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| Technical problems easy to appear in transformer test |
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| Transformer measurement accuracy is high, measurement wiring is complex, coupled with the power frequency fundamental wave measurement, environmental interference is very large, therefore, often each equipment in the metrology Bureau verification qualified, after the combination of verification of the data is quite different. The accuracy of the transformer calibrator is not the most important indicator of the instrument. In the calibration regulation of the transformer, it is required that the test error of the whole circuit should not exceed 20% of the level of the tested transformer. In fact, it is required that the data displayed by the calibrator in actual work must be true and credible. 1. Frequency selective filter performance of transformer calibrator. Transformer calibration is the measurement of fundamental wave. Because the error current waveforms of the standard and the transformer are greatly distorted (up to 500%) and modulated by high frequency interference, the calibrator must have good performance of frequency selection filtering, separation of fundamental wave and measurement. The factors causing the distortion are very complicated. In the test of the transformer with low precision (below 0.5 level) without saturated iron core compensation, the distortion is about 10%, and the influence is not obvious. The national standard requires the harmonic attenuation of the calibrator to be more than 32 dB, which is enough to meet the requirement of application. But when high precision or saturated iron core transformer is tested, this index is too low. There is no measurement in the domestic calibration of the project, and the general manufacturer often does not give indicators. When purchasing new instruments, users should compare with 2700, HGE2 or HE15 old-fashioned instruments to determine whether they are credible. 2. The load of the calibrator and the matching with the standard transformer. The additional load brought by the calibrator to the tested transformer and the load brought by the calibrator to the standard transformer are strictly stipulated in the national regulations. The domestic metrological verification does not detect these indicators, and most manufacturers do not give them. But they are one of the main reasons for the different calibration data of different units. 3, conductor load: Load Z reserves 0.06 ohm resistance for connecting wires (individual 0.05 ohm), so the sum of resistance of A, B and C wires in the diagram is 0.06 ohm. When the current transformer is rated at a small rated load (10VA), the resistance of the wire will greatly affect the data. 4. Error of load point of voltage transformer The load line and the voltage sampling line of the calibrator should be separately drawn from the transformer terminal. If a common line is used, the test error will be introduced because of the voltage drop on the wire. 5, wiring error is not standardized: With the development of the testing technology, the instrument manufacturer has developed a new standard transformer. The new standard has many advantages, such as two-stage current transformer, which can overcome the load matching problem between the standard transformer and calibrator. However, the use unit is still connected in the 1950s, but there is a greater error. 6. Ground wire: Because it is power frequency measurement, space electromagnetic field and floating potential, which have great influence on the measurement, ground wire plays an important role in the measurement. The grounding wire must be correctly grounded according to the regulation, especially in the test above 0.05 level, or in the case of a high voltage. It is suggested that when purchasing instruments, users should choose a complete set of equipment factory with a long-term industry basis rather than a single unit factory. There are essential differences in the theory and experience of transformer testing between the two. Correct selection, the first can ensure that all the indicators of the instrument are attached to the requirements of the specification, the second wiring, instrument matching are through the planning and design of manufacturers, can avoid the use of errors, affect product quality. |
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