Transformer Maintenance

Maintenance of Power Transformers • The Life of a Power transformer is 35 Years. • The Factors affecting the Life of a Transformer is :• i) Moisture. • ii) Oxidation of oil and dielectric

Maintenance of Power Transformers • Iii) Solid impurities, Varnish etc. • Iv) Repeated movements.- Slackness of windings. • Iv) Temperature- Over loading , Cooler Blocks etc.

Very Important • If a transformer trips on differential/ R.E.F/ Bucholz / Pressure Relief protection or with no indication, do not test charge. Inform the station engineer.

Factors affecting transformer life • Moisture- absorption of moisture from the air deteriorates the dielectric strength of oil and paper insulation

• Oxygen- the oxygen in the air trapped inside the transformer reacts with the cellulose of the insulation and the decomposition products cause for formation of organic acids soluble in oil and sludges which blocks the free circulation of oil

• Solid impurities- presence of impurities like particles of insulating materials etc. accelerate deterioration • Varnishes- some oxidising varnishes react with transformer oil and precipitate sludge on the windings. Synthetic varnishes which have acid inhibiting properties generally delay the natural formation of acid and sludge in the oil

• Slackness of windings- transformer windings may become slack during transportation due to coil displacements under load conditions and due to momentary short circuits which cause electrical and magnetic unbalance

Causes of failure • • • • •

Faulty design or construction Improper insulation Overloading Wear and tear, neglect, accidents etc. Deterioration of insulation due to moisture absorption, decomposition of insulating material due to ageing • Failure of auxiliary equipment

TYPICAL FAILURE DISTRIBUTION OF SUBSTATION POWER TRANSFORMERS

FAILURE OF: CORE : 3% TERMINALS : 12% ACCESSORIES: 12% TANK/FLUID : 13% WINDINGS : 19% OLTC : 19%

Maintenance of Power Transformers • Code of Practice for Installation and Maintenance of Transformers.-IS -1886 / 1967 • Code of Practice for Maintenance and Supervision of Insulating Oil in service.-IS -1866 /1978. • Application Guide for Power Transformers IS-10561 – 1983.

What is meant by Maintenance of Transformer ? • 1)Assessing the rate of ageing / deterioration taking place in the Transformer. • 2)Identification and elimination of Factors contributing to faster rate of ageing of Insulation System.

Maintenance of Power Transformers • 3)Taking measures to compensate for the deterioration. • 4)Proper care should be given to the functioning of accessories. • 5)Keep the Tank in good condition without Rust and Leak.

How can we asses the deterioration ? • Transformers are Tested at the Time of Commissioning. • Immediately after Commissioning the deterioration starts. • So the Commissioning Test results are the REFERENCE parameters.

How can we asses the deterioration ? • The following Tests are conducted and the analysis will give the amount of deterioration. • I.R.Value, P.I. Value, Testing of Oil samples in the Transformer and in the OLTC.

Maintenance of Power Transformers • Guide Lines for increasing the Life of a Transformer :• 1)A strict Oil Maintenance Schedule may be followed. • 2)Conduct all Tests Periodically • 3)Conduct DGA in every Year. • 4)No leak is permitted from the Transformer.

Maintenance of Power Transformers • 5)Dehyrate Silicagel Breather. • 6)The Maximum Top Oil Temperature will be limited to 45 0 C + Ambient. ( 45 + 30 = 75 ) • 7) Test and calibrate Protective Relays Periodically. • i) WTI, OTI, MOLG, Buchholtz Relay etc.

Maintenance of Power Transformers • 8)Clean the Insulators and Bushings Periodically. • 9)Inspect and Check Oil Level and connections regularly. • 10) Special care should be given the maintenance of OLTC. • 11) Periodic Inspection .

Oil leakage in transformers • Oil leakage is general problem in the operation of transformers • Leakage causes loss of costly oil and an ugly look • Excessive leakage may cause for low oil level in the transformer

Replacement of old gaskets • Sweating of oil in the gasket joints is the first sign of ageing • Oil level should be lowered to at least 100 mm below the position where gaskets are to be replaced • After opening the flange or valves concerned, remove the old gasket completely

• New gaskets of correct size shall be selected or prepared and fixed in position using bonding material • Longest gasket life is obtained when the rubberized cork sheets are compressed by 40 +-5% of its thickness • Gaskets of normal thickness 9.6 mm and 6.4 mm are generally used in transformers

• Use gasolene or Methyl ethyl ketone to soak the the gaskets for easy removal • Traces of gasket bonding material remaining shall be scraped and surface cleaned well • Oil traces shall be cleaned with Carbon Tetrachloride

• Gaskets grooves in the transformers are provided for a depth of 60 +-5% of the gasket thickness • By this 40 +- 5% compression is achieved by tightening • New gaskets shall be stored flat in box with activated silica gel in cool dry places

Loading and Life of Transformer • Paper insulation deteriorates rapidly if its Temperature rises above 90 0 C. • HOT SPOT :• The maximum temperature reaches at any part of the winding under given load conditions and ambient. • .

ILL-EFFECTS OF OVER TEMPERATURE. Paper or press board used as insulation in transformers, when heated beyond a certain limit for a long period of time, will lose mechanical strength, but the dielectric strength is hardly affected until the paper is actually charred to the point, when the carbon causes conduction. The lowered mechanical strength of paper may not be able to resist short circuit forces and break down of insulation occurs.

Loading and Life of Transformer • Ageing of Insulation:• When insulating materials heated under oil for long periods its mechanical strength will reduce. • Small amount of moisture and oxygen will accelerate the loss of tensile strength. • Between 80 0 C and 140 0 C the rate of loss of life due to ageing of Transformer insulation is doubled for every 6 0 C rise in Temperature

8 6 0 Law of Insulation Ageing.

7 6 5 4

Rate of 3 insulation 2 ageing NORMAL 1 80 86 92 98 104 110 116 Winding Hot Spot Temperature. 0 C Maximum Hot Spot Temperature = 98 0 C. Normal LIFE= 30 Years

Performance Under Short Circuit. • The effect of Short Circuit Currents are:• 1) Thermal • 2)Mechanical Forces.

Performance Under Short Circuit 1)Thermal :• As per IS- 2026, The duration of Short Circuit Current shall be 2 Seconds for Two Winding Transformers. • Thermal ability can be calculated from the cross sectional area of Winding

Performance Under Short Circuit • 2)Mechanical Forces :• When a transformer is subjected to Short Circuit Severe Mechanical Forces are produced due to the interaction of Fault Current and the magnetic leakage Flux density vector.

Performance Under Short Circuit • a) Radial Forces :• Mutual repulsion between HV and LV windings. • Crush LV winding. • Burst HV out ward. • Bending , Buckling inner Winding.

Inner Winding

Outer Winding Direction of forces RADIAL FORCES

Bending of inner Winding

Buckling of inner winding

Performance Under Short Circuit • b) Axial Forces :• The forces tend to bend the conductors in the axial direction and its sum total acts on the coil clamping ring.

Inner Winding

Outer Winding.

Failure of Disc Winding.

Factors affecting IR value

• Temperature • Dryness of insulation • Cleanliness • Humidity • Condition of oil

MEASUREMENT OF IR VALUES Measure Insulation Resistance between HV – LV LV – MV HV – MV HV - E LV – E MV – E Apply voltage for 60 seconds and IR Value at 300C may be computed.

MEASUREMENT OF IR VALUES • Minimum desired IR Values 11kV Winding - 300 Meg ohms for 1 Min. 33kV Winding - 400 Meg ohms for 1 Min. 66kV and above – 500 Meg ohms for 1 Min. Use 2.5 kV Megger up to 11kV. 5 kV Megger above 11 kV

Correction Factor for correcting IR Values to 300C Difference in temp. • 50 C 1.23 • 100 C 1.50 • 150 C 1.84 • 200 C 2.25 • 250 C 2.76 • 300 C 3.35 • 350 C 4.10 • 400 C 5.00

Correction factor

For every 100 reduction in temperature, IR Values increases by 50%.

Polarisation Index • It is defined as the ratio of the IR value at 10 minutes to IR value at 1 minute of the application of the test voltage. • Equipment Required :• Motorised Megger . • A low polarisation index indicates excessive moisture and contamination.

PI VALUE= IR 10 Min. ÷ IR 1 Min. PI value <1

Condition Dangerous

1.0 to 1.1

Poor

1.1 to 1.25

Questionable

1.25 to 2.0

Fair

> 2.0

Good

Dielectric Absorption Test • It is the ratio of IR value taken after one minute and IR value taken after 15 seconds of the application of the test voltage. • Dielectric Absorption = IR Value 60 Sec. IR Value 15 Sec. • A good insulation show increase in resistance. • If the reading remains the same , the insulation is contaminated.

Precaution: Remove neutral earthing before measuring insulation resistance of the Transformer. After completion of testing, remember reconnect the neutral earthing before energising the transformer.

Transformer Erection • Transformer foundation should be provided with adequate Oil Soak Pits and drains. • Fire Proof walls should be provided . • Before starting Erection works all the drawings should be made available. • Check all the accessories as per dispatch documents.

Transformer Erection • Prepare an Erection Schedule after discussion with Supplier. • The Erection shall be done as per CBIP specification 1987 December, Manufacturers Instruction Manual. • Packing and Dispatch :• Transformers are dispatched in Tractor Trailer or in Railway Wagon with Oil filled or Nitrogen filled condition.( above 50 MVA)

Transformer Erection . • A Pilot Vehicle with Tools ,should accompany the Tractor Trailer. • Unloading at site:• In the absence of Mobile crane Transformers are unloaded using Rails, Wooden sleepers, Hydraulic Jacks, Maxpuller, Chain pulley Blocks etc.

Transformer Trailer Platform

Rails Jack boss

Unloading the Transformer Using Rails

Pulling Device

Transformer Erection • Storage at Site :• Should be stored under a Shed . • Transformer should be filled with Dry Clean Oil. • While storage Breather ,Oil samples etc should be checked.

Transformer Erection • • • • • • • • • •

Equipments Required for Erection:1) Lifting Equipments. 2)OIL Filter – 1000 GPH. 3)Vacuum Pump. 4)Pressure Gauge, Vacuum Gauge 5) Oil Tanks. 6)Oil Testing Kit. 7)Motorised Megger. 8)General tools 9)Stable Power Supply.

Transformer Erection Procedure. • The Procedure is different for different voltage classes and different capacities. • If the Transformer is received in oil filled condition (10 MVA, 12.5 MVA ) the erection procedure is easy. • 1)Fit the Rollers on the Plinth. • 2)Full quantity of oil is kept ready (After filtering and testing) in a separate tank for filling. • 3)Open the upper filter valve and release the Nitrogen.

Transformer Erection Procedure

• 4)Connect the vacuum pump at the upper filter valve and the transformer is subjected to semi vacuum. • 5)Fill the oil through the bottom filter Valve through the filter. • Now the important point is that • The erection should be carried out in a dry atmosphere and the period of exposure should not exceed 8 Hours.

Transformer Erection Procedure. • 5)Drain oil from the Transformer and Start the Erection Work from the Bottom. • 6)After 8 Hours again fill oil and the process is repeated until the Erection is over. • 7)After completing erection , the vacuum pump is connected at the top of the conservator tank where breather is connected and vacuum is applied for 2Hours ( 759 mm Hg). ( For 66KV 380 mm Hg.)

Transformer Erection Procedure • Before HOC ( Hot Oil Circulation) Carry out the following Tests. • 1)Voltage ratio tests at all Taps by applying 415 V at the HV side. • 2)I.R value with 2000 V/ 5000 V Megger. • 3)BDV of oil sample from bottom of Transformer. • 4)Note down the Oil temperature. • 5)Hot Oil Circulation 60 0 C – 70 0 C , 10,000 LPH for 5-10 Days. • Finally I.R Value Graph, BDV above 60 KV, moisture content less than 10 ppm.

Oil Temperature in 0 C

I.R. Value in Meg Ohms 0

C MΏ TIME in Hours

Dry- out Curve for Power Transformer.

Maintenance Management • The following documents are to be prepared by the Maintenance Engineers for effective maintenance . • 1) Equipment Register with the specification of all Equipments, its Auxiliaries and Accessories. • 2) Allocate Code numbers. • 3) List of Spares and its Location.

Maintenance Management • • • •

4) List of Drawings. 5) Register of Drawing. 6) Manufacturers Instruction Manual. 7) Prepare the Maintenance Schedule for each Equipment with reference to Manufacturers Instruction Manual, Indian Standard Specification. • 8) Prepare a Maintenance Plan.. • 9) Prepare Bar chart before starting any Major Work. • 10)Maintain a Maintenance Register.

Maintenance Plan for 220 KV Substation for 2003-04 Sl N o

Name of April M J J A S O N D J Equipmen t

1 Tfr. No. I 2 Tfr. No.II 3 C.B. No. I 4 C.B.No.II

F M

Erection Schedule for 105 MVA, 400/220 KV Transformer Sl No

Activity

Febr March uary 22 23 4

1 Transportation 2 Unloading 3 Oil Filtering 4 Erection Work 5 Hot Oil

Circulation

5 6 7

--

1 13 2

GASES GENERATED BY DECOMPOSITION OF OIL & INSULATION

• • • • • • •

Hydrogen Methane Ethane Ethylene Acetylene Carbon dioxide Carbon monoxide

TEMPERATURE - GASES IVOLVED • • • •

Methane Ethane Ethylene Acetylene

>120°C >120°C >150°C >700°C

SAMPLE TEST REPORT OF 48 MVA ,GT, YEAR 1976 ,TESTED ON 9 /2002 ( 26 Year) ITEM

UNIT

RESULT

STD. VALUE

BDV

KV

55

> 50

Water cont. ppm.

40

20 /30

Tan Delta

0.379

< 0.1

Resistivity -90 Ohm-cm

0.05 x 10 12

> 0.1 x10 12

IFT

0.01439

> 0.018

0.2327

< 0.5

--

N /m

Total acidity mgKOH /gm