A REVIEW OF CONDITION MONITORING ASSESSMENT OF POWER TRANSFORMERS INSULATION

Tanu Aggarwal 1 , Vishavdeep Jindal 2 and Jashandeep Singh 3 . 1. Research Scholar, Electrical Engineering Department, GZSCCET, Bathinda, Punjab Technical University. 2. Electrical Engineering Department, GZSCCET, Bathinda, Punjab Technical University. 3. Electrical Engineering Department, Bahra Group of Institutes, Patiala Campus, Patiala, Punjab, India. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History

In regard to condition monitoring assessment of power transformers, so many techniques and researches have been carried out till date and some being under progress. The insulation of coils of a power transformer is employed by the Kraft paper and dipped in an insulating mineral oil. Both of these insulation terms demean with the passage of time as occurrence of fault so lead to decrease in efficiency of transformer. The paper is presented to show a widespread assessment of winding insulation provided by dielectric oil and paper. The intention is to present an extensive point of view on the winding insulation of power transformers which deals to find out an alternative to mineral oil in which paper degradation can be minimized and to characterize the Kraft paper. The presented references are broadly classified in the paper would provide rapid and practical relevance.

…………………………………………………………………………………………………….... Introduction:-
Insulation is a broad-spectrum word, exercised to illustrate the means which can diminish gain or loss in heat by giving a solution as an obstruction between the fields that are considerably diverse in temperature. This insulation is an issue that is important if dealing with power transformers for efficient operation. To assess the aged condition of transformers and additional equipments installed in plant, a power plant should entail consistent methods. Occurrence of fault affects this insulation system and gives unreliable process.
Insulating media in high voltage transformers is generally made of solid paper enfolded around the windings, insulating oil usually mineral oil and pressboard in transformers to provide insulation to conductor from earth. The paper insulation provided on windings ought to be similar in temperament with oil used for insulation in tank and must not react chemically with it.

Solid Paper Insulation:-
The structure of Paper is prepared to form a leaf after melting and treating of a fiber. The untreated fiber materials are collected from plants. Thus unadulterated cellulose fiber is the wood pulp obtained by the process known as sulphite process. This is a technique to make a Kraft paper from wood pulp by processing with a range of salts having sulphurous acid which can be sulphites (SO 3 2-)  1665 leads to formation of white liquor by entailing the pulping technique with sodium byproducts and result in splitting the chemical bonding between cellulose structure and lignin. This kind of Kraft paper is highly torn resistive, more elastic and breakdown strength, permeable and durable. The paper dipped in oil during vacuum conditions acquires sudden electrical breakdown strength. To find the applications of Kraft paper in distinct fields, permeability is an important consideration as the Kraft paper having high permeability is employed for casing the rectangle shaped Cu sheets and having moderate permeability is employed in layering the coil insulation or for bushings whereas the cable insulation is incorporated with crepe type of Kraft Paper.

Liquid Oil Insulation:-
In a high voltage transformer filled with liquid, an insulating oil acts as a key product for performing functional role of cooling by force out the produced thermal heat to ventilation system and to provide electrical insulation. But the insulation term does not matter in the transformers below 1000 V due to lower heat rejection. From a century, mineral oil obtained from fractional distillation process is practically preferred as the transformer oil worldwide due to ease of availability and higher fire point. The main tank of transformer is filled with this oil in which winding coils comprising with conductor and paper insulation are immersed. To fulfill the purpose of better transformer health and conditioning, following properties of insulating oil are considered to be appropriate with the actual standards.
Flash point is a temperature at which insulating transformer oil provides adequate vapors to form a combustible air blend which results into a spark under typical situations. This Flash point is a crucial property because it can give an idea about the probability of flame shoot. Thus higher the flash point, more the oil will be heat absorbent.
Pour point is the lowest heat point at which oil starts flowing under typical conditions. The transfer of heat to cooling system affects when temperature of the oil gets lowered than this particular standard value which reduces the efficiency.
Acidity in regard with transformer oil or neutralization number is the degree of measurement for evaluating the bitter elements of impurities. This is desired to reduce the effect of presence of acid contents in oil.
Dielectric strength of oil is also well-known as breakdown voltage of transformer oil. BDV is calculated by measuring the voltage at which arc struck between electrodes placed in oil alienated at some space. Lower Breakdown voltage shows the existence of water and conducting particles in oil.
Specific Resistance of oil is to determine the resistance of oil. It is an important parameter that is most sensitive with respect to temperature. Specific resistance of insulating oil reduces hastily with a small rise in temperature.
Dielectric dissipation factor is termed as loss angle. The tan delta is used to measure of tangent of angle between current and voltage which is displaced form 90 o when capacitive effect of dielectric is appeared.
Moisture content in insulating oil is extremely objectionable because insulating properties of oil get influenced. Paper used in solid insulation on windings aged at fast rate if it comes in contact with water. Paper soaks up surplus quantity of water present in oil due to which life of the solid insulation on windings is reduced as its dielectric properties get affected. Whereas if transformer is connected with load, then oil is heated up thus the solubility of water in oil get enhanced and paper liberates water so the water content in transformer oil is increased. Contact with oxygen results in production of acids in oil and this effect make the water more soluble in oil. This acid water mixture further decomposes the oil and oil degrades at speed.
Interfacial tension is a mean to observe force of attraction among oil water molecules. It is accurately valuable to find out the occurrence of pollutants or particles which lead to oil decomposition.
The condition monitoring assessment of insulation system of power transformers describes lifetime prediction of oilpaper insulation [1][2][3]. Extensive surveys have been carried out to monitor and diagnose the problem including laboratory experimental analysis to assess the condition of insulation used in transformers by regular maintenance [4]. These diagnostic techniques include fault gas analysis, moisture analysis, dielectric monitoring, UHF sensors, acoustic sensors, off line or on line PD testing, dielectric response measurements, time response dielectric spectroscopy, dielectric discharge method and return voltage measurement, interpretation of measurements, transfer function analysis and expert system diagnostic software. The phenomenon of understanding the ageing of Kraft paper insulation shows that paper degradation is the cause of production of furan based aldehydes or hydrocarbon gases produced in mineral oil which results in change in DP (degree of polymerization) of cellulose in insulating paper or effect of various parameters using electrical, mechanical and chemical techniques is evaluated which are relevant to assess the state of both paper and oil insulation with identifying its effective life [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Thermal degradation of Kraft paper is caused due to increase in operating temperature so this insulating paper is thermally upgraded to neglect the aging by treating it with additional additives which result in formation of a new modified insulating paper such as amine modified Kraft paper, cyan ethylated paper, and carbamate modified paper by chemical modification for better performance and some other alternative oils are used as a substitute to mineral oil like alkyl-naphthalene oil, vegetable oil to increase the life of oil-paper insulation [21][22]. Mineral oil, mostly used liquid dielectric in power transformers is a main concern because of presence of poly-nuclear aromatic hydrocarbons which are not specific for insulating paper so a considerable work have been done to find an alternative liquid dielectric for power transformer for improving the oil immersed insulating paper condition . Various alternative oils such as natural esters, vegetable oils, LN 2 , coconut oil, palm oil and many more has been experimented and has given better results for paper aging condition as compared to hydrocarbon oils.
The ambition of this paper is to provide a wide-spread review in the field of oil and paper insulation assessment in power transformers. All the referred publications are listed and presented in three categories. First division concerns with the subjects of condition monitoring of power transformers, review on monitoring condition of transformer and different diagnostic techniques, measurement and knowledge for aging of Kraft paper, effects of various parameters like air, water content, temperature, change in frequency, burden and air pressure on physical, chemical and insulating properties of paper, problems assassinated with Kraft paper like deposition of copper sulphide, presence of methanol in oil, formation of furan particles which assist degradation and their suppressive effects, thermal up gradation of paper by chemical modification to improve its condition . Second and third categories include replacement to mineral oil by another liquid dielectrics covers an alternative liquid dielectrics having esters, vegetable oils in place of mineral oil and their comparative studies, palm oil and coconut oils and challenge of using mixed liquid dielectrics for high voltage transformers .
The presentation of paper is done in five segments; First segment is about introduction whereas remaining segments depict aging effect, methods employed to assess the state of insulation, Alternatives provided to mineral oil and concluding remarks.

Ageing Effects:-
Insulation aging is an undesirable phenomenon which decreases the healthy life of a transformer. The force surrounds the insulation whether it is oil or paper insulation resulting in degradation. This force can be appeared in any form of stress like heat, pressure, electrical or chemical stress which reduces the bonding strength of molecules in oil and paper and decompose them. These stresses show the way to dreadful condition of oil and cellulose. Various factors are responsible for deterioration of oil-paper insulation as described below.
Cellulose structure is made of series of molecular bonds. This series of binding is a measure of DP (degree of polymerisation) of paper. Generally a cellulose paper should have DP value greater than 1000, but degradation of paper breaks the molecular bonds present between chains which resulting decrease in DP value. If the DP value lowers to 200 then paper is no more to use as an insulator because at that point it can be easily broken and its strength is completely goes away [21].
This breakdown process of molecular chains in cellulose paper produces H 2 O particles which go through the oil and appear as a channel between both insulations. This migration of water from paper to oil results in reduction of dielectric strength of oil with increase in moisture. The whole structure of cellulose consists of hydrogen and hydroxyl bonds at each spot. The flow of water to these spots upset the bonding chains and weakens the cellulose structure. Initially, the moisture content in insulating cellulose paper is negligible and it take up a low amount just 5-6% from oil in its initial stage to maintain the balance between oil-paper combination. If the cellulose is more hydrated than oil in which it is immersed and its surrounding air then water is taken up by oil and air from paper or vice versa. With the decrease in temperature, the moisture absorption capacity is also reduced because oil becomes more viscous at cold temperature and hinders its circulation through cellulose [14].
Another factor the breakdown voltage of oil paper insulation system in transformer is raised up if heat point is low. Crepe discharge occurs at highest temperatures due to which structure of cellulose degraded and its electric strength reduces. Thickness is one more important parameter which affects the breakdown capacity. The paper more in thickness has more breakdown capacity but it lowers with increase in temperature. Measurement beyond 170 o C is not possible because paper separates into tiny portions. Electric strength obtained is approximately 13,12,11 KV for 110 o C, 130 o C, 150 o C respectively [23].
It is observed that presence of oxygen lowers the oxidation stability of oil, deteriorates oil and form acids. Due to presence of acid contents, cellulose paper composition gets damaged. Carbon-di-oxide, carbon-mono-oxide, water content, free glucose and acids are the instant derivatives produced as a result of paper degradation. Water contents produced in oil and acid formation additionally humiliate the glucose into 5-hydroxymethyl-2-furfuryl (5H2F). This aldehyde by-product is not a balanced complex so immediately rot into various furaldehydes (furans) comprising 5methyl-2-furfuryl (5M2F), furfury alcohol (2FOL), 2-acetyl furan (2ACF) and 2-furaldehyde (2-FAL) [15].
In addition, a product DBDS (Dibenzyl disulfide) found in mineral oil which causes the layering of copper sulphide on cellulose insulating material [9]. This deposit layer on paper increases rate of deterioration of paper. Thus Sinking the DBDS amount present in liquid dielectric is very effectual for suppressing the layering of copper-sulfide [10]. It is required to minimize the presence of DBDS compound in oil to increase the life span of a transformer. Thus poorer degradation of paper engrossed in alternative oils than conventional mineral oil increases the DP of paper and life time of transformer.
Among solutions to degradation of winding insulation problems and to improve thermal ageing characteristics of paper, thermally upgraded Kraft paper is adapted by addition of additive like N-butyl carbamate with nitrogen content aged at temperatures 130-170 o C. In this modified paper contamination of transformer oil due to paper is less as compared to Kraft paper [23].Thermal ageing of Kraft paper can also be reduced by using alkyl-naphthalene oil because thermal degradation products found in alkyl-naphthalene oil are not as much of conventional mineral transformer oil [24]. Thus by modification with definite nitrogen compounds, susceptibility to thermal degradation can be transformed. The adding up of preservative suppresses the temperament of ageing progression as it reacts chemically with paper which is to be degraded.

Methods Employed to Assess State of Insulation:-
Till date a number of techniques are discussed by various researchers and experts in the field of insulation of power and distribution transformers [6, 90-91]. Various methods described in earlier researches are still exercised by power industries to minimize degradation and aging. These methods comprise measurement of partial discharges (PD), insulation resistance (IR), dielectric dissipation factor, interfacial polarization (IP) using IR and frequency dispersion of capacitance, oil quality, moisture content, dissolved gas analysis (DGA), polymerization value (DPv) of insulating paper. Others are relatively new ways over the last two or three decades, including high performance liquid chromatography (HPLC) furan analysis and Interfacial polarization spectrum (IPS) by return voltage measurements etc.

Dissolved Gas Analysis:-
The DGA is a powerful method which can significantly identify the condition of transformer. This procedure is widely used to find out the inner faults in operational transformers. In Gas chromatography, the fault identification is totally based upon the type and amount of hydrocarbon gases which present in oil under faulty conditions. So to analyze the dissolved gases by D.G.A is a trustworthy procedure to diagnose the healthy condition of transformer whose operation has been restricted by assumed false operation of differential protection as there may be inrush current or problem appeared due to wrong C.T. connections and suction difficulties in Buchholz relay can be removed and it can be in service back with more assurance on the basis of DGA outcomes [61].

Frequency Response Analysis:-
Frequency Response Analysis is analyzed to spot the faults which have an effect on impedance of transformer and also helpful to detect winding movement [6].
In Sweep Analysis of FRA, two sweep frequency responses are compared; analyzed and any considerable difference in regions of low frequency, change of existing resonance, conception of new resonance, altering the shape of plot would tell about a problem incurred with in winding and core of transformer whether it would be a mechanical or electrical fault type. With any type of structural damage in transformer, frequency response will lead to change in RLC network and by using FRA technique these changes in RLC network are tinted and problem is identified with in the transformer. The insulating system has never uniform electrical stress. Materials having low dielectric current get highly stressed. An increase in the stress beyond a specified limit results in partial insulation failure. So the discharge appeared in the part of insulation system is termed as partial discharge. Partial Discharge is a cause of inappropriate design and service provided to the transformer. It is not a term related with discharge directly between two electrodes. PD will result in production of various insignificant problems like gas suds are produced in oil and discharge trees are formed in paper which decreases the dielectric strength of insulating system [4].
Furan Analysis:-Furan analysis is used to detect the breakdown of solid insulation i.e. paper insulation due to high temperatures, moisture and oxygen present in oil and paper. If furan test is used with DGA analysis, it is proved to be best method to analyze the absolute condition of transformer. This analysis determines amount of paper degradation when it dissolved in oil. It indicates that there is presence of carbon and hydrogen compounds. The transformer is nearer to failing condition when furan result exceeds 2500 ppb. It is basically important to decide that when to replace an operational transformer working from many years with a new one [8].
A rough estimation about life of insulation is that, for every 6° to 8° Celsius rise in normal temperature of operation, it get approximately halved. Furans produced from this temperature rise affect the insulation by two ways: the first is the damage of paper insulation by local area heating, and the second is heating of complete insulation system.
Acids are produced and get reacted with insulation system and metallic structure which result in the formation of soaps and more acids, causing a sadistic cycle. Oxygen inhibitor is required element for enhancing the life of transformers. The inhibitor presently in use is Ditertiary Butyl Para cresol (DBPC). This works like a sacrificial anode in grounding circuits and protects the insulation as oxygen react with inhibitor added instead of the insulating paper. When the transformer gets aged and the inhibitor is exhausted thus needs to be replaced. With replacement of the inhibitor it requires that the oil filled should also be treated.

Corrosive Sulphur:-
Corrosive sulphur is a rickety and thermally unstable compound dissolved in oil which corrodes the copper and metallic parts of transformer. This sulfur is appeared as a compound DBDS (Dibenzyl disulfide) found in mineral oil that causes the deposition of copper sulphide on paper insulating material. When sulphur present in DBDS comes in contact with copper leads to layering of copper-sulphide on insulating paper due to which electric strength of insulation is reduced [9]. Decreasing the DBDS value partially in mineral oil will enhance life span of transformers [10].

Dielectric Dissipation factor:-
Electrical stress is also a major concern for insulation degradation. Every electrical device endures stress from mechanical vibration, in service voltage, rise in temperature, gaseous and solid metallic impurities. These stresses will lead to degradation of insulation by gradual ageing of the equipment parts and at times it will result to avalanche of Insulation breakdown. In the whole power system ageing of small insulation may cause terrible collapse of the system, leading to intense failure. It is desired to be familiar with the specified feature of insulation required for the particular device used in the power system to avoid these types of surprising collapses and for continuous service. Also it is necessary to monitor the insulation properties of all major equipments installed in power system from time to time [4].
Metal Analysis:-This is used to set off dissolved gas analysis. When DGA is applied to find out the presence of possible fault, then metal analysis will indicate which type of fault is present and where it is located. Severe faults not just degrade the oil and solid insulation of transformer but also lead to production of metal particles that go away with oil. As the oil circulates, these metals will be disseminated all over the transformer. Some of the components of transformer machinery produce certain kind of metal particles which can be found unaided or a composition of different metals based upon the type of fault. Depending upon the composition and concentration of these metals present in system will give an idea to find out which metal component is involved in which kind of fault.

Insulation Resistance:-
This test is helpful in identifying mechanical irregularities and impurities in the dielectric (may appeared during manufacturing) due to which insulation get affected. Insulation resistance should be high reliable with satisfactory insulating properties. The test is employed when connections are loose or there occur an undesired ground. To test for unplanned core grounds, take away the planned ground and Megger between the core and the grounded transformer tank. By using this test, a very high resistance will be provided which indicate that an unplanned ground is not in system. This test is done in addition to DGA that describes the production of gases and to point out the problem resulting from an unintentional core ground.

Alternatives to Mineral Oil:-
The liquid dielectric mineral oil used in transformers from many years is preferred usually because it can be used for both purposes as a coolant and an insulator, has better characteristics, easily available, less costly, and the main property is that it is freely compatible with insulating Kraft paper. But the major issue is paper degradation which increases as the temperature increases. So in regard to it various alternatives are provided from time to time for improving paper condition and transformer operation.
Ester fluids are proven to be a better alternative of mineral oil because fire points and flash points of Esters are more than mineral oil thus safe to use in transformers as insulating oil. Instead of that Ester fluids are readily biodegrade than mineral oils . The synthetic esters give up ethylene upon aging whereas natural esters give way to hydrogen and ethane. In addition, at temperatures more than 130 o C, the aging of cellulose in synthetic esters or mineral oil is more than that experimental in natural esters [46].Also paper ageing in natural ester gives lesser amounts of CO, CO 2 , 2-FAL, and 5-HMF as compared to synthetic and mineral oils. Despite the higher viscosity of natural esters, they possess higher gravity and heat than mineral oil.
Vegetable Oils are triacylglycerols. These are the products obtained by estrification of glycerol by fatty acids . Vegetable oils with lower poly instauration show better oxidation stability. These oils are suitable substitute of mineral oil in low electrical energy applications. Below 130 o C, the paper gets aged at faster speed in vegetable fluids as compared to mineral oil. However, above 130 o C, the quantity of furaldehyde products in it is less than in mineral oil. Similarly these oils possess large degree of polymerization values for cellulose insulation rather than mineral oils which shows the paper exploitation in vegetable oil is less [54][55][56][57][58][59][60][61][62][63][64][65]. The fatty acids produced in this oil with the reaction of water are transestrified by OH ionic compounds present in paper which prevents the paper deterioration. The speed of prevention of paper from deterioration is fast. The second reason for less paper aging in vegetable oil is due to a layering of paper with a thicker jelly type element which is produced as a result of aging of vegetable oil. This substance provides protection to the paper commencing aging as a safeguard of cellulose material.
Coconut Oil consist liberated fatty acids and are more conductive because of it. Refined Bleached and Deodorized (RBD) coconut oil is well again suitable for the replacement of transformer oil as compared to all its natural forms copra and virgin coconut oils available commercially. However there are some inadequacies found in its substantial behavior but still it is a better option [49].
Jatrophacurcas Seeds Oil can also be referred as an alternative to transformer oil in some research works done earlier [71]. This Jatrophacurcas oil is a non food crop. Jatrophacurcas oil is routed by catalytic estrification technique by means of potassium hydroxide to generate JMEO (jatrophacurcas methyl ester oil) whose viscous and acidic point is suitable for high power applications particularly in transformers. This oil type possess numerous benefits that suggest equally its making and practice than other vegetable oil types as palm oil and rapeseed oil as these crops are mature on sullied and trivial soil which avoid the use of more productive lands which are currently in use by some farmers to cultivate their crops. It will willingly nurture in regions where yearly precipitation levels are appreciably lesser than requisite by other vegetable oil species. For instance, this vegetation can unproductive lands are set up in various areas of eastern regions that stay intact because of the obscurity placed in the ground with different vegetation types.
Mixed Fluids are used as transformer oil by mixing two or more oil types to provide a better oil type. In earlier times, mineral oil is the only oil which could be used as insulating liquid dielectric fluid in power transformers. The reason for which mineral oil is not environment friendly is its no biodegradability. So to avoid or to conquer the difficulties arose in mineral oil, vegetable oil is found as a replacement for transformer mineral oil. These alternative fluids are considerably better biodegradable and are more compatible to the environment. In addition to this, the refined natural esters, synthetic esters and vegetable oils are selected and can be mixed in desired amounts. Thus mixed fluids obtained by this procedure gives higher values of dielectric strength as compared to other fluids [92,93].
Concluding Remarks:-An extensive review of conditioning assessment for insulation in case of power transformers is accessible to make available a comprehensible point of view on a variety of characteristics of these techniques to the research holders, assistant engineers and commercial manufacturing industrialists. The winding insulation degrades due to presence of furan degradation products in mineral oil which are not specific for the paper condition. According to various referred publications it is clear that need of alternative oils is due to inability of mineral oils to biodegrade easily. Due to which various biodegradable oils are used in place of non biodegradable conventional oils which improve the paper condition hence lifetime of power transformers.