The basic error of a measuring instrument refers to the error of the measuring instrument under reference conditions.
Since the error of a measuring instrument is related to the conditions of use, reference conditions need to be specified for comparability.
(1) Ambient temperature.
Power transformers have their own particularity and need to be used for a long time on site.
Therefore, the environmental conditions for the use of a large number of power transformers are outdoor conditions.
The three standards GB 1207-2006, GB 1208-2006, and GB/T 4703-2001 require the ambient temperature of transformers to be as low as -40°C and as high as 40°C. Due to the particularity of power supply, calibration work must sometimes be carried out at the operating temperature of the transformer.
In most cases, the winter temperature in northern my country does not exceed -25°C, while the actual summer temperature of substations in many areas can reach 55°C, which is higher than the maximum ambient temperature required by industrial standards.
However, this does not mean that these products are not suitable for use, because according to the requirements of the transformer product standard, the transformer is allowed to have a temperature rise of at least 50°C during operation.
In order to ensure accuracy, the current density of the conductor of the transformer will not be very large, and the actual temperature rise is far lower than the temperature rise limit.
Generally,it is only about 15°C. Therefore, in the case of shutdown, using the temperature range of -25°C~55°C as the temperature condition during calibration will not affect the transformer error measurement results.
The power transformer used indoors generally has a minimum operating temperature of -5°C. The calibration of this type of transformer is carried out indoors, and this temperature range also meets the operating environment requirements of indoor transformers.
(2) Relative humidity.
The relative humidity during on-site calibration is actually not for the power transformer, but for the calibration personnel and calibration equipment.
Outdoor power transformers can be used under rain conditions, but the standard equipment used for calibration is usually not rainproof and should not be used under rain conditions.
Therefore, the relative humidity cannot be taken from the humidity in the rainy environment.
If it does not rain, at a relative humidity below 95%, there will be basically no condensation on the surface of the calibration equipment, which can ensure the safe and reliable use of the calibration equipment.
(3) Power frequency.
The power frequency requirement is based on the standard frequency of my country's power grid.
Except for a few regions and local power grids, most power grids can meet the frequency requirement of 50Hz+/-0.5Hz.
The reasons for the frequency requirement are: ① The error of the capacitive voltage transformer is related to the power frequency; ② Except for the specially designed digital transformer tester, the errors of the general structure electrical and electronic testers are related to the power frequency.
