1. Relationship between transformer ratio and transformer winding

The induced potential in the primary winding and the secondary winding of the transformer is proportional to the number of turns of the winding.

The transformer winding itself has a resistance voltage drop and a magnetic flux leakage voltage drop. The primary winding input voltage U ignores the winding voltage drop, and the voltage in the winding is considered to be equal to the potential.

The ratio of the number of turns of the transformer U, the ratio K is called the transformer ratio coefficient.

2. Transformer ratio and transformation ratio

Transformer ratio K is related to the voltage U1 U2 winding turns N1 N2 current I1 I2 on both sides of the transformer. In the ideal case, the ratio K = U1 / U2 = N1 / N2 = I2 / I1.

Transformation ratio refers to the ratio of the primary voltage to the secondary voltage of the voltage transformer. The primary side voltage is higher than the upper secondary side voltage (single-phase), and the primary side line voltage is higher than the upper secondary side line voltage (three-phase). The change of transformer ratio is closely related to the change of transformer winding. The transformation ratio in ideal transformer is the ratio of turns N1 / N2 = U1 / U2. Transformation ratio is a concept of calculation error in transformer design. When the general transformation ratio is greater than 3, the error should be less than 0.5 %. When the transformation ratio is less than or equal to 3, the error should be less than 1 %.

3. The purpose of measuring the transformer ratio :

1 Check the correctness of the transformer winding turns ratio ;

2 check the status of the tap-changer ;

3 After the transformer fails, it is commonly used to measure the ratio to check whether there is inter-turn short circuit in the transformer.

4 To determine whether the transformer can run in parallel.

When the no-load voltage difference of the secondary side of the two transformers running in parallel is 1 % of the rated voltage, the circulating current in the two transformers will reach about 10 % of the rated current, which will increase the loss of the transformer and occupy the capacity of the transformer. Therefore, the difference in the ratio should be limited to a certain range. According to the relevant regulations, the transformer with a ratio of less than 3 is allowed to have a deviation of ± 1 %, and all other transformers are ± 0.5 %.

The transformer ratio test, also known as the transformer ratio measurement, is to apply a voltage to one side of the transformer winding, and then use the instrument or instrument to measure the voltage value of the other side winding, and through the calculation to determine whether the transformer meets the design requirements of the voltage transformation results.

The ratio test can check whether the number of turns of the winding is correct, whether there is a short circuit between the turns, whether the connection of the winding joint and the position of the tap changer are correct.

The variable ratio test generally adopts the automatic variable ratio speed measuring instrument. The double voltmeter method can be used for the new type of transformer. From : Home of Electrical Technology

The single-phase transformer should carry out the polarity test, and the three-phase transformer should carry out the wiring group test, which is to check whether the phase relationship of the high voltage side coil of the transformer meets the design requirements. The polarity and group test can be performed by DC method, phasor diagram method and phase table method.

4. Whether the ratio of three-phase transformer is the ratio of phase voltage or the ratio of line voltage ?

When the group is the same, the transformation ratio is the ratio.

ratio = phase voltage ratio, not line voltage.

If it is DY connection, it should be phase voltage.

5.The relationship between the transformer ratio and the quality of the transformer. Why measure transformer ratio?

The transformation ratio of the transformer is not directly related to the quality of the transformer, but it can prove whether the working condition of the transformer is normal.

1. The purpose of measuring the transformer ratio :

( 1 ) Check whether the variable ratio is consistent with the nameplate to ensure the correct transformation of the voltage ;

( 2 ) Check whether the position of the decomposition switch is correct ;

( 3 ) After the failure of the transformer, by measuring the ratio to check whether there is an inter-turn short circuit between the windings ;

( 4 ) Determine whether the transformer can operate in parallel.

2. Test methods :

The three-phase transformer ratio measurement is measured by three-phase power supply. Generally, the line voltage is 380 V three-phase power supply. When the three-phase power supply is used to measure the voltage ratio, the three-phase power supply voltage is required to be symmetrical, otherwise the measurement error will be generated.

3. Notes :

( 1 ) The accuracy of the instrument should not be less than 0.5, and the voltage transformer used for measurement should not be less than 0.5. The accuracy of Zhiwei electric ZWBB200 turn ratio tester can reach 0.1.

( 2 ) When measuring, the lead should be in good contact, and the lead of the voltmeter should not be too long ; Zhiwei electric ZWBB200 turn ratio tester also has anti-misconnection Settings.

(3) The measured voltage should not be lower than 1% of the rated voltage of the transformer being measured, and should be kept as stable as possible, and the voltage indication values on both high and low voltage sides should be read at the same time.

4. ( 1 ) Transformer ratio is unqualified, the most common fault is the tap lead shouted wrong ; the indication position of the tap changer does not correspond to the internal lead ;

( 2 ) After the fault, the transformer ratio will also change due to the inter-turn short circuit.