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KONČAR – Distribution and Special Transformers, Inc.

MAINTENANCE AND REPAIR OF MEDIUM POWER TRANSFORMERS

WAPIC, Lagos, Nigeria , 27.11.2013

Jasenko Kosorčić M.Sc.Mech. Eng.Project managerStrategic commercial departmentKončar Distribution and special transformers Inc.Address: J. Mokrovica 8; p.p. 100; HR-10090 Zagreb, CroatiaPhone:+385 1 3783 715 Fax: + 385 1 3794 051 E-mail: [email protected] Web: www.koncar-dst.hr

Content

1.Why transformers are important ?2. Why is necessary to maintaining transformers ?3. Aging of the transformers4. Parts of transformer important from maintenance point of view5. Electrical tests of transformers6. Maitenance of the transformers7. Repair of the transformers

Maintenance and repair of medium power transformers

1.Why transformers are important?

Transformers are essential pieces of electrical equipment that help to transmit and distribute electricity efficiently and reliably. They also help maintain power quality and control, and facilitate electrical networks. In the substations they are most expensive part of theequipment


2. Why is necessary to maintaining transformers?

Regardless a fact the transformers do not have any major rotaitng parts it is necessary to maintaining them. Reasons:1. Aging (internal condition of transformers degrades)2. Overvolatges and short circuits in the electical network3. Weak points in the transformers designAs transformers aging, their insulation strength can degrade to the point that they cannot withstand system events such as short-circuit faults or transient overvoltages.To prevent failures and to maintain transformers in good operating condition it is very important to perform maintenance programs combined with regular testing.


3. Aging of the transformers

Acceleration damagesTransportationHANDLINGCorrosionHumidityDegradation of paintingSun radiationOver voltageLightning

AMBIENT CONDITIONS

Over voltagesTransientsVOLTAGEMech. Winding stressShort circuitThermal expansionCyclic loadingHeatOverload

LOAD

Influence on transformerParticular causeGeneral cause

Stress on transformers



Cellulose is a polymer made from glucose. It's made from β-glucose molecules and the polymer molecules are 'straight'.

Aging of insulationDegree of polymeraization

Degree of polymerization (DP) is a number of single molecules of glucose connected in the one chain.Fresh Kraft paper will have DP around 1100 to 1200. After factory drying this value will drop up to 1000 and then decreases exponentially with time.



In IEC 60076-7 is given a chart of DP changing

over time ofnon-thermally and

thermally upgraded paper exposed to a temperature of 150 °

(Sealed tube accelerated ageing in mineral oil at 150 °C )

Aging of insulation



Mechanical strenght is determined by lenght of cellulose molecules [4]:

Aging of insulation



Aging or deterioration of insulation is a time function of temperature, moisture content, oxygen content and acid content In reaction with water the cellulose molecule is splitup and the process of depolymerization goes on, the number of glucosic units is reduced. The length of the cellulose molecule gives an objective information about the paper condition and so for the mechanical strength of thetransformer insulation system. Determination of the degree of polymerization (DP) can be done by chemical analysis of the paper.

Aging of insulation



Measurment of DP is defined byIEC 60450 ed 2.1 Consol. with am1Measurement of the average viscometric degree of polymerization of new and aged cellulosic electrically insulating materialsThis methodis not applied for transformers in operation, but it delivers good results to determine the electrical and mechanical residual strength of paper insulation.

Aging of insulation


3. Ageing of the transformers

Distribution of DP values generator step up transformer after 40 years of service [3]

Ageing of insulation



DP values for estimating remaining paper life [7]

Aging of insulation

200 DP0 life remanining

300 DP30% life remanining

500 DP60% to 66% life remaining

1.000 DP to 1.400 DPNew insulation



The main force of oil aging is oxidation.

For oxidation to take place there has to be something to oxidize(oil), oxygen, and heat to providethe activation energy.

Oxidation of oil generate a water and acids which accelerate theinsulation aging. [2]

Aging of transformer oil

[2]



Water content of oil as function of time [5]Aging of oil with open expansion systemAging of oil with closed expansion system

Aging of transformer oil without addition of solid insulation



Acid number of oil as function of time [5]Aging of oil with open expansion systemAging of oil with closed expansion system




Colour number of oil as function of time [5]Aging of oil with open expansion systemAging of oil with closed expansion system




Loss factor tan δ of oil as function of time [5]Aging of oil with open expansion systemAging of oil with closed expansion system

Temperature of measurment 90˚C f=50Hz




Break down voltage of oil as function of time [5]Aging of oil with open expansion systemAging of oil with closed expansion system

Temperature of measurment 20˚C acc. IEC

Aging of transformer oil without additon of solid insulation


4. Parts of transformer important from maintenance point of view

Content4.1 Position on the transformer4.2.Transformer oil4.3 Dehydrating breather with oil barier4.4. Buchholz relay4.5. Pressure relief device4.6. Oil level indicator4.7 Temperature indicators4.8 Cooling fans4.9. On load tap changer (OLTC)


4.1 Postion on the transformer


4.2 Transformer oil

Content:4.2.1 Function of transformer oil in transformer4.2.2 Groups of tests for mineral insulating oils according IEC 604224.2.3 Colour test4.2.4 Break down voltage test4.2.5 Water content test4.2.6. Acidity (neutralization) value test4.2.7 Dielectric dissipation factor and resisitvity test4.2.8. Desolved gas analysis test (DGA)


4.2 Transformer oil

4.2.9 Properties of insulating mineral oil for transformers during bulk delivery

4.2.10 Recommended limits for mineral insulating oils after filling in new electrical equipment prior to energization

4.2.11 Evaulation of oil in service according IEC 60422


4.2.1 Function of transformer oil in transformer

Function of transformer oil in transformer is to insulate and to serve as a coolant.


4.2.2 Groups of tests for mineral insulating oils according IEC 60422

According IEC60422Article 6.1. tests of mineral oils are divided in routine tests (1), complementary tests (2) and special investigative tests (3) Here is missing DGA test.


4.2.3 Colour test

According ISO 2049:1996, Petroleum products -- Determination of colour (ASTM scale)

NumberColour number


4.2.4 Breakdown voltage test

Test is performed according IEC 60156:Insulating liquids -Determination of the breakdown voltage at power frequency -Test method

Breakdown volatge is a measure of the abiliy of oil to withstandelectric stree

Low value of BDV indicates presence of moisture content and conducting substances in the oil.



Breakdown votlage vs. Water content [6]



Sample of BDV test


4.2.5 Water content test

Test is performed according IEC 60814 : Insulating liquids – Oil-impregnated paper and pressboard – Determination of water by automatic coulometric Karl- Fischer titration

The water content in oil is measured as mgH2O/kg oil or ppm of water

During oxidation acid are formed in the oil give rise the solubility of water in the oil. Acid coupled with water further decompose the oil forming more acid and water. In this way rate of degradation of oil increases.


4.2.5 Water content test

4.2.6. Acidity (neutralization) value test

Test is performed according IEC 62021-1 ed1.0 Insulating liquids - Determination of acidity - Part 1: Automatic potentiometric titration


4.2.6. Acidity (neutralization) value testAcidity of transformer oil, is harmful property. If oil becomes acidic, water content in the oil becomes more soluble to the oil. Acidity of oil deteriorates the insulation property of paper insulation of winding. Acidity accelerates the oxidation process in the oil. Acid also includes rusting of iron in presence of moisture. The acidity of transformer oil is measure of its acidic constituents of contaminants. Acidity of oil is expressed in mg of KOH (potassium hydroxide) required to neutralize the acid present in a gram of oil. This is also known as neutralization number.


4.2.6. Acidity (neutralization) value test

4.2.7 Dielectric dissipation factor and resisitvity test

Test is performed according IEC 60247: Insulating liquids –Measurement of relative permittivity, dielectric dissipation factor (tan δ ) and d.c. Resistivity

Test temperature is 90 °C



Dielectric Dissipation Factor is also known as loss factor or tan delta of transformer oil. If value of ′δ′ which is also known as loss angle,is smaller, means resistive component of the current IR is smaller which indicates high resistive property of the insulating material. High resistive insulation is good insulator.There is a clear relationship between tanδ and resistivity of insulating oil. If resistivity of the insulating oil is decreased, the value of tan-delta increases and vice verse



Specific Resistance ( Resistivity ) of Transformer OilThis is another important property of transformer oil. This is measure of DC resistance between two opposite sides of one block of oil. Its unit is taken as GΩm at specific temperature. With increase in temperature the resistivity of oil decreases rapidly.


4.2.8. Dissolved gas analysis test (DGA)

Test is performed according: IEC 61181 ed2.0 Mineral oil-filled electrical equipment -Application of dissolved gas analysis (DGA) to factory tests on electrical equipment



IEC 60599 ed2.0 Mineral oil-impregnated electrical equipment in service - Guide to the interpretation

of dissolved and free gases analysisIEEE C57.104 - Guide for the Interpretation of

Gases Generated in Oil-Immersed Transformers



Sample of DGA test report



According IEC 60296: Fluids for electro technical applications – Unused mineral insulating oils for transformers and switchgear



Sample of incoming oil test report

4.2.10 Recommended limits for mineral insulating oils after filling in new electrical equipment prior to energization

According IEC 60422Mineral insulation oils in electrical equipment-Supervision and matenance guidanceArticle 9.



Decisions about corrective actions


4.3 Dehydrating breather with oil barier

Light rosyLight greenSilica gel

saturated by moisture

Transparent white

Turquoise blueSilica gel moist

Orange (Salmon)

Amber yellow

Silica gelcompletelydry

Type 2. of silicagel

Type 1. of silicagel

STATUS

Required level of oil

1. The line in the middle of the container did not mark the oil level in the cup.2. Fill the oil to the specified recommendation.3. When 2/3 of silica gel becomes saturated by moisture, it has to be dried or replaced.4. Drying silica gel is performed on the way that silica-gel is placed in the oven and heated to 120-130°C for about 1 hour, after that silica-gel is dried and has recovered color to the dry.

Note:



Type 1 silicagel-dry Type 2 silicagel-dry


Dehydrating breather with too much oil in barier Dehydrating breather with

proper amount of oil in barier


4.4 Buchholz relay


4.4 Buchholz relay- ConstructionBuchholz Relay is an oil container housed the connecting pipe from main tank to conservator tank. It has mainly two elements. The upper element consists of a float. The float is attached to a hinge in such a way that it can move up and down depending upon the oil level in the Buchholz Relay Container. One mercury switch is fixed on the float. The alignment of mercury switch hence depends upon the position of the float.The lower element consists of a baffle plate and mercury switch. This plate is fitted on a hinge just in front of the inlet (main tank side ) of Buchholz Relay in transformer in such a way that when oil enters in the relay from that inlet in high pressure the alignment of the baffle plate along with the mercury switch attached to it, will change.In addition to these main elements a Buchholz Relay has gas release pockets on top.


4.4 Buchholz relay- Functions

Surge of oilTrip

Oil loss1. Alarm

2.Trip

Gas accumulationAlarm


4.4 Buchholz relay- Installation


4.4 Buchholz relay-Testing by means of function key


4.4 Buchholz relay-Sampling of gases from Buchholz relay

Defined by:IEC 60422 Oil-filled electrical equipment –Sampling of gases and of oil for analysis of free and dissolved gases – GuidanceChapter 3 Sampling of gases from gas-collecting (Buchholz) relays


4.5 Pressure relief device

Drawing of pressure relief device in open

position



Pressure relief device is spring loaded valve that automaticallyreclose following a pressure release. The spring is held in compression by the protective cover and press on a disc which selas an opening in the tank top. If pressure in the tank excides operating pressure (0,5 bar), the disc moves upward and relievespressure. As pressure decrease, the spring reclose the valve. After operating , this device leaves a brightly colored rod (yellow for oil) exposed about 5 cm above the top .The rod may be reset by pressing on the top until it is again recessed into the device. The switch must me manually reset. In a case of activation switch sends a signal for disconnection of the transformer from the grid.



Testing a signal for disconnection


4.6 Oil level indicatorREADING THE INDICATIONS OF THE OIL LEVELS- Minimum level: when the dial shows all red.- Maximum level: when the dial shows all white.- Intermediate indications between MAX and MIN: the dial shows part white and part red. Remember that the amount of red shown indicates, in proportion,the part of the conservator left without liquid


4.7 Temperature indicators

TripTrip


4.8 Cooling fans

Push buttons for start and stop fans in control cabinet


4.9. On load tap changer (OLTC)


4.9. OLTC

OLTC allows to change the ratio of a transformer by adding turns to or subtracting turns from primary windingFor this purpose transformer is equipped with”regulating or tap winding”, which is connected to the OLTC terminals


4.9. OLTC-Design principle of resistor OLTC


4.9. OLTC-Installation

4.9. OLTC-Protective relay


4.9. OLTC-Burst membrane


4.9. OLTC-Motor drive unit


5. Electrical tests of transformers

Electrical test of medium power transformers are divided in three groups:

1. Routine tests- - “a test to which each individual transformer is subjected”(performed on each produced transformer)

2. Type tests- “a test made on a transformer which is representative to other transformers” (performed on one transformer of same type /construction)

3. Special test- “a test other than a type test or a routine test, agreed by the manufacturer and the purchaser “



Sample of Transformer Test Report

- contains all measuring results


6. Maitenance of the transformers

INTRODUCTIONTransformer is very important part of the electrical power system. The primary purpose of the transformer maintenance is to ensure that the internal and external parts of the transformer and accessories are kept in good condition and able to operate safely at all times.Transformer maintenance can be done in three ways:

1. Maintenance by failure

2. Preventive maintenance

3. Maintenace by conditions



CONDITION MAINTENANCE PROGRAM Condition based maintenance is usually the most economical

way of doing maintenance. Recommended maintenance are done based on one or more of the following: visual inspections, analysis of oil samples, electrical measurements, test of equipment, measurements of temperatures, monitoring. ATTENTION:If transformer is going to be used as a spare or stand-by unit, it maintenance must be equal to the transformer in service. Special care must be taken in monitoring the conditions of the oil.

In periodic schedule, the frequency of inspection and maintenance procedures will vary with the rating of the transformer, but the intervals suggested in this presentation are recommended as minimum.



CONDITION MAINTENANCE PROGRAM

In most cases operators are familiar with the noise and behaviour that can occur during transformer normal operation, which can help to discover some defects. It is very important towrite down the readings from the meters or gauges that are generally installed. These readings can be very useful. If the readings are very different from the ones obtained in normal conditions, careful revision should be performed.In addition, you should pay attention to the following:- the temperature of the transformer is directly related to the life span of the insulating materials, so it is necessary to pay close attention,- oil level and volume changes in relationship to the temperature,- in some instances some abnormal noise can be perceived,- checking of loose pieces and valves,- oil leaks.


6. Maitenance of the transformersPOWER TRANSFORMER AND ASSOCIATED EQUIPMENT

MAINTENANCE INSPECTION INTERVALS FOR ENERGIZED TRANSFORMERS

• Check transformer load currents and voltages.

• Check transformer for possible oil leakage.

• Observe any abnormal noises from the transformer.

• Check of ambient, oil and winding temperature.

• Observe cooling fans.• Check of oil level.• Check condition of silica gel in the

breathers.• Visual check of cable connections.• Visual check of control cabinets.

Every site visit(monthly)

CHECK AND MAINTENANCE LISTINTERVAL




• Observation and registration of counter information or operation time of transformer, pumps, fans, OLTC, etc.• Check and record readings on all indicating instruments.• Examine the piping to the coolers or radiators and bolted pipe joints for signs of oil leakage.• Check the coolers or radiators for accumulation of dirt and foreign material that might obstruct air flow.• Inspect control cabinets of the transformer and the OLTC.

Quarterly





• Check bushing cleanliness in polluted environments and visual check for cracks and unscrewed hardware.• Check switchgear cable termination for pollution and electrical tracking.• Check transformer cables for deformation distance to grounded metal parts.• Protective relay trip check, calibration of monitoring equipment.• Grounding system – check for loose, damaged or corroded connections.• Dissolved gas analysis and routine analysis of oil.

Annually




MAINTENANCE INSPECTION INTERVALS FOR DE-ENERGIZED TRANSFORMERS

• For the transformer equipped with forced oil cooling, check the oil circulating pumps for any unusual noise or flutter of the oil flow gauge.• For transformers equipped with fans check functionality of the fans and ensure there is no debris covering them or are in the way of the blades.• Take an oil sample from the main tank and any other oil-filled compartment for DGA analysis.• Perform same electrical test that were made at commissioning.• Examine all bushings and interconnecting hardware for contamination and signs of electrical tracking, perform power factor and capacitance measurement on the bushings (Only for condenser type bushings).• Inspect on load tap changer according to manufacturer instructions.• Inspect breathers and pressure relief valves or devices.• Examine paint finish, particularly around welded joints.• De-energize the auxiliary power source and inspect the control equipment inside control cabinet.• General repairs e.g. tightening bolts, replacement of gaskets.

Annually (optimal)





• Complete series of transformer electrical tests that were done at commissioning.• OLTC – according to manufacturer’s instructions, if any.• Inspect alarm functionality for pressure relief device.• Cleaning of complete switch gear, inspection and lubrication of all associated devices including circuit breaker, wiring, buses.• All "annual" items not considered at one or two year interval.

3 years





•Complete service on OLTC (only certified personnel are allowed to do this). *Depends on whether tap-changer is oil or vacuum type.

6 years *



6. Maitenance of the transformersBUSHING AND JOINTSBushings generally do not require maintenance. Surface contamination is caused by natural deposits (such as early morning dew, salt fog in sea coast areas,etc.) and industrial pollution. Such contamination has often resulted in noisy substations, damage of insulating surfaces, partial discharge, tracking flashover, and loss of power. The porcelain insulators of the transformer bushings ought to be cleaned during service outages as often as necessary. Use recommended cleaning agents for cleaning the bushings. The condition of the external conductor and bus bar joints of the transformer bushings shall be checked at regular intervals. Maintenance of the bushings shall be performed according to the instructions given by the bushings manufacturer.


6. Maitenance of the transformersTRANSFORMER ACCESSORIESDepending on customer requirements transformer can be equipped with accessories (oil and winding temperture indicator, pressure relief device, oil level indicator, bladder,silicagel breather, current transformers, etc.) that are not standard for each transformer. For all transformer accessories it is recommended to be tested yearly. Relays should be cleaned, lubricated and calibrated periodically in accordance with manufacturer’s instructions. It is also necessary to check apparatus, electric cables and conductors, signal and control devices in the control room. For detailed informations please refer to manufacturer maintenance instructions..


6. Maitenance of the transformersCOOLING SYSTEMThe cooling equipment is the most important part in the normal daily operation of a transformer. Special care, maintenance and inspection is necessary, since any abnormality can reduce the life time of the transformer or cause serious defects.For radiator type cooling system (ONAN) check for oil leakage. If the radiators are detechable, check that the valves are open. If the transformer is equipped with forced oil cooling (OF), check the oil circulating pumps for any unusual noise or flutter in the oil flow gauge.If the transformer is equipped with air cooling (AF), examine the fans to ensure there is no debris covering them or that the blades are blocked. Check to make sure each fan is operational and that the blade rotation is correct. .


6. Maitenance of the transformersTRANSFORMER AND OLTC CONTROL CABINETDe energize the auxiliary power source and inspect the control devices in the control cabinet. Remove grease, oil or other contaminants. Do not soak the parts with a cleaner, but use just enough to loosen grease or dirt so that it can be wiped off. For cleaning small parts a small paint brush dipped into the cleaning solution is good for getting into corners and crevices. Repair or replace any broken or malfunctioning part, tighten all loose connections and eliminate any oil or water leaks into the compartment.ATTENTION:If the transformer is de-energized, heather inside the control cabinet should be turned on, especially in case of low ambient temperatures.


7. Repair of the transformers

1.Repair of transformers on the site2. Repair of transformers in the repair workshop



1.Repair of transformers on the site

–– Replacement of bushings–– Replacement of the sealing (gaskets)––Replacement of transformer accessories–– Maintenance of the cooling system–– Cleaning and painting of the outside surfaces and the fittings–– Stopping of oil leakage–– Replacement of oil–– Oil conditioning



2. Repair of transformers in the repair workshop

• Making new windings• Replacement of the insulation• Repair or replacement of the core• Replacement of the wiring• Replacement of the OLTC• Replacement of valves• Sandblasting and painting of tank, cover and conservator• Vacuum or Kerosene drying of windings and active parts• All electrical test of transformers according IEC standards


7. Repair of the transformersTesting a transformers after repair

Dielectric tests are defined by IEC 60076-3:2010 – article 8


Sources

[1] Moisture Determination and Degradation of Solid Insulation System of Power Transformers, C.Sumereder and M. Muhr, Graz University of Technology

[2] Properties of Mineral Insulating Oils in Service, P. Wiklund and B. Pahlavanpour

[3] Service and maintenance of transformers, M.Scaefer, Transnet BW.[4] Dejan Susa, Sintef[5] Transformerboard II , H.P.Moser and V.Dahinden[6] Transforme diagnostic –Common used and new methods , G.J.Puckel[7] Facilities instructions, standards and techniques volume 3- 31, United

States Department of Interior,Bureau of Reclamation, June 2003


Thank you for your attention!


Questions?


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