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Release time： 2021-04-22

# Measurement and Analysis of Transformer Ratio

Changhui Electric Vocational and Technical College Jie Huiping (ie voltage ratio) is the ratio of the voltage of the high voltage winding and the voltage of the low voltage winding under the no-load condition of the transformer. For a three-phase transformer, the ratio on the nameplate usually refers to the ratio of the rated line voltage U1N of the high voltage winding to the rated line voltage U2N of the low voltage winding. When it is necessary to use the transformation ratio to calculate the transformer equivalent circuit, since the equivalent circuit reflects the parameters of the transformer one-phase winding, the ratio of the phase voltage should be used to calculate, which is proportional to the number of turns of each winding. It can better reflect the winding condition of each phase.

According to the working principle analysis of the transformer, the ratio is the ratio of the number of turns of the high voltage winding of each phase of the transformer to the number of turns of the low voltage winding: n=k/u/u2=N/the meaning of the test of the transformer. The measurement of the transformer ratio is an example of the transformer. The line test project is one of the handover test items before the transformer is shipped from the factory and after installation and overhaul. It is also an important inspection item after the transformer has a winding fault. Through the measurement of the ratio, the following aspects can be checked: whether the voltage ratio of the transformer at each tap position matches the standard value of the nameplate or meets the technical requirements to ensure that the voltage conversion of the transformer can meet the requirements of the voltage regulation operation; Whether the actual position and the indicated position of the tap changer are consistent, whether the operating mechanism of the tap changer is correctly installed and working properly, whether the contacts of the tap changer have poor contact, and whether the lead connection of the tap changer is correct; Whether the turns ratio is correct, whether the three-phase transformer ratio imbalance of the three-phase transformer meets the requirements, whether the winding lead assembly and the connection between the winding segments are correct, and whether the winding has a turn-to-turn short circuit or open circuit after the transformer fails; The accuracy of the ratio is used to determine whether the transformer can run in parallel with other transformers. (If the transformers with different ratios are operated in parallel, a circulation will occur between the transformers, resulting in increased transformer losses, reduced operating efficiency, and insufficient capacity utilization.) .

2 Test method The transformer ratio measurement should be carried out at all tapping positions of each phase. For the on-load tap-changer, the electric tapping should be used to switch the tap to make the measured voltage ratio and voltage regulation during transformer operation. The resulting voltage ratio is consistent. For a three-winding transformer, when one of the windings is equipped with a voltage regulating tap changer, it is only necessary to detect the ratio between the winding with the tap changer side and the other winding at each tap position, and the third winding The ratio of the transformer is only measured at the rated tap position.

At present, commonly used measurement methods are dual voltmeter method and variable ratio bridge method.

When the double voltmeter method is measured by the double voltmeter method, the power supply voltage is generally applied to the primary side of the transformer. The stability of the test power supply should be maintained during the whole test process. The voltmeter with an accuracy of not less than 0.5 is used on both sides of the transformer. Or the accuracy is not lower than the 0.2-level voltage transformer to measure the voltage calculation ratio, and the two voltmeters should be read at the same time, especially when the voltage fluctuation is large, especially pay attention to this point. In order to ensure the accuracy of the measurement, it is preferable to test the power supply voltage to be more than 1/3 of the rated voltage of the pressurized side. The test equipment used in this method of measuring the ratio is a commonly used measuring instrument and is relatively easy to obtain.

For single-phase transformers, the ratio can be directly obtained from the test results. When measuring a three-phase transformer, you can use a single-phase power supply or a three-phase power supply as a test power supply. If a three-phase power supply is used, a symmetrical three-phase test power supply voltage should be applied to the same polarity end of the corresponding winding, and the three-phase power supply voltage used for measurement must be symmetric and stable for three phases, and the three-phase unbalanced The degree should not exceed 2%. If the three-phase power supply voltage is not completely symmetrical, the relationship between the line voltage and the phase voltage will change. When the conversion is required, the relationship between the two is no longer 姨r, so that the ratio is calculated. Errors appear, affecting the analysis and judgment of test results. The ratio of the calculated conversion ratio measured by the three-phase power supply, that is, the line voltage of the transformer.

When using a single-phase power supply to measure the ratio of a three-phase transformer, there are many measurement steps, but the measured value can directly reflect the winding turns ratio of the phase, thereby finding the phase difference in which the winding defect exists, and does not need to consider whether the measured value is received by the power supply. The effect of three-phase voltage asymmetry. When using a single-phase power supply to measure the ratio of three-phase transformers, the non-test phase windings must be short-circuited in the delta-connected windings so that no excitation current flows through the non-measured phase windings. The ratio reflects only the case of the pressurized phase. Take the transformer with Y, y (same as Y, y) as an example, and the wiring of the voltmeter is as shown.

When using a single-phase power supply to measure the three-phase transformer ratio, and the pressure terminal is UV, the transformer ratio calculation method of different wiring groups is shown in Table 1.

When wiring the test circuit in the test, make sure that the connection of the voltmeter is reliable, and the test line is as short as possible to avoid measurement errors caused by the lead being too long.

Table 1 Transformer ratio calculation method for different wiring groups Transformer wiring group Y, y (D, d) Y, d11D, y11YN, d11 measurement ratio (Ki) phase voltage ratio (K line voltage ratio (K) Ki=Uuv/Uuv

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