How to analyze transient process of transformer

As the key power conversion equipment in the power system, the transformer will not only experience the steady-state working state, but also be affected by various external disturbances and internal changes, resulting in a transient process. These transient phenomena have an important influence on the safe operation of transformers and the stability of power systems. Therefore, it is of great significance to deeply analyze the transient process of transformer for improving the reliability of equipment operation and optimizing the protection strategy.

A, the type of transformer transient process

Common transient processes of transformers mainly include the following:

1. Closing transient (inrush current): When no-load transformer is closed, a large transient excitation current, called inrush current, will be generated because the magnetic flux of the iron core cannot change suddenly. The current may reach 5~8 times of the rated current, and the duration may reach tens of milliseconds.

2. Short-circuit transient: When the secondary side of the transformer is short-circuited, the primary side current increases rapidly, resulting in short-circuit current. Short-circuit current contains periodic component and aperiodic component, and aperiodic component will cause magnetic flux saturation, which will affect the operating characteristics of protection device.

3. Sudden load transient: Sudden load changes will lead to sudden changes in the output voltage and current of the transformer, which will further affect the stability of the system.

4. Transient response caused by lightning strike or operating overvoltage: This kind of transient usually occurs on the high voltage side, which may lead to serious consequences such as insulation breakdown.

Second, the analysis method

The analysis of transformer transient process mainly includes theoretical modeling and simulation analysis.

1. Mathematical modeling

Establishing dynamic mathematical model of transformer is the basis of analyzing transient process. Commonly used models include:

-Basic voltage-current equation: Based on the law of electromagnetic induction, the voltage equations of primary and secondary windings of transformer are derived.
-Nonlinear model considering iron core saturation: The magnetization curve of iron core has nonlinear characteristics, especially in the process of inrush current and short circuit.
-Distributed parameter model: used to analyze the response of high-frequency transient processes such as lightning impulse.

2. Application of simulation tools

With the help of modern simulation software (such as EMTP-RV, PSCAD, MATLAB/Simulink), various transient conditions can be simulated more intuitively. By setting different initial conditions and disturbance sources, the response behavior of transformer under different conditions can be studied.

3. Experimental verification

In practice, the transient data obtained by laboratory simulation or field test are compared with the simulation results, which is helpful to verify the accuracy of the model and provide a basis for protection setting.

Third, the impact of transient process and countermeasures

1. Impact on relay protection device: The inrush current is easily misjudged as fault current, resulting in misoperation of protection. Therefore, harmonic braking, waveform symmetry identification and other methods are needed to identify it.

2. Impact on equipment insulation: Overvoltage transient may cause insulation damage, so lightning arrester and overvoltage protection measures need to be configured.

3. Impact on power supply quality: Transient process may cause voltage sag, harmonic distortion and other problems, affecting the normal operation of sensitive equipment.

4. Design optimization suggestion: In the transformer design stage, the thermal effect and mechanical stress caused by transient process should be fully considered to improve the anti-disturbance ability of equipment.

IV. Conclusion

The transient process of transformer is an important research content of power system stability and safe operation of equipment. Through in-depth analysis of transient mechanism, combined with mathematical modeling and simulation technology, transient effects can be effectively predicted and controlled, thus improving the reliability and security of power system. In the future, with the development of smart grid, the research of transformer transient process will be more refined and intelligent.