Analysis of the Process of Transformer Oil of Specialty Transformers Being Contaminated by Microorganisms and the Treatment Measures

1 Overview

Cases of transformer oil contaminated by microorganisms have been reported both domestically and internationally. Individual transformer manufacturers, user units, and research institutions have conducted specialized studies, but all focused on power transformers. This paper targets specialty transformers for specific applications (e.g., rectifier transformers for graphitization furnaces, submerged arc furnace transformers), exploring the process of microbial contamination of their transformer oil and the subsequent treatment measures and methods.

2 Process of Microbial Contamination of Specialty Transformer Oil

Based on literature review and the author’s experience, the process of microbial contamination of specialty transformer oil is similar to that of power transformer oil. Three basic conditions must be met: an effective invasion pathway, a viable environment for microbial survival, and sufficient reproduction time. Potential contamination routes include:

• Storing transformer oil in unclean tanks contaminated by microorganisms;
• Mixing new and qualified transformer oil with already contaminated oil;
• Inadequate tank sealing, which exposes the oil to air and allows the ingress of microorganisms and moisture;
• Failure of the transformer’s breather or rupture of the conservator’s bladder/diaphragm during operation;
• Contact with contaminated tools/PPE during final assembly, or using contaminated oil hoses during filling.

3 Characteristics of Transformers After Microbial Oil Contamination

Internal and external factors are interconnected. A transformer may have microbial - contaminated oil if exhibiting:

Low insulation resistance of the core and windings to ground, even below the conversion standards defined in Code for Acceptance Test of Electrical Equipment Installation Engineering (GB50150 - 2006) (see table below);4 Treatment of Transformer Oil Contaminated by Microorganisms

Analyzing the above phenomena shows microorganisms have staining, filterability, and certain heat resistance. Due to the complex cooling systems of specialty transformers, microbial contamination of their oil is harder to address than in storage tanks or power transformers, being a complex systems engineering task. Conventional vacuum oil purification fails to remove microbes in specialty transformer oil; treating only the oil can’t fully eliminate contamination. Thus, we must address not just the oil and transformer itself (core, tank) but also the cooling system (equipment, pipelines), using special measures beyond vacuum purification.

In general, treating microbe - contaminated specialty transformer oil involves three main steps:

• Return the transformer (with cooling system) to the original manufacturer for processing.
• Most work is done on - site. Per practice, users often use these steps: First, remove the tank, place the core in a temporary tank, and send it to a company with a vacuum drying furnace for vacuum drying (killing microbes and removing micro - water; protect with nitrogen during transport). Second, connect the tank, cooling system, heater (or vacuum purifier), and plate - frame purifier (with special adsorption plates) into a closed loop to purify the oil. If using a vacuum purifier as the heat source, control oil temp at 60±5C°; with a heater, at 70±5C°; temp can be moderately increased (refer to microbe death temp - time table) if equipment heat tolerance and oil aging allow. Always test the oil first, control moisture to ~20ppm, and act on results. Third, reinstall the dried core, remove purification system components, and use a vacuum purifier to degas the oil (including cooling system oil) after final assembly.