Changsha Electric Power Vocational and Technical College Jie Huiping (voltage ratio) is the ratio of the high-voltage winding voltage to the low-voltage winding voltage under the no-load condition of the transformer. For three-phase transformers, the transformation 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 equivalent circuit of a transformer, since the equivalent circuit reflects the parameters of one phase winding of the transformer, the ratio of the phase voltage should be used to calculate it, so that it is proportional to the number of turns of each winding. It can better reflect the winding conditions of each phase.

According to the analysis of the working principle of the transformer, it can be seen that the ratio is the ratio of the number of high-voltage winding turns to the low-voltage winding turns n2 of each phase of the transformer: k=u/u2=N/ team 1 The significance of the test The measurement of the transformer turns ratio is an example of a transformer The test project is one of the handover test projects before the transformer leaves the factory and after installation and overhaul. It is also an important inspection project after the transformer winding failure occurs. Through the measurement of the transformation ratio, the following aspects can be checked: whether the voltage ratio of the transformer at each tap position is consistent with the nameplate standard value or meets the technical requirements to ensure that the voltage transformation of the transformer can meet the voltage regulation operation requirements; each voltage tap of the transformer Whether the actual position matches the indicated position of the tap changer, whether the operating mechanism of the tap changer is installed correctly and works normally, whether there is poor contact among the contacts of the tap changer, and whether the leads of the tap changer are connected correctly; whether the transformer winding Whether the group turns ratio is correct, whether the three-phase transformation ratio unbalance of the three-phase transformer meets the requirements, whether the lead assembly of the winding and the connection between winding segments are correct, whether there is an inter-turn short circuit or open circuit in the winding after a transformer failure; Transformation ratio The accuracy of the transformer can be used to determine whether the transformer can operate in parallel with other transformers (if transformers with different transformation ratios are operated in parallel, circulating current will occur between the transformers, resulting in increased transformer losses, reduced operating efficiency, and insufficient capacity utilization).

2 Test method The measurement of the transformer ratio should be carried out at all tap positions of each phase. For on-load voltage regulating transformers , electric voltage regulation should be used to switch the taps so that the measured voltage ratio is consistent with the voltage regulation during operation of the transformer. The obtained voltage ratios are 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 transformation ratio between the winding with the tap changer side and the other winding under each tap position, and the ratio between the winding and the third winding. The transformation ratio only measures the transformation ratio when the transformer is in the rated tap position.

Currently, commonly used measurement methods include the double voltmeter method and the variable ratio bridge method.

Double voltmeter method When measuring using 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 entire test process. Voltmeters with an accuracy of not less than 0.5 are used on both sides of the transformer. Or the voltage should be measured with a voltage transformer with an accuracy of not less than 0.2 to calculate the transformation ratio, and the two voltmeters should read at the same time, especially when the voltage fluctuations are large, special attention should be paid to this point. In order to ensure the accuracy of measurement, the test power supply voltage should be more than 1/3 higher than the rated voltage of the pressurized side . The test equipment used in this method of measuring the transformation ratio are commonly used measuring instruments and are relatively easy to obtain.

For single-phase transformers, the transformation ratio can be obtained directly 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 the 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. It is also required that the three-phase power supply voltage used during measurement must be symmetrical and stable among the three phases, and the unbalance of the three phases must be maintained. The degree should not exceed 2%. If the three-phase power supply voltage is not completely symmetrical, it will cause the relationship between line voltage and phase voltage to change. When conversion is required, the relationship between the two will no longer be r , making the transformation ratio calculation Errors occur, affecting the analysis and judgment of test results. The transformation ratio measured and calculated using a three-phase power supply is the ratio of the line voltages of the transformer.

When using a single-phase power supply to measure the transformation 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 with winding defects, and there is no need to consider whether the measured value is affected by the power supply. Effect of three-phase voltage asymmetry. When using a single-phase power supply to measure the transformation ratio of a three-phase transformer in separate phases, in the delta-connected windings, the non-test phase windings must be short-circuited, so that no excitation current flows in the non-measured phase windings, and the measured The ratio reflects only the pressurized phase. Taking the transformer with Y , y (same as Y , y ) connection as an example, the wiring of the voltmeter is as shown.

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

When wiring the test loop during the test, ensure that the voltmeter is connected reliably and the test wires are as short as possible to avoid measurement errors caused by too long leads.

Table 1 Calculation method of transformer transformation ratio for different wiring groups Transformer wiring group Y , y ( D , d ) Y , d11D , y11YN , d11 Measured transformation ratio ( Ki ) Phase voltage transformation ratio ( K line voltage transformation ratio ( K ) Ki=Uuv/Uuv