Solution to Power Generation_ Transformer Oil Testing

8/12/2019 Solution to Power Generation_ Transformer Oil Testing

1/16


2/16

7/13/2014 Solution to Power Generation: Transformer Oil Testing

http://www.powervikas.com/2013/01/dga-faults-in-transformer-key-gases-insulating-oil -power-transformer-condition-monitoring-high-voltage-electrical- testing 2

2. TYPES OF FAULTS IN THE TRANSFORMERFault conditions occur primarily from the thermal and electrical

deterioration of oil and electrical insulation. Each combustible

gas level will vary depending upon the fault process.

2.1 ArcingLarge amounts of hydrogen and acetylene are produced, with

minor quantities of methane and ethylene. Arcing occurs

through high current and high temperature conditions. Carbon

dioxide and carbon monoxide may also be formed if the fault

involved cellulose. In some instances, the oil may become

carbonized.

2.2 CoronaCorona is a low -energy electrical fault. Low-energy electrical

discharges produce hydrogen and methane, w ith small

quantities of ethane and ethylene. Comparable amounts of

carbon monoxide and dioxide may result from discharge incellulose.

2.3 SparkingSparking occurs as an intermittent high voltage flashover

without high current. Increased levels of methane and ethane

are detected w ithout concurrent increases in acetylene,

ethylene or hydrogen.

2.4 OverheatingDecomposition products include ethylene and methane,together with smaller quantities of hydrogen and ethane. Traces

of acetylene may be formed if the fault is severe

or involves electrical contacts.

2.5 Overheated CelluloseLarge quantities of carbon dioxide and carbon monoxide are

evolved from overheated cellulose. Hydrocarbon gases, such as

methane and ethylene, will be formed if the fault involved an

oil-impregnated structure.

2.6 Partial DischargeThe temperature plays a less important role in the chemical

reaction occurring in the partial discharges since the vapor

temperature in the discharge zone is not higher than 60-150C.

Hydrocarbon cracking in the partial discharges occurs as a

result of excitation of molecules and their subsequent

dissoc iation by collision w ith high energy electrons, ions,

atomic hydrogen and also free radicals.

3. DISSOLVED GAS ANALYSIS (DGA)

Methods for

Calculating Short

Circuit Current of

Electrical System

Generator Rotor

Protection

Transformer Oil

Testing

Boiler Efficiency

Video Widget

Power Plant

Categories

afbc-boiler

back-emf

boilers

cfbc-boiler

drum-level-control

electrical-and-

instrumentation

heart

home

operation-and-maintenance

Power Plant
http://www.powervikas.com/search/label/Power%20stationhttp://www.powervikas.com/search/label/Power%20Planthttp://www.powervikas.com/search/label/operation-and-maintenancehttp://www.powervikas.com/search/label/homehttp://www.powervikas.com/search/label/hearthttp://www.powervikas.com/search/label/electrical-and-instrumentationhttp://www.powervikas.com/search/label/drum-level-controlhttp://www.powervikas.com/search/label/cfbc-boilerhttp://www.powervikas.com/search/label/boilershttp://www.powervikas.com/search/label/back-emfhttp://www.powervikas.com/search/label/afbc-boilerhttp://powervikas.blogspot.com/2012/12/power-plant-heart-of-every-industry.htmlhttp://www.powervikas.com/2013/01/boiler-efficiency-calculation-fuel-fired-system-ido-lpg-bagasse-rfo-combustion-properties-of-fuel-solid-fuels-liquid-fuels-viscosity-gaseous-fuels-combustion-of-carbon-combustion-air-requirement-combustion-testing-flue-gas-analysis-flue-gas-losses-thermal-efficiency.htmlhttp://www.powervikas.com/2013/01/dga-faults-in-transformer-key-gases-insulating-oil-power-transformer-condition-monitoring-high-voltage-electrical-testing-on-site-testing-insulating-oil-analysis-insulation-system-acetylene-hydrogen-ethylene-carbon-monoxide-winding-resistance-insulation-resistance-and-polarisation-index-winding-and-bushing-power-factor-low-voltage-excitation-current-test-transformer-turns-ratio-tap-changer-dynamic-resistance-measurement-Recovery-voltage-measurement-rvm-frequency-response-analysis-fra-interfacial-tension-dissolved-metal-analysis-liquid-power-factor-dielectric-breakdown-strength-moisture-neutralization-number-acidity.htmlhttp://www.powervikas.com/2013/01/generator-rotor-protection-rotor-earth-fault-rotor-overheating-protection-rotor-temperature-alarm-automatic-field-supression-and-use-of-neutral-circuit-breaker.htmlhttp://www.powervikas.com/2013/01/methods-for-calculating-short-circuit-current-of-electrical-system-short-circuit-analysis-overcurrent-protective-devices-ocpd-transformer-fault-current-generator-fault-current.html


3/16



The DGA analysis is performed in three steps:

Extraction of all the gases in the oil sample.

Measurement of the quantity of each gas in the extracted gas.

Calculation of the concentration of each gas in the oil sample.

DGA is a pow erful diagnostic and it has capability to detect

faults in the incipient stage before they develop into major

faults and cause serious damage to transformer. The

conventional Bucholtz Relay is universally used in transformer,

to protect against severe damages. However, the limitations of

this is that enough gas must be generated first to saturate the

oil fully & then to come out and collect the relay.

The DGA technique detects gas in parts per million (ppm)

dissolved oil by the use of gas extraction unit and

a gas chromatograph. It checks whether a transformer under

service is being subjected to a normal aging and

healthy or whether there are incipient defects such as hot

spots, arcing, overheating or partial discharge. Such incipient

faults otherwise remain undetected until they lead to an actual

major failure.

The most commonly measured gases are:

O2 (Oxygen)

N2 (Nitrogen)

H2 (Hydrogen)

CO (Carbon Monoxide)

CO2 (Carbon Dioxide)

CH4 (Methane)

C2H2 (Ethane)

C2H4 (Ethylene) and

C2H2 (Acetylene)

Other extracted gases are sometimes analyzed, such as C3H8,

C3H6, and C3H4 to refine a diagnostic. However this approach

is not w idely used. A high degree of success has been achieved

in the area of determining a link betw een:

Ratios of common fault gas concentration and specific fault

types:

The evolution of individual fault gases and the nature and

severity of the transformer fault.

4. INTERPRETATIONS OF DGA RESULTS ANDDIAGNOSTICS METHODS.There are many techniques of incipient fault diagnosis Review

of the most commonly used gas-inoil diagnostic method is IEEE

C57.104-1991. This method covers not only the determinations

Pow er station

products

steam-turbine

thermal-effiency

Thermal-Pow er-Station

transformer

Blog Archive

2013

( 26 )

May

( 1 )

March

( 2 )

January

( 23 )

Atmospheric Fluidised

Bed Combustion

Boilers

Alignment

Earthing Procedure in

Power Plants

Restricted Earth Fault

Protection

Transformer Oil Testing

Methods for Calculating
http://www.powervikas.com/2013/01/methods-for-calculating-short-circuit-current-of-electrical-system-short-circuit-analysis-overcurrent-protective-devices-ocpd-transformer-fault-current-generator-fault-current.htmlhttp://www.powervikas.com/2013/01/dga-faults-in-transformer-key-gases-insulating-oil-power-transformer-condition-monitoring-high-voltage-electrical-testing-on-site-testing-insulating-oil-analysis-insulation-system-acetylene-hydrogen-ethylene-carbon-monoxide-winding-resistance-insulation-resistance-and-polarisation-index-winding-and-bushing-power-factor-low-voltage-excitation-current-test-transformer-turns-ratio-tap-changer-dynamic-resistance-measurement-Recovery-voltage-measurement-rvm-frequency-response-analysis-fra-interfacial-tension-dissolved-metal-analysis-liquid-power-factor-dielectric-breakdown-strength-moisture-neutralization-number-acidity.htmlhttp://www.powervikas.com/2013/01/restricted-earth-fault-protection-of-transformer-ref-protection-schemetic-drawing-basic-operation.htmlhttp://www.powervikas.com/2013/01/earthing-procedure-in-power-plants-how-earthing-is-done-methods-of-earthing-earthing-to-water-main-pipe-earthing-plate-earthing.htmlhttp://www.powervikas.com/2013/01/motor-and-pump-alignment-problems-of-misalignment-method-of-alignment-face-and-rim-method-the-shaft-coupling-method--c-and-d-adapter-laser-method-moving-pump-driver-shims.htmlhttp://www.powervikas.com/2013/01/atmospheric-fluidised-bed-combustion-afbc-deming-cycle-pcda-cycle-mechanism-of-fluidised-bed-cobustion-time-temperature-turbulence-types-of-fludize-bed-combustion-boilers-bubbling-bed-general-arrangement-of-afbc-boiler-air-distributor-bed-and-bed-heat-transfer-surface-ash-handling-system-fuel-feeding-system-under-bed-feeding-over-bed-feeding-heat-transfer-surface-bed-temperature-bed-pressure-superficial-gas-velocity-bottom-ash-removal-fly-ash-removal.htmlhttp://www.powervikas.com/2013_01_01_archive.htmlhttp://void%280%29/http://www.powervikas.com/2013_03_01_archive.htmlhttp://void%280%29/http://www.powervikas.com/2013_05_01_archive.htmlhttp://void%280%29/http://www.powervikas.com/search?updated-min=2013-01-01T00:00:00%2B05:30&updated-max=2014-01-01T00:00:00%2B05:30&max-results=26http://void%280%29/http://www.powervikas.com/search/label/transformerhttp://www.powervikas.com/search/label/Thermal-Power-Stationhttp://www.powervikas.com/search/label/thermal-effiencyhttp://www.powervikas.com/search/label/steam-turbinehttp://www.powervikas.com/search/label/productshttp://www.powervikas.com/search/label/Power%20station


4/16


http://www.powervikas.com/2013/01/dga-faults-in-transformer-key-gases-insulating-oil-power-transformer-condition-monitorin g-high-voltage-electrical-testing 4

of the fault severity and its nature, but also offers some

suggestions regarding the follow up actions to be taken.

It is the only method that covers both the gases dissolved in oil

and the gas present in the nitrogen cover of sealed type

transformers. The method proceeds as follow s:

Calculate the total of dissolved combustible gases (TDCG):

TDCG = H2 + CO + CH4+ C2H6 + C2H4 +C2H2

Classify the condition of the transformer according to the limits

for each gas for the total of combustible gases.

Evaluate the rate of increase of combustible gases in ppm/day.

Determine what actions should be taken according to the level

of combustible gases and their rates of increase. This method

defines four possible transformer conditions:

1) TDCG < 720 PPM : Operating satisfactorily

2) TDCG = 721 to 1920 PPM: Faults may be present

3) TDCG = 1921 to 4630 PPM: Faults are probably present

4) TDCG > 4630 PPM: Continued operations could result in

failure.

These conditions are also determined accordingly to individual

gas levels. If any one of the gases exceeds a

given level (Refer Table-1) the transformer is classified

accordingly

4.1 Gas Content in Oil Due to Fault

Short Circuit Current

of E...

Variable Frequency

Drives

Generator and

Transformer

Protection

Generator Over Heaing

Protection

Generator Inter turn

fault protection

Differential Protection

for Gnerators

Generator Rotor

Protection

Boiler Efficiency

Products

Drum Level Control

CFBC Boiler

Water Chemistry

Electrical Sw itches

Coal Analysis in Power

Plants

Parallel Operation of

Transformer

Erection &

Commissioning of

Transformer

Electric Motors

Thermal Power Station
http://www.powervikas.com/2013/01/methods-for-calculating-short-circuit-current-of-electrical-system-short-circuit-analysis-overcurrent-protective-devices-ocpd-transformer-fault-current-generator-fault-current.htmlhttp://void%280%29/http://www.powervikas.com/2013/01/thermal-power-station-power-plant-steam-turbine-electrical-generator-Rankine-cycle-coal-fossil-fueled-plants-combined-heat-and-power-CH-and-P-plants-co-generation-plants-energy-efficiency-of-thermal-power-station-laws-of-thermodynamics-cooling-towers-electricity-cost-boiler-and-steam-cycle-furnace-esp-feed-water-heating-and-deaeration-conductivity-water-deaerator.htmlhttp://www.powervikas.com/2012/12/synchronous-motors-asynchronous-motors-squirrel-cage-induction-motors-slip-ring-motors-commutator-motors-single-phase-induction-motors-three-phase-induction-motors-variable-speed-motors.htmlhttp://www.powervikas.com/2013/01/what-is-the-parallel-operation-of-transformer-importance-of-phase-sequece-phase-rotation-fault-mva-calculation-transformer-installation-and-commissioning-daily-check-list-of-transformer-overhauling-of-transformer-site-preperation-for-transformer-installation-transformer-oil-oil-filling-under-vacuum-buchhloz-relay-earthing-terminals-cable-boxes-silicagel-breater-magbetic-oil-level-indicator-radiators-marchalling-box-oil-temperature-indicator-oti-winding-temperature-indicator-wti-bushings-explosion-vent-dielectric-strength-test-of-oil-lifting-lugs-jacking-pads-rollersconservator-tap-changer.htmlhttp://www.powervikas.com/2013/01/Same-phase-sequence-parallel-operation-of-transformers-same-voltage-ratio-Same-percentage-impedance-Same-polarity-Why-parallel-operation-of-transformers-is-required.htmlhttp://www.powervikas.com/2013/01/coal-properties-what-is-coal-analysis-in-power-plants-grade-of-coal-calorific-value-range-physical-properties-chemical-properties-analysis-of-coal-measurement-of-moisture-volatile-matter-carbon-and-ash-proximate-analysis-fixed-carbon-sulphur-content-ultimate-analysis.htmlhttp://www.powervikas.com/2013/01/mcb-mccb-elcbair-circuit-breaker-vacuum-circuit-breaker-rcd-residual-current-device-rccb-residual-current-circuit-breaker-elcb-earth-leakage-circuit-breaker-rcbo-residual-circuit-breaker-with-over-load-mcb-selection-difference-between-elcb-and-rccb-fuse-and-mcb-characteristics-why-fuses-can-sometimes-blow-for-no-obvious-reason.htmlhttp://www.powervikas.com/2013/01/Why-chemical-treatment-of-boiler-is-required-boiler-scale-boiler-efficiency-boiler-leakage-and-scalling-treatment-what-cause-the-high-alkalinities-cause-caustic-embrittlement-What-happens-to-all-the-mud-and-clay-build-up-Why-should-we-care-for-steam-purity-Why-feedwater-pump-fail-due-to-excessive-corrosion-What-tests-should-be-performed-on-condensate-system-Why-are-condensate-pipes-leaking-or-excessively-corroding-What-is-boiler-carryover-Why-it-is-required-to-feed-a-polymer-or-phosphate-to-boiler-What-is-boiler-blowdown-How-much-blowdown-is-required-for-boiler-What-is-a-Deaerator-How-to-improve-poor-boiler-steam-fuel-ratio-and-increase-boiler-efficiency-How-to-reduce-soot-formation-and-deposition-how-to-reduce-blow-down-rate-corrosion-problem-in-the-pressure-parts-on-F.W.-circuit-What-are-the-symptoms-of-caustic-attack-What-are-symptoms-of-Oxygen-pitting-causes-of-Oxygen-pitting-What-are-the-causes-of-acid-attack-What-are-the-symptoms-of-Stress-corrosion-cracking-What-are-causes-of-water-side-corrosion-fatigue-What-are-the-symptoms-of-Super-heater-Fireside-Ash-Corrosion-What-are-the-symptoms-of-erosion-What-are-the-causes-of-erosion-What-are-the-causes-of-mechanical-fatigue-Explain-the-common-metallurgical-problems-occur-in-the-boiler-.htmlhttp://www.powervikas.com/2013/01/boilers-cfbc-boiler-circulating-fluidised-bed-combustion-.htmlhttp://www.powervikas.com/2013/01/boiler-drum-level-control-single-element-control-feedback-control-two-element-control-cascade-control-three-element-control-cascade-feedforward-control-low-load-operation-of-boiler-and-drum-level-control.htmlhttp://www.powervikas.com/2013/01/products.htmlhttp://www.powervikas.com/2013/01/boiler-efficiency-calculation-fuel-fired-system-ido-lpg-bagasse-rfo-combustion-properties-of-fuel-solid-fuels-liquid-fuels-viscosity-gaseous-fuels-combustion-of-carbon-combustion-air-requirement-combustion-testing-flue-gas-analysis-flue-gas-losses-thermal-efficiency.htmlhttp://www.powervikas.com/2013/01/generator-rotor-protection-rotor-earth-fault-rotor-overheating-protection-rotor-temperature-alarm-automatic-field-supression-and-use-of-neutral-circuit-breaker.htmlhttp://www.powervikas.com/2013/01/differential-protection-of-generator-limitation-of-this-method-modified-differential-protection-biased-circulating-current-protection-percentage-differential-relay-protection-advantage-of-this-method.htmlhttp://www.powervikas.com/2013/01/generator-protection-stator-inter-turn-fault-protection.htmlhttp://www.powervikas.com/2013/01/generator-protection-over-heating-high-temperature.htmlhttp://www.powervikas.com/2013/01/generator-differential-protection-87-generator-transformer-differential-protection-generator-stator-earth-fault-64-generator-stator-inter-turn-fault-generator-rotor-earth-fault-protection-generator-field-failure-protection-generator-backup-protection-generator-negative-phase-sequence-protection-generator-under-power-anti-protection-generator-overload-protection-generator-over-voltage-protection-generator-field-failure-protection-lockout-relay.htmlhttp://www.powervikas.com/2013/01/electrical-and-instrumentataion-variable-frequency-drive-vfds-vfd-operation-benifits-ofvfd-energy-saving-low-motor-starting-current-reduction-of-mechanical-and-thermal-stress-on-motors-and-belts-during-starts-high-power-factor-simple-installation-lower-kva-mechanical-power-control-low-inrush-motor-starting-soft-starters-harmonics-definition-what-causes-harmonics-are-harmonics-harmful.htmlhttp://www.powervikas.com/2013/01/methods-for-calculating-short-circuit-current-of-electrical-system-short-circuit-analysis-overcurrent-protective-devices-ocpd-transformer-fault-current-generator-fault-current.htmlhttp://2.bp.blogspot.com/-FqhrDBYMtPw/UQa-A2Isz4I/AAAAAAAAAUk/7j78KW3Ch-4/s1600/dga4.JPG


5/16



Continuous Monitoring of Key Fault Gases(H2 AND CO)

Key Gas method becomes applicable to transformer w ith

developed faults w here absolute values of key gases are

considered. The key gases are acetylene, hydrogen, ethylene

and carbon monoxide.

Follow ing table illustrates the nature of faults, when key gas is

abnormally high.

KEY GAS NATURE OF FAULT

Acetylene C2H2 =Electrical arc in oil

Hydrogen H2 = Corona , partial discharge

Ethylene C2H4 = Thermal degradation of oil

Carbon Monoxide = Thermal ageing of oil

Various Types of Faults Depending on theGas Composition

2012

( 5 )

Powered by Blogger.

Home

Products

Techical

Contact Us

Join us Free

Feedback

Advertise w ith Us
http://powervikash.somee.com/advertise_with_us.aspxhttp://powervikas.somee.com/feed_back.aspxhttp://powervikas.somee.com/join_us.aspxhttp://www.powervikas.com/p/contact-us.htmlhttp://www.powervikas.com/p/blog-page.htmlhttp://www.powervikas.com/p/products.htmlhttp://www.powervikas.com/http://www.blogger.com/http://www.powervikas.com/search?updated-min=2012-01-01T00:00:00%2B05:30&updated-max=2013-01-01T00:00:00%2B05:30&max-results=5http://2.bp.blogspot.com/-qNsWUduVDYk/UQa-FKGeb0I/AAAAAAAAAUs/xAzLxXnGKDM/s1600/dga3.JPGhttp://4.bp.blogspot.com/-jAQ31QgGIuw/UQa-IIkj9RI/AAAAAAAAAU8/h_P9aROv1fk/s1600/dga5.JPG


6/16



ELECTRICAL TESTS

Preventive maintenance testing of in-service transformers has

the primary objective of monitoring conditions in the insulation

and evaluating the useful life still available in the transformer

tested.

Winding ResistanceThe w inding resistance is measured in the field to identify

shorted turns (although this is better identified in the ratio test),

poor joints, high resistance connections or contacts and open

circuits. The resistance is measured on all taps of a tapped

winding to ensure that the OLTC dose not open circuit during

the tap changing operation.

Insulation Resistance and PolarisationIndexThe perfect dielectric can be represented, at pow er frequencies,

as a lumped prefect capacitance. The application of a direct

electric field to this capac itance w ill results in a charging

current flow ing for a short time giving the capac itor sufficient

charge to support a voltage of V= Q/C. The time taken for the

capacitance to achieve this equilibrium will be determined by

the supply source resistance.

In practical dielectrics the charging current does not cease after

this short period but decreases gradually to a minimum value.

The taken for this minimum value to be reached depends upon

the dielectric and can range from seconds to days. The

insulation resistance is defined from Ohms Law as the ratio of
http://1.bp.blogspot.com/-Q4X_9qoa2v8/UQa-Gr8g8QI/AAAAAAAAAU0/KRAUHUnSJk4/s1600/dga6.JPG


7/16



the applied voltage to this residual current.

In practical applications the charging current can consist of

volume and surface currents. Therefore the insulation resistance

measurement of plant w ill be affected by the condition of the

insulation itself and the cleanliness of the insulation surfaces.

This effect can be allowed for in some plant types by the use of

guarding electrodes.

The time dependency of the insulation resistance can result

from electronic and ionic conductivity, dipole orientation

(dielectric absorption), and space charge polarisation. As the

charging current time constants are affected by the presence of

impurities, the time taken for the leakage current to settle dow n

can be used as an insulation condition indicator. The ratio of

the insulation resistance value take ten minutes after

application of the measurement voltage to that taken one

minute after voltage application is known as the Polarisation

Index. Generally insulation in good (dry) condition has a PI

greater than 1.2.

Winding and Bushing Power FactorOne of the major tests performed in the field is the

measurement of the w inding and bushing capac itance and

pow er factor.

Capacitance measurements of each of the w indings to ground

and betw een w indings is performed to provide an indication of

the condition of the w inding insulation and some indication of

the structural integrity o f the w indings. Similar measurements

are performed on the bushings to provide an indication of the

condition of the insulation in the condenser bushing and of the

pow er factor test points.

As described above, the perfect dielectric can be represented

as a lumped perfect capacitance. The charging current flow ing

in the capac itance w hen an AC filed is applied should lead the

applied voltage by 90. In practical insulating systems losses

(caused by conduction and polarisation currents) cause the

current to lead the voltage by less than 90. The complement of

the angel betw een the voltage and current vec tors is called the

dielectric loss of the angle or DLA. The tangent of this angle,

tan , provides an indication of the losses in the insulation and

is know n as the POWER FACTOR or DIELECTRIC DISSIPATION

FACTOR (DDF).

The power factor of the w indings and the bushings is usually

measured in the field as a condition assessment tool. The


8/16



pow er factor can give an indication of the moisture content of

the paper and oil in the transformer and the bushings. Major

deterioration of the insulation w ill also be detected.

Low Voltage Excitation Current TestThe low voltage excitation test is performed to identify shorted

turns or severe core damage. This method is a natural extension

of the pow er factor test and makes use of the same equipment.

The test results of a three-phase core form transformer w ill give

a pattern of tw o similar currents and one lower current. This is

usually the H2 phase of the transformer as the magnetic

reluctance of this phase is low er than the other tw o phases

resulting in a low er excitation current value.

Transformer Turns RatioThe ratio of the transformer is normally measured at

commissioning or after major refurbishment. The test is also

performed to identify incipient faults or after a transformer fault

trip to identify shorted turns. A turns ratio measurement can

show that a fault exists but does not determine the exact

locat ion of the fault.

Tap Changer Dynamic ResistanceMeasurementThe dynamic resistance measurement detects carbonized spots

and weak contacts in the mechanism of a tap changer. The

advantage of this diagnostics is that not only end positions of

the tap changer contacts can be checked but the complete

stroke of contact movement while changing between taps. This

also allow s one to diagnose the diverter switch in the tap

changer mechanism.

New Condition Monitoring ToolsMoisture, in conjunction w ith the other factors, acts on the

paper insulation reducing the papers strength and volume. This

reduces the papers, and the transformers, ability to perform its

function. Therefore two new s tests have been introduced to

determine the moisture content in the cellulose accurately and

movement of the w inding structure respectively.

Recovery Voltage Measurement (RVM)One of the critical measures of transformer condition is the

moisture content of the paper insulation. It is well know n that

an increase in the paper moisture content w ill result in a

corresponding increase in the transformer ageing rate.

One method of determining the moisture content of paper is to

use equilibrium diagrams that relate oil/water content, sample

temperature and paper moisture content. How ever, as


9/16



transformers in the field are rarely in equilibrium, this method

has varying degrees of accuracy.

A second method of determining the paper moisture content is

to drain oil and take an a paper sample from the insulation. This

method is more accurate, but costly and exposes the

transformer to the atmosphere and the possibility of moisture

ingress. RVM provides an indication of the paper moisture

content without the drawbacks of the above two methods.

RVM is non-intrusive and has proven to be accurate w hen

compared to know n paper moisture contents in oil test cells.

Moisture and the decay products from insulation degradation

are polar in nature. When an electric field is applied to a

dielectric containing polar contaminants, the polar products

become aligned w ith the electric field. If the levels of these

contaminants increase, the time required for the dipoles to align

with the applied field is reduced. This is equivalent to a

reduction in the system time constant. The RVM determines the

equivalent paper moisture content by measuring the time

constant of the insulation system. Instrumentation software

calculates the equivalent paper moisture content from the

system time constant and temperature.

Frequency Response Analysis (FRA)The conventional techniques of ratio, resistance, DDF and even

HV testing are often unable to detec t w inding deformation,

except in the most serious of cases. Any changes in the spatial

position of the w inding structrue will result in relative changes

to the internal inductive and capac itive network of the winding

structure w hich produce changes in the frequency response of

the transformer.

FRA measures the frequency response of the transformer

windings up to 10 MHz. This method involves injecting a low

voltage signal of varying frequency into each end of the

winding and measuring the response at the other end of the

winding.

The transformer under test is always disconnected from

adjacent equipment. This is done to eliminate the effect of

connect ing equipment although it is reported that short lengths

of busbar are not usually a problem. Winding movement is more

likely to occur in older, aged transformers that have reduced

winding c lamping pressure. This is particularly true when the

transformer is placed under a high mechanical load such as

experienced during fault conditions.

The advantage of obtaining a baseline signature of the

transformer is that future tests w ill be able to determine the

extent of any w inding distortion that occurs after the

measurement has been taken. This test is particularly valuable


10/16


http://www.powervikas.com/2013/01/dga-faults-in-transformer-key-gases-insulating-oil -power-transformer-condition-monitoring-high-voltage-electrical- testin 10

as a baseline reference for a new transformer prior to placing in

service as w ell as for older transformer after re-refurbishment.

A spectrum analyzer is used to excite, monitor and record the

response from the transformer. A softw are program downloads

the recorded data to a PC for analysis. Results for each phase

are then plotted against frequency. Each w inding is tested

separately. To ensure repeatable measurements, all other

windings in the transformer are left floating. The test tap

position selected to ensure that the maximum amount of

w inding is included in each measurement.

INSULATING OIL ANALYSISA regular program of oil testing is recommended to monitor for

changes in oil quality. Specialized tests are also performed that

identify specific compounds in the oil and helps determine

whether fault conditions exist inside the unit. The recommended

battery of tests include the follow ing:

Liquid pow er factor at 25o and 100o C

Dielectric breakdown strength

Moisture

Neutralization number(Acidity)

Interfacial tension

Color/Visual Examination

Sludge/Sediment

Inhibitor

Dissolved gas analysis

Dissolved metal analysis

Furanic compounds

Liquid Power FactorThe IEC standard method for this test is IEC 247. This involves

measuring the pow er loss through a thin film of the liquid being

testing.

Water, contamination, and the decay products of oil oxidation

tend to increase the pow er factor of the oil. New oil has very

low pow er factor values much less than 0.1% at 25o C and

1.0% at 90o C. As the oil ages and moisture accumulates, or ifthe unit is contaminated, the liquid pow er factor tends to

increase. This increase in liquid pow er factor is a direct

indication that materials harmful to the paper and to the

continued operation of the transformer are building up.

Many transformer ow ners make the mistake of having this test

run at only one temperature. While the 90o C test is more

sensitive, both temperatures need to be used. The relationship

between the 25o and 90o values can help in making a


11/16



diagnosis as to w hether the problem is moisture, oxidation, or

contamination.

Dielectric Breakdown StrengthThe dielectric breakdow n voltage is a measure of the ability of

oil to w ithstand electric stress. Dry and clean oil exhibit an

inherently high breakdown voltage. Free w ater and solid

particles, the latter particularly in combination with high levels

of dissolved w ater, tend to migrate to regions of high electric

stress and reduce the breakdow n voltage dramatically. The

measurement of breakdow n voltage, therefore, serves primarily

to indicate the presence of contaminants such as water or

conducting particles. A low breakdow n voltage value can

indicate that one or more of these are present. How ever, a high

breakdown voltage does not necessarily indicate the absence

of all contaminants. This test is performed in accordance w ith

IEC 156.

MoistureThe purpose for which the dielectric tests were invented monitoring moisture content can be done directly. IEC 733 is

well established and can measure moisture down to low parts

per million levels.

While acceptable values have been established by voltage class

for moisture (less than or equal to 30 ppm for voltages up to

145 kV, 20 ppm for voltages above 145 kV as used by TNBT),

these are somewhat misleading. A truer picture of moisture in

the transformer must take the sampling temperature into

account so that % saturation of the oil by moisture and %

moisture by dry w eight of the solid insulation can be

calculated. A transformer at 20o C that has 20 ppm moisture in

the oil is considerably wetter than a similar unit, w ith a similar

20 ppm moisture, but that is operating at 40o C. A new

transformer should be less than 0.5% moisture by dry w eight.

Anything over 3.0% (or 30% saturation) is considered extremely

wet. Most owners dehydrate transformers w hen the moisture

level exceeds 1.5 to 2.0% moisture by dry w eight.

Neutralization Number (Acidity)This value, measured by IEC standard method IEC 1125Areported as mg KOH/g sample, reports the relative amount of a

number of oil oxidation products, primarily acids, alcohols and

soaps. As the oil continues to oxidize, acid number increases

gradually, generally over a period of years. Running the acid

number regularly provides guidance as to how far oxidation of

the oil has proceeded. Th acceptable limit is test is usually used

as a general guide for determining w hen an oil should be

replaced or reclaimed.


12/16



Acceptable values for acid number are 0.20 and low er.

Unacceptable values are over 0.20. These are the values used

by TNBT. How ever, there are countries that use values that are

even as low as 0.05. The are reasons why. First of all, if one

examines paper from a 0.05 acid number transformer, it is

readily apparent that even at this low acid number value that

decay products are depositing in and damaging the paper

fibers. Once the damage starts, the life of the insulation is

compromised. Second, between 0.05 and 0.10, visible sludge

will start to form in operating transformers.

The short answer is that the questionable range of 0.05 to 0.10

is where the oil starts to lose its effectiveness w ith respect to

one or more of the functions that it is supposed to fulfill.

Studies have been performed that indicate that the paper w ill

lose 75-80% of its strength (and therefore be at the end of its

effective life) before the acid number reaches 0.40 mg KOH/ g

sample a value that some still consider to be below the value

where the oil needs to be serviced.

Interfacial TensionThe test method for interfacial tension (IFT), IEC 6295, measures

the strength in mN/m of an interface that w ill form betw een

service aged oil and distilled w ater. Because decay products of

oil oxidation are both oil and w ater soluble, their presence w ill

tend to weaken the interface and depress the interfacial tension

value. Brand new oil is frequently 40-50 mN/m. An acceptable

value for in-service oil is greater than 25 mN/m or greater;

unacceptable results are below 28 mN/m.

Color/VisualField examination of insulating liquids (IEC 296) includes

examination for presence of cloudiness or sediment and general

appearance as w ell as a color examination. As oil ages, it will

darken gradually. Very dark oils or oils that change drastically

over a short period of time may indicate problems. Any

cloudiness or sediment indicates the presence of free w ater or

particles that may be detrimental to continued operation of the

equipment. Taken alone, without consideration of past history

or other test parameters, color is not very important fordiagnosing transformer problems. If the oil has an acrid or

unusual odour, consideration should be given to carrying out

further tests.

Sludge/SedimentThe test in IEC 296 distinguishes betw een sediment and sludge.

Sediment is insoluble material present in the oil. Sediment may

consist of insoluble oxidation or degradation products of solid

or liquid materials, solid products such as carbon or metallic


13/16



oxides and fibres or other foreign matter.

Sludge is polymerized oxidation products of solid and liquid

insulating material. Sludge is soluble in oil up to a certain limit.

At sludge levels above this, the sludge comes out of the

solution contributing an additional component to the sediment.

The presence of sludge and sediment may change the electrical

properties of the oil and hinder heat exchange, thus

encouraging deterioration of the insulating materials.

Inhibitor ContentInhibited oil deteriorates more slow ly than uninhibited oil so

long as active oxidation inhibitor is present. However, once the

oxidation inhibitors are consumed, the oil may oxidise at a

greater rate. The determination of residual oxidation inhibitor in

in-service transformer oil is carried as per IEC 666.

Dissolved Metals AnalysisDissolved metals analysis (in particular, for three metals: iron,

copper, and aluminum) can be of use in further identifying the

location of transformer faults discovered by dissolved gas

analysis. For example, dissolved metals analysis indicating the

presences of conductor metals may indicate a fault is occurring

in the w inding or at a connection w hile the presence of iron

indicates involvement of the core steel.

Furanic CompoundsWhen paper breaks dow n, the cellulose chains are broken and

glucose molecules (which serve as the building blocks of the

cellulose) are chemically changed. Each of the glucose monomermolecules that are removed from the polymer chain becomes

one of a series of related compounds called furans or furanic

compounds. Because these furanic compounds are partially

soluble in oil, they are present in both the oil and the paper.

Measuring the concentration in the oil can tell us quite a bit

about the condition of the paper.

The standard method typically tests for five compounds that

are normally only present in the oil as a result of the paper

breaking dow n. Those five compounds, and their probable

causes, are:5-hydroxymethyl-2-furaldehyde (5H2F), typically formed by

oxidation of paper.

2-furyl alcohol (2FOL), typically formed in connection with a

high moisture content.

2-furaldehyde (2FAL), very common, formed by all overheating

and aging conditions.

2-acetyl furan (2ACF), very rare, may be related to electrical

stress.

5-methyl-2furaldehyde (5M2F), typically formed as a result of


14/16



overheating.

These are typically present in very low concentrations,

microg/kg or parts per billion, requiring detailed extraction

methods and analysis using a very sophisticated instrument: a

high performance liquid chromatograph. Typically, we find that

total furan concentrations relate w ell to the following

conditions:

i) 25 parts per billion (ppb) is a new transformer with only

background presence of furans.

ii) Up to 100 ppb is an in service transformer that has aged

normally (Acceptable level).

iii) 100 to 1000 ppb is a unit that may have accelerated ageing

(Questionable level).

iv) Over 1000 ppb has significantly aged and should be

investigated (Unacceptable level).

Very high levels, 1000 ppb and above starts to enter the

danger zone. Transformers with total furans 1000 ppb and

above have a much higher failure rate because they are starting

to reach their end of life or because small areas of the paper

have been destroyed by localized overheating.

Interpretation of Test ResultsTypically, the justification for running transformer oil tests is to

provide the maintenance program w ith information to allow the

efficient and safe continued use of the equipment. In this

context, transformer oil tests listed in the table above (except

for DGA, furans, metals, and PCBs) are run on a regular basis

usually annually or every six months. Trends of the oil quality

are monitored so that w hen the oxidation inhibitor nears

depletion and/or when one or more of the other test

parameters enter the questionable range, the oil can be

serviced to restore it to new oil quality before any lasting

damage to the insulation system is done.

Oil servicing includes reclamation by processing the oil through

filtering (to remove solid materials), through heat and vacuum to

remove moisture and dissolved gases, and through a chemical

adsorbent such as fullers earth to remove acids, sludges, and

decay products. Oil can generally be reclaimed, however far the

oxidation process has proceeded, and it can generally be

reclaimed any number of times to like new oil quality. Oil that

has aged in a transformer, how ever, has caused degradation

products to build up inside the transformer, particular on and

inside the structure of the solid insulation. Removing and

replacing the oil regardless as to whether the replacement oil

is new or reclaimed has little effect w ith regard to cleaning up


15/16



the inside of the transformer. Reclamation of the transformer oil

in the transformer, frequently referred to as hot oil cleaning,

cycles the oil from the transformer through a processing rig

where the oil is cleaned up. Because the oil passing over the

internal structures of the transformer has been heated, it

redissolves the acids and sludges, even those that are inside

the solid insulation. Depending on how far oxidat ion of the oil

has been allow ed to proceed, a reclaiming project may require

a volume of oil passing through the transformer that is

anywhere from 4 to 20 times the liquid volume capacity of the

transformer. If done properly, reclaiming can frequently be done

without deenergizing the transformer. Energized reclaiming

saves on equipment dow ntime, and the loading and vibration

of the energized equipment actually makes the cleaning of the

internal structures proceed more effectively. Limiting factors on

whether reclamation can proceed on an energized basis include

the moisture content, voltage class of the equipment, volume

and access to the oil, and presence of incipient fault conditions.

Faults identified and diagnosed by DGA, furans, and/or metals

analysis must be corrected to ensure that the unit can continue

to operate safely. These faults typically require an outage to

repair as they are related to electrical or mechanical problems

with the internal components. Since it is not always practical to

immediately schedule an outage (and if the fault is not

immediately destructive of the equipment), the monitoring

interval betw een DGA tests or furan analyses may be decreased

normal intervals for DGA may be 3 months to one year

sometimes to daily retests w here problems are particularly

severe. Except for highly critical units, furans and metals are run

only w hen they w ill be useful to help diagnose fault conditions.

The key issue behind testing is to use the information to

improve operations. Too frequently, limited funds are spent on

testing units w here no remedial act ion will ever be taken. It

would be much more cost effective to reallocate those funds to

more critical units perhaps shortening testing intervals where

testing results are a determinant in the continuing maintenance

of that equipment.

Conclusion:An attempt has been made in this paper to review modernchemical and electrical diagnostic methods for proper

transformer maintenance. DGA is the most w idely used method

for investigating incipient faults. So, w ith this case study we

can analyze the higher concentration of C2H4 indicates thermal

Fault, maximum fall in condition 3 in which fault probably

present and then monitor the rate of rise of individual gases,

indicates higher concentration of key gases. But there is no fault

and transformer presently in service condition.


16/16


Newer Post Older Post

New comments are not allow ed.

Home

The transformer just like human beings needs a physical check-

up, for a clean bill of health. No single test procedure is

adequate to supply all the necessary information needed to

properly evaluate a transformer, resulting in the various test

performed by the condition monitoring unit.

The frequency of the tests w ill be determined by many factors

such as the age, loading and history of operation. These tests

may fulfill three distinct but general functions:

i) Prove the integrity of a piece of equipment at the time of

acceptance.

ii) Verify the continued integrity of the unit at periodic intervals

of time.

iii) Determine the nature of the extent of the damage w hen a

unit has failed.

Recommend this on Google

0 comments:

Copyright 2013 Solution to Pow er Generation and Customized byCyberOceanZ

Terms Privacy Policy Sitemap Contact Us.

From Twitter Recent Posts Custom content Get in Touch

- Template by SoraTemplates
http://www.soratemplates.com/http://www.cyberoceanz.com/http://www.powervikas.com/http://www.blogger.com/share-post.g?blogID=3471583257878549326&postID=8115254755881664195&target=facebookhttp://www.blogger.com/share-post.g?blogID=3471583257878549326&postID=8115254755881664195&target=twitterhttp://www.blogger.com/share-post.g?blogID=3471583257878549326&postID=8115254755881664195&target=bloghttp://www.blogger.com/share-post.g?blogID=3471583257878549326&postID=8115254755881664195&target=emailhttp://www.powervikas.com/http://www.powervikas.com/2013/01/methods-for-calculating-short-circuit-current-of-electrical-system-short-circuit-analysis-overcurrent-protective-devices-ocpd-transformer-fault-current-generator-fault-current.htmlhttp://www.powervikas.com/2013/01/restricted-earth-fault-protection-of-transformer-ref-protection-schemetic-drawing-basic-operation.html

Documents

Solution to Power Generation_ Transformer Oil Testing