Measuring transformers are important instruments for current, voltage, power, and electrical energy testing and metering. Their accuracy directly affects the reliability of measurement data. In order to ensure accurate and reliable testing and metering, verification must be performed prior to use. The special instrument used to test the current transformer and voltage transformer is a transformer calibrator. At present, the types and types of transformer calibrators used in China are numerous, but whether they are based on the principle of difference method or the principle of balance of current comparators, their correct use or not have affected the measurement results to varying degrees. Therefore, we must pay attention to the following issues in the verification process of transformers.
1 Selection of verification environment
The environmental conditions for the transformer verification must meet the requirements of the verification regulations, that is, the ambient temperature is 10 to +35°C and the relative humidity is not more than 80%. The measurement error caused by the electromagnetic field existing around the workplace should not be greater than 1/20 of the allowable error of the tested transformer. The measurement error caused by the riser, voltage regulator, and high-current cable used for verification work should not exceed 1/10 of the allowable error of the tested transformer. For this reason, in the laboratory, the relevant measurement and power supply equipment should be arranged rationally, and even the current-carrying conductors of large currents should be arranged rationally. Otherwise, their calibration of the transformers will produce measurement errors that cannot be ignored. In general, at least the distance between the current riser and the high-current conductor and the transformer calibrator should be greater than 3m. In order to reduce the measurement error caused by high-current cables, a cable with a large cross-sectional area should be selected as much as possible.
2 correct choice of wiring
The vast majority of transformer calibrators are designed according to the differential measurement method. Therefore, when connecting the tested transformer and the standard transformer to the transformer calibrator, the polarity of the wiring must be ensured. Otherwise, the difference circuit may take the sum of two currents (voltages) rather than the difference between the two currents (voltages). In this way, the calibrator may be burned out. Some circuit breakers of the transformer calibrator are burned out. The main reason is that the wiring is in the wrong way, but it is caused by a large amount of current or a higher voltage. In the wiring must also take into account the high and low potential side of the transformer, for the current transformer, only when the L1 end of the primary circuit and the K1 end of the secondary circuit are close to the ground potential, the measurement is injected from the L1 end Current and output current at terminal K1 is the true error of the transformer. For the voltage transformer, its X and x terminals are at low potential, and the A and A terminals are at high potential. In the verification, the a-side of the standard transformer is shorted to the a-terminal of the tested transformer. The x terminals of the two transformers take the secondary voltage difference. If the current is reversed, leakage errors may occur.
In summary, in the verification of transformers, we should avoid the alignment of the current transformers L1, K1 and L2 and K2; the voltage transformers A, a and X, and x must be reversed.
3 Grounding Problems During Verification
The transformer calibrator is used for transformer detection. The circuit of the calibrator must be kept at a low potential to reduce the leakage current to the ground. However, for the current transformer, the difference is in use. When the method is used to check the time, it is not allowed to ground the K1 terminal. Therefore, we need to select the grounding point reasonably according to the actual situation of the specific circuit during the verification process of the transformer. The usual effective grounding method is to reliably ground the grounding terminal on the panel.
4 load matching
The error characteristics of current transformers and voltage transformers are very sensitive to load impedance (or admittance). During the verification process, due to the mismatch in the load selection of the standard transformers, misjudgment may result. Therefore, it is necessary to load-match the standard transformer and the tested transformer separately so that the actual load assumed by the verification circuit is equal to the rated load of the transformer. Since the test circuit has formed part of the load, the test circuit should be tested for internal load. Combine the parameters of the load box, select the appropriate wire, and match it before working. Prior to each verification, be sure to crimp each terminal to prevent loosening and disconnection.
5 reasonable choice of calibrator range switch
Because the transformer calibrator has many functions, it is necessary to correctly select the function switch and correctly select the proper range to check the transformer. This can avoid human error caused by misoperation and reduce the measurement error caused by the calibrator.
6 appearance inspection
The appearance inspection is the surface visual inspection of the examined transformer by the verification personnel. Although very simple, it is an indispensable part. The main purpose of this link is to discover the problems that exist on the surface and handle it properly. That is, first check the integrity of the nameplate mark in order to provide the correct parameters for verification. Next check the integrity of the terminal buttons, as well as the polarity markings. For multi-variable ratio transformers, the connection methods of different ratios should also be checked.
7 Determination of insulation resistance
Measure the insulation resistance between each winding and the winding to ground with a megohmmeter.
8 power frequency withstand voltage test
Power frequency withstand voltage test, including power frequency withstand voltage test and induced voltage test. When frequency voltage withstand test is conducted, the relevant procedures must be strictly observed.
9 Polarity check
Whether it is a current transformer or a voltage transformer, if the polarity is incorrectly connected, it is easy to burn the instrument. Therefore, the correctness of the polarity should be checked before the formal verification error. The inspection method can be compared by the comparison method or the direct current method. In general, the calibrator has the transformer polarity test and display function. When the connection is correct, the polarity indicator is still active, indicating that there is a problem with the internal polarity of the tested transformer. At this point, reverse the polarity and try again. For any transformer test, this step can not be omitted, otherwise it will easily lead to human accidents.
10 Demagnetization
Current transformer core generally has two kinds of materials, namely iron-nickel alloy and silicon steel sheet. For different materials and different types of current transformers, their methods and requirements for demagnetization are different. For iron-nickel alloy core current transformers, if a secondary open circuit demagnetization is used, excitation current cannot often be generated. The phenomenon of good use of closed-circuit demagnetization. The current transformer using a silicon steel core as a core can be closed loop demagnetization or open circuit demagnetization. For current transformers of class 0.2 and above, closed-circuit demagnetization is preferred.
11 Sensitivity check
When verifying or measuring with a transformer calibrator, ensure that the measuring line has sufficient sensitivity. During the test, in order to protect the galvanometer from excessive impact, the sensitivity of the galvanometer should be gradually increased and tested until the line sensitivity reaches the required verification.
The above-mentioned sensitivity is essentially different from the sensitivity of the commonly used instrument to be inspected. As discussed here, it is not the sensitivity of the sensed transformer, but the sensitivity of the measuring circuit.
12 Error Determination
When measuring the error, according to the accuracy level of the transformer to be inspected and the requirements of the regulations, select the appropriate calibrator and adjustment and measurement equipment. The wiring must be correct. The rise and fall of the current (voltage) must be performed smoothly and slowly.
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