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  1. #1
      sayed saad
    Sep 2005
    6,249





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  2. #21
    Junior Engineer
    Sep 2006
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  3. #22
    Junior Engineer   mohammed ahmed azab
    Oct 2005
    cairo
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  4. #23
    Junior Engineer   khaled ahmed
    Oct 2006
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  5. #24
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    Sep 2006
    saudi arabia
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    POWER TRANSFORMER
    CONDITION ASSESSMENT

    Presented by:- Mohammed Farhan Al-Ghamdi, Plant Manager, SWCC, Jeedah
    Prepared by:- 1) Osama Mahjoub, Electrical Engineer, Maintenance Division, SWCC, Jeddah
    and 2) Raj Kumar, Training Specialist, Operation Division, SWCC, Jeddah

    Abstract

    Though the evolution of transformer dates back to the early 1800s but commercially introduced in 1885. Before the invention of the transformer, electric power was distributed as direct current (D.C.) at low voltage which limited its use to urban areas. With the introduction of transformer it became feasible to generate electric power at locations best suited for generation and transmit it many kilometers away at load demand point.

    Transformer main purpose is to convert one voltage-ampere ratio of electric energy into other voltage-ampere ratio. Power transformers have to carry very heavy currents which results in load losses and no-load losses. These losses in the transformer are converted to heat which heats up the conductor, core and insulation. The long lasting life of the transformer requires the proper dissipation of heat to prevent excessive temperature rise and injury to insulation on the conductors.

    The heat is dissipated to surrounding oil of core and insulated conductor. The main purpose of oil is cooling and providing insulation. The life of transformer is the life of the oil and paper insulation. Once the oil gets degraded it starts attacking insulation. The fluid may be reclaimed or changed during the service life of transformer but nothing can be done to insulation- the cellulosic paper. The condition in which the insulation system is maintained determines the life of the power transformer between 20 to 50years.. A properly maintained power transformer may have a practical life of 50-75years.

    Phase-4 Generator-transformer of capacity 147MVA started failing after giving a service of 20 to 22 years. In depth study was made for the cause of failure which were requiring special tests on transformer oil to know the condition of oil and of insulation-cellulosic paper. These special tests like FURAN analysis & Degree of Polymerization and other tests revealed the degradation of oil and insulating paper. Due to oxidation of oil, peroxides, high acidity and sludge formation were to such an extent that cellulosic paper was badly attacked by the oil. The insulation- cellulosic paper was deteriorated to a degree that the jolt and high fault current due to system fault and surges in the grid resulted complete failure of conductor insulation and flash over inside the transformer which led to premature failure and explosion of some transformer.

    To increase the life expectancy of the transformer now a program of reclaiming of oil has been taken of Phase 3 transformers. For this, purification machine of PEDCO Ltd. Filter Vac RSM-5000-14 is employed. It purifies by means of a set of clay columns that incorporates the latest PERMASORB process.
    INTRODUCTION

    The electrical energy is transmitted from generating station to load demand point by high voltage system so that by reducing current a lot of saving in current carrying, copper- conductor is made. While designing the high voltage system a economical balance is made between level of high voltage and current reduction.. The function of stepping up and stepping down voltage is done by a transformer.

    A transformer is a static machine which has no moving part but its wear and tear depends completely on maintaining quality of oil and insulation. Transformer does not die of old age. As the Power transformer has to stand high voltage and is subjected to heavy electrical energy so the maintenance of insulation on the conductor is of prime importance. The process of deterioration of oil-impregnated cellulose, through the loss of mechanical and electrical strength, throughout its useful life, can be slowed down by cleaning the mineral oil, making it as good as new oil or by replacing with the new oil.

    Main Features of Jeddah Plant
    Jeddah plant has 4 phases.
    1) Phase 1 has no generation but it has two RO plants. The electrical energy is drawn from Desalting Plant 380kv bus bar by means of two 60MVA transformer.
    2) Phase 2 has two Generators each of 42.5MW and one Gas turbine of 18MW. The power is exported to 110kv grid by stepping-up voltage from 13.8kv to 110kv by means of 3 power transformers.
    3) Phase 3 has four generators each of 64MW and Phase 4 has five generators each of 118MW. The Power of phase-3 and 4 is exported to 380kv grid by means of 9 step-up transformers.
    4) 380kv and 110kv of Desalting plant sub-station are interconnected by means of three inter-bus transformer each of 250MVA.

    Features of Phase # 4 Generators
    1) Make ErcoleMarelli, Italy
    2) Capacity: 118MW at 0.8 p.f. with hydrogen cooling
    3) Output voltage 13.8kv
    4) Phase current : 6171 amp
    5) Excitation Exciter of brushless type with rotating diode
    6) Winding connection Star with neutral grounding through a transformer.
    7) Generator Transformer Generator connected to 13.8kv, LV winding of Tx.

    Main Features of J-4 Generator-Transformer which failed/ exploded
    1) Make Industrie Elettriche di legnano, S.p.A.
    2) Year of Manufacture 1979
    3) Capacity 147,500KVA (with 2 coolers in service)
    (Cooling water temp. =350C, Ambient temp.=500C).
    4) Type of Cooling OFWF
    5) Vector Group YNd11 (380kv neutral solidly grounded)
    6) Rated voltage LV= 13.8kv/ HV= 403Kv (at no-load, mid. tap)
    7) Current Ratio 6171amp/ 211.3amp
    8) Type of winding H.V :- Inter-leaved disc, L.V :- Helicoidal
    9) Insulation a) Winding:- Pure cellulose tape
    b) Core bolts & washers Bakelite,
    c) Core lamination Carlyte.
    10)Thickness of Tank a) Side:- 8mm, b) Bottom:- 10mm
    11) Total weight 206,000Kg
    12) Oil weight 68,000Kg (77Klt).
    13) No. of taps on 380kv wdg:- 8 11.33 %

    DISCUSSION

    A) MINERAL OIL
    1) Function of Mineral Oil
    Mineral oil in the oil filled transformers is intended to:-
    1) Provide dielectric strength of the transformer insulation system
    2) Provide efficient cooling
    3) Protect transformer core and coil assembly from chemical attack
    4) Prevent the buildup of sludge in the transformer

    Continue to operate a transformer with bad oil, which has lost the properties to perform above function, significantly reduces the transformer life expectancy.

    2) Goal of Mineral oil:-
    Based on the functions of good oil, the goals to be attained are
    1) Operating the transformer in sludge free range
    2) Preventing premature transformer failure due to oil oxidation products
    3) Conserving two very valuable resources- oil and transformers

    3) Oil Decay:-
    Deterioration begins as soon as the oil is placed in the equipment at the factory itself. Decay is in two forms
    1) Contamination:- When oil contains moisture or other foreign substances that are not products of oxidation of the oil
    2) Deterioration:- When the oil is effected by its oxidation.
    The oxidation of oil begins when oxygen combines with unstable hydrocarbon impurities under the catalytic effect of the other materials in the transformer. Certain other factors accelerate heavily the sludge reaction.

    If a used oil , unfit for further use is examined , it may be found that 80% of the hydrocarbons present in used oil are unchanged and can be reused if the oxidized products are completely eliminated. It means that the oxidation of an oil is that of the impurities present in the oil and not that of the hydrocarbons. Pure hydrocarbons are not easily oxidized.



    4) Factors responsible for oil deterioration:-
    a) Oxygen
     Oxygen is derived from the air inside the transformer. Even by vacuum filling, all oxygen can not be removed. At least 0.25% by volume remains inside.
     With the dissolved air in oil, the oxygen is present in higher ratio as it has higher solubility as compared with nitrogen (16:7). Transformer oil has special affinity for oxygen.
     Degrading of cellulose especially via heat is also a source of oxygen..
     The natural oxygen inhibitors in new insulating oil are gradually depleted with time, increasing the rate of oxidation throughout the in-service life of the oil.

    b) Catalysts:-
    Catalysts is the primary substances that increases the rate of oil degradation without being consumed in the process
    Moisture :- Externally , it can enter into oil via a leak as condensation, or internally through the chemical process of oxidation. Water is a major catalyst in oil oxidation. All forms of water contain additional oxygen.
    Copper and Iron:- All the prior discussion shows the beginning stages of deterioration. This is a long term process, catalyzed also by copper (windings) and iron (core)

    c) Accelerators:-
     Heat
     Vibration (120cycle mechanical vibration due to 60Hz power)
     Shock loading
     Surge voltages and high electrical stresses

    Several secondary factors called accelerators increase oil oxidation. Like:
    I. Heat Depending upon transformer temperature , time taken for oxygen to combine chemically with oil varies.. At 750C, e.g. it takes about 5 days to combine , in contrast at 500C, it takes several months.
    II. Vibration and shock- Electromechanical vibration and sudden electrical or mechanical shock also speed up reaction
    III. Electrical stress- In newer transformer designs, because of weight and dimension restriction, oil ducts are narrower and the electric stress, expressed in KV/ cm, is increased. See the table for oxidation acceleration under low electric stresses.
    IV. Hydrogen and light hydrocarbons, like methane and carbon oxides are observed in relatively large amounts in the gaseous oxidation state

    d) Cellulosic Materials:-
    These exerts an additive effect on the oil ageing process. This is just one of the natural effects within the transformer.





    Table- Acceleration of oxidation under electric stress
    Oxidizability of transformer oils under electric stresses
    Oil test method Electrical stress KV/ cm
    0 49 0 49
    (Fatty) (Heavy) acid number mg/KOH/gm 0.1 0.13 100% 130%
    H2O soluble acid mg/KOH/gm 0.032 0.049 100% 153%
    H2O weight in % 0.003 0.017 100% 566%
    Tan  at 700C 5.5  100% 195%
    O2 absorption ml/100g oil 28.5 48.5 100% 170%

    5) Other phenomena of oil under electric stress:
    a) Visible particles in the deposits are much larger
    b) Accumulation of contamination deposits in zone of maximum intensity is characteristic.
    c) Deposit is not uniform but forms separate elongated sections
    d) Even formation of additional water is accelerated under electric stress.

    6) Increased electrical stresses thus:
    a) Interferes with heat transfer
    b) Enhance aging of cellulose insulation through loss of mechanical strength
    c) Forms conducting bridges in transformer insulation leading to a decrease in electrical resistance.

    7) Transformer Oil Decay
    It is generally agreed that peroxides, acids, alcohols, ketones and sludge are formed as the major stages in the deterioration process.
    i) Early Deterioration Stages and ii) Final Visible Stage (Sludge)

    i) Early Deterioration Stages
    Once a series of PEROXIDES are formed , these unstable compounds start a chain reaction. The cellulose insulating paper (paper, cotton and so forth) reacts with peroxides, resulting in OXYCELLULOSE. This compound lacks in mechanical strength, with embrittled insulation materials as the result.. Such embrittled insulation can not withstand the shock produced by surge voltages. Because of the porous structure of insulating paper they are considered absorbents. Decay products are Peroxide gas, acids (solubles & fatty), water, Alcohols, ****llic soaps (copper & ferrous naphthenate), Aldehydes, Ketones, Lacquers, Sludge of asphaltene

    ii) Final Visible Stage (Sludge)
    Sludge is the visible sign that the oxidation process has long been at work. It is partially conductive substance which is moderately soluble in oil and it increases even under low electric stress of 10kv/cm.
    Sludge comes when acids attack iron, copper, varnish, paints etc. This sludge precipitates out of solution and forms a heavy tarry substance , adhering to insulation, side of wall of the tank, lodging in ventilating ducts, cooling fins and so on.

    The sludge (collection of decay products) has formed in the cellulose insulation even before precipitating into oil. This deposition works to shrink the insulation. Shrinkage causes the transformer to loose the ability to absorb shock loading, so coil movement under load brings about premature failure. A deposit of 1/8 inch to inch thick on the core and windings necessitates to de-rate the transformer as operating temperature may increase by 10 to 150C
    Important is that oil can be used again for original purpose after removal of oxidation products.

    8) Mechanics of sludge Formation
    As per ASTM study oil oxidation consists of two main cycles of reaction:
    I. Formation of oil soluble decay products such as acids. It begins as soon as oil is put in operation.
    II. Change of soluble oxidation products into insoluble compounds.

    Sludge precipitates first on cold parts then onto the hot parts and it takes place periodically rather than continuously. Build up progresses layer by layer. Inner layer, in contact with coil is thin and as progressing to outer layer it becomes thicker and thicker. Layers have varying degree of hardness depending upon the extent of oxidation. The innermost layer having contact with heat source continues to oxidize and finally becomes insoluble forming sludge.


    B. INSULATION CELLULOSIC PAPER
    1) Insulating Materials:-
    Various insulating material to insulate transformer windings are available like press board, Kraft board, hemp paper, pressboard, laminated board, enamels, porcelain, epoxy coating, vulcanized rubber, cotton, plastics and glass fiber bands etc.

    A practical insulation performs four major function:_
    a) Withstand high voltages of normal service , dielectric strength, including impulse and transient surges.
    b) Withstand mechanical and thermal (heat) stresses including due to short circuits
    c) Prevent excessive heat accumulations (heat transfer)
    d) Maintain desired characteristics for the service life, when given proper maintenance

    The effect of temperature upon insulating material is so significant that the thermal characteristic has become the basis for the classification.
    Considering the above factors cellulosic insulation and mineral oil form the strongest electrical-mechanical insulating system for power and distribution transformers.

    2) The benefits of cellulose includes:
    i. Availability and low cost
    ii. Relative ease with which it can be applied
    iii. Flexibility and simplicity of handling and shaping
    iv. Moderate strength combined with lightness
    v. Facility with which these materials absorb impregnating material

    3) The oil impregnation is used in order to
    i. wet the fibrous components to maintain higher dielectric and chemical stability.
    ii. Seal the cellulose materials from moisture absorption
    iii. Fill any void to eliminate air pocket in high voltage to prevent dielectric breakdown
    iv. Prevent contact with oxygen

    4) Cellulose Structure:-
    Cellulose is a vegetable substance formed from repeated glucose units. Its molecular formula is (C6H10O5)n.The degree of polymerization depends upon material source and method of formulation. Cellulosic material used for electrical paper is manufactured from coniferous wood pulped by Kraft process. Kraft process consists of boiling chips of wood with alkaline solution of sodium sulfide and sodium hydroxide using sodium sulfate make-up solution.

    Cellulose consists of carbon, hydrogen and oxygen while transformer oil is predominantly made of carbon and hydrogen. The observation of cellulose structure indicates there are three hydroxyl groups (OH) in each glucose unit. Presence of these hydroxyls is ******ionable characteristic as these are potential water formers. Moreover around hydroxyls various polar molecules (e.g. acids, alcohols and water) are attached by the hydrogen bonds.
    These two factors constitute natural seeds of destruction.
    Chemical structure of cellulose






    The cellulosic absorption of water weakens the interacting forces between the secondary hydroxyls. Thus, as the cellulosic structure becomes unstable, a rapid decrease in mechanical properties takes place.





    5) Factors considered in selecting paper insulation:-
    i. Mechanical strength
    a) Tensile strength (and elongation) b) Bursting strength
    c) Tear strength d)Toughness
    Tensile strength has formed the basis for the loss of life of winding insulation and overload guides.
    ii. Density (compactness)
    iii. Impermeability and Porosity
    iv. Uniformity

    6) Three major steps taken place to produce improved insulation paper:-
    1) Modification of the cellulosic fiber
    2) Use of stabilizing chemical additive
    3) And improvement in the paper making process.
    Most recently, standard additive dicyandiamide, is added at the paper mill for the cost measures. These dicyandiamide treated (Amine) Kraft paper, thermally upgraded, gives 25 to 30 % longer life in addition to better heat resistance.

    7) Cellulosic Degradation:-
    Aging or gradual degradation is the result of several chemical reactions within the transformer. The solid insulation is one part of the transformer that has an ageing characteristic which is irreversible. When the cellulosic paper is neglected, any loss in mechanical and electrical strength can never be regained. Therefore the life of transformer is life of the cellulosic paper. But in contrast mineral oil can be cleaned, making it as good as new or can be replaced.
    The process of deterioration of oil-impregnated cellulose starts at the paper mill and continues throughout its useful life.
    Block diagram- Degradation of insulation-Cellulosic paper:-

























    8) Following specific effects of cellulosic deteriorating can lead to premature transformer failure:-

    i) Cracking of the cellulose:-
    Embrittlement occurs when moisture is driven out from paper to oil. Embrittled cellulose without excessive moisture is good electrically but failure occurs as cellulose falls apart or mechanically distorted by vibration, short circuit or switching operation.

    ii) Loss of Cellulosic Mechanical Strength:-
    When cellulosic insulation is heated for any length of time, a proportional decrease of mechanical strength occurs. Insulation resistance reading increases as moisture is driven out, however high current of short circuit of six times rated current or even shock loading can not be bear by insulating cellulose paper.

    iii) Shrinkage of the Cellulose:-
    Embrittlement because of heat, causes shrinkage of insulation. This shrinkage makes the winding to get loosen and it grows progressively loosen and loosen over the service life. The windings compressing screws get loosen which permits additional coil movement by vibrations or surge voltages.

    iv) Incomplete Maintenance Procedures:-
    Improper inspection may fail to find the faults in transformer. For example in a sealed transformer empty nitrogen bottle or leakage from the pressure gauge may result entry of oxygen.

    9) Insulation characteristics affected by following factors
    The factors of influence on insulation are:-
    1) Thermal, mechanical, chemical and electrical stresses
    2) Moisture and oxygen *******
    3) Impact loading (short circuit duty)
    4) Degree of overloading

    10) Dielectric stresses
    Transformer operates under 60Hz excitation. This results 120cycle mechanical vibration of the structure under influence of 60Hz power which results in voltage stress.

    A second major dielectric stress is lightning surges. When lightning hits directly electrical system or indirect surges when hitting in the vicinity, surges reaches its peak value in 1.2 microseconds and decays to half value in 50microsecond

    The third stress is due to switching surges, caused by either line switching of system operations or by abnormalities. Typical switching surges reaches maximum value in 230 seconds and decays to half value in 2000seconds.. In contrast 60Hz time requires 4167microseconds to reach its peak value``.


    11) Short Circuit Stresses:-
    Through faults gives rise an excessive current which are not only more severe but more numerous. Transformer failure as a result of short circuit is not because of direct thermal damage of insulation but the mechanical forces produced in the windings. The interaction between windings results in both horizontal and vertical electromagnetic forces.

    The vertical or axial forces, causing the LV and HV windings to shift with respect to each other, a condition called telescoping, increases the magnetic flux of the system . If two windings are in series, the electromagnetic force varies as square of current. Means a short circuit current of 20 times normal will produce (20)2 or 400times the normal stress.

    The vertical force between primary and secondary windings results because the low and high voltage electrical center lines can not be exactly balanced. It can be eliminated if two coils occupy same space at same time, which is impossible. It can be minimized if manufacturer is diligent in design and workmanship.

    Horizontal force (radial or hoop) is the major force but vertical component is most difficult for which to design and manufacture. Horizontal repulsive radial forces tend to make the low voltage coil buckle inward and the high voltage coil buckle outward.

    All three stresses dielectric, mechanical & thermal occur simultaneously. Fault current can cause a hot spot temperature of 5000F due to I2R losses.

    Indeed the transformer does more than hum. Designer must recognize these various stresses effects.

    12) Failure in service or in test can result from:-
    1) Inadequate insulation as a design error
    2) Improper installation
    3) Presence of partial discharge, corona
    4) Long time deterioration due to thermal and voltage stress
    5) Improper or no maintenance
    6) Excessive and overloading.

    13) 60Hz Transient Over voltage:-
    Normal operation of 3-phase transformer results in a phase to ground stress of 58% of the phase to phase stress. In a single fault condition, the stress on the major insulation may be higher by 30% for a grounded system or 73% for an ungrounded system.

    14) Lightning Impulse:-
    A lightning impulse is not more than 50microseconds but sufficient to cause failure because of oil stress produced by the voltage oscillation. The very rapid increase and collapse of voltage stresses the bushing, the winding leads and the first few turns of the winding.

    Lightning (Surge) arresters are the means to protect against surges. They should be located as close as possible to the terminals of transformer, preferably mounted directly on transformer.
    15) Switching surges:-
    The front of this wave is slow enough so that voltage distribution is approximately linear. The impulse is transferred to other winding in proportion to turn ratio between two windings. The surge can creep around the major insulation and high voltage can puncture the primary layer insulation The resultant failure is in the major or phase to phase insulation.

    ANALYSIS:-
    The above discussion shows that transformer mineral oil and cellulosic insulating materials, like other organic materials, are changed chemically while in service under the influence of moisture, oxygen, heat and catalyzed by copper and iron building materials. This process is termed ageing..

    Oil, if not vacuum processed into the equipment, can dissolve 10 to 12% gas by volume. The oxygen in the gas leads to oxidation. Copper, water and iron are catalysts in oil deterioration. Cellulose when heated also provides a source of moisture and oxygen.

    Prevention of sludge formation is much easier than the removal . Oils should be serviced before they reach the critical acid number, i.e. the sludging stage, in order to assure long life.

    Main Tests for oil and insulating paper:-
    OIL analysis:-
    1) Dissolved gas analysis (DGA)
    2) Acidity
    3) Tan delta
    4) Dielectric strength test
    5) Moisture
    6) Interfacial tension test
    7) Oil power factor
    8) 2-furaldehyde (Furan)
    9) Dissipation factor
    10) Dielectric strength test.
    11) Water ******* test.
    PAPER analysis
    1) Furan analysis
    2) Degree of polymerization.

    Other Tests on Oil:- These tests are not essential but can be used to establish type identification
    1) Oxidation stability
    2) Flash point
    3) Compatibility
    4) Pour point
    5) Density
    6) Viscosity

    Diagnostic methods:-
    Following are the main diagnostic methods:-
    Main diagnostic methods
    Type Method ****** Online Offline
    Electrical Resistance measurement Detection of breaks and/or bad contacts for all taps ●
    Insulation resistance measurement Determination of dielectric strength ●
    Transmission ratio measurement Detection of winding- or layer short circuits ●
    FRA (Frequency Response Analysis) Detection of winding buckling and coil displacement ●
    RVM (Recovery Voltage Measurement) Determination of humidity of the paper insulation material / Aging ●
    FDS (Frequency Domain Spectroscopy) Determination of humidity of the paper insulation material ●
    PDC (Polarization Depolarization Current) Determination of humidity of the paper insulation material ●
    Partial Discharge gaging Detection of weak spots inside the insulation system ●
    Chemical DGA (Dissolved Gas Analysis) Integral determination of electrical and/or thermal faults ● ●
    Oil parameter Determination of oil quality ● ●
    Furan analysis Estimation of paper aging ● ●
    DP (Degree of Polymerization) Determination of paper aging ●
    Optical Visual inspection Detection of leakages, contaminations etc. ●
    Thermography Detection of heat sources and hot spots ●
    UV-Camera Detection of Corona impulses ●

    Table- Condition of insulation signified by presence of water in oil
    Percent Saturation Water in oil Condition of cellulosic insulation
    0 - 5 % Dry insulation
    6 - 20 % Moderate wet, low numbers indicate fairly dry to moderate levels of water in the insulation. Values toward the upper limit indicate moderately wet insulation
    21 - 30% Wet insulation
    > 30 % Extremely wet insulation
    Transformer oil color and classification
    Pale Yellow Yellow Bright yellow Amber Brown Dark Brown Black
    GOOD PROP A MARGINAL BAD VERY BAD EXTREMLY BAD CLASS 7 OILS
    Effect on transformer
    Providing these functions
    1. Efficient cooling
    2. Preserving insulation Polar compounds (sludge) in solution (products of oil oxidation) causes the drop in IFT Fatty acids coat the windings. Sludge in solution ready for initial fall-out. Sludge in insulation voids highly probable. In almost 100% of transformers in this range sludge are deposited on core and coils. Sludge are first deposited in fin areas Deposited sludge continue to oxidize and harden.. Insulation shrinkage is taking place.. Premature failure a good possibility Sludge insulate cooling fins, block vents causing higher operating temperature
    Acid (Neutralization) Number mg. KOH/ g
    0.03 to 0.10 0.05 to 0.10 0.11 to 0.15 0.16 to 0.40 0.41 to 0.65 0.66 to 1.50 1.50 and higher
    Interfacial Tension Dynes/ cm
    30-45 27-29 24-27 18-24 14-18 19-14 6-9




    A precise classification of transformer insulation oils involves the unique relationship between interfacial tension (IFT) and neutralization number (NN). Several 1940 studies show that an increase in NN should normally be followed by a typical drop in IFT.


    If the values of IFT versus NN for a given oil sample do not fall within a reasonable range on either side of the median line , further investigation is in order.



    Interfacial Tension (IFT) versus Neutralization Number (NN)
















    Fig:- The role of acid build up,
    where the critical neutralization number is 0.25 mg KOH/ gram

    How to avoid transformer failure:-
    Transformer life is the life of solid insulation-cellulosic paper. The degradation process can only be minimized though can not be completely stopped. Once degraded or failed nothing can be done to recover it.. The degradation can be slowed down by maintaining the fluid- the mineral oil condition. The fluid may be reclaimed or changed during service life depending on the condition of oil.
    Table- Reconditioning or reclaiming
    CORRECTIVE ACTIONS
    Type of contamination Symptoms Corrective actions
    Physical High water ******* Reconditioning
    Low breakdown voltage value
    High particles *******
    Turbid (not clear)
    Chemical High color value Reclaiming
    High acidity value
    High dissipation factor value
    Presence of sediments and sludge
    Note.- In some cases, if the chemical contamination is extremely high, may be economically more effective to replace the oil. A reclaiming viability test acc Std is recommended.





    Table- Showing difference of Diagnostic tests and monitoring tests
    Monitoring Diagnostic
    Off - line On - line
    Status Trends
    Cyclic analysis Permanent acquisition
    Oil, DGA, Furanes, FRA, FDS, RVM, PD DGA, H2O, load current
    2-18 kCHF/ transformer without PD measurement 15-30 kCHF / transformer

    JEDDAH PLANTS, PHASE#4 GENERATOR-TRANSFORMER FAILURE

    1) Technical details of transformer
    1) Make Industrie Elettriche di legnano, S.p.A.
    2) Year of Manufacture 1979
    3) Capacity 147,500KVA
    4) Type of Cooling OFWF
    5) Rated voltage LV= 13.8kv/ HV= 403Kv (at no-load, mid. tap)
    6) Type of winding H.V :- Inter-leaved disc, L.V :- Helicoidal
    7) Insulation a) Winding:- Pure cellulose tape
    b) Core bolts & washers Bakelite,
    c) Core lamination Carlyte.
    8)Thickness of Tank a) Side:- 8mm, b) Bottom:- 10mm
    9) Total weight 206,000Kg
    10) Oil weight 68,000Kg (77Klt).
    11) No. of taps on 380kv wdg:- 8 11.33 %


    2) Failure date and test result of Furan

    S.No. Transformer Name Date of Failure 2-furaldehyde Furan Limit values as per ABB
    1 Generator-Tx 13 23.01.2004 260
    2 Generator-Tx 12 15.08.2003 1720 Acceptable:- <100ppb
    3 Generator-Tx 11 28.01.2005 2130 Questionable:- 101 to 249ppb
    4 Generator-Tx 10 27.01.2002 540 Unacceptable:- >249ppb
    5 Generator-Tx 9 switched off / 03.12.2005 290


    Phase -4 has 5 numbers of such Power Transformers which failed in very short span of 3 years consecutively one after another, after giving a service of about 23 -25 years.

    Transformers were continuously monitored and thought to be maintained by carrying out regular maintenance program. The tests on oil and transformer were carried out as recommended by the manufacturer and by the general practice. Transformer oil used to be tested for dissolved gas analysis and dielectric strength test etc. The transformer oil was not filtered or regenerated as the tests performed were not suggestive of doing it.

    Note that in failure of all transformers, system fault in the grid has taken place a little before the instance of failure. After the fault current flown through the transformer it failed. The short circuit fault current must have aggravated the deterioration of transformer winding insulation severely.




















    This experience has shown that the routine tests on oil and filtering of oil were not just sufficient to know the deterioration at very early stage. Special tests like Acidity, Tan delta, Interfacial tension test, Furan analysis and Appearance test must have been done to help in evaluating the condition of oil and insulation-cellulosic paper. The filtering of oil removes the insoluble impurities like sludge but not the soluble impurities like acid, oxygen and sludge forming impurities.. Moreover filtering will not remove sludge 100% in spite of the best efforts taken. The sludge lodged in narrow space like between windings and coil turns is very difficult to remove. So it is necessary to stop formation of sludge by removing oil soluble impurities.

    Many Electrical giants were contacted and then found out the solution. For removal of oil soluble impurities the best is to regenerate the oil and make it as good as new oil. If oil can not be regenerated then it must be replaced with new oil. For regenerating the oil the ECOIL Mobile machine, Model RS-M5000 of FilterVac International Inc. has been employed for Phase # 3 generator transformers



    The Table below gives difference of oil condition before and after regeneration.
    Test De******ion Method Unit Initial Condition RS-M5000 single pass quality
    Acidity IEC 296 mg KOH/ g 0.25 <0.03
    Color appearance -- Visual Brown/ cloudy Clear Light Yellow
    Breakdown Volt IEC 156 KV <30 >70
    Moisture IEC 733 ppm <2000 5
    Tan Delta (900C) IEC 247 <0.01 <0.005
    Interfacial Tension ASTM Dynes/ cm <15 >35
    Gas ******* GC % v/v 8 0.01
    Oxidation Stability IEC 74/ 164hrs -- Depleted Restored


    3) Reasons for failure
    a) Moisture
    b) Operating at high temperature
    c) Presence of oxygen
    d) Short circuit fault currents mechanical forces
    e) Electrical switching surges from the system

    4) Furan Analysis & Degree of Polymerization (D.P.)
    i) 2-furaldehyde (Furan) analysis detects the thermal oxidation and hydrolytic breakdown of paper insulation.
    ii) D.P. (Degree of Polymerization):- gives assessment about mechanical strength of the paper insulation

    5) Importance of Monitoring of transformer:-
    It is important to monitor. The reasons why it should be done and when it should be done are given below:-
    a) Increase of uptime of the transformers
    b) Reduce maintenance
    c) Priorities for maintenance actions
    d) Early detection of hidden defects
    e) Planning of replacement
    f) Planning of investments



    OIL REGENERATION:-

    During oil oxidation the following products are produced - peroxides, acids, alcohols, ketones and sludge. This is the major stage of deterioration.

    Once a series of PEROXIDES are formed, these unstable compounds start a CHAIN REACTION. The cellulose insulation (paper, cotton and so forth) reacts with peroxides, resulting in OXYCELLULOSE. This compound lacks in mechanical strength, with embrittled insulation materials as the result.. Such embrittled insulation can not withstand the shock produced by surge voltages. Because of the porous structure of insulating paper they are considered absorbents.

    This deterioration of paper insulation is indicated by special test on oil - Furan analysis and on paper - Degree of Polymerization. Once the insulating paper cellululosic paper is deteriorated , nothing can be done to insulation. As it starts from deterioration of oil, so oil must be taken care not to get it oxidized, which is practically not possible so better is to slow down the process of oxidation. With the service life of transformer, when the test results on oil and on paper suggest that the results are going out of limit then oil must be regenerated.

    Oil regeneration has good advantages over oil replacement like, transformer remains in service and transformer continuously under oil, cleans active part, long term stability, economical and cost effective as very low volume of oil (<<1%) is disposed off.

    For regeneration of oil of Phase# 3 Power Transformer, mobile transformer oil reclaiming unit of FilterVac International Inc. was employed and after regeneration oil has become like new oil.


    1) Filter Vac RSM 5000-14 ECOIL mobile transformer oil reclaiming:-
    a) Introduction:-
    The FilterVac RSM 5000-14 is composed of a transformer oil purification system coupled to a set of clay filled column that incorporate the latest PERMASORB process. The plant is designed to maximize both reliability and ease of operation. The latest in Programmable Logic Controller (PLC) personal computer (pc) and SCADA Software technology is utilized to ensure proper operation of all systems. An operator initiated Remote Access system allows PC to perform diagnostics and limited system upgrade.

    b) Operation of Plant:- The plant has three main operating modes
    i) Regeneration:-The contaminated oil from transformer tank is pumped onto the truck, heated to 700C, through the clay columns and the vacuum purification system (degasifier). Clean treated oil is discharged back to the transformer. The set of columns are used to process oil or can be reactivated.
    ii) Reactivation:- Once the clay columns become saturated with contaminants stripped from the dirty oil the clay must be reactivated. Columns are isolated from the vacuum purification loop. Hot air @ 900C is blown through the clay which burns contaminants. Any residual contaminants collected from the clay are pumped into the dirty oil storage tank. The reactivation mode is fully automated and run unattended.
    iii) Purification:- In this cycle clay columns are isolated and by-passed so that oxidation inhibitors are not removed by clay columns. The oil is processed through a vacuum purification loop to degasify and to add oxidation inhibitor. During this additive reconstitution cycle 0.3 % Degussa Antioxidant = DBPC ADDITIVE (2,6-Di-Tert-Butyl-P-Cresol) is added after getting desired sample values of IFT, color & moisture to improve oil life time to avoid oxidation.

    The Filter RSM 5000-14 will not absorb PCBs. PCB level in oil remains same as before regeneration.

    2) Advantage of using ECOIL method over conventional method:-
    In modern practice the oil regeneration is done by ECOIL Oil regeneration Mobile system. In this system oil is passed through the CLAY columns.. Clay columns can be cleaned and reactivated to utilize it again.
    Table- Advantage of ECOIL compared with conventional method
    De******ion Unit Conventional ECOIL
    Amount of waste as a % of treated oil % 10 0.05
    Amount of oil loss during reclamation as a % of treated oil % 6 0.05
    How many times Same Fuller's Earth utilized # 1 400
    Number of man hours required per 24hrs. # 48 1
    Degree of Automation Limited Full
    Estimated amount of New Fuller's Earth required for 2years operation Kg/ Pounds 576,000 / 1,250,000 1500 / 3300
    Amount of oil processed with one charge of clay L/ gallons 3000 / 800 4,000,000 / 1,060,000
    Running cost per 1L/ 1 Gallon of oil treated (Excl. labor) USD 34 US cents 0.8 cents
    Field self sufficiency W/O logistic support One week 6 months


    LATEST TECHNOLOGY OF HYBRID INSULATION:-
    High temperature insulation materials provide the key to unlock the door to the many advantages of higher temperature rise systems
    a) Nomex Thermal Protective Hybrid Insulation Technology
    i) Protect the insulation system by selective replacement of cellulose with an aramid insulation

    ii) Is used only where it is needed to prevent rapid degradation

    iii) Provides the protection of an aramid insulation, where the thermal limitation is removed from the windings, and the top oil temperature becomes the new thermal limit

    b) Why Thermal Protecting Insulating Materials ?
    i) Classic cellulose materials, when heated above 105oC, degrade prematurely and emit Water, CO and CO2, which reduces the oil quality, and limits the system capability. This results in 65 K average winding rise .
    For given quantities of active materials (core steel and conductor), the throughput can be increased if the allowable average winding temperature rise is increased beyond 65 K rise

    ii) When aramid insulation ages, it breaks down the long chain molecules into shorter chains. It does not give off water or other gases until 7500C.
    The result is more power per unit size and weight or more compact, lighter systems

    c) Advantages
    Higher reliability of the insulation
    No gas emission until 7500 C
    No furan emission
    Less degradation of the oil
    No thermal degradation at operating temperatures (< 150oC)
    No water emission
    Reduce sludge formation for better thermal conductivity
    Higher operational flexibility of the transformer
    Increase in service life
    Over-loading : continuous, peak, emergency (with oil change)
    Reduced impact on environment
    Reduced waste generation by re-use of most parts (core, tank..)

    Table- Advantage of Hybrid insulation over cellulose
    Characteristic Unit Insulation System
    Cellulose Cellulose Hybrid
    55grC 65grC 95grC
    Rated Power MVA 10 12.5 17
    Average wdg-ambient temp rise 0C 55 65 95
    Total mass Ton 24 24 24
    Total losses pu 1.00 1.4 2.1
    Relative price $/ kVA 1.00 0.84 0.70
    Relative price $/ kVA xYear 1.00 0.84 0.35

    Table- Advantage of Hybrid insulation over cellulose
    Factor Units Cellulose Hybrid
    Top oil temperature rise K 60 60
    Average winding temp. rise K 65 95
    Maximum ambient temp. 0C 40 40
    Maximum Hot Spot temp. 0C 118 170
    CONCLUSION

    Mineral oil starts deteriorating from the moment it is introduced in the transformer, means oxidation of insulating fluid begins. Nothing can be done to insulation i.e. cellulosic paper but certainly something can be done to insulating fluid i.e. mineral oil..
    Insulating oil serves two primary functions in power transformers:
    a) Provide a dielectric medium b) Provide a cooling medium
    The first purpose requires that the oil be free of water and suspended organic or inorganic matter thus acting as a good electrical insulator. The second function requires an oil of relatively low viscosity, volatility, and good heat transfer capability. Deterioration of the oil through either oxidation or contamination, can cause the transformer to overheat and prematurely fail. Only simple corrective maintenance of reclamation of oil and de-sludging can rectify the situation.

    The oil completely unfit, for further use in the transformer contains at least 80% of the hydrocarbons which are unchanged and can be reused, if the oxidized products are completely eliminated. Oxidation of oil is that of impurities present in the oil and not that of the hydrocarbons. Pure hydrocarbons are not easily oxidized under normal conditions.
    Sludge forms when acid attacks iron, copper, varnishes and paints, etc. and these materials coming into solution, combine together to form sludge. This sludge ultimately precipitates and immediately go to work to suck off the insulation, varnishes, and cellulose materials, resulting in severe shrinkage of the insulation.

    So on the basis of special test on oil like Furan analysis, Degree of Polymerization, Interfacial Tension, moisture *******, appearance, etc. the oil must be reactivated, called as regenerated. If regeneration is not possible then the oil must be replaced.

    The transformers must be protected from the high voltage of lightning and switching surges.
    To reduce the effect of surges the SURGE arrestors must be installed.

    Operating of transformer within the design capacity and keeping oil and winding temperature within prescribed limit increases the transformer life.

    The shock loading, over-voltage and over current should be avoided to have a long life.

    RECOMMENDATION
    It is highly recommended that in all over SWCC a routine should be adopted to perform special test on transformer oil like - Furan analysis, Interfacial Tension, Moisture *******, Appearance and Dissolved Gas Analysis etc. and on paper- Degree of Polymerization. Based on the result oil should go under regeneration.

    All the transformers must be protected against surges of lightning and switching, by providing Surge Arrestors as close to transformer as possible.

    REFERENCES
    1) A Guide to Transformer Maintenance (Insitu- Invivo) by S.D.Myers, Kelly & R.H.Parrish
    2) ABB Handbook on Transformer.
    3) Guidance from FilterVac International Inc.

    * * * *

  6. #25
      /
    Sep 2006
    saudi arabia
    501

    :






    TRANSFORMER OIL PURCHASE SPECIFICATION
    COMPOSITION
    Insulating oils for general application which are covered by this specification are manufactured from predominantly naphthenic base crudes. Distillates from these crudes may be acid refined, hydrogen treated, solvent extracted, or processed by other suitable refining methods to yield acceptable mineral insulating oils which meet the test requirement at the point of delivery.
    Oils from paraffinic crudes may also be covered by this specification with the exception of the requirement for aniline point and performance at low temperatures. Other requirements may be needed for these products to ensure proper function at low temperatures.
    Uninhibited oils must be free from additives of every kind, either natural or synthetic.
    Inhibited oils are insulating oils which have been supplemented with either 2,6 ditertiary-butyl phenol, 2,6 ditertiary-butyl paracresol, or any other specified and acceptable oxidation inhibitor. Inhibited oils should otherwise be free of additives.
    PRODUCTION AND SHIPPING
    Insulating oils covered by this specification shall be produced from proven crudes by suitable refining methods, both of which shall have been approved by the purchaser. After such approval, no change in crude source, processing and refining methods, or shipping containers shall be made without prior approval by the purchaser.
    Shipping containers should be dedicated to new transformer oils.
    TRANSFORMER OIL PURCHASE SPECIFICATION - TEST LIMITS
    TYPE OF TEST ASTM METHOD UNINHIBITED INHIBITED
    TYPE I TYPE II
    Aniline Point, C D 611 63-84 max 63-84 max 63-84 max
    Carbon Type Composition
    % Aromatics, Naphthenics,
    Paraffinics D 2140 No limits No limits No limits
    Color (a) D 1500 0.5 max 0.5 max 0.5 max
    Corrosive Sulfur D 1275 noncorrosive noncorrosive noncorrosive
    Dielectric Breakdown, kV D 877 30 min 30 min 30 min
    Dielectric Breakdown, kV D 1816
    (0.04" gap) 20 min 20 min 20 min
    Water *******, ppm D 1533
    (as received) 30 max 30 max 30 max
    Flash Point, C D 92 145 min 145 min 145 min
    Furanic Compounds(b)
    (optional test) D 5837 25 g/L max 25 g/L max 25 g/L max
    Impulse Breakdown Voltage,
    kV @ 25C (c) D 3300 145 min 145 min 145 min
    Interfacial Tension,
    dynes/cm, 25C D 971
    (unfiltered) 40 min 40 min 40 min
    Neutralization Number,
    mg KOH/g D 974
    (modified) (d) 0.015 max 0.015 max 0.015 max
    Pour Point, C D 97 -40 max -40 max -40 max
    Power Factor, 100C, % D 924 0.30 max 0.30 max 0.30 max
    Power Factor, 25C, % D 924 0.05 max 0.05 max 0.05 max
    Specific Gravity, 60/60 D 1298 0.910 max 0.910 max 0.910 max
    Viscosity: Kinematic cSt,
    100C
    40C
    0C
    D 445 max.
    3.0
    11.0
    76.0 max.
    3.0
    11.0
    76.0 max.
    3.0
    11.0
    76.0
    Oxidation Inhibitor
    *******, % by wt. D 2668 or
    D 4768 (e) 0.00 max 0.08 max 0.3 max
    Sludge-Free Life (SFL)
    measured at 8-hr periods, hrs,
    +8 hrs. (Doble Procedure) (f) 40 min 64 min 80 min
    and
    Power-Factor Valued
    Oxidation (PFVO) (optional test) (Doble Procedure) (f) See attached graph for limit curve. See attached graph for limit curve. See attached graph for limit curve.
    and
    Oxidation Stability (acid sludge)
    72 hours: % sludge by wt.
    Total acid no., mg KOH/g D 2440 (f)
    0.15 max
    0.5 max
    0.15 max
    0.5 max
    0.1 max
    0.3 max
    164 hours: % sludge by wt.
    Total acid no., mg KOH/g - 0.3 max
    0.6 max 0.3 max
    0.6 max 0.2 max
    0.4 max
    and
    Oxidation Stability
    (Rotating Bomb), minutes D 2112 (f) - 195 min 220 min
    Polychlorinated Biphenyls D 4059 ND (g) ND (g) ND (g)
    OPTIONAL TEST
    Gassing Tendency Under Electrical
    Stress, micro L/min @ 80C, hydrogen D 2300 negative (h) negative (h) negative (h)
    NOTES
    (a) Oil must be clear and bright.
    (b) The test is for five furanic compounds, 5-hydroxymethyl-2-furfural, furfuryl alcohol, 2-furfural, acetyl furan, 5-methyl-2-furfural. The limit of 25 g/L maximum applies to each compound.
    (c) Needle negative to sphere grounded, 1-in (25.4-mm) gap.
    (d) Neutralization number is measured by using 1/100 normal potassium hydroxide standard alcoholic solution.
    (e) Both 2,6-ditertiary butyl-paracresol and 2,6-ditertiary butyl-phenol have been found to be suitable oxidation inhibitors for use in oils meeting this specification.
    (f) Sludge-Free Life and Power Factor Valued Oxidation (PFVO) tests are performed utilizing Doble Methods. The Sludge-Free Life of an oil, sampled at 8-hour intervals, is the number of hours which have elapsed between the start of the test and the time of taking the last sample which showed a sludge-free precipitation test.
    (g) ND = none detected.
    (h) The characteristic is positive if gas is evolved under the conditions of the test, and negative if gas is absorbed.

    DOBLE TRANSFORMER OIL PURCHASE SPECIFICATION CHART

    SIGNIFICANCE OF TESTS
    The following comments and interpretations, based on both technical understanding as well as empirical knowledge, emphasize those properties which are functionally important to transformer oils:
    Aniline Point (D 611) The aniline point is the temperature at which a mixture of aniline and oil separates. It provides a rough indication of the total aromatic *******, and relates to the solvency of the oil for materials which are in contact with the oil. The lower the aniline point, the greater the solvency effect.
    Carbon Type Composition (D 2140) The carbon type composition characterizes an insulating oil in terms of the percentage of aromatic, naphthenic, and paraffinic carbons. It can be used to detect changes in oil composition and to relate certain phenomena that have been demonstrated to be related to oil composition.
    Color (D 1500) The color of a new oil is generally accepted as an index of the degree of refinement. For oils in service, an increasing or high color number is an indication of contamination, deterioration, or both.
    Corrosive Sulfur (D 1275) This test detects the presence of ******ionable quantities of elemental and thermally unstable sulfur-bearing compounds in an oil. When present, these compounds can cause corrosion of certain transformer ****ls such as copper and silver.
    Dielectric Breakdown (D 877, D 1816) The dielectric breakdown is the minimum voltage at which electrical flashover occurs in an oil. It is a measure of the ability of an oil to withstand electrical stress at power frequencies without failure. A low value for the dielectric-breakdown voltage generally serves to indicate the presence of contaminants such as water, dirt, or other conducting particles in the oil.
    Method D 1816 is more sensitive than Method D 877 to contaminants that lower the dielectric-breakdown voltage and is the preferred method for assessing the intrinsic breakdown strength of an oil.
    Water ******* (D 1533) A low water ******* is necessary to obtain and maintain acceptable electrical strength and low dielectric losses in insulation systems.
    Flash Point (D 92) The flash point is the minimum temperature at which heated oil gives off sufficient vapor to form a flammable mixture with air. It is an indicator of the volatility of the oil.
    Furanic Compounds (D 5837) Furanic compounds are generated as byproducts of the degradation of cellulosic materials such as insulating paper, pressboard, and wood. These compounds serve as indicators of insulation degradations. Because they are dissolved in the oil, furanic compounds can readily be sampled and tested by high performance liquid chromatography (HPLC). No significant quantity should be detected in new oils.
    Impulse Breakdown Voltage (D 3300) The impulse breakdown voltage is the voltage at which electrical flashover occurs in an oil under impulse conditions. It indicates the ability of an oil to resist transient voltage stresses such as those caused by nearby lightning strokes and high-voltage switching surges. The results are dependent on electrode geometry, spacing, and polarity.
    Interfacial Tension (D 971) The interfacial tension of an oil is the force in dynes per centimeter required to rupture the oil film existing at an oil-water interface. When certain contaminants such as soaps, paints, varnishes, and oxidation products are present in the oil, the film strength of the oil is weakened, thus requiring less force to rupture. For oils in service, a decreasing value indicates the accumulation of contaminants, oxidation products, or both. It is a precursor of ******ionable oxidation products which may attack the insulation and interfere with the cooling of transformer windings.
    Neutralization Number (D 974) The neutralization number of an oil is a measure of the amount of acidic or alkaline materials present. As oils age in service, the acidity and therefore the neutralization number increases. A used oil having a high neutralization number indicates that the oil is either oxidized or contaminated with materials such as varnish, paint, or other foreign matter. (A basic neutralization number results from an alkaline contaminant in the oil.)
    Pour Point (D 97) The pour point is the lowest temperature at which oil will just flow. A low pour point is important, particularly in cold climates, to ensure that the oil will circulate and serve its purpose as an insulating and cooling medium. It may be useful for identifying the type (naphthenic, paraffinic) of oils.
    Power Factor (D 924) The power factor of an insulating oil is the cosine of the phase angle between a sinusoidal potential applied to the oil and the resulting current. Power factor indicates the dielectric loss of an oil; thus the dielectric heating. A high power factor is an indication of the presence of contamination or deterioration products such as moisture, carbon or other conducting matter, ****l soaps and products of oxidation.
    Specific Gravity (D 1298) The specific gravity of an oil is the ratio of the weights of equal volumes of oil and water determined under specified conditions. In extremely cold climates, specific gravity has been used to determine whether ice, resulting from the freezing of water in oil-filled apparatus, will float on the oil and possibly result in flashover of conductors extending above the oil level. The specific gravity of mineral oil influences the heat transfer rates. Oils of different specific gravity may not readily mix when added to each other and precautions should be taken to ensure mixing.
    Oxidation Inhibitor ******* (D 2668, D 4760) These tests provide a method for the quantitative determination of the amount of oxidation inhibitor (2,6-ditertiary butyl-paracresol or 2,6 ditertiary phenol) present in an inhibited oil. Control of the inhibitor ******* is an important factor in maintaining long service life of inhibited insulating oils.
    Power Factor Valued Oxidation (PFVO) This test, developed by the Doble Engineering Company, periodically measures the power factor of an oil while it is being aged at 95C in the presence of copper and air. Consequently, it indicates the dielectric-loss characteristics of insulating oil as a function of accelerated aging conditions. The resulting graph of power factor versus time characterizes a given oil. It is applicable as a continuity test, as well as a measure of oil quality. The test is run concurrently with the Doble Sludge-Free Life test which measures the time until the oil forms sludge.
    Oxidation Stability (acid/sludge) (D 2440) The acid/sludge test is a method of assessing the oxidation resistance of an oil by determining the amount of acid/sludge products formed when tested under certain prescribed conditions. Oils which meet or exceed the requirements tend to preserve insulation system life and ensure acceptable heat transfer. The test may also be used to check the performance consistency of this characteristic of production oils.
    Oxidation Stability (D 2112) This test is a rapid method for the evaluation of the oxidation stability of new insulating oils containing an oxidation inhibitor. It is used as a control test for evaluating the response characteristics of new oils to oxidation inhibitors. It may also be used to check the performance consistency of production oils. Good oxidation stability is a principal requirement for long service life of transformer oils.
    Gassing Under Electrical Stress (D 2300) The gassing tendency is defined as the rate of gas evolved or absorbed by an insulating oil when subjected to electrical stress of sufficient intensity to cause ionization. The characteristic is positive if gas is evolved and negative if gas is absorbed. Correlation of results with equipment performance is limited at present.
    Polychlorinated Biphenyls (D 4059) Regulations prohibiting the commercial distribution of polychlorinated biphenyls (PCBs) mandate that insulating oils be examined for PCB contamination levels to assure that new products do not contain detectable amounts.
    Viscosity (D 445) Viscosity is the resistance of oil to flow under specified conditions. The viscosity of oil used as a coolant influences heat transfer rates and consequently the temperature rise of an apparatus. The viscosity of an oil also influences the speed of moving parts in tap changers and circuit breakers. High viscosity oils are less desirable, especially in cold climates. Standard viscosity curves can be generated using Method D 341 by measuring two or three data points and plotting the data on special chart paper. The resulting curve can be used to interpolate or extrapolate values at temperatures where the viscosity is not measured directly


    DISSOLVED GAS ANALYSIS
    This is the single most important test performed on oils from transformers. As the insulating materials in a transformer break down due to thermal and electrical stresses, gaseous by-products are formed which are characteristic of the type of incipient-fault condition
    WATER IN OIL ANALYSIS
    The dielectric breakdown voltage of insulating material is a function of the water *******. The water migrates between the solid and liquid insulation in a transformer with changes in temperature. The water ******* is reported in parts per million and percent of relative saturation.
    OIL QUALITY SCREENS
    Once in service, the dielectric liquid should be tested periodically to make sure it retains its important properties such as good dielectric breakdown voltage, low acidity and no sludge formation. The rate of deterioration of the insulating oil should be relative slow. Accelerated aging may indicate an equipment or operating problem. The Doble Laboratory offers a number of Package Screen Tests to suit your testing needs for new and service-aged insulating liquids. We are equipped to perform a comprehensive set of purchase specification tests, such as those for the Doble Transformer Oil Purchase Specification (TOPS) or ASTM D 3487 for new mineral oils. A variety of Package Screen Tests are available for testing of in-service oils. Let us help you select the one most appropriate for your equipment and purposes. We can also tailor a package for your requirements, or perform individual tests.
    FURANIC COMPOUNDS IN OIL
    As the cellulosic insulation in a transformer ages, oil-soluble by-products of the cellulose chain, called furanic compounds are produced. High concentrations of 2-furfural, the predominant compound, are a clear indication of cellulose degradation, as this is the only type of material in transformers that yields this byproduct. When cellulosic materials are exposed to extreme temperatures, which results in charring, furanic compounds can be destroyed and the carbon oxides may be the only by-products remaining in significant quantities. Experience is required in evaluating the furanic compound data, as there are factors such as type of insulation preservation/oil expansion system, type of conductor wrapped insulation, family of transformer, and treatment of the oil or the transformer, which can influence the interpretation. Tests for furanic compounds should be performed initially for all power transformers to have a baseline, for important or older transformers, when high carbon oxides are generated, for highly loaded transformers, and when other tests indicate accelerated aging.
    DEGREE OF POLYMERIZATION
    The degree of polymerization (DP) test is another means for assessing insulation aging and is performed on paper samples. The DP provides an estimate of the average polymer size of the cellulose molecules in materials such as paper and pressboard. Generally, paper in new transformers has a DP of about 1000. Aged paper with a DP of 150-200 has little remaining mechanical strength, and therefore makes windings more susceptible to mechanical damage during movement, particularly during extreme events such as through-faults. As insulation aging in transformers can be uneven due to thermal, moisture, oxygen, and byproduct concentration gradients samples from various locations are needed to provide the best diagnosis of the overall insulation condition. This test in recommended when there is other evidence of very accelerated aging of the insulation, the transformer is >20 years old and an internal investigation is being performed, for condition assessment of older transformers for possible refurbishment, when a partial rewind is being considered, to assess cause of failure, for condition assessment of insulation when purchasing a service-aged transformer, to assess the condition of a transformer after an extreme overheating event.
    ****LS-IN-OIL
    There are two types of ****l-in-oil tests that are commonly done on insulating oil.
    Wear ****ls: Pumped cooling systems are susceptible to bearing wear, which when excessive can create ****l particles which are deleterious to the insulation system. To detect such problems an oil sample is filtered and the particles are analyzed by atomic absorption spectroscopy for excessive amounts of copper, lead, zinc, and iron.
    Dissolved ****ls: The high temperatures associated with some incipient-fault conditions will cause the amount of dissolved ****ls associated with the problem to increase along with the dissolved gases in oil. Comparison of the ****l-in-oil ******* with baseline values before the occurrence of the incipient-fault condition can help locate the source of the gassing and the problem

    POLYCHLORINATED BIPHENYLS (PCBS)
    PCBs are highly regulated therefore insulating liquids that may contain PCBs should be tested to ensure proper handling and disposal. Doble offers a testing service to quickly and accurately quantify the amount and determine the type of PCBs present in various insulating liquids and solids or other materials such as soils. The Doble Laboratory is certified by the State of Massachusetts Department Of Environmental Protection. Ask about our extensive quality assurance program. International Customers please note that Federal Regulators prohibit the importation of oil samples with known PCB. We are unable to accept in-service oil from oversees that need PCB testing or are known to contain any detectable quantity.

  7. #26
    V.I.P Member   .
    Sep 2006
    K.S.A
    67

    :


  8. #27
    Junior Engineer
    Oct 2006
    38

    :


  9. #28
      nourdeen
    Apr 2006
    - -
    329

    :




  10. #29
      ehabezzeden
    Jun 2006
    204

    :



  11. #30
    Junior Engineer   rafat
    Sep 2006
    iraq
    20

    :




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