Electrical faults in oil-filled transformers usually generate gases, some of which are combustible. Many transformer faults in their early stages are incipient and deterioration is gradual, but sufficient quantities of combustible gases are usually formed to permit detection and allow corrective measures to forestall a serious outage.

Depending on the transformer oil preservation system, the gas may either be dissolved in the oil or enter the gas space above the oil. In certain types of transf

These relays are usually set to alarm for the presence of gas. It is common practice to draw off samples of oil or gas for periodic analysis of combustible gas content. If there is a gas space in the oil preservation system, it is possible to directly draw off a sample of the gas and perform an on-the-spot analysis with a portable gas analyzer.

If there is no gas space in the transformer, it is necessary to analyze an oil sample for dissolved gas content by gas chromatography (see Bean and Cole [B69]).

The presence of key gases is an indicator of the location of the source of the gas

a) Hydrogen is generated by corona or partial discharges. The presence of other key gases can indicate the source of the discharge.

b) Ethylene (C2H4) is the key gas associated with the thermal degradation of oil. Trace generation of associated gases (ethane and methane) may start at 150 °C. Significant generation of ethylene begins around 300 °C.

c) Carbon monoxide and carbon dioxide are generated when cellulose insulation is overheated.

d) Acetylene (C2H2) is produced in significant quantities by arcing in the oil. To interpret the results of the analysis, the relative ratios of key gases are used.

There has been substantial work to define the best methods for interpreting the results and guidelines have been published in IEEE Std C57.104-1991 [B74] and IEC 60599: 1978 [B8].

Gas analysis on transformers should be made periodically by manual or automatic methods. The interval between tests may be varied according to size, importance, loading, and exposure to faults.

This test should also be made after protective relay or relief diaphragm operation and before reenergizing, if practical. It should be made on new transformers after installation and original loading.

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