Very Good Condition Monitoring Transformers

Working principle of a transformer A transformer is static (or stationary) piece of apparatus which: 1.Transfers electric power from one circuit to another. 2.It does so without a change in frequency. 3.The principle is based on mutual induction between two circuits linked by a common magnetic flux. 

Basic parts of a transformer Basically a transformer consists o f a : 1.A primary coil or winding. 2.A secondary coil or winding. 3.A core that supports the coils or the windings 

Transformer construction Main constructional elements of Transformers are A) Magnetic circuit

Core & clamping structure

B) Electric Circuit

Winding,Insulation, Bracing devices.

C) Terminals

Tapping, Tapping switches, Terminal Insulator, Leads , Bushings

D) Tank

Oil, Cooling devices, conservator, piping, Breather

E) Protective Circuit & Monitoring

Buchholz relay, WTI, OTI, Oil surge relay pressure relief device, MOG

Transformer construction A)Magnetic Circuit: The core provides closed path for flux. It is made up of CRGO insulated laminations. (CRGO has iron loss of about 1.3 W / Kg at 1.6 Tesla ) B)Electric Circuit: Winding, insulation & bracing are constructional parts of electrical circuit of transformers. This is the most vulnerable part of transformer because of direct association with power system. Must be designed to withstand voltage stress resulting from system fault, transient over voltage and thermal stresses (lightening or switching surges)

Transformer construction 

Insulation: Commonly used material are Paper or press board Oil is used as insulating medium Insulating varnish applied to make coil mechanically strong.

C) Terminal: Leads : Connection of winding are (copper rod or bus bar) taken to bushing. Bushing: Up to 33 kV porcelain bushing are used. Above 33 kV, condenser & oil filled terminal bushings are used.

Transformer construction D) Transformer tank  Cooling: Small transformers are air cooled whereas large transformers are provided with oil or oil & air cooling . The transformer tank is designed to withstand full vacuum. Types of Cooling: Air insulated AN AF ANAN

air cooled Air Natural Forced Air Cooling Natural Air cooling inside and outside transformer

Transformer construction Oil immersed air cooled ONAN ONAF OFAN OFAF

Natural Oil Circulation. Natural Air flow Natural oil and forced air flow Forced Oil & Natural Air Circulation Forced Oil and Forced Air Circulation

Oil immersed & water cooled ONWF OFWF

Natural Oil, water (internal) cooler Forced oil, water (external) cooler

Transformer construction E) Protective devices : Various protective devices mounted on transformer are as follows: Bucchholz Relay Gas actuated relay, Transformer Internal fault. WTI/OTI Provided for alarm and / OR trip against over load PR Device To release internal pressure generated in the transformer during fault. MOG Alarm when oil level is low Oil Surge Relay To release actual pressure generated during fault in OLTC

Accessories & their functions Terminal & Bushing : Type of Bushing, terminal is selected depending on voltage, currents & operating conditions. Porcelain, condenser type of oil filled bushings are used as per requirement. 

Cable Boxes : Cable boxes are primarily designed for receiving and protecting cable ends and ensure effective sealing of cable against ingress of moisture. 

Conservator : It is provided to accommodate change in oil volume caused due to change in loads or ambient conditions. 

Breather :Whenever there is change in ambient temperature or load , there is a change in oil temperature and hence the volume of oil. Increase in volume causes the air above oil level in conservator to be pushed out and decrease causes air to be drawn in. Thus the transformer breathes. 

Accessories & their functions  

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When air is breathed in, moisture and dust from atmosphere is sucked in . Silica gel crystals absorb moisture. Color of silica gel is blue when dry and turns pink when absorbs moisture. Oil cup at the bottom is filled with oil which acts as coarse filter and removes dust form outside air. Magnetic Oil Level gauge:

This is a dial instrument operated by magnetic coupling from a float on oil surface. It is normally fitted with contact to give alarm for low oil level.

Accessories & their functions 

Oil temperature indicator:

Bourdon tube with a pointer arrangement mounted in a case comprising of a reading dial and a glass cover. There is a temperature sensing bulb which communicates to the bourdon tube through the armored capillary. 

Winding temperature indicator: It comprises of following: WTI pot :mounted at top of transformer tank. Oil in pot is temp. of top oil. Imae coil:Heater coil and develops additional hear raising temperature of oil incide heater coil. WTI CT: Mounted on one of the line lead with secondary connected to image coil WTI: The bulb of the WTI is immersed in oil inside image col .Temperature of this oil is dependent on top oil temperature and load on transformer. 

Accessories & their functions Buchholz Relay : Gas & oil operated relay detects formation of gas or development of sudden pressure inside the oil of transformer. Any electrical fault inside the transformer is accompanied by evolution of gas. 

Pressure Relief Device: This is provided to relieve the internal pressure in the event of major fault within the transformer. 

Tapping Switch: To maintain secondary voltages reasonably constant at load end when incoming voltage and/or load on transformer changes, it is necessary to change the voltage ratio (I.e, turns ratio of the winding) of the transformer. This is achieved by changing the number of turns ( HT Side) by operating a switch called as tapping switch. Depending on the requirement, off circuit or on load tap changer is installed in the transformer. 

Accessories & their functions Radiators : The function of radiator is to limit the temperature of oil and winding by dissipating heat that is generated due to losses within transformer while in service.  When transformer is in operation warm oil rises and enters the radiator from the top valve cools and then descend to enter the bottom of the tank. 

Other Accessories: Inspection cover, jacking lugs, Earthing terminals, Rating Plate, Filter valve, Drain Valve, Terminal marking, Rollers etc. 

DGA 

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Insulating materials within transformer breakdown to liberate gases. The identity of these gases indicate the type of fault and the rate of gas generation indicate the severity of fault. Causes of fault gases can be divided into three categories: Corona or partial discharge

DGA 

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Pyrolisis or thermal heating Arcing Arcing is the most severe fault (intensity of energy that is dissipated per unit time per unit volume of the fault) ,less with heating, least with corona. One of the most important technique to indicate the health of a transformer.

Major (Minor) fault gases under various fault conditions

Sampling And Labeling Procedure Sampling 

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Dry Weather, avoid contamination Clean, dry, leak proof glass or stainless steel container. Equipment operating normally Take safety precautions. Sample bottle must be full without any air trap completely sealed and should be properly labeled.

FREQUENCY OF SAMPLING NEW TRANSFORMERS 

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FIRST

DGA TEST : BEFORE HEAT RUN TEST ON TRANSFORMER SECOND DGA TEST : AFTER HEAT RUN TEST ON TRANSFORMER (2&24 HRS) THIRD DGA TEST : BEFORE ENERGIZING TRANSFORMER (B.M.) FOURTH DGA TEST : WITHIN THREE MONTHS OF SERVICE

FREQUENCY OF SAMPLING IN-SERVICE TRANSFORMERS  

ANNUALLY AS A ROUTINE CKECKING BEFORE (LATEST DATA) & AFTER FILTRATION /TOPPING UP

TRANSFORMERS AFTER OVERHAULING, REPAIRS, MAINTANANCE 

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BEFORE ENERGISING TRANSFORMER WITHIN THREE MONTHS OF SERVICE

Interpretation of Results 

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Ensure that gas concentrations are high enough to warrant further investigation. Gas being present even in normal operating condition without any fault being present. Gases might have formed on the occasion of previous faults or during repairs by brazing, welding etc., and not completely removed. Since gases are produced in normal ageing also, the service duration of the oil has to be taken into account.

PERMISSIBLE LIMITS OF DISSOLVED GASES IN OIL OF A HEALTHY TRANSFORMER GAS <4YRS IN PPM) 4-10 YRS (PPM) >10 YRS (PPM) H2

100-150

200-300

200-300

100-150

200-300

GAS <4YRS IN PPM) 4-10 YRS (PPM) >10 YRS (PPM) H2

100-150 50-70200-300 CH4

CH4

50-70

100-150

C2H2 20-3030-50

C2H2 20-30

C2H4 100-150

150-200

C2H4 100-150 C2H6 30-50 100-150 CO

200-300 C2H6 30-50400-500

CO2

3000-3500

CO

4000-5000

200-300

CO2 3000-3500

200-300 200-300

30-50

100-150 200-400

150-200

800-1000 600-700

100-150 200-400

100-150

800-1000

400-500

600-700

4000-5000

9000-12000

9000-12000

Condition monitoring of transformer oil   

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Role of transformer oil:

It is used as coolant. It is used as insulating material.

Reasons for deterioration of transformer oil Physical contamination: Release of fibrous impurities by paper, pressboard, wood and cotton tapes in contact with oil for longer period at elevated temperatures. Due to dissolution of varnish . Due to foreign matters like dust, metallic particles and other solid impurities Due to moisture

Condition monitoring of transformer oil 

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Effects: Life is reduced by high sludge formation. Electrical properties of insulating oil get disturbed due to conductivity of suspended particles. Chemical deterioration: It is due to oxidation. Effects of oxidation: Results in acids, sludge .Acid attack solid insulation and metal. Sludge causes poor thermal conduction and mechanical hindrance to proper oil circulation.

Condition monitoring of transformer oil 

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Contamination of gases: Gases are present in oil due to following: Those which dissolve in the oil from atmosphere. Those which are generated inside due to thermal decomposition of oil, decomposition of oil by arcing. Effects of gases: The ignition of inflammable gases can be causes by corona occurring in th air space or arcing.

Transformer oil testing

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Transformer oil testing is carried out to detect abnormalities in transformer and based on test results corrective actins can be taken before actual failure takes place. To evaluate quality of oil To decide periodic maintenance (filtration. reclamation etc) To know health of transformer (by DGA) To estimate remaining life of transformer

Oil Testing  

New oil (IS-335)- 15 tests Oil in service (IS-1866)- 8 tests        

Dielectric strength (BDV) Dielectric dissipation factor (Tan delta) Resistivity Neutralisation value (Acidity) Flash point Water content Sludge Interfacial tension

Transformer oil testing 1.Physical condition of the oil:

Color ,clarity and odour gives information regarding quality of oil and presence of certain contaminants in oil.

2.Electrical strength:

Important parameter as used as insulating medium. This test gives conductive contaminants and moisture present in oil.

3.Water content:

Reveals total water content ,leak or cellulosic deterioration

Transformer oil testing 4.Specific resistance (Resistivity): This test provides a measure of the total soluble contaminants and ageing products .It is numerically equal to the resistance between opposite faces of a centimeter cue of the oil and is expressed as ohm com.

5.Dissipation factor:

This test provides a measure of the total soluble and ageing products.

6.Neutralisation value:

contaminants

This test gives acid present in the oil. It is the no. of milligrams of potassium hydroxide required to neutralise completely the acids produced in one gram of oil.

Transformer oil testing 7.Interfacial tension test: This test provides a measure of sludge and polar component present in oil. It is expressed as molecular attractive force between the molecules of water and oil at oil-water interface.

8.Flash point: Sudden drop in flashpoint is indicative of unsafe working condition of transformer.

Scheduled of oil characteristics for transformer in service as per IS : 1866-2000 Property

Highest Voltage of equipment, kV

< 72.5

72.5 to 170

> 170

Breakdown voltage (kV), Min.

More than 30

More than 40

More than 50

Water content (ppm), Max.

Max. 95

Max. 40

Max. 20

Neutralization value (mg KOH/g), Max.

Max. 0.3

Max. 0.3

Max. .0.3

Sediment & Sludge, % by mass

ND

ND

ND

0.1 x 1012

0.1 x 1012

Resistivity @ 90°C x 1012 0.1 x 1012 (ohm-cm), Min.

Property

Highest Voltage of equipment, kV

< 72.5

72.5 to 170

> 170

Dielectric dissipation factor @ 90°C, Max.

1.0

1.0

0.2

Interfacial tension (mN/m), Min.

15

15

15

Flash Point, (°C), Min.

125

125

125

Condition Monitoring - Oil testing Sr. No.

Test

Remedial Action for Deviations from permissible limits

1

Electric strength 2.5

Oil filtration

mm gap (Break down voltage) 2

Water content

Oil filtration

3

Sediment and / or

Oil filtration

precipitable sludge

Condition Monitoring - Oil testing Sr.

Test

No. 4

5

Remedial Action for Deviations from permissible limits

Specific resistance

Oil filtration if the tan  value

(Resistivity at 90 deg.C

permits other wise replace the oil

Dielectric dissipation factor (

Replace the oil

tan  ) 6

Neutralisation value (total

Replace the oil

acidity) of the oil 7

Inter facial tension of the oil against water at room temperature

Replace the oil

Furanic compound test 

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Life of transformer is dependent on life of solid insulation and life limit is determined by thermal degradation of winding paper. Kraft insulation paper is used as solid insulation.

Furanic compound test 

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When oil soaked paper is damaged by heat, some oil soluble compound are released into oil called furans. Paper is made of cellulose consisting of log chains of glucose rings joined by glycosidic bonds. During degradation bonds are broken and glucose rings are opened. Glucose is unstable which further degrades which are more stable and oil soluble called furans.

Furanic Compounds 

The most commonly found furanic derivative is 2furfuraldehyde(2 FAL) and other derivatives are

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2- furfuryl alcohol (2 FOL) , 2- Acetyl furan (2 ACF), 5- hydroxymethyl furfuraldehyde,(5HMF)

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5 methyl furfuraldehyde(5MEF).

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FURANS AND GASES   

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Cellulose degradation=Glucose+H2O+CO+CO2+Organic acids. With DGA and furan test extent of paper damage can be seen. CO and CO2 are determined by DGA and are considered as level indicator for cellulosic degradation. In case of severe localised paper damage ,high furans and high gas content can be seen. In case general heating slow building of furans without necessarily seeing an increase of gas content.

Degree of polymerization (DP) 

Degree of polymerization (DP) is another way of expressing the molecular weight. Physical properties of the paper depends on the degree of polymerization of paper

M = Dp x m Where M= molecular weight of the polymer Dp = the degree of polymerization and m = the molecular weight of monomer.

Degree of polymerization (DP) 

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DP value 1000 to 1500 i.e,1000 to 1500 glucose units are present in cellulose molecule. Degradation of paper is due to temperature, water ad oxygen. When DP value<300,paer becomes brittle and more suspectible to failure. DP relates directly to cellulosic degradation

Correlation between Dp and Furan 

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The absolute correlation of Furan to DP is difficult, but can be related fairly accurately extent through an empirical formula. Dp = - 100 ln (2 FAL) +709

INSULATION RESISTANCE AND POLARISATION INDEX TESTS 

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These tests are performed to verify state of dryness if insulation. These tests are intended to check overall cleanliness ,dryness ,localized defects and general condition of insulation system. When DC voltage is applied across insulation, the current flows is the resultant of three currents: Capacitive charging current Absorption current Leakage current

S C DC Voltage source

RL

Absorption current

RA

Conduction or leakage current

Capacitance charging current

INSULATION RESISTANCE AND POLARISATION INDEX TESTS 

Capacitance leakage current: The current lasts for a few seconds as DC voltage is applied and drops out after the insulation is charged to its full voltage. The time depends on the size and capacitance of the test object. Larger time for larger capacitance objects.

INSULATION RESISTANCE AND POLARISATION INDEX TESTS  

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Absorption leakage current: It is caused by polarisation of molecules within dielectric material. In low capacitance equipments the current is high for first few seconds and decrease slowly to nearly zero. In high capacitance equipment or wet and contaminated insulation ,there will be no decrease of absorption current for long time.

Conduction or leakage current: Conduction or leakage current 

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This is the current that normally flows through the insulation ,between conductors or conductors to ground. It increases quickly and becomes stable. This current increases as insulation deteriorates and becomes predominant after absorption current vanishes. It is steady and time independent .Hence the important current for measuring insulation resistance.

Interpretation of results: 

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If IR value shows a decreasing trend it shows gradual deterioration of insulation quality due to humidity ,dust accumulation etc.. A very sharp drop indicates insulation failure.

IR MEASUREMENTS 

THE TEST VOLTAGES FOR IR MEASUREMENT OF TRANSFORMER ARE AS UNDER. RATING

TEST VOLTAGE

415/440 V

500 V

3.3 kV

1000 V

6.6 kV

2500 V

11 kV & ABOVE 

5000 V

THE MINIMUM ACCEPTABLE LOWER LIMIT FOR IR VALUES IS GIVEN IN STANDARDS AS V L-L + 1 M

Polarisation index measurements 

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Polarisation Index=600sec IR value/60 sec IR value PI<1 , for bad insulation and PI in the range of 1.2 to 2 can be considered as an indication of good insulation. A very high value of PI is also not advisable since it shows the brittleness of insulation .

STEP VOLTAGE TEST 

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The DC voltage is applied in various steps and in each step the leakage current is noted. Step duration:60 seconds. The variation of this leakage current (or IR) with test voltage gives the condition of insulation. If insulation is dry, clean and with out physical damages shall show the same value at all voltage levels. If insulation value decreases at higher voltage levels ,may be due to dirt, moisture, cracking, aging etc. The application of increased voltage creates electrical stresses on internal insulation cracks. This can reveal aging and physical damage in relatively clean and dry insulation which would have not been apparent at lower voltages.

CAPACITANCE MEASUREMENTS 

THE CAPACITANCE VALUE IS DEPENDENT ON  

THE CHARACTERISTICS OF THE DIELECTRIC MATERIAL THE PHYSICAL CONFIGURATION OF THE ELECTRODES  

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 A C = ----

d Void /impurities may discharge partially during a voltage apication and the effective distance between eectrodes ncreases.

HENCE C WILL INCREASE WITH INCREASE IN VOLTAGE, WHICH INDICATES PRESENCE OF PD AND THE DETERIORATION OF INSULATION A

ELECTRODE

DIELECTRIC

d ELECTRODE

TAN  MEASUREMENTS 

THE TAN  VALUE DIRECTLY INDICATES THE POWER DISSIPATED BY THE INSULATION Ic

Ic

 90

v

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v

IT INCREASES WITH INSULATION DETERIORATION AND SERVES AS AN EARLY INDICATOR OF FAILURE HAZARDS

Concept of Tan  

In an ideal capcitor the voltage and current are phase shifted by 90 deg.and current through insulation is capacitive. If there are impurities in insulation,the resistance of insulation decrease resulting in increase of resistive current. Thus the total current I = Ic + Ir which leads the voltage by a phase angle < 90°. And lags the Ic by an angle . 20.4 30.6 45.9

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The tangent of this angle directly indicates the heat dissipation that takes place inside the dielectric material. The values obtained on new insulation forms the reference value for periodic measurements.

Partial discharge: 

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It is an electrical discharge that occurs across a portion of the insulation between two conducting electrodes without completely bridging the gap. This results in localized, nearly instantaneous release of energy. The most convention unit for quantifying the PD quantity is Pico coulomb

Interprtation of results: 

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High value of tan-delta at low voltage gives an indication of contamination and presence of moisture content. Tan-delta tip up gives an indication of void content (variation of tan delta with applied voltage). Generally tan delta values shall not increase as applied voltage increase. A higher tan delta tip up at a applied voltage indicates presence of voids/moistures and the inception of partial discharges at this voltage. Increase in tan delta above passing of time also indicates deterioration of insulation

Transformer protection

Transformer - Protections   

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Over load Capability: Working life of transformer – dependent on life of insulation Rate of deterioration of insulation – increases with increasing winding temperature  W Winding temperature – dependent on loading Transformer has substantial over load capability IF  W < 80 deg.c – use of life negligible If transformer is operated @ 104 deg. For every hour of operation = 2 hrs of life is lost. If transformer is operated @ 116 deg. For every hour of operation 8 hrs of life is lost Rate of using transformer life doubled for every temperature increase of 6 deg.c

Transformer - Protections . Relative Rate of using Life in Hours

Transformer Life Vs Temperature 100 10

98

1

104

2

110

4

Accelerated Ageing

1 Normal Ageing

0.1 80

92

104

116

128

Winding Temperature in Deg C

140

Transformer - Protections Period1

Period2

Period3

Total Life Lost

Loss Hrs./Day

24 Hrs @80oC

24 X 0.125

3

24 Hrs @98oC

24 X 1

24

9 Hrs @80oC 7 Hrs @80oC 8 Hrs @80oC 9 X 0.125 + 7 X1 +8 X 2

24

24 Hrs @104oC

48

24 X 2

Transformer - Protections Criteria for overload operation For normal duty cycle, current shall not exceed 150% I RAT  For emergency duty, current can exceed 150% I RAT provided associated cables, switch gear, tap changers, bushings etc. are suitable rated. Under no circumstances,   Winding shall exceed 140 deg.c   Oil shall exceed 115 deg.c.

Transformer - Protections  

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Buchholz Protection: Relay installed in the pipe line between transformer tank and conservator Two Floats 

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Upper float = responds to slow accumulation of gas due to mild or incipient faults – for alarm Lower float (Vane) – responds to oil surge caused by major internal faults – for trip

Relay mounting precautions   

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Gas shall freely pass up the pipe work Extra turbulence shall not be induced in oil stream Relay shall be mounted on straight run of pipe line which should slope from transformer to conservator at an angle of 5 deg. Operating time: 100 to 200 milliseconds Petcock provided on top of housing to draw accumulated gas for analysis

Transformer - Protections    

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Buchholz Protection: Gas actuated relay Popularly used in all countries except USA Used to detect incipient faults which may lead to major damage if allowed to continue Some Examples:    

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Hot spots on the core due to short in lamination insulation Core bolt insulation failure Faulty joints Inter turn faults Loss of oil due to leakage

Depends for its operation on the fact that most internal faults generate gases.

Maintenance Schedule Frequency of Inspection Daily

Half yearly

Item to be inspected

Remarks

Amb. Temp. Oil temp. Winding temp. Load current Voltage Oil level / leakage Bushing Cable boxes Breather Tap changer operation WTI/OTI IR value

Check whether temp. rise is reasonable Check against rated values Take corrective action if abnormality is noticed

Maintenance Schedule Frequency of Inspection Yearly

Item to be inspected Remarks

Yearly

DGA analysis

Five yearly or condition monitoring report based

Over hauling of transformer

Oil testing

Take corrective action (oil filtration / oil replacement) as per the test report Over haul the transformer if abnormality is indicated in the report Take corrective action if abnormality is noticed