(4) Waveform distortion.
The waveform distortion of the power supply also affects the verification results of the transformer in the following two aspects:
1) The standard instrument and the transformer under test have different frequency characteristics, which increases the harmonic content in the differential pressure and current signals.
However, the calibration instrument has limited ability to suppress harmonics, and excessive harmonics will affect the error indication of the calibration instrument.
When the power waveform distortion coefficient is controlled within 5%, the harmonics in the primary voltage can be offset to less than 1/10 of the original value in the comparison bridge under normal conditions, that is, the harmonic content becomes 0.5%.
JJG169-2010 "Verification Procedure for Transformer Calibrators" requires that the accuracy level 2 calibrator should have a harmonic suppression capability of not less than 26dB, which is equivalent to more than 20 times attenuation of high-order harmonics.
In this way, the 0.5% harmonic in the difference signal has only a 0.025% impact on the verification results, which can meet the verification needs.
2) Excessive harmonics will affect the error indication of the tester.
This situation mainly occurs when testing capacitive voltage transformers and using capacitive voltage dividers to test voltage transformers.
The voltage divider capacitor and the compensating reactor of the capacitive voltage transformer resonate at around 50Hz, which has a frequency-selective effect on the fundamental signal and has a relatively large attenuation on the frequency components below 50Hz and above 50Hz.
The bandwidth of the traditional electromagnetic voltage transformer reaches more than 2.5kHz, resulting in that most of the harmonic components in the differential pressure cannot be offset.
The capacitive voltage divider has a frequency band of more than 1MHz, which has a small phase shift on the harmonic components, and also causes a large number of harmonics in the differential pressure.
Therefore, limiting the harmonics in the test power supply, that is, limiting the waveform distortion, is necessary for the reliability and accuracy of the test results.
(5) External insulation contamination.
The influence of the degree of external insulation contamination on the error mainly occurs in capacitive voltage transformers with multi-section structures.
Under normal circumstances, the current flowing through the external insulation of the electromagnetic transformer will not enter the primary or secondary circuit of the transformer, so it will not have a substantial impact on the error.
However, if the capacitive voltage transformer has more than one coupling capacitor, the current flowing through the external insulation of the upper capacitor will flow into the lower capacitor and flow into the voltage divider circuit together with the capacitor current.
The external insulation leakage current is mainly resistive current, which will cause abnormal phase error of the capacitive voltage transformer and easily cause out-of-tolerance.
This situation is more obvious in areas with serious pollution. If the phase out-of-tolerance is found during measurement and the porcelain sleeve is obviously dirty, the outer surface of the porcelain sleeve can be cleaned to restore the normal phase error value.
(6) Electromagnetic environment.
The environmental electromagnetic field reference condition given in this regulation is that the electromagnetic field interference generated by the calibration equipment is not greater than the electromagnetic field interference generated by the normal working wiring.
Since the electromagnetic current and voltage transformers have low loop impedance, the electric field generated by the calibration equipment induces very small current and voltage in the loop.
Both theory and experiment have proved that the working electric field basically does not affect the error of the electromagnetic transformer.
When calibrating the voltage transformer, the loop current is small and the magnetic field interference is relatively small, so it can be roughly ignored.
When calibrating a current transformer, it is important to note that if there is a high-current busbar near the transformer, 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 influence of the electric field under working conditions on the capacitive voltage transformer is mainly the influence of the primary busbar of the substation.
The capacitive coupling between the high-voltage busbar and the coupling capacitor will affect the error of the capacitive voltage transformer.
The magnitude of the influence can be estimated based on the lead-to-ground capacitance.
