component shall not exceed ±3% of the rated load.
For every 10℃ change in ambient temperature, the load error shall not exceed ±2%.
The additional error limit of the current load box below 20% of the current percentage is: for every 5% change in the current percentage, the error increases by 1%.
【JJG314-2010】5.1.6 Voltage load box
When the rated frequency of the voltage load box is 50Hz (60Hz), the rated voltage is 20%~120%, and the ambient temperature is (20±5)℃, the error of the active and reactive components of the voltage load shall not exceed ±3%.
When cos=1, the residual reactive component shall not exceed ±3% of the rated load.
The error change of the load shall not exceed ±2% for every 10℃ change in ambient temperature.
【Article Interpretation】The conditions that the load box should meet when using it should be determined based on the impact of the secondary load change on the transformer error.
For a current transformer with a grade index a, at a certain current percentage, the error change from the rated load to the lower limit load should be controlled within 1.4a%.
In this process, the excitation impedance can be approximately considered unchanged, and the impedance in the loop can be ignored compared with the load.
In this way, the impact of the load change on the error is uniform, and the lower limit load is calculated to be equal to 1/4 of the rated load.
If the load changes by 3%, the error changes to 0.056a%, if the load changes by 5%, the error changes to 0.09a%, if the load changes by 6%, the error changes to 0.11a%.
The verification procedure usually regards the interference that affects the verification result less than 1/10 as a secondary component.
Therefore, it is reasonable to choose the allowable error of the load box as 5% or 6%.
In addition, the allowable error of the transformer is related to the current and voltage percentage.
At low current and voltage percentages, the error is relaxed by multiples, so the error of the load box can also be relaxed by multiples in principle.
For example, the error limit of the 1% point of the 0.2S-level current transformer is 0.75%, which is more than 3 times that of the 0.2% point.
Therefore, relaxing the error of the load box from 3% to 6% will not have a significant impact on the verification result.
Taking these factors into consideration, JJG313-2010 and JJG314-2010 put forward the above provisions for the error of current and voltage load boxes.
This provision fully takes into account the additional errors that may exist in the design and manufacture of the load box, so that there is actually no need to measure and check the additional errors, which greatly simplifies the manufacturing process and verification work.
The load box only needs to be verified at (20±5)℃, and it is qualified if the error does not exceed 3% at the two points of 20% and 120% of the rated current (or 20%~120% of the rated voltage).
