4.3 Stability
The change of the error of the power transformer in two consecutive calibrations shall not be greater than 2/3 of the basic error limit.
Stability is an important metrological performance of the transformer.
As a measuring instrument, it must be ensured that the error remains basically unchanged in two periodic calibrations, otherwise the calibration cycle must be shortened.
The error stability of the transformer in two periodic calibrations can be judged by comparing the results of the two calibrations.
4.4 Operation variation
The operation variation of the power transformer is defined as the change of the transformer error under the influence of the operating environment.
It can be caused by operating conditions such as ambient temperature, residual magnetism, and proximity effect, or by operating mode, such as changing the voltage of the primary conductor of the high-voltage current transformer to the ground, changing the primary conductor loop of the large current transformer, etc.
The power transformer is installed on the power grid and measures the current, voltage and power under the environmental conditions of the power grid operation.
The operating conditions have a considerable impact on their actual errors.
If the operating conditions do not meet the reference conditions in Table 1, the error of the transformer cannot be guaranteed.
In this case, the error of the power transformer during verification is different from the error during use.
Therefore, how to standardize the error limit of the power transformer under operating conditions becomes a problem that needs to be considered and solved in this regulation.
The error obtained during verification is the controllable part of the transformer error component, that is, the reference part.
This part of the error is defined as the basic error.
The error of the power transformer when it exceeds the reference condition is generally impossible to be reflected in the verification result, so the actual operation may cause an electric energy metering error that exceeds the control range.
For example, the operation of the switch in the power line will cause the current transformer core to produce residual magnetism; the transformer is installed near the high current bus, and the environmental magnetic field will induce voltage and current in the winding and change the magnetic field distribution in the core.
In severe cases, the core will be partially saturated; the electric field generated by charged and uncharged objects near the test piece will affect the transformer error; when verifying the high-voltage current transformer, a low-voltage measurement line is used, and the capacitive current between the high and low voltage windings under operating conditions is not considered; the error of the capacitive voltage transformer is affected by the power supply frequency, but during the test, it can only be tested at the grid frequency at that time, etc.
This regulation adopts the method of controlling operation variation when dealing with this problem, that is, first determine the influence of environmental conditions and operating conditions on transformer errors through experiments, and limit their error range.
This treatment can effectively control the errors in the use of power transformers.
According to many years of experience in field inspection of transformer errors, if the operation variation is controlled according to 1/2 of the basic error, most power transformers can pass the field inspection.
Operation variation is not suitable for synthesis and can only be assessed according to a single impact quantity.
Some operation variation items can be tested under laboratory conditions, but some are related to the installation environment, such as the proximity effect of capacitive voltage transformers.
Only the deviation between the laboratory verification results and the field verification results can be used as the actual value of the operation variation.
