How to calibrate the transformer, the detection of current transformer and voltage transformer
The detection transformer is an important instrument for the detection and measurement of current transformer, voltage transformer, power and electric energy. Its accuracy directly affects the reliability of the detection data. In order to ensure accurate and reliable inspection and measurement, it is necessary to calibrate before putting it into use. The special instrument used to calibrate the current transformer and voltage transformer in the combined transformer is the transformer calibrator. At present, the types and types of transformer calibrators used in my country are complicated, but whether it is the difference method principle or the current comparator balance principle, its correct use or not affects the detection effect to varying degrees. Therefore, in the verification process of the combined transformer, we must pay attention to the following issues.
1 Selection of the verification environment SF6 gas qualitative leak detector, smaller size, higher sensitivity, and more convenient to use. The sensor head has improved sensitivity, reliability and service life compared with other products; streamlined design, beautiful and comfortable.
The environmental conditions for transformer calibration must meet the requirements of the calibration regulations, that is, the ambient temperature is +10~+35℃, and the relative humidity is not more than 80%. The detection error caused by the electromagnetic field around the workplace should not be greater than 1/20 of the allowable error of the transformer being tested. The detection error caused by the booster, voltage regulator, high-current cable, etc. used for calibration work should not be greater than 1/10 of the allowable error of the transformer being tested. For this reason, in the laboratory, the relevant detection and power supply equipment should be arranged reasonably, and even the current-carrying wires of high current should be arranged reasonably, otherwise, their calibration of the transformer will cause non-negligible detection errors. Generally speaking, the distance between the booster, high-current wire and the transformer tester should be at least greater than 3m. In order to reduce the detection error caused by the high-current cable, the cable with a larger cross-sectional area should be selected as much as possible.
2 Correctly select the wiring method
Most transformer testers are designed according to the difference detection method. Therefore, when connecting the transformer to be tested and the standard transformer to the transformer tester, it is necessary to ensure that the wiring is correct. Otherwise, the difference circuit may take the sum of the two currents (voltages) instead of the difference between the two currents (voltages). In this way, the tester may be burned out. The main reason for the burnout of some transformer tester circuit components is the incorrect wiring method and the mistaken addition of a larger current or a higher voltage. In the wiring, it is also necessary to consider the convex and concave potential ends of the transformer. For the current transformer, only when the L1 end in its primary circuit and the K1 end in the secondary circuit are close to the ground potential, the current injected from the L1 end and the current output from the K1 end are detected, which is the real fault of the transformer. For voltage transformers, its X and x terminals are at low potential, while A and a terminals are at high potential. During the test, the a terminal of the standard transformer is short-circuited with the a terminal of the transformer to be tested, and the secondary voltage difference is taken at the x terminals of the two transformers. If the current terminals are reversed, leakage faults may occur.
In summary, in the test of transformers, we should avoid swapping the L1 and K1 terminals of the current transformer with the L2 and K2 terminals; swapping the A and a terminals of the voltage transformer with the X and x terminals.
3 Handling of grounding problems during verification
When using a transformer tester for transformer testing, it is necessary to keep the circuit of the transformer tester in a low potential state to reduce its leakage to the ground. However, for current transformers, when using the difference comparison method for testing, the K1 terminal is not allowed to be grounded. Therefore, we need to reasonably select the grounding point according to the actual situation of the specific circuit during the test of the transformer. Generally, the effective grounding measures are: reliably ground the grounding terminal set on its panel.
4 Load matching
The error characteristics of current transformers and voltage transformers are very sensitive to load impedance (or admittance). In the process of verification, the load selection of the standard transformer is not matched, which may lead to misjudgment. Therefore, the standard transformer and the transformer to be tested should be load matched respectively, so that the actual load they bear in the verification circuit is equal to the rated load of the transformer. Since the verification circuit has formed part of the load, the internal load test of the verification circuit should be carried out. Combined with the parameters of the load box, select the appropriate wire, and only after accurate matching can it work. Before each verification, be careful to turn each terminal button to prevent loosening and disconnection.
5 Reasonable selection of the range switch of the calibrator
Since the transformer calibrator has many functions, when inspecting the transformer, it is necessary to correctly select the function switch and the appropriate range to avoid human accidents caused by misoperation and reduce the detection errors caused by the calibrator.
6 Appearance inspection
Appearance inspection is an intuitive inspection of the combined transformer by the inspectors. Although it is very simple, it is an indispensable and important link. The primary purpose of this link is to find problems on the surface and deal with them correctly. That is, first check the integrity of the nameplate symbols so as to provide correct parameters for inspection. Secondly, check the integrity of the terminal buttons and the polarity symbols. For multi-ratio transformers, the wiring methods of different ratios should also be checked.
7 Determination of insulation resistance
Use a megohmmeter to detect the insulation resistance between each winding and between the winding and the ground.
8 Power frequency withstand voltage test
Power frequency withstand voltage test, including power frequency withstand voltage test and induced voltage test. During the power frequency withstand voltage test, it is necessary to strictly abide by the relevant regulations.
9 Polarity check
Whether it is a current transformer or a voltage transformer, if the polarity is connected incorrectly, it is easy to burn the instrument. Therefore, before formally verifying the fault, the correctness of its polarity must be checked first. The inspection method can be comparison method or DC method. Generally, the tester has transformer performance test and flashing function. When the connection method is correct, the performance indicator is still found to be active, indicating that there is a problem with the internal performance of the tested transformer. At this time, the performance can be reversed and tested again. This process cannot be omitted for the verification of any transformer, otherwise it is easy to cause human accidents.
10 Demagnetization
The core of the current transformer generally has two materials, namely iron-nickel alloy and silicon steel sheet. For current transformers with different materials and different structural types, the demagnetization methods and requirements are different. For current transformers with iron-nickel alloy as the core, if the secondary open circuit demagnetization is used, the excitation current will often fail to open. Closed circuit demagnetization is used. For current transformers with silicon steel sheets as the core, closed circuit demagnetization and open circuit demagnetization can be used. For current transformers of level 0.2 and above, closed circuit demagnetization is appropriate.
11 Checking the sensitivity
When using a transformer tester for verification or testing, the detection circuit should be guaranteed to reach a satisfactory sensitivity. During the test, in order to protect the galvanometer from excessive impact, its sensitivity should be gradually increased for testing until the circuit sensitivity reaches the required calibration.
The above sensitivity is essentially different from the sensitivity of the commonly-mentioned instrument under test. What is being discussed here is not the sensitivity of the transformer under test, but the sensitivity of the detection circuit.
12 Fault measurement
When testing faults, the appropriate standard instrument and dispatching and testing equipment should be selected according to the accuracy level and specification requirements of the combined transformer under test, and the wiring must be correct. The rise and fall of current (voltage) must be carried out smoothly and slowly.
13 Preventing secondary open circuit of current transformer
For most current transformers, the number of turns of the secondary winding is large. Under the condition of operating with rated current, once the secondary open circuit occurs, a very high open circuit voltage will be generated in the secondary winding, endangering the safety of equipment and personnel. Therefore, when testing the current transformer, secondary open circuit must not occur.
14 Periodic verification and rotation
The high-voltage transformer in operation should be rotated regularly and laboratory verification should be carried out. The high-voltage transformer can be periodically verified by on-site inspection. According to the requirements of "DL448-91", the high-voltage transformer should be rotated or inspected on-site at least once every 10 years; the low-voltage current transformer should be verified or rotated at least once every 20 years.
