Transformer Oil Analysis and Maintenance
Transformer Oil Degradation
Internal faults cause oil decomposition due to:
- Oxygen (producing sludge)
- Temperature (active oxidation)
- Light metals like copper (catalyzing aging reactions)
Oxidation leads to sludge deposits on windings. Water contamination, even in small amounts (parts per million), significantly reduces the oil’s dielectric strength.
Transformer Oil Preservation
Small transformers are completely filled with oil. Expansion is accommodated by flexible cooling fins. Large transformers either maintain an 8% expansion space or are hermetically sealed. This isolation from heat and air slows oil aging and improves cooling efficiency.
In tanks with air contact, the reduced surface area and cooler oil temperature help maintain a chemical balance, limiting oxygen to around 800 ppm. Desiccants like silica gel absorb moisture, but require replacement or drying after saturation (indicated by a color change).
Buchholz Relay Operation
The Buchholz relay, located in the pipe connecting the main tank and the expansion tank, detects gas accumulation and abnormal oil flow. It contains a float and a diaphragm immersed in oil, both connected to a disconnect circuit.
Gas generation from oil decomposition displaces the oil, lowering the float and triggering an alarm. A large, rapid gas release, indicative of a major fault, moves the diaphragm, activating the disconnect switch. Manual reset is required after activation.
Oil Analysis
Physicochemical analysis assesses oil aging. Factors include:
- Color (new oil is light yellow, old oil is dark)
- Water content
- Dielectric strength
- Tangent delta
The tangent delta measures dielectric losses. Polar particles in the oil align with the applied electric field, generating heat due to rotation and collisions (dielectric hysteresis). This heat represents active power loss. A high tangent delta indicates the need for oil replacement or reconditioning (filtering or Fuller’s earth treatment).
Gas Analysis
Oil decomposition produces gases like methane, ethane, ethylene, and acetylene. Gas concentrations and ratios, compared to standards (e.g., UNE), indicate potential faults. A high CO/CO2 ratio suggests paper degradation. Regular gas analysis helps track fault evolution.
Partial Discharge and Gas Bubbles
Partial discharges, often occurring in gas bubbles within the oil, are symptomatic of insulation issues. Gases, having lower dielectric permittivity than oil or paper, experience higher electric fields, leading to discharges within the bubble and against the oil. These discharges further decompose the oil.
Here are some images that illustrate the concept: 
Alternative Insulating Fluids
Mineral oil, while a good insulator and coolant, has a low flash point. Alternatives include:
- Dry transformers (cast resin insulation): Suitable for indoor, low-voltage (up to 30kV), and small power (up to 10MVA) applications. Bubbles in the resin can lead to partial discharges.
- Silicone oil: Good insulator but high viscosity limits cooling capacity.
- Vegetable oils: Good insulators with high flash points, but prone to oxidation. Used in sealed transformers with rubber preservers.
Transformer Maintenance
Regular maintenance is crucial to prevent costly failures and supply disruptions. Key tests include:
- Winding capacitance measurement
- Excitation current measurement
- Transformation ratio measurement
- Frequency Response Analysis (FRA)
- Sweep Frequency Response Analysis (SFRA)
- Dielectric Spectroscopy Diagnosis (SDS) for moisture content estimation