condition monotoring

7/29/2019 condition monotoring

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DRUM TRAINING PROGRAMME

DISTRIBUTION EQUIPMENT TECHNOLOGY & APPLICATIONS

24th OCTOBER,2007,TORRENT,SURAT

Instrument Transformers


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CONDITION BASED MAINTANANCE

OFINSTRUMENT TRANSFORMERS

24th OCTOBER 2007

PRESENTAION BY

G.V.AkreDirector Production

Hivoltrans Electricals Private LimitedHalol, Gujarat,India


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INTRODUCTION

-Instrument transformers play vital role in power system andtheir operational reliability is very important.

-There are frequent cases of catastrophicfailure ofInstrument Transformers.

-Explosive failure may cause damages to adjoining equipmentscausing considerable loss of asset & injuries (sometimesfatal) to the personnel.

-Failure of ITs leads to malfunctioning of system protection,controls, instrumentation and costly power outage.

Explosive failure is caused as the result of sudden pressureand heat developed due to huge arc formation, burning andvaporisation of oil/paper in small confined space.


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INTRODUCTIONINTRODUCTION

Large population of equipments commissioned during last 2 toLarge population of equipments commissioned during last 2 to3 decades are ageing out which is cause of concern as the3 decades are ageing out which is cause of concern as thefailure of aged CTs is not predictable.failure of aged CTs is not predictable.

-Looking at the consequences of failure, the focus has now-Looking at the consequences of failure, the focus has nowchanged from Conventional Maintenance to Condition Basedchanged from Conventional Maintenance to Condition BasedMonitoring.Monitoring.


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BATHTUB CURVE


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BATHTUB CURVE

The curve is a general representation of failure pattern of electrical

equipments from the initial years of commissioning to the end ofoperating life.

Failure during first few years are usually due to manufacturingdefects,transit damages,installation problems,leakages etc asexplained in detail ahead.

Random failure mode includes failures due to service condition,systemswitching serges,lightening impulses,ineffective maintenance etc.Therate of failure during this period is quite low.

Wearout failures includes natural aging process resulting intodielectric degradation. Failures during this period are sudden andunpredictable.

The failure rate during this period is quite high.This period as

experienced by utilities starts after two decades.


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L1 CATASTROPHIC FAILURE


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Failure


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Pressure buildup in metallic bellow


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Partially burnt primary insulation Fractured insulator

HV creepage inside the insulatorHV creepage inside the insulator


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Factors responsible for failure

-Life of IT is expected to be around 20-25 years.

-Early failures due to manufacturing defects are lowcompared to aged out transformers.

- Failure of aged out transformers is often suddenand catastrophic.

-State of deterioration in such equipments has to beproperly diagnosed and timely action be taken.

Following factors are some of the main causesresponsible for degradation of insulation.


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Factors responsible for failure

Manufacturing defects

Residual Moisture-inadequate drying of insulation

Over stressing- electrical stress Partial Discharges

Dielectric loss overheating of insulation Selection of R.M. Improper insulation design Overheating due to I2R loss in primary

Poor quality control Poor hermetic sealing

-moisture ingression-air ingression


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FACTORS RESPONSIBLE FOR FAILURE

Service Condition

Mishandling and damage during transit/installation Contacts of terminal connectors Unattended leakages-moisture ingress Leakage of N2 gas

Leakages through aged out pressure release device Old installation, frequent interruption / over-currents

Lightening and switching surges Ferroresonance

Polluted atmosphere Poor maintenance program


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FACTORS RESPONSIBLE FOR FAILURE

Natural Ageing

-Stresses like dielectric, electromechanical,thermal, mechanical and chemical are continuouslyacting on paper and oil insulation.

-The stresses cause the natural aging process ofdegradation of insulation.

-THE EFFECT OF ABOVE FACTORS ONINSULATION SYSTEM IS EXPLAINED INDETAIL AS FOLLOWS.


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DEGRADATION PROCESS OF INSULATION

Oil impregnated paper (OIP) is standard and proven

insulation system used in design of HV equipments.

The combination has much more better insulationstrength.

Insulation paper is made from cellulose fibre. Paper is very hygroscopic by nature & readily absorbs

moisture from the surrounding.

Aging is influenced by degradation of cellulose and oilas the result of different stresses.

Aging accelerate in presence of oxygen and moisture.


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Thermal degradation causes oxidation & reducesstrength of paper (degree of polymerization).

Degrdetion process produces mainly H2, CO2, CO,Methane (CH4), H2O, acids and sludge.

Water formed acts as catalyst to accelerate furtherdegradation

Oxygen mainly affects the oil causing oxidation


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Effects -

Production of components like H2, CO2, CO, methane,H2O,sludge and acids

Reduces tensile strength of paper (degree of

polymerization) Affects resistivity & insulation properties of oil & paper

Increases tan delta, which in turn increases dielectricheating

Tan delta of 0.5% produces 20 watts in 245 kV insulation &100 watts in 420 kV CT insulation as dielectric loss(heatloss)



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With low dissipation rate of heat, there may bethermal breakdown of insulation.

Thus, it can be seen that presence of moisture andO2 can cause speedy degradation of insulation.

A poorly dried transformer with higher moisture

content and gas (particularly O2) drasticallyreduces life expectancy.

Initial moisture and O2 causes early degradation

reducing life expectancy. A well-dried but leaking transformer can easily

absorb moisture & O2 from atmosphere.


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MOISTURE TRANSITION

BETWEEN OIL & PAPER INSULATION

- Moisture present in CT is absorbed by paper and oil in a

proportion depending on temperature

- There is always equilibrium between moisture in celluloseof paper and oil in an insulation system at any given

constant temperature.

- With change in temp, the equilibrium is disturbed andtransition of moisture takes place between oil & paper.

- At higher temperature, the water absorption in oilincreases and that of paper reduces


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MOISTURE TRANSITION cont.

- As temperature reduces moisture migrates from oil

to paper.

- It is also established that oil to paper migration isfaster than that from paper to oil

- During sunny days with higher loads, temp is high andmigration of water takes place from paper to oil

- During cool nights the reverse takes place & moisturemigrates from oil to paper

- With high variations in the temperature theconcentration of the moisture in any of the mediamay become critical and break down may take place.


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Moisture equilibrium in OIP insulation


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MOISTURE TRANSITION contd.Illustration from the experimental graph.

- At 60C with moisture of 20 ppm in oil, the paper contains2.5% moisture by weight

- At 20C with moisture of 20 ppm in oil, the paper cancontain

7% of moisture by weight.- The paper with higher percentage of total moisture present

in CT may cause reduction of dielectric strengthsignificantly and leads to insulation breakdown.

- Above experimental finding is also experienced by fact thatmax failures are taking place during cool nights of hot

summers in India,when max variation in temp takes place.


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MOISTURE TRANSITION cont.

Thus, it is important to observe following:

- In new transformers,the the paper and oil must be perfectly dried tominimise the initial moisture content. (preferably 5 ppm in oil and max

0.5% in paper.) The moisture accelerate the degradation process duringinitial years.

- ITs are minimum oil equipment and oil is not changed during itslifetime,hence the must be hermetically sealed,preferably with metallic

bellows.

- If nitrogen cushioned sealing is provided, the required pressure shouldbe monitored and maintained during operating life.

- Leakages must be attended immediately. Even minor leakage can absorbsubstantial moisture and O2 from atmosphere in course of time.

- With above precautions, the risk of absorption of external moisture willbe minimum.

DIAGNOSTIC TESTS AND CONDITION MONITORING


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Condition monitoring may be defined as a process of

monitoring the characteristics during operation ofequipment and find out the changes and trends of thecharacteristics of the insulation system which can beused to predict the need for maintenance beforeserious deterioration occurs.

Thus it determines the health of the equipment and

routine maintenance can be rescheduled asrequired.This reduces the unnecessary cost ofmaintenance.

DIAGNOSTIC TESTS AND CONDITION MONITORING

Diagnostic tests


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Diagnostic tests Electrical Diagnostic Tests

Test for Insulation Resistance Polarization Index

Tan Delta and Capacitance Measurement Resistivity, tan and BDV of oil Partial Discharge Test on site

Chemical Analysis

(A) -Test for Water content

-Dissolved Gas Analysis (DGA)-IFT

(B) -Degree of Polymerization (DP)-Furan Analysis

Other Tests and Inspection Infrared Thermograph Visual inspection for leaks, polluted insulators, corona

discharges.

I l ti R i t T t


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Insulation Resistance Test Initial screening test for equipments. To use high range 1000,000 m, 5 to 10 kV IR tester Test during fair weather condition.

Frequency of test may be high for better results. Gives initial warning to engineers to decide further

investigation. Normally useful for medium and low voltage trans upto72.5

kV.

Polarization Index

Ratio of IR values at end of 600 seconds and 60 seconds. Possible only with high range meggar. Value of 1.3 to 2.0 indicates good insulation for instrument

transformers.

Tan and capacitance Test


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Tan and capacitance Test

One of the best diagnostic test to monitor insulation condition.

Concept of tan can be explained by considering insulation ascapacitor.

An ideal capacitor carries only capacitive current (Ic) which leadsthe voltage by 90.

But actual capacitor formed by transformer insulation conductsresistive current (Ir) due to moisture and impurities. The resultantcurrent (I) is vectorial sum of Ic and Ir and leads voltage by slightly

less than 90. The angle between Ic and I is called loss angle andtangent called tan .


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Tan-delta Test

Higher tan indicates degraded insulation.

Comparison between periodic measurements reveal

trend in deterioration of insulation. Increase in tan may be due to Moisture, contaminated

oil, internal PDs etc.

Rise in tan with rise in voltage indicates highmoisture /deterioration and steeper rise indicatesmajor defects.

Higher tan increase dielectric losses producing losses

and heat, thus causing more degradation. Normal value for new CTs can be 0.15 % to 0.4%


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Tan-delta Test --cont.. Old equipment with more than 0.8% should be closely

monitored.

Higher tan produces higher dielectric loss and if paper-oilinsulation do not dissipate the heat it will lead to thermalbreakdown.

Partial Discharge Test at Site More advanced site testoff line or online.-Direct online method receives signals from PF terminal of

CT.-Signals are mixed with unwanted external discharges.-Special filters and microprocessor base instruments for

separating internal discharges from external.-Identifies fault in insulation.

Resistivity tan and BDV IFT Acid Number of oil


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Resistivity, tan and BDV ,IFT,Acid Number of oil

Tan-delta increase indicates oxidation,contamination

and suspended water particals.Low BDV indicates moisture, contamination due to

oxidation.

Low resistivity indicates presence of suspended water,

acidic oxidation etc.-Decrease in Resistivity with increase in tan indicates soluble contaminants and aging.

-Satisfactory resistivity at 90 compared with low

resistivity at ambient temperature indicates moisure.

IFT detects contaminants and oxidation products ininsulating oils.

CHEMICAL ANALYSIS TESTS


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CHEMICAL ANALYSIS TESTS

Water Content Test water content in oil should be less than 10 ppm,

target to be 5ppm.

High content results into lower BDV and higher tan and conductivity.

Moisture in oil may provide some information aboutmoister content in paper insulation, but it may not

be always true.Temperature at which samplestaken is important due toTransition Theory ofmosture

Above tests in addition to DGA and Furan can givemore reliable information about insulationcondition.


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Dissolved Gas Analysis (DGA)

- Detection of incipient faults in equipment.

Incipient faults cause PDs, Corona, & Arching and generatesheat with very high range of temperatures in affectedsection.

Oil and cellulose insulation decomposes and produces

different gases at different temperatures.

Significant gases due to oil decomposition are H2, methane(CH4), ethane (C2H6), ethylene (C2H4)and acetylen(C2H2).

CO2, CO and O2 are produced as the result of degradation ofcellulose during aging process and due to hotspot ininsulation.


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Dissolve Gas Analysis -- cont

IEC 60599, IEEE C. 57.104 and other standardsprovides method to establish nature of faults onbasis of gasses generated.

By taking periodic tests, trend in degradation of

insulation can be determined.

In hermetically sealed ITs,the oil movement isvery slow and the dissolve gas may move to sample

collection point after several days. Hence the testresults may not give the status of presentinsulation condition.

Degree of Polymerization Measurement (IEC 450) DP


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Degree of Polymerization Measurement (IEC 450) DP

Determines thermal aging of solid cellulose insulation. Quality of cellulose is measured in DP.

Measures indicate tensile strength of paper. New Kraft paper has DP = 1000 to 1500. With long service DP maybe = 200 to 250. DP value of 150 to 200 indicates mechanical strength of

only 20% of initial strength and is considered to be theend of insulation life.

Most accurate test, but sample of paper must beobtained which is impossible for IT.

Use to verify remaining life of the equipment. May be performed to establish cause of failure in failedequipments.

F A l i


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Furan Analysis

Furans form as degradation product of solid insulation

and are soluble in oil. Furan analysis is performed by drawing sample from oil

from operating transformer.

Easy test as compared to DP test as transformer is not

required to be opened. Interpretation of results are not as reliable as DP.

No universal correlation established between DP andFuran yet.

Furan analysis, DGA Test & DP Test combined offersvery reliable conclusion.

MAINTENANCE PROGRAM


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A simple program may be devised for conditionmonitoring by users as under:

1. Regular measurement of IR, PI, Tan, andcapacitance be made at suitable intervals in fairdry weather and recorded.Their correlation canbe determined.

2. The variations from each periodical test be

recorded in a computerized database and thetrend of deterioration determined.3. If the trend is indicative of progressive

deterioration, then oil sample should be drawnfor Moisture content,IFT,Acid numbers, DGAand Furan test.

4. If oil tests confirm the deterioration, theinstrument transformer should be removed fromservices as early as possible to avoid

catastrophic failure.

MAINTENANCE PROGRAM

Maintenance program cont.


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Ma ntenance program cont.

For small and medium voltage equipments where population is inthousands, IR, PI, Tan tests may be taken as indicator to

replace them. The transformer up to 145 kV may be sent for reconditioning

to departmental workshop. The transformers of 245 kV andabove should be sent for further investigations to

manufacturer of instrument transformers where PartialDischarge test and tan-delta at operating voltage can beperformed. The transformer may be re-processed only ifthere is reasonable remaining life left.

For 420 kV class transformers, where population is not highcomparatively, should be regularly tested on-site for Moisture,DGA and Furan along with electrical tests listed in step 1.


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ONLINE CONDITION MONITORING

As a result of compelling need felt by many utilities, online

monitoring systems have been developed for transformersin more advanced countries to avoid unexpected failures.On-line monitoring of critical network assets providesinformation previously unavailable. This in turn allows betterasset management. It is fast maturing into a serious and

reliable network tool.

Normally dissipation factor and partial discharge tests areconducted on line using specially developed instruments.

Technique of on-line monitoring of DGA and evolved gases isalso developed, however it is mostly used for very costlyequipments like power transformers presently.

ONLINE CONDITION MONITORING--cont


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ONLINE CONDITION MONITORING cont

The key advantages are:

1. Relevant data collected and made visible on network.

2. Major help in delaying routine maintenance as long aspossible hence driving down costs.

3. Costly and not easy to replace equipments are primecandidates for on-line monitoring.

4. Damage to the asset is minimized.

5. Equipment need not be taken out of service

INFRARED THERMOGRAPHY


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INFRARED THERMOGRAPHY

Infrared systems can measure temperature ofexposed parts of equipment online.

Can be used on routine basis by maintenance engineers. Infrared thermal imaging system is a technique

involving infrared camera, software and computer. Camera senses infrared radiation from heated

components of equipment in yard. A computer

processes this information and displays the images ofcomponents with different colors depending upon thetemperatures.

By comparing the difference in similar parts in theequally loaded CTs in different phases, abnormallyheated component is pointed out.

Further investigation, corrective action is takenavoiding further damages.


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CONCLUSIONS


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CONCLUSIONS

The degradation and subsequent failure of transformers is

the result of aging of cellulose from paper insulation atelevated temperatures.

Use of proper material and best manufacturing practice is

the first step to minimize the degradation of insulation.

However, for the instrument transformer to live its full lifeof 25-30 years, it is important to adopt efficientmaintenance management by monitoring the condition ofinsulation at site by the user.

Conclusions


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In our country, very few utilities may be using some or all ofthe diagnostic techniques.

Therefore, increased awareness and adoption of these essentialtechniques is highly desirable

Effective condition-based maintenance practices for substation

plant assets will result in reduced controllable operating costsand improved utility performance.

Proper investigations and analysis of failure of the equipments,carried out worldwide has resulted into improvement in

electrical equipment design.

It has also revised systems and developed new equipments forhealth monitoring.

CONTD


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Presently On-line monitoring is becoming main focus areafor utilities mainly in developed countries to evolve newtechniques for reliable operation of power equipments.

It is highly desirable that the utilities and equipment

manufacturers in our country have collaborativeefforts in bringing together the resources in form oftheir experiences/data on failures, ideas, personnel,skill and fund to have common guidelines on the subject

of condition monitoring.


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THANK YOU

G.V.AKRE

HIVOLTRANS ELECTRICALS PRIVATE LIMITED,GUJARAT,INDIA.

HIVOLTRANS

RATIO TEST


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RATIO TEST

Processinfg of Insulation


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Processinfg of Insulation

MOISTURE IN PAPER AND OIL

1) Paper = 3 - 7 % MoistureAfter drying - < 0.5% Moisture.

2)Raw Oil = 30 - 55 ppm MoistureAfter Processing- < 10 ppm of moisture.

3)Processed Oil BDV = 65-70kV.

Oil from Inst trans. = 45 - 55 kV.4)Gas in Oil apprx.10% to 12 %

After Processing and Degassing - 0.1% GasContent.

D si n f S cl ss m t in CTs


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0.5S AND 0.2S CLASS CTs

These special class metering CTs require the stipulatedaccuracy limits to be maintained upto 1% of the operatingprimary current.

Normal CTs require this limit to 5% of primary current.

IS 2705, 1992 specifies these CTs for 5 Amps secondarycurrent and ratios of 25/5, 50/5, 100/5 & their decimalmultiples.

However, IEC in their amendment in 2002,included 1.0 AmpCT but with observation that there is uncertainty in accuracymeasurement at low current.

Design of S class meterin CTs

0 5S AND 0 2S CLASS CTs


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0.5S AND 0.2S CLASS CTs

The design criteria for accuracy is same as discussed butthe flux density at 1% of the load current falls well below

ankle point.

The behavior of magnetic materials below ankle point isvery erratic.

To design core for errors within limit, the operating fluxdensity has to be reduced, which increases the corecross-section

Nickel-Iron cores must be used for low current CTs (below100/1) which nearly costs 20 times than that of CRGO.

Even in case of high ratio CTs of 145 kV and above ni-ironcore is ued due to limitation in operating amp-turns assystem fault current limits number of turns..

Voltage Factor


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Voltage Factor

The voltage factor is dependent upon the system earthingcondition.

To achieve high VF core flux density has to be reducedsubstantially, which increases the cost.

Adequate VF must be provided OR there is danger of failure during

fault on the system which produce transients of high voltagenature.

VTs with low VF are prone to Ferro-resonance under favorableconditions.

Under fault conditions in isolated neutral system, the voltageappearing across open delta winding is 3 times the rated secvoltage.

I t t S it f t (ISF)


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It is desirable that the ISF should be as low as practicable

such that the metering core saturates during fault current.The secondary current is restricted due to saturation andthe instrument connected does not get damaged at faultcurrent.

Multiple ratio CT with ratio selection by primary re-

connection can have same ISF for all ratios as secondaryturns are same on all ratios.

For CTs with secondary taps, same ISF on all ratios cannotbe obtained.

In such CTs, if ISF is specified on lowest ratio, sameshould be proportionately high on higher tappings.

Instrument Security factor (ISF)


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Instrument Security factor (ISF)

ISF has direct relation with VA burden connected. If ISF of 5is assigned at 30 VA, and actual burden is 10 VA, ISF will be

modified to 15.

If actual burden is too low there may be burning of instrumentconnected.

However, it is important to note that all the meters are capableof carrying 10 Amp for 5 sec. as confirmed by the metermanufacturers.

Thus it can carry 22 Amps for 1 sec.

ISF of 5 on higher taps on multi-ratio CT means proportionallyless ISF on lower taps.This will require very costly Nickel Ironcores.

ISF of 10 to 15 is safe for all practical purposes.

RECONDITIONING OF INSTRUMENT TRANSFORMERS


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Life may be increased by reprocess if life of insulation is not over.

Purpose

clean insulation of carbon particles, wax, etc.

Replace components like gaskets, pitted terminals, explosion vents,secondary terminals.

Remove moisture, replace degraded oil.

Improve IR, Oil Resistivity, etc.

Process

Remove all oil, dismantle and reassemble with parts as above. Heat the trans in oven for 24 hours at around 80-90 dgree under vacuum.

Fill fresh treated hot oil (65-75 deg.cent)under vacuum so as to penetratethe bulk of insulation.

After 24 hrs, remove this oil which washes out trapped oil in insulation.

Repeat the process twice or thrice by observing drained oil for color, BDV. Treat the washed equipment in oven under vacuum at 70 80 C for

sufficient time depending upon voltage class.(3 to 7 days for 33 kv to 220kV)

Remove finally the oil residue before filling finally with dry hot oil.

Heat Loss due to tan-delta


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Dielectric Dissipation Factor

If an alternating voltage V of frequency f is applied across aninsulation system comprising capacitance C and equivalent series

loss resistance RS, then the voltage VR across RS and thevoltage VC across C due to the resulting current I are:VR = IRSVC = IXCV = (VR2 + VC2)

The dielectric dissipation factor of the insulation system is the

tangent of the dielectric loss angle d between VC and V:tand = VR / VC = RS / XC = 2pfCRSRS = XCtand = tand / 2pfCNote that an increase in the dielectric losses of a insulationsystem (from an increase in the series loss resistance RS)results in an increase in tand. Note also that tand increases withfrequency.

The dielectric power loss P is related to the capacitive reactivepower QC by:P = I2RS = I2XCtand = QCtand


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A)ELECTRICAL POWER RESEARCH INST.(EPRI-1)

EPRI has sponsored extensive research targeted atunderstanding the dynamic behavior and effects ofmoisture in transformer insulation systems.

High Voltage Instrument Transformers & Bushings A project has been completed to monitor a large

number of HVCTs and bushings in laboratories and inservice, including on-line tan delta, partial discharge(pd) and other available monitoring methods. Unitswere tested to failure to evaluate failure modes,sensitivity of monitoring and to develop "end-of-life"criteria for interpretation of field monitoring data.


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EPRI-4

About the Electric Power Research Institute The Electric Power Research Institute (EPRI), with major

locations in Palo Alto, California, and Charlotte, North Carolina,was established in 1973 as an independent, nonprofit center forpublic interest energy and environmental research. EPRI bringstogether members, participants, the Institute's scientists andengineers, and other leading experts to work collaboratively onsolutions to the challenges of electric power. These solutionsspan nearly every area of electricity generation, delivery, anduse, including health, safety, and environment. EPRI's members

represent over 90% of the electricity generated in the UnitedStates. International participation represents nearly 15% ofEPRI's total research, development, and demonstration program.

Oil tests significance-1/3


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Oil tests significance-1/3

Significance of the key parameters Gas concentration/development: Tells us the actual condition and

performance of the transformer. Faults such as corona, arcing, hot

spots, partial discharges. (IEC 60599)

Acid Number (TAN): Acidic compounds in the transformer oil. Yieldsinformation on the deterioration level (acidic byproducts) of the oil andcellulose. (IEC 60296)

Water content: Tells us how critical the condition of the cellulose is. High

water content results in lower breakdown voltage which in turn cancause partial discharges. (IEC 60814) Particle Content: Particles can cause accelerated wear and reduction in

breakdown voltage (ISO 4406) Anti-oxidants: Inhibitor that prevents oxidation. The residue tells us how

deteriorated the oil is. Produces water. BHT (or DBPC) (IEC 60666) Temperature: Tells us something about the actual load. Figures should be

compared with a gas analysis. High temperature + presence of acetyleneis an indication of a faulty transformer.

Oil test significance-2/3B kd V l T ll hi b il l i ll d i


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Breakdown Voltage: Tells us something about oil electrically conductioncontamination (particles, sludge, water). Particles may be wet cellulosefibers. Low dielectric breakdownvoltage indicates the presence ofelectrically conductive contaminants in oil. (IEC 60156)

Tangens delta (power factor): Gives information on dielectric losses.Important to new oil quality as well as regenerated oil. The dissipationfactor is a measure of the power lost when an electrical insulating liquidis subjected to an ac field. The power is dissipated as heat within thefluid. A low-value dissipation factor means that the fluid will causelittle of the applied power to be lost. The test is used as a check on the

deterioration and contamination of insulating oil because of itssensitivity to ionic contaminants. (IEC 60247) Content,

Oil test significance-3/3


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- Surface Tension (IFT): Gives information on the level of

impurities in the oil. Interfacial tension and acid number(sometimes called neutralization number or acidity) are affectedby oxidation and contamination. IFT is an excellent means ofdetecting oil-soluble polar contaminants and oxidation productsin insulating oils. (ISO 6295)

-Furfuraldehyde (Furans): Oil soluble oxidation products fromdegradation of cellulosic insulation. Can be used to estimate theDP-value (IEC 61198)

- Color/Appearance: General indicator of the condition of the oil.(ISO 2049)

- Degree of Polymerisation: Mechanical strength of cellulose.- Other parameters: Flaming point, Density, Viscosity, Pour Point,Resistivity, Sulfur


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From www.sayedsaad.com /highvoltage/--

In practical applications liquids are normally used atvoltage stresses of about 5060 kV/cm when theequipment is continuously operated. On the otherhand, in applications like high voltage bushings, where

the liquid only fills up the voids in the solid dielectric,it can be used at stresses as high as 100200 kV/cm.

.
http://www.sayedsaad.com/http://www.sayedsaad.com/


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Fromhttp://www.usbr.gov/power/data/fist/fist3_30/fist3_30.pdf

It should be noted that small amounts of H2, CH4,and CO are produced by normal aging. Thermaldecomposition of oil-impregnated cellulose producesCO, CO2 , H2, CH4, and O2. Decomposition of

cellulose insulation begins at only about 100 C or less.Therefore, operation of transformers at no morethan 90 C is imperative. Faults will produce internalhot spots of far higher temperatures than these,

and the resultant gases show up in the DGA.

http://www.usbr.gov/power/data/fist/fist3_30/fist3_30.pdf


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a)4.6.2 Interfacial Tension (IFT). This test (ASTM D-791-91) [21], isused by DGA laboratories to determine the interfacial tension between

the oil sample and distilled water. The oil sample is put into a beaker of distilledwater at a temperature of 25 C. The oil should float because its specific gravityis less than that of water, which is one. There should be a distinct line betweenthe two liquids. The IFT number is the amount of force (dynes) required to pull asmall wire ring upward a distance of 1 centimeter through the water/oilinterface. (A dyne is a very small unit of force equal to 0.000002247 pound.)Good clean oil will make a very distinct line on top of the water and give an IFTnumber of 40 to 50 dynes per centimeter of travel of the wire ring.

-As the oil ages, it is contaminated by tiny particles (oxidation products) of the oiland paper insulation. These particles extend across the water/oil interface lineand weaken the tension between the two liquids. The more particles, the weakerthe interfacial tension and the lower the IFT number. The IFT and acid numberstogether are an excellent indication of when the oil needs to be reclaimed. It isrecommended the oil be reclaimed when the IFT number falls to 25 dynes percentimeter. At this level, the oil is very contaminated and must be reclaimed toprevent sludging, which begins around 22 dynes per centimeter. See FIST 3-5[20].

-If oil is not reclaimed, sludge will settle on windings, insulation, etc., and causeloading and cooling problems discussed in an earlier section. This will greatlyshorten transformer life.


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Fault processes such as arcing, pyrolysis andpartial discharge differ tremendously in theirenergy content. Partial discharge is the lowestenergy process and will characteristically produce

significant amounts of only hydrogen. Thermalprocesses (pyrolysis) will produce methane, ethane,ethylene, as well as hydrogen. Arcing will produceall of the fault gases. It is the only fault process

that will produce acetylene.


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IFT

IFT Significance:

The magnitude of the IFT is inversely related to theconcentration of the hydrophilic degradation products from thedeterioration of the oil. Since the hydrophilic materials areusually highly polar and thus not very soluble in the non-polar oil,the presence of these species can result in sludge formation.These materials that remain dissolved in the oil can affect thedesired electrical properties of the oil. They will reduce thedielectric strength and increase the dissipation factor of the

oil. Sludge buildup can also affect the heat transfercharacteristics of the oil by slowing or perhaps even blockingcirculation of the oil.

IFT


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There is usually an inverse relationship between theneutralization number of an oil and its IFT. As an oil

sample undergoes oxidative degradation, itsneutralization number will increase while its IFT valuewill decrease. It should also be recognized that adecrease in the IFT does not imply that the aciditymust also be high, since there are other non-acidic

contaminants that could be present in the oil that arehydrophilic and will lower the IFT but not raise theacidity. An example of such a situation might be thatof a free breathing transformer near salt waterwhere a salt water mist might be able to enter the

unit. Such an event will not affect the acidity butwould markedly affect the IFT and dielectricstrength of the oil.

orre ations- insu ation testsCT MAIN INSULATION


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No. IR Value 10 min. PI tan %M

1. 34,000 1.619 3.84

2. 21,400 1.685 5.70

3. 72,000 2.075 2.05

4. 28,000 1.333 4.75

5. 39,000 1.393 4.01

6. 43,000 1.536 2.50

7. 67,000 2.018 2.93

8. 33,000 1.571 3.24

9. 1640,000 2.485 0.36

PRIMARY WINDING OF CT- Cross section selected considering rated thermal current


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Cross section selected considering rated thermal currentand short time current requirement

- Example :Ratio = 150 / 1, STC = 40 KA / 1.0 SecCross Section for 150 A = 150 / 1.65 = 91 mm2

Cross Section for 40 KA / 1.0 sec = 40000/180 = 223 mm2

Thus, Primary Winding section required = 223 mm2

- Heat generated (I2R) due to passage of STC is notdissipated stored as latent heat

- Temp is increased momentarily before dissipation

- Design should absorb such heat shocks- STC current peaks are approx 2.5 times RMS value- Peak current (say 100 KA) develops high mechanical forces

inside primary winding- Primary must be reinforced to withstand such dynamic

forces

REFERENCES-1


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References:

Internal Insulation Failure Mechanism of HV Equipment under

Service Condition GIGRE, Session 2002, by A.K. Lakhanin,V.V.Sokolov.

IEEE Guide for Interpretation of Gases Generated in oilimmersed transformers. Std. C.57.104-1991

Review of Modern Diagnostic Techniques for Assessing

Insulation Condition of Aged Transformers. By Tapan K. Saha. Diagnostics of Paper and Oil CT Insulation CIGRE International

Conference on large H.V. Electrical Systems 1998.

Live Assessment of TransformersBy M.J. Patel, ERDA.

B.Buereschaper, O.Kleboth - Lugova and T.Leibfried.

The electrical strength of transformer oil in a transformerboard-oil system during moisture non-equilibrium.

Terry Krieg, ElectraNet SA, Jeff Benach, AVO International.


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REFERENCES-2 Managing High voltage current Transformers and Bushings using

On-Line Insulation Monitoring Technique. The AVO SOS system is designed to provide continuous online

monitoring of insulation condition by comparative measurementof insulation capacitance and dielectric dissipation factor.

Powerlink Queensland, Australias research and development(R&D) team developed innovative condition-monitoringtechniques. Initially, the team produced the InstrumentTransformer Testing Device (ITTD), which led to thedevelopment of the continuous online monitoring system in theearly 1990s.

LCMSEA- Life Cycle Management of Substation Equipment andApparatusInterest Group of CEA Technologies Inc (CEATI).CEATI is focused on coordinating research and cost-sharing

efforts among electrical utilities. It has published researchreport on Instrument Transformer Condition Assessment andDiagnostics. The interest group has sponsored the report whichconsists of literature review and worldwide survey of utilities toidentify the best practice)http://www.ceatech.ca/psearch.php.
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