RESIDUAL LIFE ASSESMENT & LIFE EXTENTION OF

TRANSFORMER

Submitted By:Rajat Kumar Meher

Guided By:Mr. Prabhas Ankuri

(Asst.Director-NPTI),Guwahati.

CONTENTS INTRODUCTION

MAIN COMPONENTS OF TRANSFORMER

RESIDUAL LIFE ASSESSMENT OF TRANSFORMER

CAUSES OF FAILURE OF TRANSFORMER

LIFE EXTENSION OF TRANSFORMER

FACTORS INVOLVED WITH OIL FILLED TRANSFORMER

MAINTENANCE SCHEDULE

DO’S AND DON’T FOR POWER TRANSFORMER

CONCLUSION

REFRENCE

INTRODUCTION In an power system, power transformer are used from

generating station to utilization for stepping down and stepping up the voltage level, for transmission within low losses and throughout over long distance

Transformer is a vital device in power system and costliest too. It is most efficient device it’s efficiency is around 95% to 99% .So as it is costliest equipment in power system network the major concern is to keep it away from the all trouble and problems. so to achieve it, satisfactory and trouble free life maintenance is required.

So for trouble free life of transformer regular planned checks and monitoring is important ,so the term come preventive maintenance

MAIN COMPONENTS OF TRANSFORMER Oil Core and lamination Winding Insulation Auxiliaries component:- Pressure relief vent or explosion vent Silica gel breather On load tap changer Buchholz relay Conservator tank Winding temperature indicators Oil temperature indicators Arcing horn Impact recorder Nitrogen filling system ladder Cooling system Lightning arrester mulsifier system

By giving proper maintenance to these main parts, we can extend the life of transformer.

OIL Oil is the most vital organ of transformer; The condition of oil

determines the life and efficient operation of transformer. Transformer is immersed in oil because it provide following two function.

It is a pure hydrocarbon mineral oil. Provide the Insulation between the conductors and coils. Provide cooling medium, extract the heat without losing the insulating

capacity. To meet these function oil should have following properties.

Electrical properties Chemical properties Physical properties

Oil appearance

CORE AND LAMINATIONLaminated core is also vital organ of the power

transformer It is made up of CRGO (3% silicon steel)It is a magnetic circuit to transfer the electrical

energy from one circuit to another circuit with the help of winding.

WINDING AND INSULATIONS

Transformer winding depends upon the short circuit strength, current rating, temperature rise and overload condition, impedance and voltage strength of the transformer for which it is be used

Insulation used Cellulose paper Cotton tape Pressboard Varnishes Bakellite tube Sleeving tube

OTHER AUXILARIES MOUNTED ON TRANSFORMER

RESIDUAL LIFE ASSESSMENT OF TRANSFORMER

Residual life assessment of transformer means to predict the life of transformer on the basis of rate of degradation of main & auxiliary components of transformer.

To find out these degradation rate ,we performs following analysis, these are:-

1. Dissolved gas analysis 2. Furfural analysis 3. Frequency response analysis 4. Partial discharge 5. Infra- red thermal image scanning

DISSOLVED GAS ANALYSIS

It is a powerful diagnostic technique for monitoring the internal condition of transformers before they develop in to faults and serious damage to the transformer. It is not only a science but also an art.

Fault associated with different gases:- Gas formed Associated fault C2H4,C2H6,CH4 Oil overheating CO2,CO cellulose

overheating H2,CH4 Partial discharge C2H2,H2 Arcing in oil

C P R IDISSOLVED GAS ANALYSIS FOR

TRANSFORMER FAULTS

SAMPLING

Extraction of gases

ANALYSIS BY GAS CHROMATOGRAPH

INTERPRETATION

Acceptable limits of individual fault gases (as per IEEE)

Fault gases Volume(in PPM)

H2 100

CH4 120

C2H6 65

C2H4 50

C2H2 35

CO 350

CO2 2500

FURAN ANALYSIS• It is use for the finding out the condition of the

cellulose paper• The degradation of paper give rise to the CO

and CO2 their measurement of CO and CO2 can not used the un-ambiguous indication of paper degradation

• Other complexity are collection of paper sample, location of specific nature of test, complexity and accuracy involved in test

• That is why furan compound are used to asses thermal degradation of cellulose insulation of transformer.

In addition to the CO and CO2 the pressboard in the oil produce the several oil soluble by products, one of the significant compound related to the degradation of paper is 2-furfural ad other are present in undetectable level

If the furan compound in oil reaches to 10PPM AT DP of 150-200 then the life of cellulose paper is ended

After analysis of many transformer , it was found that the highest value of furfural was 3 PPM and other furan compound was being <0.3 PPM

FREQUENCY RESPONSE ANALYSIS Frequency response analysis technique is used for assessment of

the mechanical integrity of transformer windings. The loss of mechanical integrity occur due to , Large electromechanical forces due to fault currents. Winding shrinkage due to loosening of clamping pressure. During transformer transportation and relocation.

PARTIAL DISCHARGE A partial discharge is defined as a localized electrical

discharge in insulating media , which only partially bridges the insulation between conductors.

It initiates due to following reasons,1. Improper processing or drying of insulation.2. Over stressed insulation due to a lack of proper

recognition of the voltage limitation of the insulation.3. High stress areas in conducting parts, which can

cause by sharp edges on either the conducting part or the ground plane.

INFRA-RED THERMAL IMAGE SCANNING For detection of local hot spots, infra red thermal image scanner is

used on transformer and their connections. It is a powerful condition monitoring tool. Incipient faults caused by:

Poor connections, Corrosion, Blockage of radiators, Abnormal surface heating due to internal faults, Insulation failure and equipment defects, Can be detected by the

equipment.

CAUSES OF FAILURE OF TRANSFORMER

The causes of the breakdown of transformer can be due to the following reason

In correct installation, improper commissioning, lack of operation and maintenance

Lack of adequate supervision / negligence Ageing and other deterioration Accidents Repeated feeding of fault current

Design 36% Manufacturing problem 28%

Material defect 13% Poor maintenance 7%

Lightning surge 4% Short circuits 2%

LIFE EXTENSION OF TRANSFORMER Transformer is vital device in power system so the life of

transformer is plays very important role The condition of insulation determines the life of transformer which

can be monitor by several test if we found the appropriate result then we take some suitable action

General life of transformer is 25 years but we can extend the life of transformer by performing the following step:-

Proper maintenance Protection from faults and voltage surge

Reducing the fault current feeding to winding by installation of reactor

By increase of dielectric strength of insulation By transformer monitoring

Keep insulation in good condition Proper grounding

PROTECTION OF TRANSFORMER Gas actuated relay (buchholz relay ) Restricted earth fault relay Differential relay protection Unrestricted earth fault and over current protection Over fluxation relay Oil temperature indication Winding temperature indication Lightning protection Short circuit protection Effective Grounding Other Auxiliary Protections Are Pressure relief valve (PRV) Silica gel breather Impact recorder Oil charge relay Mulsifeir system Nitrogen filling system Thermo siphon filter

By effective protection scheme we can reduce the chances of breakdown due to the internal and external fault

FACTORS INVOLVED WITH OIL FILLED TRANSFORMER

The following factors are the enemies of transformer:- Moisture Sediments/solid particle Fibers Oxygen Acid Sludge Voltage stress Overvoltage Temperature rise Low frequency varnishes

New insulating oil characteristicsSR.NO CHARACTERISTIC STANDARD VALUE

01. Appearance

Oil should be transparent and free from suspended particles or sediments

02. Density at 27 degree centigrade 27 g/m3

03. Viscosity at 27 degree centigrade 27 center stroke

04. Interfacial tension at27 degree centigrade 0.04 N/M

05. Flash point 140 degree centigrade

06. Pour point -6 degree centigrade

07. Neutralization value 0.03 mg KOH per gram of oil

08. Electric strength or break down voltage 30 KV before filter 60KV after filter

09. Dissipation factor 0.05

10. Water content <50 PPM before filter <20 PPM after filter

11. SK value 4% to 8 %

12. Co efficient of expansion at 60 degree cent. 0.00781/degree centigrade

13. pecific heat at 60 degree 2.06 KJ/Kg degree

14. Thermal conductivity 0.12 W/m degree centigrade

BASIC CONCEPTS OF MAINTENANCE

Generally the term Maintenance may be defined as, Combination of many actions carried out to retain

equipment in or to restore it to an acceptable operating condition.

Maintenance is carried out after some operating hour or calendar months as the failure and deterioration processes are dependent on the operating time

“prevention is better than cure” So, in order to ensure the trouble free services over long

period, all vital components of the transformer and its auxiliaries need the planed program of checks, visual inspection and preventive maintenance

Maintenance schedule Hourly maintenance

Oil and winding temperature Load current and voltage

• Daily maintenance Oil level in conservator and main tank Oil level in bushing Oil level in OLTC tank Leakage of water in cooler Check pressure relief vent Oil seal of dehydrating breather

Monthly /Quarterly maintenance

Check the color of active agent of dehydrating breather Examine the bushing Check the circuit of cooler fan and pumps bearing , motors OLTC driving mechanism

• Half yearly maintenance

Measurement of winding PI values BDV and water contents of oil in transformer and OLTC Check earth resistance DGA of oil OLTC driving mechanism check

Yearly maintenance

Check transformer oil Check oil filled bushing Check gasket joints Check surge diverter Relay and alarms circuit Paint works

• Another maintenance after 10,000 or 15,000 operation

Internal inspection of conservator tank Mechanical inspection of buchholz relay Overall inspection including lifting of core and winding

DO’s FOR POWER TRANSFORMER Check the diaphragm of the relief vent, if

cracked or broken, replace it. Examine the bushings for dirt deposit. Check the oil in transformer and OLTC for

dielectric strength, moisture content, acidity, sludge.

Check the protection circuit periodically. Clean the oil conservator before erecting. Check all the operating mechanism, bearings

and lubricate them. Check the buchholz relay and adjust the flats,

switches etc.

Don’ts for Power transformer

Do not leave tertiary terminals unprotected outside the tank.

Do not energize the transformer, until the reason for buchholz relay operation is investigated.

Do not leave any connection loose. Do not move EHV transformer with bushings

mounted. Do not allow conservator oil level to fall below ¼

level. Do not leave C.T secondary terminals open. Do not continue with pink silicate.

CONCLUSION By using the new techniques like DGA,FFA,FRA we

can predict the condition of insulation and if we keep insulation in good condition then automatically the life of transformer extended

Also proper maintenance means periodic maintenance and planned checks and monitoring we can reduce the breakdown and improve the life of transformer

By improving the protection scheme and protection from overloads , lightning surge we can reduce the breakdown due to the faults which are more savior

REFRENCE Maintenance of power transformer by NPTI

publication

Condition monitoring of power transformer by NPTI publication

Theory and performance of electrical machine by J B GUPTA

Transformer manual by BHEL

THANK you