How Do You Reinhibit an Oil-Filled Transformer?

This article covers oxidation inhibitor management in oil-filled transformers. Here we will address what oxidation inhibitors are, how inhibitor concentration is tested and interpreted, and how the reinhibiting process is performed.

What Is an Oxidation Inhibitor?

Most mineral oil dielectric fluids are formulated with an added oxidation inhibitor — a chemical additive that functions as a preservative by interfering with the reaction between oxygen and the oil. By slowing oxidation, the inhibitor extends the service life of both the fluid and the solid insulation system within the transformer.

The two most widely used oxidation inhibitors in transformer oils are 2,6-ditertiarybutyl para-cresol (DBPC) and 2,6-ditertiary-butyl phenol (DBP). DBPC is also commercially known as butylated hydroxytoluene (BHT).

Role of Oil Preservation Systems

In transformers equipped with an oil preservation system designed to maintain dissolved oxygen below 1,000 ppm — such as an active nitrogen system (pressurized tank or generator) or a conservator tank with a rubber bladder diaphragm — inhibited oil is not strictly required. However, maintaining inhibitors in the oil provides an additional layer of protection against oxidation in the event the preservation system fails.

Testing Inhibitor Concentration

Inhibitor concentration is measured as the combined total of DBPC and/or DBP by weight percentage. SDMyers quantifies this using a Fourier Transform Infrared (FTIR) spectrometer, following ASTM method D2668.

Concentration Thresholds and Action Criteria

• Below 0.1% by weight: Considered unacceptable. At this level, the inhibitor concentration is insufficient to adequately protect the oil from oxidation. Reinhibiting is recommended, with the target of restoring concentration to 0.3% by weight.
• 0.3% by weight: The target post-reinhibiting concentration.

The Reinhibiting Process

Reinhibiting is performed by a qualified contractor with demonstrated experience in transformer oil processing. The procedure involves several sequential steps.

Oil Circulation Through Adsorbent Material

Once the contractor connects the oil processing equipment to the transformer, the oil is heated and circulated — referred to as passes — through an adsorbent material, typically Fuller’s Earth or activated alumina. These passes serve to remove depleted inhibitors, acid precursors such as peroxides, low-level organic acids, and other polar contaminants that have accumulated in the oil.

Removing these contaminants prior to reinhibiting is critical: their presence accelerates degradation of both the oil and the solid insulation, significantly shortening transformer service life.

Inhibitor Addition and Moisture Control

Following the cleaning passes, the contractor introduces a concentrated DBPC inhibitor solution into the oil return stream. Moisture content of the inhibitor-oil mixture must be continuously monitored and maintained below 10 ppm during this stage. The final inhibitor concentration in the transformer must reach 0.3% by weight.

When Additional Oil Processing Is Required

In cases where oil degradation has advanced beyond the threshold addressable by reinhibiting alone, hot oil cleaning must be performed first before inhibitor is added. The specific trigger conditions are:

• Acid number ≥ 0.06 mg KOH/g (questionable range): Indicates acid formation that requires adsorbent processing of both the oil and solid insulation before reinhibiting.
• Interfacial tension (IFT) ≤ 31.9 mN/m (questionable range): Indicates the presence of polar contaminants and oxidation by-products at a level that warrants hot oil cleaning prior to reinhibiting.

Hot oil cleaning — oil processing through adsorbent media — targets both the fluid and the insulating paper/pressboard, making it a prerequisite when either of these conditions is present.

Energized vs. De-Energized Processing

Reinhibiting and hot oil cleaning are typically conducted with the transformer de-energized. Both procedures can be performed on an energized transformer under appropriate conditions, but this requires a contractor with specific experience in energized oil processing.

Energized processing demands a thorough risk evaluation, proper equipment rated for live-unit work, and demonstrated expertise in safely executing the procedure without compromising the transformer or personnel safety. Selecting an unqualified contractor for energized work carries significant risk and should not be treated as a cost-saving shortcut.

Summary:

• Oxidation inhibitors (DBPC/DBP) slow oil aging by preventing oxygen from reacting with the dielectric fluid and solid insulation.
• FTIR testing per ASTM D2668 is used to determine total inhibitor concentration; levels below 0.1% by weight require corrective action.
• The reinhibiting target is 0.3% by weight; moisture content of the oil-inhibitor mixture must remain below 10 ppm during inhibitor addition.
• Pre-reinhibiting oil circulation through Fuller’s Earth or activated alumina removes depleted inhibitor, peroxides, acids, and polar contaminants that would otherwise accelerate insulation degradation.
• Hot oil cleaning is required before reinhibiting when the acid number reaches ≥ 0.06 mg KOH/g or IFT drops to ≤ 31.9 mN/m.
• Energized reinhibiting and hot oil cleaning are viable but high-risk — contractor selection must be based on verified experience with energized transformer processing.