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On November 30, 1876, Yablochkov Pavel Nikolayevich patented first transformer, which allows this date to be considered the birth of this equipment. Nowadays, large power transformers are an integral part of energy supply system without which transmission of electricity from the power plants to consumers would be impossible. Power transformers are expensive and their design elements must be carefully checked during their work. Current paper discusses and illustrates what paper insulation is, the problems of paper deterioration in electrical power plant items, methods for determining paper deterioration, and future trends in this field.
What is Paper and Why it is Important in Power Plant Insulation
Insulation is always necessary 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 (Oommen & Prevost 2006). The distance between the conductors is not an effective preventive measure to split the differences of potentials in power transformers. Large power transformer insulation system consists mainly of hydrocarbon oil and paper. Large power transformer life largely depends on insulation, its state, material composition, geometry, etc. (Navamany & Ghosh 2003). 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.
Isolation in high-voltage transformers requires a lot of attention during the design phase (Prevost 2006). 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 (Ziomek, Vijavan, Boyd, Kuby & Franchek 2011). 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 (Tapan 2003). 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 (IEEMA Journal 2006).
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 (Blue, Uttamchandani & Farish 1998). 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.
Electronic nose technology is a method to determine the degree of insulation degradation (Lessard, Noirhomme, Larocque & Vienneau 2012). 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.
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 ethane, methane, ethylene, acetylene, hydrogen, carbon monoxide, carbon dioxide, nitrogen, and oxygen are emitted during transformer operation. In properly working transformers, the concentration level of hydrogen, methane, ethane, ethylene, and acetylene 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.
DGA Method for Determining Paper 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 several advantages over classical dissolved gas analysis, which requires sampling and transportation to a laboratory.
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.
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. 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.
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.
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 (Prevost & Oommen 2006).
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.
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. There are three lines that need to be developed in this area. Firstly, technical design of transformers needs improvement. Secondly, use of new advanced materials in insulation should be encouraged. Thirdly, 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.