Dissolved Gas Analysis and natural ester transformer oil
Samples of natural ester transformer fluid for dissolved gas determinations are taken and analysed using the same procedures and techniques as those used for
The data are interpreted in much the same way as for gases in mineral oil. The combustible gases generated by faults in natural ester fluids are similar to those in mineral oil: high levels of hydrogen may be an indication that partial discharge is occurring; carbon oxides in certain ratios suggest overheated paper; hydrocarbon gases could result from a thermal fault in oil; acetylene points to arcing.
Always, the first step is to determine if a fault exists using the amounts and generation rates of dissolved gases before trying to further interpret the gas data.
The most useful approaches to dissolved gases in natural ester fluid use the gas generation rates combined with the IEEE Key Gases method or the IEC Duval method.
DIFFERENCES FROM MINERAL OIL
The solubility of gases in natural ester transformer fluid differs slightly from their solubility in mineral oil. The volume of gases generated by some faults, most notably arcing faults, can also be different. Low current arcing faults in natural ester oils generate smaller volumes of gas (tests yield gas volumes of about 75% the volume generated in mineral oil). These differences might affect the utility of some ratio analysis methods and estimates of combustible gas content in the headspace.
Ethane and Hydrogen
Many (but not all) otherwise normally operating transformers using natural ester transformer oil have higher ethane content than their mineral oil counterparts. Other hydrocarbon gases remain low – only ethane is elevated.
Occasionally, an unvarying but slightly elevated level of hydrogen is found in otherwise normally operating natural ester transformers. This may incorrectly indicate a partial discharge fault. These anomalies require additional study in order to suitably explain them.
Throughout the adaptation of gas chromatography and analysis for natural esters, we often see a peak (identity unknown) with an elution time close to the elution time of acetylene.
At times this peak is no more than a baseline rise that quickly levels off and can easily be distinguished from acetylene. In other cases, the peak appears to be genuine (more than a baseline rise) and elutes so closely to acetylene that it can be mistaken for acetylene.
Because the presence of small quantities of acetylene prompts additional transformer scrutiny, the chromatographer should be aware of the possible occurrence of the misleading peak.
More work must be done to identify this substance and develop criteria to reliably distinguish it from acetylene.
Methods of Interpretation
The IEC gas guide basic ratio and simplified ratio methods use various ratios of hydrogen and hydrocarbon gases to help identify fault types. The IEC Duval method looks at the relative proportions methane, ethylene, and acetylene to identify the type of fault, assuming one is present. The Duval method plots the data on a ternary graph divided into areas of fault types. This has so far been the most reliable fault identification method for
As with the IEEE guide, the user must determine if a fault condition exists for the interpretation methods to be meaningful. The user establishes the presence of a fault using the gas generation rate and typical gas levels of normally operating transformers. Duval reviews the IEC methods development and application.
Rates of gas increase
According to the IEC guide, an increase in gas concentrations of more than 10% per month above typical concentration values is generally considered a prerequisite for pronouncing the fault as active, provided it is clear that the precision of DGA values is better than 10% after one month.
Much higher rates of gas increase, such as 50% per week, and/or evolving towards faults of higher energy (e.g. D2 or T3), are generally considered very serious, especially if they exceed alarm concentration values. In the case of power transformers, typical rates of gas productions in milliliters per day are also reported. Special attention should be given to cases where there is acceleration in the rate of gas increase.
IEC uses broad classes of detectable faults: partial discharge, low or high-energy discharges, and thermal faults in oil and/or paper. The basic and Duval methods subdivide these into more specific types. The simplified method identifies only the main fault type.