Transformers: what degrades the insulation and how to inspect it

In one line

In a transformer, the insulation system (oil and paper) is the component that demands the most care — and four concrete factors (moisture, oxygen, heat and external contamination) explain almost everything that can go wrong with it.

Why the diagnosis changed over time

For many years, transformer preventive maintenance relied almost exclusively on insulation resistance and the oil's dielectric strength. Today it is known that tests such as the insulation's power factor, moisture content, interfacial tension or acidity are just as important for an accurate diagnosis. Dissolved gas analysis by gas chromatography has also become a key tool for knowing a transformer's internal condition without having to take it out of service.

A transformer with a well-maintained insulation system withstands overvoltages from switching or lightning, and internal short circuits, far better.

The parts of a transformer

The equipment has six main components: the core, the insulation system (oil and paper — the most important one, the one that demands the most care), the windings, the bushings, the valves and the radiators.

The 4 factors that degrade the insulation

Moisture. It can be dissolved, in the form of a water/oil emulsion, free at the bottom of the tank, or as ice at the bottom of the tank. The effect depends on its form: a water/oil emulsion reduces the oil's dielectric strength, while dissolved water has little or no effect on that same strength.

Oxygen. It reacts with the oil, forming organic acids, water and sludge. It comes from the atmosphere or is released by the paper insulation's cellulose when it heats up.

Heat. 90% of cellulose deterioration is thermal in origin. Degradation depends on time, temperature and how dry the insulation is. High temperatures age the cellulose, reduce its mechanical and electrical strength and depolymerize it — and also generate water, acidic materials and gases (CO2, CO) as a byproduct.

External contamination. It shows up as "dandruff" coming from the transformer's own manufacturing process, when it was not properly removed when filling it with oil; or as particles that come loose from the cellulose while in service.

Which test applies to each component

| Component | Inspection / test | |---|---| | Windings | DC resistance, turns ratio, insulation resistance, insulation power factor | | Bushings | Insulation resistance, temperature (thermography) | | Dielectric oil | Oil level, dielectric strength, gas chromatography, interfacial tension, water content, power factor, sediment | | On-load tap changer | Temperature (thermography), turns ratio at every tap, oil dielectric strength, contact inspection (continuity) | | No-load tap changer | Temperature (thermography), turns ratio at every tap, contact inspection (continuity) | | Core | Core-to-ground, core-to-tank insulation resistance | | Tank | Visual inspection (leaks, corrosion), temperature (thermography) | | Radiators | Temperature (thermography), visual inspection (leaks, cleanliness, corrosion) |

What this table is for

Without it, an inspection can easily keep focusing on the same things (oil, a visual check of the tank) and leave out less visible components — the tap changer or the core, for instance, have their own tests that are not "more of the same" but checks specific to that component. Thermography shows up repeated across several components because, together with oil analysis, it is the tool that gives the most information without needing to de-energize the transformer.

Benefits

• Understanding the 4 degradation factors lets you interpret a lab result (say, high interfacial tension or the presence of certain gases) in terms of cause, not just symptom
• The per-component table keeps the inspection from depending on whoever is doing it and their habits, and ensures coverage of the less visible parts
• Dissolved gas analysis allows diagnosis without taking the transformer out of service, something that previously required more invasive tests

Limitations to keep in mind

• Several tests (gas chromatography, interfacial tension, power factor) require a specialized lab, not a plant-floor inspection
• The four degradation factors interact with each other — heat and moisture together, for example, degrade insulation faster than either one alone — so a diagnosis that looks at only one factor in isolation can be incomplete
• The per-component test table does not include frequencies: each company has to set how often each test is repeated based on the transformer's criticality

In summary

A transformer's insulation degrades through four concrete pathways — moisture, oxygen, heat and external contamination — and each component of the equipment has its own set of tests to catch it in time. Dissolved gas analysis and thermography are, today, the tools that allow the most diagnosis without disconnecting the equipment.