Dissolved gas analysis

Dissolved Gas Analysis or DGA is the study of dissolved gases in insulating fluid. ^[1]

Insulating materials within transformers and related equipment break down to liberate gases within the unit. The distribution of these gases can be related to the type of electrical fault and the rate of gas generation can indicate the severity of the fault. The identity of the gases being generated by a particular unit can be very useful information in any preventative maintenance program. ^[2]

The collection and analysis of gases in an oil-insulated transformer was discussed as early as 1928. Many years of empirical and theoretical study have gone into the analysis of transformer fault gases.

DGA usually consists of three steps: Sampling, extraction, analysis. Modern technology is changing this process with innovation of DGA units that can be transported and used on sight as well as some that come directly connected to the transformer its self. Online monitoring of electrical equipment is an integral part of the smart grid. Though this new technology is promising often oil quality labs are still utilized as third party verification. Also upgrading all equipment to meet the goals of the smart grid can be cost prohibitive.

Transformer oil

Transformer oil is used as a coolant and insulator in a transformer. It baths every internal component and contains a lot of diagnostic information in the form of dissolved gases. Since these gases can reveal the faults of a transformer, they are known as Fault Gases. They are formed in transformer oil, due to natural ageing and as a result of faults inside the transformer. Formation of fault gases is due to oxidation, vaporization, insulation decomposition, oil breakdown and electrolytic action.

Sample container

Oil sample tube

An oil sample tube is used to draw, retain and transport the sample of transformer oil in the same condition as it is inside a transformer with all fault gases dissolved in it.

It is a gas tight borosilicate glass tube of capacity 150 ml or 250 ml, having two airtight Teflon valves on both the ends. The outlets of these valves have been provided with a screw thread which helps in convenient connection of synthetic tubes while drawing sample from transformer. Also this provision is useful in transferring the oil into Sample oil burette of the Multiple Gas Extractor without any exposure to atmosphere, thereby retaining all its dissolved and evolved fault gases contents.

It has got a septum arrangement on one side of the tube for drawing sample oil to test its moisture content.

Thermo foam boxes are used to transport the above Oil Sample Tubes without any exposure to sunlight

Glass syringe

As with the sample tubes it is important that the syringe is clean and free from defects that could introduce error into the sample.

Oil syringes are another means of obtaining an oil sample from a transformer. The volume of the syringes have a large range but can be commonly found in the 50ml range. The quality of the syringe is important as it maintains the integrity of the sample before the analyses. One should check the syringe construction for defects before using. One common place of syringe failure is the stopcock which after multiple uses can deteriorate.

Extraction

Extraction of the gases can be done is several ways but they share the common goal of removing the dissolved gas from the oil. The solubility of the gases in oil changes depending on the gasses and the insulation matrix used. Temperature and pressure have effects on how well the gasses remain in the oil and by manipulating those variables one can extract the gas from oil.

Multiple gas extractor

A Multiple Gas Extractor is a device for sampling transformer oil. During 2004, Central Power Research Institute, Bangalore, India introduced a novel method in which a same sample of transformer oil could be exposed to vacuum many times, until there was no increase in the volume of extracted gases. This method was further developed by Dakshin Lab Agencies to provide a Transformer Oil Multiple Gas Extractor.

In the apparatus a fixed volume of sample oil is directly drawn from a sample tube into a degassing vessel under vacuum, where the gases are released. These gases are isolated using a mercury piston to measure its volume at atmospheric pressure and subsequent transfer to a gas chromatograph using a gas-tight syringe or auto-sampler.

Head space extraction

Head space extraction is explained in ASTM D 3612-C. The extraction of the gasses is achieved by agitating and heating the oil to release the gasses into a 'head space' of a sealed vial. Once the gases have been extracted they are then sent to the gas chromatograph.

"Rack" method

The “Rack” method is an older method of extraction explained in ASTM D 3612-A. Samples are vacuum extracted which separates the gases from the oil. Next the gases are compressed to ambient pressure and the volume measured and compared using the Ostwald solubility coefficients and then ran into a chromatograph.

Fault gases

When gassing occurs in transformers there are several gasses that are created. Enough useful information can be derived from nine gases so the additional gasses are usually not examined. The nine gasses examined are:

• Atmospheric Gases: hydrogen, nitrogen and oxygen
• Oxides of Carbon: carbon monoxide and carbon dioxide
• Hydro Carbons: acetylene, ethylene, methane and ethane

The gases extracted from the sample oil are injected into Gas Chromatograph where the columns separate gases. The separated gases are detected by Thermal Conductivity Detector for atmospheric gases, by Flame Ionization Detector for hydro carbons and oxides of carbon. Methanator is used to detect oxides of carbon, when they are in very low concentration.

Types of faults

Thermal faults are detected by the presence of by-products of solid insulation decomposition. The solid insulation is commonly constructed of cellulose material. The solid insulation breaks down naturally but the rate increases as the temperature of the insulation increases. When an electrical fault occurs it releases energy which breaks the chemical bonds of the insulating fluid. Once the bonds are broken these elements quickly reform the fault gases. The energies and rates at which the gases are formed are different for each of the gasses which allows the gas data to be examined to determine the kind of faulting activity taking place within the electrical equipment.

• Insulation overheating

When transformer is overloaded it generates more heat and deteriorates the cellulose insulation. In this case DGA results show high carbon monoxide and high carbon dioxide. In extreme cases methane and ethylene are at higher levels.

• Insulation liquid overheating

The overheating of insulation liquid results in breakdown of liquid by heat and formation of high thermal gases. They are methane, ethane and ethylene.

• Corona

It is a partial discharge and detected in a DGA by elevated hydrogen.

• Arcing

Arcing is the most severe condition in a transformer and indicated even by low levels of acetylene.

Application

In a new transformer the levels of hydrocarbons in transformer oil after vacuum filtration shall be 5 ppm. After commissioning a new transformer DGA shall be done every month or earlier depending on the DGA results observed.

In a overhauled and repaired transformer, DGA is to be done a week after re-commission. Subsequently DGA is required every month or earlier depending the DGA results.

Results

In interpretation of the results obtained for a particular transformer, due regard should be given to the following factors before arriving at a specific conclusion:

• Date of commissioning of the transformer
• Loading cycle of the transformer
• Date on which the oil was last filtered

External links

• http://www.dissolvedgasanalysis.in
• http://www.morganschaffer.com/

References

1. ^ Herbert G. Erdman (ed.), Electrical insulating oils, ASTM International, 1988 ISBN 0803111797, page 108
2. ^ "DISSOLVED GAS ANALYSIS OF MINERAL OIL INSULATING FLUIDS". 2005 [last update]≤. http://www.nttworldwide.com/tech2102.htm. Retrieved November 2, 2011.