Understanding the Working Principle of Online Transformer Oil Filtration

In power systems, the stable operation of transformers is crucial, and the purity of insulating oil directly impacts transformer performance and lifespan. Online transformer oil filtration, as an advanced maintenance method, continuously purifies insulating oil during transformer operation, effectively extending its service life and improving system reliability. This article details the working principles of online transformer oil filtration to help users better understand this critical technology.

What is Online Transformer Oil Filtration

Online transformer oil filtration is a process that removes moisture, gaseous, and solid impurities from transformer oil without interrupting transformer operation. Unlike traditional filtration methods, this system connects directly to the transformer’s oil circuit, continuously purifying the oil while the equipment is energized. This method is particularly well-suited for: Utilities managing large substations; Industrial plants that rely on uninterruptible power supply; and Critical facilities where downtime is unacceptable.

Working Principle of Online Transformer Oil Filtration

The principle of Online Transformer Oil Filtration is based on a closed-loop circulation process that continuously draws contaminated oil from the transformer, purifies it through multiple treatment stages, and then returns the clean oil back into the transformer.

Oil Extraction and Circulation

In Online Energized Transformer Oil Filtration, an oil pump draws transformer oil from the main tank or conservator, pipes it through a filtration system for treatment, and then returns it to the transformer, forming a continuous, closed-loop process. This process gradually removes impurities and contaminants from the oil without stopping the system, ensuring consistently high oil quality and preventing sudden performance degradation. For example, in a large 220kV transformer, the system can circulate 1,000–3,000 liters of oil per hour, achieving stable and efficient purification results.

Solid Particle Removal

In the solid impurity removal process of Online Transformer Oil Filtration, the oil first passes through a coarse filter to intercept large particles such as dust, paint flakes, and metal particles. It then passes through a 1-5 micron precision filter element to remove fine impurities such as carbon residue. This effectively prevents conductive particles from causing partial discharge and slows the oil’s oxidation process, ultimately restoring the oil’s clarity and chemical stability. According to the IEC 60422 standard, particulate contamination is one of the main reasons for the decrease in breakdown voltage of aging transformer oil. Therefore, this filtration step is particularly critical for improving insulation performance.

Moisture Removal

During the dehydration process of Online Transformer Oil Filtration, even just 50 ppm of water can cause the breakdown voltage of transformer oil to drop by more than 30%. Therefore, vacuum dehydration (heating under low pressure to evaporate the water and condense it out) or adsorption (using molecular sieves or silica gel to directly absorb the water) must be used to reduce the moisture content in the oil to below 10 ppm, significantly restoring its insulation properties. According to the IEEE C57.106 standard, the water content of oil in critical equipment should be controlled below 20 ppm to ensure safe operation.

Gas Removal

During the degassing process of online transformer oil filtration, gases such as oxygen, nitrogen, hydrogen, and carbon dioxide dissolved in the transformer oil can form bubbles under high temperature or load, creating insulation weaknesses and posing a risk of breakdown. Therefore, a vacuum degassing chamber is required to reduce the oil pressure, allowing the dissolved gases to be released and safely discharged or collected for analysis. Dissolved gas analysis (DGA) is also a standard diagnostic method recommended by IEEE and IEC, providing an important basis for assessing transformer health.

Dielectric Property Restoration

After Energized Transformer Oil Filtration removes particles, moisture, and gases, transformer oil significantly recovers its performance: breakdown voltage is typically increased from less than 30 kV in its aged state to 60–70 kV, maintaining dielectric strength. Thermal conductivity is also improved, facilitating heat dissipation from the windings. Chemical stability is enhanced, slowing the formation of acid and sludge. This results in safer and more efficient transformer operation, extending the insulation system life by 5–10 years and reducing overall maintenance and replacement costs.

Key Components of an Online Filtration System

An Online Transformer Oil Filtration system is composed of several critical modules, each designed to address a specific type of contamination or ensure stable operation. These components work together in a closed-loop process to continuously restore the oil’s dielectric strength and chemical stability while the transformer remains energized.

Component	Function
Vacuum System	Creates low-pressure conditions to efficiently remove dissolved moisture and gases.
Heating Unit	Preheats oil to lower viscosity, enhancing dehydration and degassing efficiency.
Cooling Section	Stabilizes oil temperature before the purified oil re-enters the transformer.
Filter Assemblies	Multi-stage filters that capture solid particles and sludge of varying sizes.
Control & Monitoring Panel	Continuously tracks vital parameters such as oil temperature, moisture content, and breakdown voltage.

Conclusion

Online transformer oil filtration is more than just a maintenance technique; it’s a long-term investment in power reliability and safety. By understanding how it works, plant operators and engineers can make informed decisions that reduce downtime, lower costs, and ensure transformers maintain performance.

For companies looking to enhance their transformer maintenance programs, adopting an online transformer oil filtration system is a reliable step toward greater efficiency and reliability.