Dissolved gas analysis (DGA) is an examination of electrical

transformer oil Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers (wet transformers), some types of high-voltage capacitors, fluorescent lamp b ...

contaminants. Insulating materials within electrical equipment liberate gases as they slowly break down over time. The composition and distribution of these dissolved gases are indicators of the effects of deterioration, such as

pyrolysis The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...

partial discharge In electrical engineering, partial discharge (PD) is a localized dielectric breakdown (DB) (which does not completely bridge the space between the two conductors) of a small portion of a solid or fluid electrical insulation (EI) system under hig ...

, and the rate of gas generation indicates the severity. DGA is beneficial to a

preventive maintenance The technical meaning of maintenance involves functional checks, servicing, repairing or replacing of necessary devices, equipment, machinery, building infrastructure, and supporting utilities in industrial, business, and residential installa ...

program. 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 sampling the oil and sending the sample to a laboratory for analysis. Mobile DGA units can be transported and used on site as well; some units can be directly connected to a transformer. Online monitoring of electrical equipment is an integral part of the smart grid.

Oil

Large power transformers are filled with oil that cools and insulates the transformer windings. Mineral oil is the most common type in outdoor transformers; fire-resistant fluids also used include

polychlorinated biphenyls Polychlorinated biphenyls (PCBs) are highly carcinogenic chemical compounds, formerly used in industrial and consumer products, whose production was banned in the United States by the Toxic Substances Control Act in 1979 and internationally by ...

(PCB)s and silicone. The insulating liquid is in contact with the internal components. Gases, formed by normal and abnormal events within the transformer, are dissolved in the oil. By analyzing the volume, types, proportions, and rate of production of dissolved gases, much diagnostic information can be gathered. Since these gases can reveal the faults of a transformer, they are known as "fault gases". Gases are produced by

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

vaporization Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenomenon ...

, insulation

decomposition Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is e ...

, oil breakdown and electrolytic action.

Sampling

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 Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...

tube of capacity 150 ml or 250 ml, having two airtight

Teflon Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemo ...

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 Dissolved gas analysis (DGA) is an examination of electrical transformer oil contaminants. Insulating materials within electrical equipment liberate gases as they slowly break down over time. The composition and distribution of these dissolved gas ...

without any exposure to atmosphere, thereby retaining all its dissolved and evolved fault gases contents. It has 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

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 and cleanliness of the syringe is important as it maintains the integrity of the sample before the analyses.

Extraction

The DGA technique involves extracting or stripping the gases from the oil and injecting them into a gas chromatograph (GC). Detection of gas concentrations usually involves the use of a flame ionization detector (FID) and a thermal conductivity detector (TCD). Most systems also employ a methanizer, which converts any carbon monoxide and carbon dioxide present into methane so that it can be burned and detected on the FID, a very sensitive sensor.

"Rack" method

The original method, now ASTM D3612A, required that the oil be subjected to a high vacuum in an elaborate glass-sealed system to remove most of the gas from the oil. The gas was then collected and measured in a graduated tube by breaking the vacuum with a mercury piston. The gas was removed from the graduated column through a septum with a gas-tight syringe and immediately injected into a GC.

Multi Stage Gas Extractor

A ''Multi Stage Gas Extractor'' is a device for sampling

. During 2004, Central Power Research Institute,

Bangalore Bangalore (), List of renamed places in India, officially Bengaluru (), is the Capital city, capital and largest city of the Indian state of Karnataka. It has a population of more than and a metropolitan area, metropolitan population of a ...

India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...

introduced a novel method in which a same sample of

could be exposed to

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...

many times, at ambient temperature, until there is no increase in the volume of extracted gases. This method was further developed by Dakshin Lab Agencies, Bangalore to provide a Transformer Oil Multi Stage Gas Extractor. This method is an improvised version of ASTM D 3612A to do multiple extraction instead of single extraction and based on Toepler principle. In this apparatus a fixed volume of sample oil is directly drawn from a sample tube into a

degassing Degassing, also known as degasification, is the removal of dissolved gases from liquids, especially water or aqueous solutions. There are numerous methods for removing gases from liquids. Gases are removed for various reasons. Chemists remove ga ...

vessel under vacuum, where the gases are released. These gases are isolated using a mercury piston to measure its volume at

atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...

and subsequent transfer to a

gas chromatograph Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substance, ...

using a gas-tight syringe. An apparatus, in very similar design and in principle providing a multiple gas extraction, using vacuum and Toepler pump has been in service in Sydney (Australia) for more than 30 years. The system is in use for power and instrument transformers, as well as cable oils.

Head space extraction

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

. Specialized techniques exist such as Headspace sorptive extraction (HSSE) or stir bar sorptive extraction (SBSE).

Analysis

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

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

*oxides of carbon:

carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...

and

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

*hydrocarbons: acetylene, ethylene,

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...

and

ethane Ethane ( , ) is an organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is isolated on an industrial scale from natural gas and as a petroc ...

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

The gases extracted from the sample oil are injected into a gas chromatograph where the columns separate gases. The gases are injected into the

chromatograph In chemical analysis, chromatography is a laboratory technique for the separation of a mixture into its components. The mixture is dissolved in a fluid solvent (gas or liquid) called the ''mobile phase'', which carries it through a system (a ...

and transported through a column. The column selectively retards the sample gases and they are identified as they travel past a detector at different times. A plot of detector signal versus time is called the chromatogram. The separated gases are detected by

thermal conductivity detector The thermal conductivity detector (TCD), also known as a katharometer, is a bulk property detector and a chemical specific detector commonly used in gas chromatography. This detector senses changes in the thermal conductivity of the column eluent an ...

for atmospheric gases, by

flame ionization detector A flame ionization detector (FID) is a scientific instrument that measures analytes in a gas stream. It is frequently used as a detector in gas chromatography. The measurement of ion per unit time make this a mass sensitive instrument. Standalo ...

for hydrocarbons and oxides of carbon. A methanator is used to detect oxides of carbon by reducing them to methane, 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 gases which allows the gas data to be examined to determine the kind of faulting activity taking place within the electrical equipment. * Overheating windings typically lead to thermal decomposition of the

cellulose insulation Cellulose insulation is plant fiber used in wall and roof cavities to insulate, draught proof and reduce noise. Building insulation in general is low- thermal-conductivity material used to reduce building heat loss and gain and reduce noise tr ...

. In this case DGA results show high concentrations of carbon oxides (monoxide and dioxide). In extreme cases methane and ethylene are detected at higher levels. *Oil overheating results in breakdown of liquid by heat and formation of methane, ethane and ethylene. * Corona is a

and detected in a DGA by elevated hydrogen. *

Arcing An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma; the plasma may produce visible light. An ...

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

Application

Interpretation of the results obtained for a particular transformer requires knowledge of the age of the unit, the loading cycle, and the date of major maintenance such as filtering of the oil. The IEC standard 60599 and the ANSI IEEE standard C57.104 give guidelines for the assessment of equipment condition based on the amount of gas present, and the ratios of the volumes of pairs of gases.Martin J. Heathcote (ed)., ''The J&P Transformer Book Thirteenth Edition'', Newnes, 2007 pages 588-615 After samples have been taken and analyzed, the first step in evaluating DGA results is to consider the concentration levels (in ppm) of each key gas. Values for each of the key gases are recorded over time so that the rate-of-change of the various gas concentrations can be evaluated. Any sharp increase in key gas concentration is indicative of a potential problem within the transformer.,Lynn Hamrick, "Dissolved Gas Analysis for Transformers" Dissolved gas analysis as a diagnostic technique has several limitations. It cannot precisely localize a fault. If the transformer has been refilled with fresh oil, results are not indicative of faults.

References

{{Electric transformers Electric transformers Maintenance