Insulation Deterioration

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Curso de Aislamiento de Transformadores

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  • Mechanism of Insulation Degradation

    Dangerous effect of degradation Factors

  • AGENTS OF DEGRADATIONWater, oxygen, oil aging products and particles

    of different origin are agents of degradation which can shorten transformer Life significantly

    under impact of thermal, electric, electromagnetic

    and electrodynamics stresses

  • MAIN SOURCES OF WATER CONTAMINATTION Residual moisture in the thick structural components not removed during the factory dryout

    Ingress from the atmosphere

    Aging (decomposition) of cellulose and oil.

  • Cold thin structurecoolerMoisture Model

  • Upper Estimation the Rate of Water ContaminationDirect exposure of oil-impregnated insulation to air:RH = 75%, 20 C :13,500 g in 16 hours

    Operation with open-breathing conservator6,000 g per year or 0.2% per year

    Operation with membrane sealed conservatorapp. 0.06 % per year

    Insufficient sealing with rain water present200 g in an hour as free water

  • Ageing water in the tested models

  • Cellulose Microstructure

  • Where oil and agents of degradation are within cellulose?

    OIL

    Within macrocapillaries (30-35%

    Viscouse movement under pressure gradient (capillary strength + external pressure)

    WATER

    Within macrocapillaries and partly microcapillaries

    Diffusion movement

    GASES

    Basically within macrocapillaries

    Diffusion similar to water, but less active.

    OIL AGING PRODUCTS

    Macrocapillares, outer layers, near to surface.

    Diffusion still less active.

  • Moisture content in cellulose depend on:Moisture concentration(relative humidity)TemperatureInsulation ThicknessOpen surface

  • Oil-barrier insulation structure

  • Oil-barrier insulation structureTHIN STRUCTURE

    Turn coil, conductors (paper) Barrier, end cap (pressboard) Lead (paper)

    Diffusion time constant a few months

  • INSULATION STRUCTURE COMPONENTS

    THICK STRUCTURE

    SupportLaths, padsLead (paper, warnished cloth)

    40-55% of the mass and 4-8% of the surface

    Diffusion time constant some years

  • Transformer

    25 MVA110kV167MVA500kV471 MVA750 KVThickPassive

    38%

    41%

    57.2%ThinMoisture

    22%

    42%

    18%

    ThinHeated

    40%

    19.3%

    24%Hot spot

    4.0%-70kg

    2 .0%-80kg

    2.5% -430kgShare of Deteriorated Insulation

  • Involve slow diffusion of water, gases, andaging products

    Affect basically only the thin structure which comprises typically 40-60% of the total mass.

    Heated mass of conductor insulation that subjectedto accelerated deterioration comprises typically 2-10 % of the total massProcesses of insulation deterioration

  • Hottest-spot insulation Conductor wrapped insulation- top of transformer Conductor wrapped insulation- bottom of transformer Bulk insulation - top of transformer Bulk insulation - bottom of transformer

    Moisture distribution based on temperatureThermal zones

  • The main source is atmospheric water The main mechanism is viscous flow of wet air or free water via poor sealing Solid Insulation is water accumulator; Thin cold structures operating at bulk oil temperatures comprise 20 30% of the total but retain a large portion of the waterMOISTURE DISTRIBUTION WITHIN A TRANSFORMER

  • MOISTURE DISTRIBUTION WITHIN A TRANSFORMER Parameters of moisture equilibrium depend on temperature, water-in-oil solubility Influence of temperature makes non-uniform distribution of water in thr turn insulation The moving force of water transfer is moisture potential which is greater for turn insulationad weaker fo major insulation

  • EquilibriumThermo diffusion of moisture between microcappilaries

  • Moisture equilibrium in pressboard and paper

  • 12Distribution of temperature across winding insulation

  • Temperature distribution of water in turn insulation

  • For oil immersed transformer water content In cellulose insulation is a function of relative saturation of the oil

  • The Assessment of the Aromatic Carbon Content, CA, is important for Determining the Solubility of Water in Mineral Oils

  • Solubility of moisture Solubility of moisture in oils

  • Transformation of Bound Water to Soluble Water from Aged Oil

    Type of Oil

    Properties

    Moisture Content, ppm

    Before Heating

    After Heating

    Used oil from

    750 kV CT

    Acidity=.064mgKOH/g

    IFT=32 dynes/cm

    PF90 =5.32%

    26.3

    85

    Used oil from

    750 kV CT

    CA=18%

    Acidity=0.064mgKOH/g

    IFT=32 dynes/cm PF90=6.1%

    23.5

    132

  • High oil temperature can add soluble water

  • Transformation of Bound Water

  • High temperature can add soluble waterOil contaminated with particles

  • Dangerous Effect of Insulation Degradation Factors

  • Critical conditioin: Bubbles in oil Residual air trapped in insulation after oil refilling Residual air in reserve cooler Burned out oil pump motor Arcing in oil (acetylene evolution) Oil oversaturation + mechanical shock

    Evolution of vapor bubbles out of heated conductor insulation + Water

  • DANGEROUS EFFECT OF WATERFREE WATER

    May kill transformer Immediately at rated voltage

    Drops of water in viscous oil works as particles generator

  • Critical condition: Ingress of Free WaterRapid drop of temperature+ rapid drop of pressure+ insufficient sealing+ rain water

    The top seal of draw-lead bushings, The seals in explosion vents, Poor sealing of nitrogen blanketed transformers.

  • UbdParticles50 g/toCritical condition: high oil relative saturation+particles

  • Particles are responsible for dielectric state

  • Moisture increases particles conductivity

  • Sharp reduction of temperature can results in dramaticreduction of dielectric strength of oil

  • DANGEROUS EFFECT OF WATER WATER IN TURNS INSULATION

    Accelerates aging decomposition, and depolymerization of cellulose is proportional to the water content. This process becomes much more dangerousin presence of acids and non-acid polars

    Bubbles evolution

    Water in turn insulation relates to Hot problems

  • Model of bubbles evolution

  • Bubbles Evolution

  • The nature of the cellulose fibres used in the transformer insulation changes the dimensions of the insulating parts with the temperature and the moisture content. The drying and stabilisation process, as well as the clamping fixture will influence the final behaviour during the operation.

    Effect of moisture on clamping compression

  • Dimensional changes - Insulation material

  • An increase of 3 % moisture can double the clamping pressure when initial pressure is in the range of 2.5 N/mm2 a later loss of clamping pressure is to be expected when these insulating parts are submitted to additional drying process.

    Therefore, the drying of moist solid transformer insulation can be critical if the coil is losing too much clamping force

  • Particles are most dangerous contaminants

    Origins

    Particles mode

    Manufacturing process

    Installation

    Repair

    Cellulose fibres

    Iron, copper, aluminum

    Clay, dust

    Aging

    Sludge particles, fibres, metals

    LTC divertor

    Localized overheating

    Wear of bearing

    Carbon

    Metals

  • Manufacturing contaminantsCellulose fibres, iron,copper,aluminum and otherresulting from manufacturing process

    Non-conductive mode particles in a 5 to 50 micron range

    Copper < 10 ppbIron

  • This type of contaminant gets in the transformer tank during bushing installation, oil filling, from cooling system, etc. Size range probably from 5 to 100 microns. Sometimes, the filter itself can supply particles, especially if the paper and the oil are somewhat wet.

    Dress and test dirt :

  • During utilization at normal and overload temperatures oil slowly forms sludge particles, "polymeric" in nature.

    Based on Velcon Filters research these could be one to five microns in size and this contamination is difficult to remove by common filtration medias

    Aging destruction of cellulose insulation would result in fibers partition.

    Aged oil

  • Over 500C would be a symptom of forming carbon.

    Any transformer (shunt reactor) that has a source of localized oil heating may be at a time a source of carbon generation.

    Clay particles as well as carbon are difficult to remove using conventional filter medias.

    Oil leaks from LTC Diverter Switch cause carbon contamination

    Localized oil overheating

  • Trapping effect of Transformer Components Electrical and electromagnetic fields attracts the conductive particles and deposits them on the winding surfaces, barriers, and bushing porcelain.

  • Converter transformers effect of DC voltage;

    Shunt reactors effect high electromagnetic field

    EHV power transformers

    HV bushings that operate in contaminated oil

    HV LTC

    Most sensitive Components

  • Dormant Incipient fault: residue of oil aging products on insulation surface

  • Dangerous Effect of Degradation Factors