5 General technical requirements
5.1 Nameplate and marking
The power transformer should have a nameplate and marking on the body.
The nameplate should have the product number, date of manufacture, wiring diagram or wiring method description, rated current ratio or (and) rated voltage ratio, accuracy level and other obvious markings.
The primary and secondary terminals should have current or (and) voltage wiring symbols, and the grounding terminal should have a grounding mark.
5.2 Insulation
The insulation level of the power transformer should comply with the provisions of GB1207, GB1208 and GB/T4703.
6 Measuring instrument control
Measuring instrument control includes initial calibration, subsequent calibration and inspection during use.
6.1 Calibration conditions
6.1.1 Environmental conditions
6.1.1.1 Ambient temperature and relative humidity
Ambient temperature -25℃~55℃, relative humidity not more than 95%.
[Article interpretation] The current and voltage standards currently used in field calibration include precision current transformers, precision voltage transformers and capacitor voltage dividers.
Due to the structural characteristics of precision current and voltage transformers, they can still maintain the same accuracy when used in the temperature range of -25℃~55℃, but they must be careful to prevent oil and gas leakage caused by low temperature.
Capacitor voltage dividers have a relatively large temperature coefficient, but they can be used after field calibration, and will not introduce additional errors caused by ambient temperature in actual use.
Current and voltage load boxes are also used after on-site calibration.
The calibration department prepares low-temperature, normal-temperature and high-temperature load boxes according to the local conditions.
They are used in combination according to the seasons, and the impedance and admittance measurement circuits of the transformer tester are used to check whether the load value is qualified before use.
Transformer testers can also be designed and produced according to the temperature range.
Low-temperature and normal-temperature products.
The key technology of low-temperature testers is the use of low-temperature resistant LCD screens and low-temperature resistant impedance elements with small temperature coefficients.
Laboratory-use normal-temperature testers can be used in most parts of my country for most of the year. According to experience, this type of tester can meet the use conditions of 55°C.
6.1.1.2 Environmental electromagnetic field interference
The error change of the standard instrument caused by environmental electromagnetic field interference is not greater than 1/20 of the basic error limit of the transformer under test.
The error change of the transformer under test caused by the calibration wiring is not greater than 1/10 of the basic error limit of the transformer under test.
[Article Interpretation] The limit and measurement of the degree of electromagnetic field interference under the calibration environment conditions is a difficult problem.
In the two calibration procedures JG313-2010 and JJG314-2010, only the working electromagnetic field and the external electromagnetic field are restricted respectively, and the measurement or estimation method of these two interference quantities is not explained.
The JJG1021-2007 calibration procedure also faces this problem.
If it is not limited, the uncertainty of the measurement cannot be guaranteed; if it is limited, it is difficult to give a calculation method.
This problem can only be handled by the calibration personnel based on the actual calibration experience. Under normal circumstances, once the measurement result is found to be abnormal, the interference factor should be considered.
Experienced calibration personnel can judge the size and direction of the interference by moving the equipment (changing the distance and direction) and comparing the changes in the errors measured under different equipment arrangements.
6.1.2 Test power supply
The power supply frequency is about 50Hz, and the waveform distortion coefficient is not more than 5%.
[Article interpretation] For the requirements of the power supply frequency and waveform distortion of the standard instrument, please refer to the interpretation of the test power supply in 4.2 Basic error.
