Looking At The Transformer Kraft Paper

Transformers play an important role in providing a reliable and efficient electricity supply – and represent the most critical equipments in electric power transmission and distribution systems. In the early days, only a few insulation materials were known for low temperature superconducting power applications. Paper was one of the first insulation materials used in high voltage technology. Of all the materials used in the construction of a transformer, the most vulnerable is the paper insulation. It comprises a combination of cellulose fibers of Kraft paper and an impregnating material, which may be a compressed gas, a liquid or vacuum. Kraft paper (a wood-pulp paper) is employed extensively in the electric power industry for insulating HV apparatus, such as transformers, capacitors and cables. The dielectric properties of such a system depend on the distribution of moisture, the applied voltage and its frequency, temperature and the density of the paper. In order to obtain the required electric strength, several sheets are stacked or rolled together. The thermal contraction of paper is low enough and elastic elongation at low temperatures is reasonable without leading to mechanical problems. Moreover, paper is very cheap compared to plastic tapes. Consisting of cellulose, the paper insulation begins to degrade from the day of installation. Once the paper degradation has occurred, it cannot be reversed. It can however be slowed down.

Paper life = Transformer life

Heat and moisture are the most commonly known factors in the degradation of the paper insulation, for each rise of 8 deg. C – the life of the paper insulation is halved (due to ageing), similarly each time the moisture content doubles the paper life is reduced by half (due to loss of mechanical strength).

During the life of transformers, contaminants also build up in the oil and are deposited in the paper, these are the by-products of the oxidation process and the effect on the paper life becomes more aggressive as the buildup increases.

As paper ages due to heat, it polymerizes, (the molecules become shortened as chemical / physical changes take place). As polymerization occurs compounds known as Furans are released and from this an assessment of the overall condition of the paper (cellulose) can be made.

New paper installed in a transformer has a DP (Degree of Polymerization) of about 1000. The degree of polymerization has a directly proportional relationship to the mechanical strength of the paper. 1000DP = 17,500 psi (+/-120MPa). As paper loses its mechanical strength, it becomes more likely to fail. It is widely accepted that the loss of 75% of the tensile strength = the end of paper life. The effects of accelerated trials on transformer insulating paper subjected to a constant heat, which shows the non linear deterioration of the paper to slow down over time. In normal operation, most transformers have a fluctuating heat profile and this, as well as other factors such as moisture and contaminant build up in the oil / paper, seriously affects the life of the paper. This is because these other factors cause the loss of mechanical strength leading to premature failure: –

• Moisture content of the paper
  • Build up of oxidation by products

At any given point in the life of the paper insulation, the level of moisture and contaminant build up will have a detrimental effect on the paper’s mechanical strength, and thus the transformer’s life. The effects of moisture and contaminant build up on paper strength. As the buildup of contaminants increases so the rate of paper deterioration accelerates. Although an analysis may show a reasonable residual life in terms of the DP (Degree of Polymerization) the actual life remaining will be much less due to the loss of mechanical strength. As the paper becomes weaker, the probability of failure increases and the effect of failures can be more dramatic. Maintaining the paper insulation in a healthy condition is the surest way to maximise the life of a transformer and minimise the risk of failure!

“Restore the oil and decontaminate the paper = extend the transformer life”

Process of manufacturing of kraft paper

In general, the following steps are involved in Paper Making process:

• Raw Material Preparation
  · Wet Washing
  · Pulping (Cooking of Raw Material)
  · Washing, Screening and Cleaning
  · Refining
  · Blending and Stock Preparation
  · Paper Making
  · Finishing and PackingA. Raw material preparation

In the case of agricultural residue, raw material received from the sources requires conditioning for preparation of pulping. Straws and Bagasse received from the fields contained dust, fine particles (other than raw material), pith and so many other undesirable materials. These non-fibrous materials require to be separated from the raw material before pulping. In case of Bagasse, dry depithing is carried out to remove pith which is non-fibrous material. For this process, Depither is installed. For Wheat Straw, Deduster is being used to remove dust, sand etc. In case of Sarkanda, a Cutter is installed to cut it in small pieces of around 10-12 mm in length to have proper penetration of the Chemicals. Other undesirable materials like Bitumen/Plastic Bags, Laminated, rotten etc. which are harmful for paper making have also been removed.

B. Wet washing

Agro based materials like Wheat Straw, Bagasse and Sarkanda came for Wet Cleaning after thorough cuttings and dry Dedusting through Deduster, Depither and cutter. Even after dry dedusting, lot of dust, sand, solids etc. remain adhere to the raw materials. Even some of the inherent Silica and Chemical like chloride which are soluble, get dissolved and taken out through washing processing. In wet cleaning System, raw material is passed through water vat and mechanically beating/thrashing is done to de-attach sand, soil and kept for some time to get Chemicals in the raw-materials dissolved and some of the inherent silica and then wet raw material is passed through Equa separator to remove free flowing water and further passed through screw press to get desired solid percentage of raw-material suitable for cooking process.

C. Pulping

It is proven that for Paper making only fibrous material – particularly cellulose – is required and there is a need to remove non-fibrous constituent like lignin, pentasons, Resin, Pectin etc. For this, all non-fibrous components require delignification by cooking. Generally, cooking is done by soda process in which mainly Caustic Soda is used in the percentage of 10-12% for Kraft Paper and 14-16% for White Grade of Paper. Raw-Material combined Wheat Straw, Bagasse; Sarkanda etc. are cooked in Globe Digester under steam pressure of 6-7 Kgs /Cm2 and temperature of 160-165 Deg C.

The Globe Digester is rotated for 4 Hours for re-action of this process – and then it is blown in Blow Tank to segregate the excess steam and Pulp. In further steps Pulp is screened to remove undesirable material including uncooked fiber, if any. After screening, the Pulp is passed through Single Disc Refiner to give mild refining to make homogenous slurry to have better and proper washing in washing section.

D. Washing, screening and cleaning

In Washing Section, all undesirable components; which are in liquid form including spent caustic liquor; are removed through filtration process. Then pulp is passed through Double Disc Refiner to make homogeneous slurry for smooth further processing and sent for Screening.

E. Refining

After thorough Washing, Pulp is collected in a Storage tank and finally Pulp is refined through Double Disc Refiners and Tri Disc Refiners to make the pulp suitable for paper making and to impart better fiber bonding condition – which improves the physical strength of the paper. Similarly cut and dedusted jute is cooked in Globe Digester in the same manner to remove lignin, wax, oil etc. and to make the jute softer for pulp making. Cooked Jute is chopped through Chopper Breaker and some refining is also given in the same process as well. Washing also takes place applying drum washer in between. This pulp is again screened through 6 mm hole Johnson Screen and passed through number of Centricleaners to remove sand and other impurities – which remain with the Pulp. This Pulp is thickened and refined through Tri Disc Refiner to impart desired physical strength to the Pulp. The Waste Paper does not require any cooking before pulping. It is directly slushed in Hydro Pulper in presence of Water and Mechanical thrashing. This material is also passed through Screening System to two stage i.e., turbo separator and refiner. All three types of Pulp which are kept separately sent for blending in Stock Preparation.

F. Blending and stock preparations

Different types of Pulp i.e., Agro, Jute and Waste Paper are taken in the measuring tank in desired proportion according to the quality of Paper to be made. In the same Chest some Chemicals and additives are added like around 0.4% Rosin, 5% Alum and 0.2% additives to impart physical and chemical properties to the Paper. The Pulp is kept for 30-35 minutes for re-action before sending for further processing. Next step is taken to make pulp more homogenous using Refiners i.e., refining is done through Tri Disc Refiner, Cleaning through Nos. of Centricleaners Bottles in three stages to remove uncooked, undesirable, sand, fines etc. from the pulp if any. Screened Pulp is passed through Pressure type Screening Equipment which has got 2.6 mm perforation which works on centrifugal force to remove longer fiber which is undesirable for Paper making. The accepted Pulp is taken on Paper Machine and rejected Pulp is taken separately for further processing to remove the rejects from the System.

Paper making section

For Paper making Paper Machine is having a number of different sections, which has got different functions like: –

A. Head box

Head Box is Equipment which receives cleaned pulp from the Stock Preparation and discharge on fourdrinier machine which has got wire mesh for sheet formation and the filtration of the fiber. The main function of the Head Box is to deliver the Pulp on the fourdrinier in such a manner so that the fiber can be spread across the width and on Machine direction uniformly.

B. Fourdrinier

Fourdrinier section consist a forming board, Hydro Foils, Suction Boxes, Table Rolls etc. Pulp poured on the Wire in the consistency of 1% fiber and 99% Water. Over and above, this fourdrinier is supported with wire mesh and number of drainage elements. During formation of Sheets, Water drainage takes place in control manner to have uniform and strong paper sheet. Basically, fourdrinier work is to form a proper uniform fiber sheet by draining the Water. The Pulp slurry at inlet is 1% consistency and at the outlet of the fourdrinier the consistency is raised to 20 to 22% by using gravitational discharge of Water by employing hydro foil and table rolls. More Water is removed through Vacuum Suction Boxes.

C. Top wire

It is used to impart extra strength by fine sheet formation and lamination in wet condition. Top Wire consists of all Components similar to the Fourdrinier. By using top wire, low strength of raw-materials is being used which contribute in keeping lower cost of production and giving better quality of product.

D. Presses

Formed sheet with the consistency of 22% passed through in between two heavy rolls under 70 Kgs. linear pressure to make paper sheet compact and to remove Water to make it drier. Similar type of Presses are installed in 3 stages, where application of load goes on increasing up to 200 Kgs. of linear pressure at the third Press – and finally Paper dryness is achieved about 42 – 43%.

Dryer Pressed paper sheet with the moisture of 57-58% is passed through a number of cylindrical dryer cylinders that are heated through steam to evaporate water to make the Paper dry. At the moisture of around 45%, the paper sheet is passed through M.G. Cylinder to impart one side glaze to the Paper, which is basic requirement of Semi Kraft Paper. At last, numbers of Dryers are used to dry the Paper to 6-8% moisture contents.

Multi paper covered copper conductor for transformers…

E. Dryer section

Pressed paper sheet with the moisture of 57-58% is passed through a number of dryer cylinders which are heated through steam to evaporate water to make the Paper dry. At the moisture of around 45% the paper sheet is passed through M.G. Cylinder to impart one side glaze to the Paper, which is a basic requirement of Semi Kraft Paper. At last, numbers of Dryers are used to dry the Paper to 6-8% moisture contents.

F. Finishing and packaging

Finally the Paper is wound on Pope Reel and sent for Slitting Rewinder to cut it in small sizes in Reel Form as per the requirements.

Types of paper

A. Epoxy – impregnated paper

As per Atkinson & Thomas in April, 1967, Epoxy-impregnated paper offers a solid insulation system having good thermal stability, high dynamic and static strengths both through and between laminations, high dielectric strength and little change in power factor with temperature and voltage. It has good chemical and moisture resistance, and high impermeability to gases and liquids. It is a resin-rich (approximately 70%) material offering one of the few solid insulation systems that is void-free, resulting in high corona-inception voltages.

B. Polypropylene paper

Headings, Buntin & Wesselhoft checked the properties of polypropylene paper in December, 1972 for possible use as an insulating material for power transmission cables carrying electric stresses excess of 500 kV for replacement of Kraft Paper. It has been recognised for some time that oil-impregnated cellulose paper will not be a useful insulating material for cables carrying in excess of 500 kV because of prohibitive dielectric loss heating and excessive increase of insulation thickness. In order to reduce dielectric losses, increase dielectric strength, and hence reduce insulation thickness in oil-filled cables, the cellulose paper has to be replaced by some properly chosen synthetic material that exhibits better dielectric properties than cellulose paper does. Such material should also have sufficient mechanical strength – and it should be compatible with liquid impregnants. Paper-like structures obtained from such polymeric materials as polyphenylene oxides and polyolefin’s have shown considerable promise as replacements for cellulose paper in cable insulation. While polyphenylene oxides exhibit many attractive properties for the contemplated use in oil filled cables, they are not yet available as an industrial product with fixed specifications.

C. Thermally upgraded insulating paper

Headings As per Akira Miyoshi in September, 1976 Kraft paper is used for insulation in oil-immersed equipment such as transformers. While it has given satisfactory performance on electrical and mechanical properties, thermal degradation occurs due to temperature rise in operation. Examples of these kinds of the Kraft paper are cyanoethylated Kraft paper and amine modified Kraft paper. But Miyoshi tested the kraft paper for upgradation by immersing it in new synthetic insulation oil, alkyl-naphthalene oil, and they concluded that the Kraft paper and cyanoethylated Kraft paper are less aged in alkyl-naphthalene oil with air present than in conventional mineral transformer oil with air present.

D. Multilayer stack dry kraft paper

Paper has a cellular structure, and the thickness of each sheet varies across the sheet, several sheets are stacked or rolled together to obtain the required electric strength. A sheet of paper is a bonded layered network of randomly-arranged short fibers, lying mainly in the plane of the sheet, with a denser core and rough outer layers of less-dense material. When first a mechanical load is applied to a stack of sheets at atmospheric pressure, the total thickness decreases, as some of the air between the fibers is expelled and the fibers at the interfaces between sheets bed down in the mat. This reduction in thickness (compression) increases with increasing mechanical pressure and increasing dwell time of the load. Under vacuum, the rate of compression increases, as air and adsorbed moisture are extracted.

E. Oil impregnated insulation paper

Nasrat & Kassem focused on the electrical and physical properties of oil impregnated insulation paper under different aging conditions in January 2013, and conclude that AC breakdown voltage of insulation paper used for power transformers slightly increased at low temperature. According to them Oil filled power transformers have a composite liquid-solid insulation structure, where the oil-impregnated papers are used for both mechanical and electrical purpose. Oil impregnated papers gradually age due to thermal stress, moisture and acidity; and their degradation affects the lifetime of the power transformers. Water content is an effective parameter in the mechanical properties of insulation paper and is inversely proportional to the tensile strength. Thermal aging at 110°C, 130°C and 150°C drastically affected the electrical and mechanical performances of mineral oil impregnated papers. Breakdown voltage decreased by 14.11% for 0.4 mm thickness of paper insulation, when temperature increased from 110°C to 150°C. Tensile strength resisted to a certain extent to impregnation of paper for long times at low temp, but it is very much affected by high temperatures.

Power cables with paper-impregnated insulation with lead sheath…

Conclusion

In the research work, it is concluded that Kraft paper is the backbone of Transformer Insulation system. It passes through different processes from raw material to finished product.

Researcher have developed different types of papers for the alternate of kraft paper but still kraft paper is the best choice because of its cost and easily availability.

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