Dissolved Gas Analysis Results of Transformer Oil

For the results of the chromatographic analysis, the procedure for diagnosis should be:

1. First look at the content of characteristic gases. If H₂, C₂H₂, and total hydrocarbons, one of which is greater than 20% of the specified attention value, should be roughly judged according to the characteristic gas content. The main corresponding relationship is:

(a) If acetylene (C₂H₂) is present, arc or spark discharge should be suspected;

(b) Hydrogen (H₂) is very large, and the possibility of ingress of moisture should be suspected;

(c) An excess of alkanes and alkenes and little or no alkynes in the total hydrocarbons is characteristic of superheating.

2. Calculate the generation rate and evaluate the speed of fault development.

3. Through the analysis of the gas component content, carry out the three-ratio calculation to determine the fault category.

4. Check the historical operation of the equipment, and make comprehensive judgments through other tests to exclude various possibilities that the characteristic gas comes from non-body faults (such as tap changer barrel leakage, submersible pump failure, oil filtration machine failure, etc.).  click here to know the source of dissolved gas in transformer oil.

So why should we pay special attention to the acetylene content in transformer oil?

Acetylene (C₂H₂) is one of the products of transformer oil cracking at high temperature. Others include methane and ethane with monovalent bonds, and ethylene and propylene with divalent bonds. Acetylene is a trivalent hydrocarbon that requires temperatures as high as a thousand degrees or more to form. Therefore, when the transformer oil contains acetylene, it indicates that the internal fault temperature of the oil-filled equipment is very high, and there is a high probability of arcing, so special attention is required.

In general, how to judge the type of equipment failure with characteristic gas based on chromatographic analysis data?

The content of acetylene (C₂H₂) and hydrogen (H₂) is mainly used to distinguish overheating and discharge failures. Generally speaking, overheating failures, acetylene (C₂H₂) and hydrogen (H₂) and other gas contents are low or even zero, while discharge failures of ethylene and hydrogen and other gases with higher content. This is because:

(a) Overheating failures are caused by the deterioration of the insulation performance of the equipment and the cracking and decomposition of insulating materials such as oil. It is further divided into two categories: bare metal overheating and solid insulation overheating.

In the event of an overheating fault, the only cause of oil decomposition at the fault point in the equipment is the overheating of the bare metal. This kind of failure, due to the decomposition and cracking of the transformer oil, increases the hydrogen and hydrocarbon gases in the oil, especially methane and ethylene, which generally account for more than 80% of the total hydrocarbons. When the temperature of the fault point is low, the proportion of methane is relatively large. With the increase of the temperature of the fault point, the proportion of ethylene also increases, and the composition of hydrogen also increases. In severe overheating, a small amount of acetylene gas will also be produced.

When overheating, only the solid insulating material changes, such as the carbonization of insulating paper, wooden pads, etc., which is called solid insulation overheating. The gases produced by this fault are mainly CO and CO₂. In short, the difference between bare metal overheating and solid insulation overheating is the content of CO and CO₂, the former has a lower content and the latter has a higher content. The latter mainly occurs in transformers.

(2) The discharge fault is the deterioration of the insulation performance of the equipment caused by the electrical effect (ie discharge) inside the equipment. It can also be divided into three types: high-energy discharge (arc discharge), low-energy discharge (spark discharge) and partial discharge according to the strength of the electrical effect.

When arc discharge occurs, the gases produced are mainly acetylene and hydrogen, followed by methane and ethylene. Due to the large energy in the event of failure, the total hydrocarbon content is high, and a large amount of gas generated in a short time is often accumulated in the gas relay before it dissolves in the oil, causing the relay to operate or have obvious discharge sounds. This kind of fault exists for a short time in the equipment, and the omen is not obvious, so the general chromatography method is difficult to predict. The usual method is to perform chromatographic analysis on the transformer oil and the gas in the relay immediately to accurately judge the fault.

Spark discharge is an intermittent discharge fault. The gas produced is mainly acetylene and hydrogen, followed by methane and ethylene gas, but due to the low failure energy, the total content is generally not high.

Partial discharge mainly occurs in transformers and bushings. Partial discharge can be caused by equipment damp, poor manufacturing process or improper maintenance. Failures occur in gas cavities in oil-impregnated paper insulation or in spaces where charged bodies are suspended. The gas produced is mainly hydrogen, followed by methane.

When the discharge energy is high, a small amount of acetylene gas will also be produced.

It is worth noting that there is an issue of aromatic content. Because it has good "anti-gassing" performance. Different grades of oil contain different amounts of aromatic hydrocarbons, and the amount of gas generated under the action of electric field is different. Oil with less aromatic content has poor "anti-gassing" performance, so it is easy to generate hydrogen and methane under the action of an electric field, and in severe cases, waxy substances will be formed; while insulating oils with more aromatic content have better "anti-gassing" performance , producing less hydrogen and methane. Therefore, the influence of this factor should be considered when making specific judgments.

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