ELEC5205 Chap.8 Transformer Condition Monitoring
Transformer Condition Monitoring
Interpret trend and mixture together
Dissolved gas analysis measures products created when electrical or thermal faults decompose transformer oil and paper. Hydrogen supports low-energy discharge, methane and ethane lower-temperature oil heating, ethylene hotter oil and acetylene high-energy arcing. Furfural reflects cellulose-chain deterioration and helps assess paper ageing.
Sampling delay, oxidation, laboratory variation and operating history limit the certainty of any single result.
Map products to energy without forcing certainty
Different fault energies favour different decomposition products, but a transformer rarely produces a perfectly isolated signature.
Hydrogen can rise with lower-energy discharge, methane and ethane support lower-temperature oil heating, ethylene becomes more prominent with hotter oil, and acetylene is important evidence for high-energy arcing. Carbon monoxide and dioxide add context about cellulose involvement.
Read the whole mixture, absolute concentrations and change over time before choosing a fault family.
Build ratios only from defensible measurements
Gas ratios can emphasise relative production and reduce the influence of oil volume, but they become unstable when a denominator is near the detection limit.
Report the underlying concentrations beside any ratio, preserve units and note results below quantification limits. Compare samples obtained with consistent methods and similar operating conditions.
A mathematically large ratio caused by one tiny uncertain denominator is not stronger evidence than a clear absolute trend across several gases.
Treat sampling as part of the diagnosis
Air ingress, poor flushing, contaminated vessels, delayed transport, temperature and laboratory variation can change results.
Check sample point, date, oil temperature, load, handling and analysis method before interpreting a difference. Repeat sampling is valuable when the proposed rate of change is surprising or a decision would be costly.
Consistency across laboratories or repeated samples strengthens a trend; disagreement should widen uncertainty rather than be averaged into a false precise value.
Combine oil and paper indicators deliberately
Dissolved gases primarily record recent electrical and thermal decomposition in the oil-paper system.
Furfural products provide additional evidence about cellulose degradation and the transformer's thermal history. Neither measurement uniquely locates a defect. Add load records, moisture, electrical tests, partial-discharge measurements, thermal inspection or outage examination as the hypothesis demands.
Conclude with an action proportional to persistence and energy: routine resampling, closer monitoring, planned investigation or urgent removal from service.
What this chapter covers
- 01
Dissolved Gas Analysis
- 02
Fault Gas Pattern
- 03
Furfural
- 04
Cellulose Ageing
- 05
Sampling Error
- 06
Trend Diagnosis
Worked example · free
Read a changing gas mixture
- 2Compare absolute concentrations and total listed gas.
- 3Calculate the largest relative and share change.
- 2State a fault hypothesis and confirmation step.
Key terms
- Fault Gas
- Fault Gas is a decomposition product whose concentration pattern can support a transformer fault mechanism hypothesis.
- Furfural
- Furfural is a cellulose-degradation product measured in transformer oil as evidence related to paper ageing.
- Degree of Polymerisation
- Degree of Polymerisation describes average cellulose chain length and is related to the mechanical strength of paper insulation.
Transformer Condition Monitoring FAQ
Why are gas proportions useful?
Different electrical and thermal energies favour different decomposition products, so the relative mixture carries mechanism information. Absolute concentration still matters because a stable proportion with rapidly rising total gas can indicate increasing activity.
What does furfural add to DGA?
Furfural is produced as cellulose paper degrades, so it adds evidence about solid insulation ageing. DGA more directly reflects gases from oil and paper faults; the methods answer related but not interchangeable questions.
Why should one unusual sample be repeated?
Sampling, transport, oxidation, instrument resolution and laboratory variation can change measured concentration. A controlled repeat distinguishes a persistent trend from a handling anomaly before an expensive operational decision is made.
Exam move
Memorise the direction from lower-energy discharge through overheating to high-energy arcing, then practise reading a full mixture rather than a single gas. Pair each diagnosis with one alternative and one confirming measurement. Keep paper-ageing evidence separate from immediate electrical-fault evidence. For every ratio, write the two raw concentrations beside it and flag a denominator near the reporting limit.
Build a sampling checklist covering point, temperature, load, vessel preparation, delay and laboratory method. End each case with a proportional trigger for repeat sampling, closer monitoring, planned outage tests or urgent intervention rather than a fault label alone. Revisit the diagnosis after adding load history or a repeat sample, and note exactly which conclusion strengthens, weakens or remains unresolved.
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