3.1 Basic error
[JJG313-2010] When the ambient temperature is 0℃~40℃, the relative humidity is not more than 80%, and the environmental electromagnetic interference and mechanical vibration are negligible, the error of each accuracy level of the current transformer for measurement shall not exceed the limit value when the rated frequency, rated power factor and secondary load are any value between 25%~100% of the rated secondary load.
For S-class current transformers manufactured to meet special use requirements, the error of each accuracy level shall not exceed the limit value.
The actual error curve of the current transformer shall not exceed the range of the broken line formed by the error limit line.
[JJG314-2010] When the ambient temperature is 0℃~40℃, the relative humidity is not more than 80%, the environmental electromagnetic interference and mechanical vibration are negligible, and the rated frequency, rated power factor and secondary load of the voltage transformer for measurement are any value between 25%~100% of the rated secondary load, the error of each accuracy level shall not exceed the limit value.
The actual error curve of the voltage transformer should not exceed the range of the broken line formed by the error limit line.
[Interpretation of the Article]
1. Ambient temperature and relative humidity
The basic error of the measuring instrument needs to be defined under certain environmental conditions.
The two obsolete regulations JJG313-1994 and JJG314-1994 did not put forward requirements for the basic error of the measuring transformer, but only stipulated the error limit that should be met during the calibration under the conditions of 10℃~35℃, which actually implied the reference condition of 10℃~35℃ as the basic error.
The manufacturing standards for instrument current and voltage transformers JB/T 5472-1991 and JB/T 5473-1991 both stipulate that the environmental conditions for the normal use of instrument transformers are ambient temperature 0℃~40℃ and relative humidity not more than 80%.
The manufacturing standards for miniature current and voltage transformers, JB/T 10665-2016 and JB/T 10667-2016, stipulate that the environmental conditions for the normal use of miniature transformers are ambient temperatures of -25℃~60℃ and relative humidity of no more than 95%.
JB/T5472-1991 and JB/T5473-1991 stipulate that the accuracy of instrument transformers is defined at 0℃~40℃. Even if the calibration device is not suitable for this temperature environment, there is no need to reduce the applicable range of the basic error of the transformer.
In other words, transformers that have passed the calibration according to JJG313-2010 and JJG314-2010 are also qualified for use at 0℃~40℃, which is conducive to the full use of transformer resources.
2. Rated load of measuring transformers
The error of the transformer is related to the secondary load, so the secondary load value during verification must be specified.
JB/T5472-1991 and JB/T 5473-1991 both stipulate that the error of the transformer should be measured when the secondary load is between the rated load and 114 rated load.
JB/T10665-2016 stipulates that the load of the miniature current transformer is between the rated load and the external zero load, and JB/T10667-2016 stipulates that the load of the miniature voltage transformer is between the rated load and 4 times the rated load.
The standard values of the rated load of instrument current and voltage transformers given in JB/T5472-1991 and JB/T5473-1991 are 1VA, 2.5VA, SVA, 7.5VA, 10VA15VA, 20VA, and 25VA.
There is no standard value for miniature current and voltage transformers.
Instrument transformers are widely used in instruments and equipment.
With the application of electronic technology, the actual load of many transformers is very low, and often fixed.
The use of large-capacity transformers is not conducive to the development of measurement technology and resource conservation.
Therefore, on the one hand, it is encouraged to use small capacity for instrument current and voltage transformers, but too small capacity will also affect the verification and use, so there must be some restrictions.
JJG 313-2010 clearly states: For current transformers with a rated secondary current of 5A and a rated load of 7.5VA or less, the lower limit load is specified by the manufacturer; if the manufacturer does not specify the lower limit load, the lower limit load is 2.5VA.
At present, the reference current loop of the current transformer tester generally uses a 0.1 sampling resistor, and the load is 2.5VA.
If the current transformer is used as a standard for the verification device, the lower limit load should not be less than 2.5VA.
The regulations for current transformers with rated secondary currents of 5A, 1A or less than 1A are expressed in the form of load resistance, that is, the lower limit load of current transformers with rated load resistance less than 0.22 is 0.12.
In the case of incomplete product nameplate markings, there are cases where the manufacturer intentionally or unintentionally does not specify the lower limit load.
For the above situation, the provisions of the regulations can only be handled in a rigid and unified way to eliminate the opportunity to use the nameplate to take shortcuts.
In addition, some current transformers specify that the secondary load is a fixed load on the nameplate.
Most of these current transformers are installed in instruments (such as electric energy meter calibration stations).
The nameplate generally does not specify the allowable deviation value of the secondary load, which can easily cause disputes in the verification results.
Therefore, the regulations need to explain the fixed load and clarify that the error within the +10% range of the fixed load should meet the accurate limit requirements.
When the voltage transformer is used as a sensor, the secondary load can be 0VA, while the current transformer needs to be connected in series with a sampling resistor in the secondary circuit and is generally not used at 0VA.
At present, voltage transformer testers mostly use a load of 0.2VA in the circuit with a rated reference voltage of 100V, and the load is not changed in the range below 100V, and the actual load is less than 0.2VA.
In view of this feature, the rated secondary load of the standard voltage transformer designed for transformer verification equipment is generally not greater than 0.2VA.
If such a small load is still taken as 25% of it as the lower limit, it is very inconvenient to operate, and the difference between the error of the transformer under such a small load and 0VA is not large, so it is not necessary to select it according to 25% of the rated load.
Taking these factors into consideration, the JJG314-2010 regulations uniformly and clearly assess it according to 0VA.
The regulations do not explain how to implement it when the secondary load is 0VA.
According to convention, the influence amount deviating from OVA can be selected according to the secondary influence amount, for example, according to the influence of the tester or the working current bus on the error, it is selected as 1/10 of the error limit.
