Transformer Analysis

Transformers are theoretically designed to last 300 years; the limitation to their life span is the deterioration of the insulation paper. Today majority of transformers are operating at a 25 % higher work load than recommended, this leads to gas production and faster deterioration of the insulation paper.

Changing role of transformer analysis in SA

• Condition monitoring (monitoring or checking of elements on a regular basis) is now essential for transformer owners.
• Movement away from doing just classic moisture, acid and dielectric (MADS) tests to more “specialist tests”
• Environmental legislation/Insurance purposes
• Transformer laboratories have become better equipped and the focus is now heavily on quality

Transformer test methods

• Furanic analysis
• Polychlorinated Biphenyl (PCB) analysis
• Dissolved gas analysis (DGA)
• Corrosive sulphur
• Moisture content
• Total acid number (TAN)
• Dielectric strength

Transformer tests
Furanics
Furancis are degradation products of the insulation paper found in transformers.
Analysing furanics is important in predicting the degradation of the insulating paper.

The following will destroy insulation paper

• Water (moisture)
• Oxygen
• Heat
• Oil oxidation

Polychlorinated biphenyl (PCB) analysis
PCB’s where realised in the 1970’s during the oil crises to bulk up the transformer oils. Due to the high toxicity of PCB’s it is now legislation to know the PCB content of your transformer oil.

Dissolved gas analysis (DGA)
Dissolved gases in transformer oils are inherent gases and gases that form due to the breakdown of the paper or oil under stress or degradation.
With regular DGA testing the following problems can be detected months in advance

• Overheating
• Loose connections
• Breakdown in cellulose
• Arcing
• Partial discharge
• Corona 

Corrosive sulphur
Corrosive sulphur forms acidic conditions in Transformers. This is a vital test as DGA, MAD and Furanic tests may indicate normal operation even when failure is imminent. If corrosive sulphur oil is found, oil will need to be replaced.

Moisture analysis
Why is important to determine the moisture content in transformer oil?

• To determine if there are any leaks in the transformer
• Increase in moisture indicates insulating paper degradation
• Determine if decrease in insulating strength is due to high water content in the transformer oil.

Total acid number (TAN)
TAN is the quantity of base (mg of KOH) that is required to neutralise acid constituents in 1g of sample. An increase in acid indicates that sludge formation is beginning to occur or is occurring.

Dielectric Strength
Dielectric strength is the voltage at which breakdown of the oil occurs. Insulating power decreases as the amount of contaminants in the transformer oil increases so the insulating quality of the oil can be predicted.