Transformer Oils

Transformer Oil Functions

Transformer oils perform at least four functions for the transformer. Oil provides insulation, provides cooling, and helps extinguish arcs. Oil also dissolves gases generated by oil degradation, moisture and gas from cellulose insulation, deterioration, and gases and moisture from exposure to atmospheric conditions. Close observation of dissolved gases in the oil, and other oil properties, provides the most valuable information about transformer health. Looking for trends by comparing information provided in several DGAs and understanding its meaning is the most important transformer diagnostic tool.

Dissolved Gas Analysis

After 1 month of service and once each year, and more often if a problem is encountered, complete a DGA. This is, by far, the most important tool for determining the health of a transformer. A DGA is the first indicator of a problem and can identify deteriorating insulation and oil, overheating, hot spots, partial discharge, and arcing. The Ahealth@ of the oil is reflective of the health of the transformer itself. A dissolved gas analysis consists of sending transformer oil samples to a commercial laboratory for testing. The most important indicators are the individual and total combustible gas (TCG) generation rates based on the International Electrotechnical Commission (IEC) 60599 and IEEE C 57-104™ standards.

CAUTION: DGA is unreliable if the transformer is de-energized and has cooled, if the transformer is new, or if it has had less than 1 to 2 weeks of continuous service after oil processing.

This section provides guidance in interpreting DGA and suggests actions based on the analysis. There are no “quick and sure” easy answers when dealing with transformers. Transformers are very complex, very expensive, and very important to Reclamation; and each one is different. Decisions must be based on experienced judgment founded on all available data and consultation with experienced people. Periodic DGA and proper interpretations, along with thorough periodic inspections (covered earlier), are the most important keys to transformer life. Each DGA must be compared to prior DGAs to recognize trends and establish rates of gas generation. Although examples will be presented later, there are no universally accepted means for interpreting DGA data.
Transformers are very complex. Aging, chemical actions and reactions, electric fields, magnetic fields, thermal contraction and expansion, load variations, gravity, and other forces all interact inside the tank. Externally, through-faults, voltage surges, wide ambient temperature changes, and other forces (i.e., the earth’s magnetic field and gravity) affect the transformer. There are few, if any, “cut and dried” DGA interpretations; even experts disagree. Consultation with others, experience, study, comparing earlier DGAs, keeping accurate records of a transformer’s history, and noting information found when a transformer is disassembled will increase expertise and provide life extension to this critical equipment.
Keeping accurate records of each individual transformer is paramount. If a prior through-fault, overload, cooling problem, or nearby lightning strike has occurred, this information is extremely valuable to determine what is going on inside the transformer. Baseline transformer test information should be established when the transformer is new, or as soon as possible thereafter. This must include DGA, Doble, and other test results.
Table on Picture 1 represents a summary of DGA transformer analysis. Table on Picture 2 is an actual example of a Reclamation transformer.

Picture 1: Transformer DGA Condition Summary Table

Picture 2: 345-kV Transformer Example


CAUTION: Transformer engineering expertise is needed in the use of this table. The action required may be as simple as sending an additional oil sample to the lab or as complex as conducting extensive testing on the transformer. It may also include repairs, internal inspection, and/or complete replacement of the transformer. The condition ranges shown represent a composite of IEEE C57-104, IEC 60599, Delta X Research’s Transformer Oil Analysis, and many years of transformer experience.