Paper is a thin material made from plant fibers. It is used for writing, printing, drawing, and other purposes. Paper is a multifunctional material. It is also widely used in industrial and construction processes, and in a variety of cleaning products. Paper is considered to be one of the early forms of insulation as it ishygroscopic and absorbs moisture. One of the industrial uses of paper is paper insulation. As insulation is usually provided by the material that reduced electricity as flow, paper is a suitable material for this purpose. Paper insulation is typically impregnated with resin or oil. Synthetic resin bonded paper (SRBP) insulated bushings are usually used up to the voltages of 72.5 kV. Therefore, paper insulation is a plant fiber that is used in roof and wall cavities in order to fraught proof, insulate and reduce noise.

Being among the expensive assets of distribution systems and power generation, power transformers are considered to be the key elements in the electrical power sector. Transformers are defined as static electromagnetic devices, which consist of several windings links with a magnetic field that is used in order to provide electrical power distribution from one level of voltage to another. The main components of a transformer are a tank, windings and solid or liquid insulation. Transformers are used to convert the alternating current from one voltage to another. They ensure trouble free and long life current as they have no moving parts. The main functions are transferring of electrical current and regulating the current voltage before it reaches its destination. Insulation is considered to be a significant transformer point. It influences transformer’s reliability, operation and loading. Life of transformers is decided by the isolation condition. The two most common materials for transformer insulation are paper and mineral insulating oil. 

The transformer is also considered to be the main component in the electricity network. Thus, it is of vital importance to monitor the condition of the transformer. Transformer insulation consists of paper insulation, mineral oil and other cellulose materials. The integrity of these materials influences the performance of the transformer. Due to this fact, a lot of attention is focused on the estimation of the transformer residual life by studying the degradation of paper insulation. For reliable transformer operation, it is essential to use different maintenance strategies and various sensing techniques of degradation, including the furan analysis and infrared spectroscopy. Their purpose is to identify the property and concentration change of by-products, which are relevant to the degradation of transformer insulation. Problem prediction contributes towards the removal of transformers, in which insulation deteriorates as it prevents from catastrophic failures and extends the transformer service life.  

Paper is often used in power plant insulation due to the benefits it provides. Thermal and electrical properties of paper present a variety of advantages of its usage. Paper is often used to provide a kind of wrapping for conductors. Paper insulation eliminates the necessity of a vapor barrier. This kind of insulation can completely prevent moisture problems in wall cavities.  Moreover, it may distributemoisture throughout the cavity and in the same way prevent the moisture buildup. Thermal performance is another advantage of paper insulation. The thermal conductivity of cellulose is almost the same as of rock wool or glass wool. Paper is widely used, while fitting around items in wiring and pipes as it leaves few air pockets that serve to reduce the overall wall efficiency. In addition, cellulose structure has shown to use about 27 % less energy to heat. One more advantage is long-term cost savings. These savings depend on the thickness, performance and use of insulation, but it can save up to 50 percent on the utility bills (Cellulose Insulation n.d.).

The advantages of paper insulation are:

1. It is eco-friendly as it is used from recycled materials.

2. It has no adverse effects on health.

3. Paper does not decrease the insulation effectiveness as it is highly resistant to fluctuations and air flows.

4.  In spite of the fact that chemicals that are used to make the cellulose fire retardant repels insects, mold and rodents, they are completely safe for humans.

5. Paper settles better than any other material.

The usual operation lifetime of transformers is 40 years, and they operate at temperatures of 60-90° C. Such temperatures are the reason of gradual degradation of paper insulation, as well as degradation of by-products, including CO2, CO, furans, and water. However, in order to be a good material for insulation, paper should have a high resistivity, high polymerization degree, good breakdown voltage, low extract conductivity, and low factor of dissipation.

Paper is valued for its mechanical strength. In case this strength reduces to 50%, it indicates the end-of-life. The paper that is used for insulation consists of cellulose (80%), hemicelluloses (12%), lignin (8%), and other substances. Mechanical strength of insulation is influenced by its solid aging. The main factor that leads to degradation of cellulose is acids, water, hydrolysis, as well as temperature, products of oil degradation and possible electrical strength. It is rather difficult to determine life expectance of paper insulation and the transformer (Schaut & Eeckhoudt 2012, p. 1).

Paper insulation has the characteristic feature to degrade with time depending on such factors as air, moisture and temperature. In case it is heated to above 100° C, it degrades rather quickly, while reducing the transformer service life. The materials, which constitute a transformer, have a usual process of aging under the normal conditions of operation. Hence, identification of insulation degradation and operation in specified conditions and environment will provide a reliable service of the transformer for many years.  

The causes of paper degradation can be electrical or mechanical stress, type and quality of paper, operating condition, type and quality of oil, and others. Mechanical stresses, design weakness, internal mechanical vibration, as well as equipment overloading, are important criteria of reducing mechanical tension of paper insulation. Electrical stresses (arching, high loading, overheating, and partial discharges) pose a threat to the operating transformer, which may cause a loose of paper insulation and polymerization integrity. High moisture content and humidity in the surrounding environment of the transformer, changes of hot and cold climate during the day and at night, and the transformer location also make a contribution to the deterioration of cellulose paper insulation. Chemical and mechanical strength of the paper insulation is often affected by high ladings or operations at maximum loading. Generated heat accelerates the cellulose oxidation process, in which the chains of carbon-carbon bonding of cellulose polymer have the feature of braking into their furanic compounds. Furanic compounds may be dissolved in oil, then extracted and studied to detect the compound decay. However, such processes have a negative effect on the insulation as they weaken the mechanical strength of paper insulation. In case it is possible to take a paper sample, it is better to identify the Degree of Polymerization. Although there is almost no relationship between paper strength and polymerization degree. The furanic materials that are usually analyzed are aromatic compounds, which arise from the cellulosic materials degradation within a transformer. Thus, the condition of paper insulation is identified by the amount of products, which are present in the oil (Hasan 2006).

Determination of the significant levels of these compounds and type characterization of degradation products from paper insulation provide crucial information that is exceedingly useful for monitoring conditions of transformers. The paper insulation survival may also be determined by paper type and oil that is used in the transformer. Due to the fact that synthetic silicon oil is different from mineral oil according to the properties, the type of oil contributes to the processes of paper deterioration in power transformers.  

Paper is made of furanic compounds, which enables the analysis of dissolved furanic compounds to provide significant information on the deterioration extent of the paper insulation. Furanic compounds, which are found in power transformers, are solely caused by the degradation of the paper insulation. The most commonly found furanic compound is 2-furfural (Hasan 2006). Furanic derivatives are specific characteristics of the paper and cannot be produced by the general oil oxidation. The amount of furan compound concentration in the oil indicates the condition of the paper insulation. The method of a high performance liquid chromatography is sometimes applied to assess the concentration of furan within oil samples extracted from the transformer (Blue, Uttamchandani & Farish 1998, p. 165).

Thus, a power transformer consists of components, which are constantly yielded to thermal and electrical stresses. The paper insulation of the power transformer is considered to be the major component, which has the characteristic of degrading under these stresses. The life of the power transformer is determined by insulation paper. Paper insulation degrades over the time of its usage. Reduction of its polymerization degree affects the mechanical strength of paper. Electrical faults lead to the acceleration of the paper insulation degradation (Samsudin et al. 2010). Paper insulation in a power transformer degrades at a higher operating temperature. Moisture and oxygen have the capability to accelerate degradation. Due to the fact that power transformers are expensive items for power plants, it is necessary to monitor them throughout their operation. Paper and hydrocarbon oil are the elements that constitute the system of a power transformer to a considerable degree. A number of modern diagnostic techniques, which are used to assess conditions of isolation, are widely used by practicing engineers, including dissolved gas analysis (Kelly 1980), moisture analysis in transformer oil, furan and measurement analysis by high performance liquid chromatography (Unsworth & Mitchell 1990; Emsley et al. 2000), as well as degree of polymerization. It is vital to monitor the paper insulation condition in order to avoid the catastrophic transformer failure. Measurement of mechanical strength is considered to be the best technique, which is used to determine the ageing state of the paper, due to the fact that this is the only property that is affected by ageing and not electrical or other properties. As it is impossible to get samples of paper from in-service transformers, non-intrusive techniques are usually used to determine the level of paper degradation. The method of a high performance liquid chromatography is also used to detect paper degradation. It is highly significant to be aware of the condition of power transformer paper insulation in order to make optimum decisions of asset replacement in the power industry.

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