View
236
Download
0
Embed Size (px)
DESCRIPTION
Curso de Aislamiento de Transformadores
Citation preview
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