How to deal with DC bias of transformer

As an important electrical equipment in power system, the running state of transformer directly affects the security and stability of power grid. In actual operation, transformers are often affected by DC bias, which leads to problems such as core saturation, increased loss, intensified vibration and increased noise. Therefore, it is of great significance to study the causes and countermeasures of transformer DC bias for ensuring the stable operation of power system.

First, the causes of DC bias

The DC bias means that the DC component appears in the winding of the transformer, which makes the magnetic flux in the iron core no longer symmetrical to the time axis, which leads to the saturation of the iron core in one and a half weeks, thus causing a series of adverse consequences. The main sources of DC bias include:

1. Influence of Geomagnetic Induced Current (GIC): Geomagnetic storm caused by solar storm will induce Geomagnetic Induced Current (GIC) on long-distance transmission lines, and these currents will flow into the earth through the neutral point of transformer, resulting in DC magnetic bias.
2. Asymmetric operation of HVDC transmission system: Under the operation mode of unipolar earth circuit, the earth current will flow into the system through the transformer neutral point of AC system, resulting in DC magnetic bias.
3. Influence of power electronic devices: Some power electronic devices, such as rectifiers and frequency converters, may introduce DC components during operation.
4. Improper neutral grounding: If the neutral grounding of the transformer is unreasonable, DC current may also be introduced.

Second, the harm of DC magnetic bias

DC magnetic bias will cause the working point of transformer core to shift and enter the nonlinear region or even the saturation region. The specific hazards include:

-The excitation current increases sharply after the core is saturated, which increases the copper loss and causes local overheating;
-Flux waveform distortion, resulting in voltage waveform distortion, affecting power quality;
-Causing mechanical vibration and noise increase, affecting the service life of the equipment;
-The protection device may be misoperated, threatening the security of the system.

Third, the treatment measures of DC magnetic bias

Aiming at the problem of DC magnetic bias, the following treatment methods can be adopted:

1. Installation of neutral point isolation device: This is the most widely used method at present. By installing a capacitor or a solid-state DC isolation device at the neutral point of the transformer, the path of DC current can be effectively blocked and the DC component can be prevented from entering the transformer.
2. Optimize the grounding mode: reasonably select the grounding position and mode of transformer neutral point to avoid the formation of DC loop.
3. Adopt symmetrical operation mode: avoid unipolar earth loop operation as far as possible in HVDC transmission system, and give priority to metal loop operation mode to reduce earth current.
4. Adopt transformer design with strong DC bias resistance: for example, increase the cross-sectional area of iron core and adopt new materials such as amorphous alloy materials to improve the saturation resistance of transformer.
5. Real-time monitoring and early warning: establish an online monitoring system for transformer DC bias, grasp the change of DC current in real time, and take timely countermeasures.

IV. Conclusion

With the continuous expansion of power system scale and the wide application of DC transmission technology, the problem of DC magnetic bias faced by transformers is increasingly prominent. Comprehensive measures must be taken to prevent it from design, operation and maintenance. Through the combination of scientific management and technical means, we can effectively meet the challenge of DC bias tape and ensure the safe and stable operation of power system.

In a word, the DC magnetic bias of transformer is a complex but controllable problem. Only by deeply understanding its causes and taking practical and effective countermeasures can the transformer work safely and reliably in complex operating environment.