TNB LIBERTY POWER LTD.

Transformer Bladder Replacement work Report

Transformer Tripped on Low Oil Level

Submitted to: Mr. Maqsood Ahmed Mangi (Sr. Engr. EIC&IT) Submitted by: Danish Aslam (A.E Electrical)

2/4/2014

Abstract: GT main transformer 11BAT01 was tripped due to low oil level in main tank. The transformer was tested and found to have a damaged bladder in conservator tank causing the low oil level indication even when filled with sufficient quantity of oil. Bladder was removed, replaced with new one and oil level maintained after treatment of oil and transformer is now in service since Feb. 01, 2014.

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Table of Contents

Problem statement: .................................................................................................................................. 2

Activity plan: ............................................................................................................................................... 3

Findings: ..................................................................................................................................................... 6

Actions taken: ............................................................................................................................................ 7

Project Team: .......................................................................................................................................... 15

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Transformer (11BAT01) Low oil level indication Issue

Problem statement:

On Jan 16, 2012 breaker opening event of bay E-07 and E-06 was reported by Ops. Dept. On

investigation it is found that both breakers opens to isolate G.T Transformer 11BAT01 as it is showing

low oil level in the transformer tank. Electrical test (tripping contact) reveled that tripping was true as

well as oil level indicator gauge clearly showing low level.

Steps were taken and test made to counter the problem following addition of oil in the conservator tank

but no marked difference observed in the oil level indication. Studies were made which shows problem

in either air bladder or the oil level gauge itself.

In order to determine the exact fault two tests were made. First the oil level in conservator was

measured by means of glass gauge (Transparent plastic pipe) which shows oil level above 50% of the

conservator capacity.

Secondly the bladder was pressurized with nitrogen to check leakages and it clearly shows problem with

the air bag as the oil level starts rising in the level indicator.

For rectification of this problem opening of conservator tank is required.

Oil Level noted after tripping

The oil and winding temperatures were noted at the same level reading and found to be 20° C

and 22° C respectively.

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Activity plan:

On 23rd January 2014, the PTW was submitted so as to start conservator opening/ Inspection work. The work was planned as:

1. Arrangement and transportation of Transformer oil storage tanks near transformer area so that removed oil from conservator tank could be stored safely.

2. Transportation of the oil treatment plant near transformer area so that the removed transformer oil could be treated before refilling into the conservator tank.

3. Arrangement of hose pipes, fitting flanges and valves for ease of drainage. 4. Scaffolding work to access the conservator lateral cover and inspection cover. 5. Connection of hose pipe and related equipment for drainage work. 6. Drainage of the conservator oil. 7. Removal of the inspection flange to check condition of both the air membrane and the level

indicator float. 8. Damaged Bladder replacement work. 9. Testing of Level Indicator gauge and its Float.

Accordingly the work was started on Jan 24, 2014 as per the plan, outlined above.

Transportation of Transformer oil storage tanks

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Transportation and Cleaning of Oil Treatment plant

Arrangement of hose pipes, Fitting flanges and valves

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Scaffolding work to access the conservator tank

After completion of scaffold fixing work protection covering of the whole conservator tank was

performed with the aid of Plastic sheets and special protection cover so that ingress of foreign

matters (dust, moisture) would be avoided in to the conservator tank once its lateral cover will

be removed.

Afterwards the drainage of conservator oil was performed while the main tank was isolated

through the isolation valve on the bucholz relay.

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Findings:

Once the drainage activity was over the inspection cover of conservator was removed and it

was found that the Bladder was filled with oil hence sink into the conservator oil and resting

on the float of the level indicator as opposed to normal behavior of floating on top of oil.

This causes the oil level gauge to forcefully show low oil level even though conservator contain

sufficient amount of oil.

Possible causes of this damage were;

the adoption of wrong oil filling procedures

Bladder by-pass valve remained open during purging

In both the cases oil will be introduce in the bladder and damages its interior surface, hence

allowing oil of the conservator tank to ingress into the bladder.

Furthermore the bladder was found brittle as its interior was contaminated with oil which is

usually designed to remain in contact with the atmospheric air.

Actions taken:

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The Bladder was drained using hand pump after opening of the top flange and the oil collected

was just above three drums (approx… 700 Ltrs).

Drainage of Bladder oil using hand pump

After the bladder was emptied it was removed. The bladder present in the ware house was

pressure tested, cleaned and made ready to install on the conservator.

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Bladder inspection, Cleaning and Pressure Testing

Aside from these test, all the dimensions of the bladder were noted and compared to that of

the defected one. This includes measurement of length of bladder from center of the fixing

flange to both extremities and sideways measurements.

Also the tags of both bladders were compared for further assurance and clarification.

COEMSA BLADDERS

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Preparation of Bladder installation

The interior of the conservator tank is protected against paint damages by the use of protection

sheet as shown below.

The interior surface of the conservator tank was also properly cleaned using lint free clothes

prior to the bladder installation work.

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Bladder shifting and installation

Bladder was shifted to the site and installed on the conservator tank.

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The floater and indicator assembly of gauge was tested and found working satisfactorily.

The Transformer oil was put in to the treatment plant (approx.. 3300 Ltrs) and its values of

Dielectric strength tested before and after circulation as:

Before purification/ Treatment: 82.16 KV (Avg. Value)

After purification/ Treatment: 83.3 KV (Avg. Value)

Dielectric Testing

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Also the main tank bottom oil sample was collected for Dielectric strength test and results

were found to be: 79.2 KV (Avg. Value)

The bladder was filled with nitrogen and the purified oil is taken into the conservator tank. The

bladder was inspected and is now found floating on the oil. Simultaneously the oil level gauge

reading was found to be increasing with the rise in oil level in the conservator tank.

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The oil is filled upto slightly above the correct temperature reading on the guage so that

purging can be facilated.

Finally the purging of the conservator is performed so as to release the air trapped inside the

conservator tank after settling period of more than 12 hrs.

The purging is continued till only oil (without bubbles) starts flowing through the vent valve.

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The reading obtained after overnight setlling time on the guage is:

Where as the reding noted on the Oil And winding temperature indicators were 22 °C and

23°C respectively.

Also the Oil level gauge reading of the OLTC compartment was noted and found same as that of

Conservator tank Oil level.

Finally the transformer was energized on 01-02-2014 and its level observed preodically and

found no abnormality in it now.

Note that 03 new (unused) oil drums by PAK-HY OILS marked as “PARAFINIC BASE OIL” were

shiffted to storage area whereas 02 new oil drums “PARAFINIC BASE OIL” were added to the

conservator along with the treated oil to make up the corrected oil level.

Whrere as 02 Oil drums “PARAFINIC BASE OIL” which were removed from baldder and

collected as spil oil during filling and drainage activity have also been shifted to the storage area

and properly marked as “Not Usable Oil”.

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Project Team:

Special Thanks to TNB Management, unit and section Head their confidence and believe on the

project staff and the rest of the project staff for dedicatedly putting their focused effort and

time as well as cautiously executing this apparently simple but in actual extremely Critical and

an unusual Job.

The following TNB employs as well as contract employs (daily wedges staff) play key role in the

successful execution of the Job.

Mr. Maqsood Ahmed Mangi (Sr. Engr. EIC&IT)

Danish Aslam ( A.E Elect. )

Muhammad Zaman (Elect. Tech)

Sujawal Hussain (Elect. Tech)

Zeeshan Iqbal (Elect. Fitter)

Other Satff:

Elect. Technicians (03 members)

Elect. Fitters ( 02 members)

Scaffolding installation staff ( 04 members)

Helpers (06 members)