How to test the short-circuit impedance of transformer

Transformer is an important electrical equipment in power system, and its performance directly affects the safe and stable operation of power grid. Among the parameters of transformer, short-circuit impedance (also called short-circuit voltage percentage) is a key parameter, which not only reflects the voltage drop characteristics of transformer in short-circuit state, but also has an important influence on the short-circuit current level of the system, the stability of parallel operation and relay protection setting. Therefore, it is of great significance to accurately test the short-circuit impedance of transformer for equipment operation and system analysis.

First, the definition of short-circuit impedance

Short-circuit impedance of transformer refers to the percentage of applied voltage to rated voltage when one winding of transformer is short-circuited at rated frequency and rated tap position and voltage is applied at the other side, so that the current flowing through short-circuit winding reaches rated value. Usually expressed in percentage (%), that is:

$$ Z_{sc} (%) = frac{U_{sc}}{U_{n}} imes 100% $$

Among them, $ U_{sc} $ is the voltage applied during the short-circuit test, and $ U_n $ is the rated voltage.

Second, the purpose of short-circuit impedance test

1. Evaluate the short-circuit performance of the transformer: understand the voltage drop and current tolerance of the transformer in short-circuit state.

2. Provide basis for system short-circuit calculation: short-circuit impedance directly affects the short-circuit current, which is an important basis for selection and setting of protection equipment.

3. Judging transformer winding state: Abnormal short-circuit impedance may reflect winding deformation or mechanical damage.

4. Verify whether the transformers meet the requirements of parallel operation: When different transformers run in parallel, the short-circuit impedance should be as consistent as possible, otherwise it will lead to uneven load distribution.

Third, the test method of short-circuit impedance
　　 1. Laboratory short circuit test method (standard method)

This is the most commonly used standard test method, which is carried out under factory or laboratory conditions. The specific steps are as follows:

-Connection mode: select one side winding (usually low voltage side) of the transformer for short circuit connection, and the other side (high voltage side) is connected with an adjustable voltage source.

-Step-by-step boosting: slowly increase the high-voltage side voltage until the current passing through the winding reaches the rated current.

-Record data: at this time, measure the voltage $ U_{sc} $ and current $ I_n $ applied on the high voltage side, and calculate the percentage of short-circuit impedance.

-Note: The experiment time should not be too long to prevent the winding from overheating; When testing, disconnect other loads and ensure that the short circuit connection is firm.
　　 2. Field Portable Instrument Testing Method

Portable transformer tester (such as short-circuit impedance tester) can be used for measurement under substation or field conditions. Its principle is similar to the laboratory method, but through digital signal processing technology, data can be obtained more quickly and safely. Advantages include:

-There is no need to disconnect the transformer from the system (supported by some instruments);

-High degree of automation and simple operation;

-Data can be stored and exported for subsequent analysis and comparison.

# 3. Software simulation aided analysis

In the case that the field test cannot be carried out, the transformer model can also be established by electromagnetic simulation software, and the short-circuit impedance can be estimated by short-circuit simulation after inputting the rated parameters. Although there is a certain error between the simulation results and the actual measurement, it can be used in the preliminary design stage or auxiliary analysis.
　　Fourth, the analysis and judgment of test results

After the test is completed, the obtained short-circuit impedance value should be compared with the factory value or historical data:

-If the deviation is within 3%, it belongs to the normal range;

-If the deviation exceeds 5%, it is necessary to further check whether there is winding deformation, looseness or other mechanical failure;

-Especially after the transformer suffers short-circuit impact, if the short-circuit impedance changes obviously, it may indicate that the internal structure is damaged.

V. Conclusion

In a word, as an important parameter of transformer, the test of short-circuit impedance is not only helpful to evaluate the performance of the equipment itself, but also provides a key basis for the safe operation of power system. With the development of testing technology, the convenience and accuracy of on-site testing are constantly improved, which makes transformer condition evaluation more scientific and efficient. Therefore, power workers should master the correct testing methods and conduct regular testing to ensure the safe and stable operation of transformers.