Dissolved gas analysis

Last updated

Dissolved gas analysis (DGA) is an examination of electrical transformer oil contaminants. [1] 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 or partial discharge, and the rate of gas generation indicates the severity. [2] DGA is beneficial to a preventive maintenance program.

Contents

The collection and analysis of gases in an oil-insulated transformer was discussed as early as 1928. [3] As of 2018, 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 (PCB)s and silicone. [4]

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, vaporization, insulation decomposition, 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 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

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. [5]

"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 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, 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 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.

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 gas chromatograph.

Specialized techniques exist such as Headspace sorptive extraction (HSSE) or stir bar sorptive extraction (SBSE). [6]

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:

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 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 for atmospheric gases, by flame ionization detector 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.

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. [7]

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. [8]

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. [7]

Related Research Articles

<span class="mw-page-title-main">Inductively coupled plasma mass spectrometry</span> Type of mass spectrometry that uses an inductively coupled plasma to ionize the sample

Inductively coupled plasma mass spectrometry (ICP-MS) is a type of mass spectrometry that uses an inductively coupled plasma to ionize the sample. It atomizes the sample and creates atomic and small polyatomic ions, which are then detected. It is known and used for its ability to detect metals and several non-metals in liquid samples at very low concentrations. It can detect different isotopes of the same element, which makes it a versatile tool in isotopic labeling.

In electrical engineering, partial discharge (PD) is a localized dielectric breakdown (DB) of a small portion of a solid or fluid electrical insulation (EI) system under high voltage (HV) stress. While a corona discharge (CD) is usually revealed by a relatively steady glow or brush discharge (BD) in air, partial discharges within solid insulation system are not visible.

<span class="mw-page-title-main">Gas chromatography</span> Type of chromatography

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, or separating the different components of a mixture. In preparative chromatography, GC can be used to prepare pure compounds from a mixture.

<span class="mw-page-title-main">Gas chromatography–mass spectrometry</span> Analytical method

Gas chromatography–mass spectrometry (GC–MS) is an analytical method that combines the features of gas-chromatography and mass spectrometry to identify different substances within a test sample. Applications of GC–MS include drug detection, fire investigation, environmental analysis, explosives investigation, food and flavor analysis, and identification of unknown samples, including that of material samples obtained from planet Mars during probe missions as early as the 1970s. GC–MS can also be used in airport security to detect substances in luggage or on human beings. Additionally, it can identify trace elements in materials that were previously thought to have disintegrated beyond identification. Like liquid chromatography–mass spectrometry, it allows analysis and detection even of tiny amounts of a substance.

<span class="mw-page-title-main">Switchgear</span> Control gear of an electric power system

In an electric power system, a switchgear is composed of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is directly linked to the reliability of the electricity supply.

<span class="mw-page-title-main">Electronic component</span> Discrete device in an electronic system

An electronic component is any basic discrete electronic device or physical entity part of an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not to be confused with electrical elements, which are conceptual abstractions representing idealized electronic components and elements. A datasheet for an electronic component is a technical document that provides detailed information about the component's specifications, characteristics, and performance.

<span class="mw-page-title-main">Total organic carbon</span> Concentration of organic carbon in a sample

Total organic carbon (TOC) is an analytical parameter representing the concentration of organic carbon in a sample. TOC determinations are made in a variety of application areas. For example, TOC may be used as a non-specific indicator of water quality, or TOC of source rock may be used as one factor in evaluating a petroleum play. For marine surface sediments average TOC content is 0.5% in the deep ocean, and 2% along the eastern margins.

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 wet transformers, some types of high-voltage capacitors, fluorescent lamp ballasts, and some types of high-voltage switches and circuit breakers. Its functions are to insulate, suppress corona discharge and arcing, and to serve as a coolant.

In electric power distribution and transmission, a Buchholz relay is a safety device mounted on some oil-filled power transformers and reactors, equipped with an external overhead oil reservoir called a "conservator".

<span class="mw-page-title-main">Solid-phase extraction</span> Process to separate compounds by properties

Solid-phase extraction (SPE) is a solid-liquid extractive technique, by which compounds that are dissolved or suspended in a liquid mixture are separated, isolated or purified, from other compounds in this mixture, according to their physical and chemical properties. Analytical laboratories use solid phase extraction to concentrate and purify samples for analysis. Solid phase extraction can be used to isolate analytes of interest from a wide variety of matrices, including urine, blood, water, beverages, soil, and animal tissue.

<span class="mw-page-title-main">Electrical steel</span> Iron alloy optimized for magnetic properties

Electrical steel is speciality steel used in the cores of electromagnetic devices such as motors, generators, and transformers because it reduces power loss. It is an iron alloy with silicon as the main additive element. The exact formulation is tailored to produce specific magnetic properties: small hysteresis area resulting in low power loss per cycle, low core loss, and high permeability.

<span class="mw-page-title-main">Flame ionization detector</span> Type of gas detector used in gas chromatography

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. Standalone FIDs can also be used in applications such as landfill gas monitoring, fugitive emissions monitoring and internal combustion engine emissions measurement in stationary or portable instruments.

A gas detector is a device that detects the presence of gases in an area, often as part of a safety system. A gas detector can sound an alarm to operators in the area where the leak is occurring, giving them the opportunity to leave. This type of device is important because there are many gases that can be harmful to organic life, such as humans or animals.

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 and compares it to a reference flow of carrier gas. Since most compounds have a thermal conductivity much less than that of the common carrier gases of helium or hydrogen, when an analyte elutes from the column the effluent thermal conductivity is reduced, and a detectable signal is produced.

Perfluorocarbon tracers (PFTs) are a range of perfluorocarbons used in flow tracers and other tracing applications. They are used by releasing the PFT at a certain point, and determining the concentration of that PFT at another set of points, allowing the flow from the source to the points to be determined.

Condition monitoring of transformers in electrical engineering is the process of acquiring and processing data related to various parameters of transformers to determine their state of quality and predict their failure. This is done by observing the deviation of the transformer parameters from their expected values. Transformers are the most critical assets of electrical transmission and distribution systems, and their failures could cause power outages, personal and environmental hazards, and expensive rerouting or purchase of power from other suppliers. Identifying a transformer which is near failure can allow it to be replaced under controlled conditions at a non-critical time and avoid a system failure.

Methanizer is an appliance used in gas chromatography (GC), which allows the user to detect very low concentrations of carbon monoxide and carbon dioxide. It consists of a flame ionization detector, preceded by a hydrogenating reactor, which converts CO2 and CO into methane CH4. Methanizers contain a hydrogenation catalyst to achieve this conversion. Nickel is commonly used as the catalyst and there are alternatives available.

Transformer oil, a type of insulating and cooling oil used in transformers and other electrical equipment, needs to be tested periodically to ensure that it is still fit for purpose. This is because it tends to deteriorate over time. Testing sequences and procedures are defined by various international standards, many of them set by ASTM. Transformer oil testing consists of measuring breakdown voltage and other physical and chemical properties of samples of the oil, either in a laboratory or using portable test equipment on-site.

Headspace gas chromatography uses headspace gas—from the top or "head" of a sealed container containing a liquid or solid brought to equilibrium—injected directly onto a gas chromatographic column for separation and analysis. In this process, only the most volatile substances make it to the column. The technique is commonly applied to the analysis of polymers, food and beverages, blood alcohol levels, environmental variables, cosmetics, and pharmaceutical ingredients.

This glossary of electrical and electronics engineering is a list of definitions of terms and concepts related specifically to electrical engineering and electronics engineering. For terms related to engineering in general, see Glossary of engineering.

References

  1. Herbert G. Erdman (ed.), Electrical insulating oils, ASTM International, 1988 ISBN   0-8031-1179-7, p. 108
  2. "DISSOLVED GAS ANALYSIS OF MINERAL OIL INSULATING FLUIDS". Archived from the original on April 25, 2012. Retrieved November 2, 2011.
  3. Vahidi, Behrooz; Teymouri, Ashkan (2019), Vahidi, Behrooz; Teymouri, Ashkan (eds.), "Dissolved Gas Analysis (DGA)", Quality Confirmation Tests for Power Transformer Insulation Systems, Cham: Springer International Publishing, pp. 65–73, doi:10.1007/978-3-030-19693-6_4, ISBN   978-3-030-19693-6, S2CID   191166554 , retrieved 2022-06-01
  4. "Dissolved Gas Analysis". 2005. Retrieved November 21, 2011.
  5. "Using Dissolved Gas Analysis to Detect Active Faults in Oil-Insulated Electrical Equipment". Archived from the original on April 15, 2012. Retrieved November 21, 2011.
  6. Headspace sorptive extraction (HSSE), stir bar sorptive extraction (SBSE), and solid phase microextraction (SPME) applied to the analysis of roasted Arabica coffee and coffee brew. Bicchi C1, Iori C, Rubiolo P and Sandra P, J Agric Food Chem., 30 January 2002, volume 50, issue 3, pages 449-459, PMID   11804511
  7. 1 2 Martin J. Heathcote (ed)., The J&P Transformer Book Thirteenth Edition, Newnes, 2007 ISBN   978-0-7506-8164-3 pages 588-615
  8. "Dissolved Gas Analysis for Transformers" (PDF). Retrieved November 21, 2011., Lynn Hamrick, "Dissolved Gas Analysis for Transformers"