Detection of Paper Degradation in Large Power Transformers
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Transformer is a static electromagnetic device that has two or more inductively coupled windings on some magnetic core. The transformer was designed to convert by electromagnetic induction of one or more systems (voltage) AC power to one or more other systems (voltage) AC without changing the frequency of the system (voltage) AC. Today, large power transformers are an inseparable part of energy supply system without which transmission of electricity from the power plants to consumers would be impossible. Large power transformers are specially designed equipment, which carries a large capital investments and long terms due to the complicated procurement and production. The costs and the prices depend on the manufacturer and size; large power transformers can cost millions of pounds and weigh hundreds of tons. Power transformers design elements must be carefully checked during their work. What paper insulation is, methods for determining paper deterioration, the problems of paper deterioration in electrical power plant items and future trends in this field are questions that discussed in this paper.
What is Paper and Why it is Important in Power Plant Insulation
Failure of the transformer can cause underproduction of electricity power station and stop equipment of power unit at the time of repair or replacement of the transformer, and costs a lot of money. Power transformers are essential component power networks as they regulate the voltage to an acceptable level for each segment of the transmission of electricity from generation to the end user. In other words, the power transformer increases the voltage for efficient transmission of electricity over long distances, and reduces the voltage for distribution to the level used by customers. Main stuff required for the production of power transformers with copper conductors, silicon, iron/steel, oil and insulating materials.
The cellulose insulation winding is potentially the most dangerous element of the power transformer and at the same time the most prone to development of aging process that actually determines its resource. Insulation is always necessary element when there is a difference between potential of two points. In three-phase transformer, insulation between the conductors is not necessary as air separation unit is used as an insulator that prevents the flow of current. The range between the conductors is not an effective preventive measure to split the differences of potentials in power transformers. Isolation of transformer is a constituent of the dielectric system that located between the electrodes of the winding wire and grounded parts of the transformer. Large power transformer insulation system consists mainly of hydrocarbon oil and paper. Power transformer life mainly depends on insulation, its state, material composition, geometry, etc. Isolation in high-voltage transformers requires a lot of attention during the design phase. Degradation of cellulose insulation windings reduces mechanical strength of paper and development of dehydration leading to an increase in local concentration of water in solid insulation and reduction in voltage breakdown resistance of oil transformers. Wear of paper insulation is accelerated in the presence of oxygen and moisture. The development of these processes in cellulose insulation in combination with a possible weakening of mechanical fixing winding leads to increased risk of circuit and damage to the transformer as well as the impact on short-circuit currents and storm switching overvoltage.
Methods of Determining the Degree of Insulation Deterioration
Regular control of the condition of paper insulation of electrical windings is needed to ensure safe and reliable operation of large power transformers. Engineers use a range of modern diagnostic techniques to assess transformer insulation. Condition monitoring methods to determine the degree of paper insulation deterioration and paper degradation include furan analysis, dissolved gas analysis, partial discharge measurement, frequency response analysis (FRA), recovery voltage measurement (RVM), thermo vision measurement, and condition assessment levels.
Oil tests. Studies have shown that over 60% of the concealed damage of high power transformers were ientified through analysis of oil. Changing much of the characteristics of the oil in the transformer makes it difficult to determine the type of possible damage. Increase of the test efficiency takes place during differentiation of diagnostic parameters in four groups: parameters of oil that does not change during the term of service, determining the characteristics referring to the process of aging, the definition of the dielectric parameters, tests using oil as a diagnostic medium.
Electronic nose technology is a method to determine the degree of insulation degradation. This technology is used in various industries for the detection of degradation products such as wine, coffee and cheese. Now, tools for determining the state of transformer insulation require taking samples for laboratory analysis. In electronic nose technology, the sensor can be set to detect various types of paper degradation, thus serving as an effective diagnostic tool.
However there are two main reasons why the electrical methods do not provide good basic measure of degradation of cellulose insulation. The first is the dominant moisture effect on most electrical appliances. The second is that the electrical properties of the oil-impregnated paper and board are higher features than impregnating oil from the pulp. Thus, the electrical results are not sensitive measures a state of isolation.
Concerning the mechanical properties of cellulose insulation only test of tensile strength measurements was studied as well as other measures, such as insulation test for bursting which also can be relevant. However, the mechanical testing of insulation paper has a number of disadvantages such as the need to stop the operation of the transformer during the measurement, the inability to online monitoring of the mechanical properties of insulation.
Dissolved gas analysis was recognized worldwide as a diagnostic method for detection of incipient faults and early diagnosis of paper degradation in transformers (Tapan 2003). Gases are emitted as a result of decomposition of transformer oil and solid insulating materials, such as paper, cardboard and transformer board, which is made of cellulose. Wear rate of cellulose and oil significantly increases if a fault occurs in the transformer. Such gases as methane, ethane, acetylene, ethylene, carbon monoxide, hydrogen, carbon dioxide, oxygen, and nitrogen are emitted during transformer operation. In properly working transformers, the concentration level of ethane, hydrogen, acetylene, methane, and ethylene, shall not exceed 0.05 ml for 100 ml of oil, while insignificant level of higher hydrocarbon gases is acceptable. Gases that are generated when a fault occurs in the transformer can be divided into three groups: 1) gases that are released during corona or partial discharge; 2) gases released during thermal heating; 3) and gases that are released when arcing.
Furan analysis is one of the methods to determine the degree of insulation deterioration by assessing the concentration of furanic compounds. Furanic derivatives that occur in the aging process of paper insulation due to thermal effects are specific to the paper. Such furanic compounds are not formed in the process of oil oxidation. Concentration of furanic compounds in the oil gives a precise picture of the state of paper insulation. High performance liquid chromatography allows determining the amount of furanic compounds in oil samples taken from the transformer.
DGA Method for Determining Paper Degradation
Cellulose insulation is a mixture of three components such as cellulose polymer of high molecular weight, hemicellulose copolymers with lower molecular weight, and lignin, which are based on aromatic polymers (IEEE Standard Association, 2010). Wear isolation depends on the environmental conditions and can include hydrolytic, oxidative and thermal degradation.
Dissolved gas analysis (DGA) has been successfully used for many years for the diagnosis and monitoring of mineral oil transformers and paper degradation specifically. Yet, two types of DGA currently exist, namely classical laboratory and online analysis. Online analysis of emitted gases that can damage insulation and cause paper degradation in transformer has severaal advantages over classical dissolved gas analysis, which requires sampling and transportation to a laboratory (Muhamad, Phung, Blackburn, 2011).
Characteristics of the emitted gases in a transformer are usually unique to each individual transformer because the conditions of transformer’s operation and its internal characteristics are unique. Online monitoring can provide the history of gas emissions, which allow determining the type of transformer (Feilhauer, Werner, Handschin, Edmund, 2006).
Analysis of specific gas emission trend line allows seeing gas-generation events as they occur disregarding high levels of gas accumulations, which is not possible with the help of laboratory DGA due to the variance in sampling, testing and specific conditions of the transformer during the time when samples are taken (Su, Lai, Austin, 2000). Thus, online monitoring of oxygen can detect air leaks and, thus, warn of potentially dangerous ingress of water, while classic DGA would often provide varying amount of air in the transformer. In addition, 24/7 online monitoring of gas emissions transformer allows assessing possible composition dynamic behavior of gases in power supply transformers (IEEMA Journal, 2006).
However, dissolved gas analysis has disadvantages as well. The main disadvantage of DGA is that it is impossible to establish the location and phase of the fault if it occurs (Tapan, 2003)
An Indication of Challenges and Future Trends in the Field
Improving characteristics of material, especially core material, and development of advanced design tools are two factors of impressive progress of transformer technology in recent decades. Yet, one of the most important tasks aimed at increasing the life of large power transformers is improvement of diagnostic methods used for assessing insulation and preventing paper degradation in transformers. These methods include dissolved gas analysis, moisture analysis, degree of polymerization measurement, furan analysis in high performance liquid chromatography, chemical techniques, electrical diagnostic methods, time domain polarization methods, and frequency domain polarization measurement. Also, one of the future trends in the production of transformers is the use of new materials for insulation that would decrease probability of paper degradation and, thus, increase service life of paper insulation and transformer itself. A good example of the new trend is using silicone materials in high temperature transformers. The use of silicone materials and aramid fiber in solid insulation has been proved to be technically feasible; but commercial acceptance is limited due to reluctance to change from traditional designs for many power applications. Also, the properties of the paper insulation at cryogenic temperatures are a prospective area for further research and application in transformers.
Mathematical modeling of processes occurring during operation of transformers gains particular significance in increasing their service life. However, appropriate software is needed to implement such modeling. Modeling paper degradation processes occurring in paper insulation will solve the problem of timely diagnosing possible faults in transformers and help implement preventive measures to prevent transformer failure.
During operation power transformers expose a wide range of electrical, mechanical and thermal loads, which can produce damage to the insulation. This type of denial figures among the most costly mistakes in the distribution network, as it produces disconnection of electric machines and power interruption. Thus, transformers are an important and integral part of electricity networks not only on country-level but in the world. Transformer is a very expensive facility; and therefore, it requires proper operation and timely maintenance. An important task is to increase the service life of transformers. The technical design of transformers needs improvement,use of new advanced materials in insulation should be encouraged. Since paper is currently best insulation material available due to its high dielectric properties, creation of new and improvement of existing methods of diagnosing possible insulation problems in transformers need to be considered.
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