Dissolved gas analysis
Encyclopedia
Dissolved gas analysis is the study of dissolved gases in insulating fluid such as transformer oil
.
Insulating materials within transformers and electrical 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.
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 site 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.
Major power transformers are filled with a fluid that serves several purposes. The fluid acts as a dielectric media, an insulator, and as a heat transfer agent. The most common type of fluid used in transformers is of a mineral oil origin. Other types that are not as common include the askerals and silicone types.
The insulating fluid is in contact with most internal components and by evaluating the dissolved gases much diagnostic information can be gathered. 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.
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 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
. 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.
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.
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.
The overheating of insulation liquid results in breakdown of liquid by heat and formation of high thermal gases. They are methane, ethane and ethylene.
It is a partial discharge
and detected in a DGA by elevated hydrogen.
Arcing is the most severe condition in a transformer and indicated even by low levels of acetylene.
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. It is recommended that values for each of the key gases be trended over time so that the rate-of-change of the various gas concentrations can be evaluated. Basically, any sharp increase in key gas concentration is indicative of a potential problem within the
transformer.
Transformer oil
Transformer oil or insulating oil is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage...
.
Insulating materials within transformers and electrical 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.
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 site 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.
Oil
Major power transformers are filled with a fluid that serves several purposes. The fluid acts as a dielectric media, an insulator, and as a heat transfer agent. The most common type of fluid used in transformers is of a mineral oil origin. Other types that are not as common include the askerals and silicone types.
The insulating fluid is in contact with most internal components and by evaluating the dissolved gases much diagnostic information can be gathered. 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
Vaporization
Vaporization of an element or compound is a phase transition from the liquid or solid phase to gas phase. There are three types of vaporization: evaporation, boiling and sublimation....
, insulation decomposition
Decomposition
Decomposition is the process by which organic material is broken down into simpler forms of matter. The process is essential for recycling the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death...
, oil breakdown and electrolytic action.
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
Borosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion , making them resistant to thermal shock, more so than any other common 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 a septum
Septum
In anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.-In human anatomy:...
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. In the present modern day laboratory, however, mercury is not a favorite material of chemists.Multiple gas extractor
A Multiple Gas Extractor is a device for sampling transformer oilTransformer oil
Transformer oil or insulating oil is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage...
. During 2004, Central Power Research Institute, Bangalore
Bangalore
Bengaluru , formerly called Bengaluru is the capital of the Indian state of Karnataka. Bangalore is nicknamed the Garden City and was once called a pensioner's paradise. Located on the Deccan Plateau in the south-eastern part of Karnataka, Bangalore is India's third most populous city and...
, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
introduced a novel method in which a same sample of transformer oil
Transformer oil
Transformer oil or insulating oil is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage...
could be exposed to vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
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
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
piston to measure its volume at atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
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.Analysis
Chromatographic Analysis is a method of separating the different gases. The gases are injected into the chromatograph and transported through a column. The column selectively retards the sample gases and they are identifi ed as they travel past a detector at different times. A plot of detector signal versus time is called the chromatogram.Fault gasses
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: hydrogenHydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, nitrogenNitrogenNitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and oxygenOxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition... - Oxides of Carbon: carbon monoxideCarbon monoxideCarbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
and carbon dioxideCarbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom... - Hydro Carbons: acetyleneAcetyleneAcetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...
, ethyleneEthyleneEthylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
, methaneMethaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
and ethaneEthaneEthane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
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
Thermal conductivity detector
The thermal conductivity detector , also known as a Katharometer, is a bulk property detector and a chemical specific detector commonly used in gas-liquid chromatography. This detector senses changes in the thermal conductivity of the column effluent and compares it to a reference flow of carrier gas...
for atmospheric gases, by Flame Ionization Detector
Flame ionization detector
A flame ionization detector is a type of gas detector used in gas chromatography. The first flame ionization detector was developed in 1957 by scientists working for the CSIRO in Melbourne, Australia....
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
Partial discharge
In electrical engineering, partial discharge is a localised dielectric breakdown of a small portion of a solid or fluid electrical insulation system under high voltage stress, which does not bridge the space between two conductors...
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
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.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. It is recommended that values for each of the key gases be trended over time so that the rate-of-change of the various gas concentrations can be evaluated. Basically, any sharp increase in key gas concentration is indicative of a potential problem within the
transformer.