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On the Principle of Transformer
Transformer is a critical device used for measuring current, voltage, power, and energy. Its accuracy plays a vital role in ensuring the reliability of measurement data. To guarantee precise and trustworthy results, it must be calibrated before being put into use. A specialized instrument used to test current and voltage transformers is called a transformer calibrator. In China, there are various types of these calibrators available, but regardless of whether they operate on the difference method or the current comparator balance principle, improper usage can significantly affect the measurement outcomes. Therefore, attention must be paid to several key aspects during the calibration process.
First, **choosing the right test environment** is essential. The ambient temperature should be around 10°C to 35°C, with humidity below 80%. Electromagnetic interference from the surroundings should not exceed 1/20 of the allowable error of the tested transformer. Additionally, errors caused by the riser, voltage regulator, and high-current cables should not exceed 1/10 of the allowable error. To minimize such errors, equipment should be arranged carefully in the lab, and large current conductors should be placed at least 3 meters away from the calibrator. Using thick cables also helps reduce measurement errors.
Second, **correct wiring is crucial**. Most calibrators use the differential measurement method, so the polarity of connections must be strictly observed. Incorrect wiring may cause the calibrator to measure the sum instead of the difference, potentially damaging the device. For current transformers, ensure that the L1 terminal of the primary and K1 terminal of the secondary are grounded. For voltage transformers, the X and x terminals are low potential, while A and a are high potential. Miswiring can lead to leakage errors.
Third, **grounding issues must be handled properly**. The calibrator’s circuit should remain at low potential to minimize ground leakage. However, for current transformers, the K1 terminal should not be grounded during testing. Grounding points should be selected based on the circuit configuration, and reliable grounding is usually achieved through the panel’s grounding terminal.
Fourth, **load matching is important**. Transformer errors are highly sensitive to load impedance. Mismatched loads can lead to incorrect readings. It is essential to match the standard and tested transformers’ loads to their rated values. Before testing, check the internal load and select appropriate wires and connections.
Fifth, **choose the correct range on the calibrator**. With multiple functions, selecting the right range prevents human errors and reduces measurement inaccuracies.
Sixth, **perform a visual inspection**. This includes checking the nameplate, terminal buttons, and polarity markings. For multi-ratio transformers, verify the connection methods for different ratios.
Seventh, **measure insulation resistance** using a megohmmeter between windings and ground.
Eighth, **conduct a power frequency withstand test**, including both power frequency and induced voltage tests, following strict procedures.
Ninth, **check polarity**. Incorrect polarity can damage the instrument. Use the calibrator’s built-in polarity test function to confirm correctness.
Tenth, **demagnetize the core**. Different materials require different demagnetization methods. Iron-nickel alloy cores need closed-circuit demagnetization, while silicon steel cores can use either closed or open circuits.
Eleventh, **ensure sufficient sensitivity** in the measuring circuit. Gradually increase galvanometer sensitivity during testing to avoid damage.
Twelfth, **accurately measure errors**. Select the appropriate calibrator and equipment based on the transformer’s accuracy class and follow proper wiring and adjustment procedures.
Thirteenth, **never open the secondary winding of a current transformer**. An open circuit can generate dangerously high voltages, risking both equipment and personnel.
Fourteenth, **schedule periodic verification and rotation**. According to DL448-91, high-voltage transformers should be verified every 10 years, and low-voltage current transformers every 20 years. Regular checks ensure continued accuracy and safety.