Abstract: Power Transformers are the central and most vital components in a substation and are essential to

efficient operation of Transmission & Distribution System of a power utility. To ensure that power

transformers provide long & trouble-free service, varieties of diagnostic tests are carried out and

remedial actions initiated throughout their operational lifetime. One such procedure that is necessary to

ensure that a power transformer continues to serve, as it should, is to do dehydration. The paper

presents our experience with on-site dehydration & refurbishment of power transformers.

1.0 INTRODUCTION:

Power Transformers are one of the most important components of electric power distribution & transmission

systems. Therefore their reliable and efficient operation for many years is of basic importance for a reliable

power utility. The reliability of a power transformer is limited by the reliability of the winding insulation system

mainly, which operates additionally as a heat transfer medium. From the age-old days, the most frequently used

insulations in these devices are the oil immersed paper & pressboard insulations. Due to cost aspects, a

combination of cellulose paper and mineral oil has emerged as the favourite choice, although the longevity of

the insulation depends on the operating conditions, presence of catalysts, oxygen and most importantly water.

Ageing of the oil/paper insulation system of power transformers is influenced by various stresses – thermal,

electromechanical and chemical. Thermal stress leads to a major degradation process of both oil and cellulose

paper. Under the effect of all the above conditions, the paper becomes brittle and durability against mechanical

stress is sharply reduced. The process of breaking cellulose molecule chains produces water in the solid

insulation, which acts as a catalyst for further breakdown. Further, the breakdown voltage of the insulating oil

and paper is reduced with increasing moisture content of the oil. The conductivity of a material is a property,

which can be directly related to the moisture content. Thus the knowledge about the conductivity and hence the

moisture contents of the oil and paper can be used as an important basis for onsite drying and oil reclamations.

2.0 ASSESSMENT OF MOISTURE IN TRANSFORMER WINDING: The paper / pressboard insulation system is very hygroscopic and can still contain moisture, even if the oil has

been completely dried out. As of now indirect methods are available to estimate moisture content of paper.

However, paper ageing and moisture can only be reliably measured by paper samples collected at critical

locations (leads, outer windings etc.) and analyzing them in laboratory. Therefore, solid insulation components

can be examined by opening the transformer and taking samples from the insulation. Obviously this is not

possible in most cases. As of now there is no commonly accepted method for a non-destructive evaluation of

solid insulation.

At present there is cluster / group of methods based on the measurement & analysis of dielectric properties of

insulation available. Diagnosis with these methods is based on the fact that paper and oil change their dielectric

properties with change in moisture levels. Therefore, by measuring parameters like dissipation factor,

polarization & depolarization currents (PDC) and residual voltage (RVM) in the frequency and time domains, it

is possible to relate these changes to water concentration in paper oil system. In our system we are following the

age old methods like dielectric dissipation factor (tan delta) and simple IR & PI values. We have initiated steps

in our transformer preventive maintenance programme, to record the water content in the oil as well as the paper

insulation. 3.0 ADVANTAGES OF ON SITE DRYING OUT

There had been lot of discussions & debates on whether to carryout repairs of power transformers involving

removing entire oil and opening tank covers- exposing the active components. Loss in tariff may be too high to

allow for the application of traditional drying out involving shutting down the transformer for several months

and shifting the same to manufacturer’s / repairers’ premises. The transformer insulation may age to the point

where it becomes risky to move the unit. The transformers are risky to be moved because aged insulation

becomes brittle. Shipping the transformer may cause winding displacement and permanent insulation failure. As

far as CESC is concerned, evacuation of bulk power from recently commissioned Generating Station at Budge

CESC’s EXPERIENCE WITH ON-SITE DRYING &

REFURBISHMENT OF LARGE POWER TRANSFORMERS S. Kar Chowdhury

Assistant Manager, CESC Limited, Kolkata.

Budge to the city centre had been a difficult proposition, considering paucity of space and non-availability of

free hold land within the city precints for installation of power transformers. The existing power transformers in

the 132 kV substations forming the N-S power corridor had nearly outlived their useful life and required major

refurbishment to infuse fresh lease of life. Considering the nature of consumers being served from these

substations, the transformers could not be taken out of service for more than a month at a stretch. On-site drying

out has proved to be beneficial in our case where most of the power transformers have attained 20 years of age

or more.

4.0 SELECTION OF THE TRANSFORMER

Station: Titagarh Receiving Station

Transformer: 50 MVA, 132/33 KV, % Impedance- 10.2, Year of Commissioning – 1983

Pre-dehydration parameters:

IR Values: HV/LV/HV-LV= 10/15/10 M-ohms at 55 deg C.

PI Values(600 sec reading / 60 sec reading): HV=1.0; LV=1.0

The oil test results conducted just before dehydration and 5 months prior are given below.

Date of Test 01/01/2004 30/07/2003

Break Down Voltage (KV) 60 66

Acidity 0.355 0.35

Water Content (at 65 deg C) 55 48

Resistivity 0.03x1012

0.055x1012

DDF (tan delta) 0.45 0.4

Table 1: Oil test result just before & 5 months prior to drying out

Although the health of the insulating oil of the transformer was meeting the requirement of prescribed standards,

the acidity & resistivity were however not up to the acceptable limit. DGA results, carried out 15 days prior to

shutdown indicated no abnormalities. The transformer with total oil volume of approximately 27 KL was

subjected to nearly 25 cycles through a 3000 litre/hr. high vacuum on-line oil filter machine about 5 months

back, without any tangible improvement in IR values of the windings, leaving us with no options but to go in for

complete dehydration for improvement the health of the liquid insulation to a respectable level.

This transformer serves an important link in N-S power corridor. The power from Budge Budge Generating

station is wheeled to the extreme northern limits of the city via Prinsep Street Substation, B.T. Road Switching

Station to Titagarh Receiving Station. The transformer, as the network demands, could only be taken out of

service during the peak winter for about a month. Taking cognizance of the network requirements and system

demands, our team of engineers putting their head together came out with a solution for carrying out thorough

dehydration & complete refurbishment taking into consideration the age of the transformer. 5.0 METHODOLOGY

We, in CESC, during the last 5/7 years have developed our own methodology for on-site drying out of power

transformers of 145 KV class having capacity 50 MVA. Over the years the methodology has been fine tuned to

reduce the shutdown period and at the same time suit our requirement. A few case studies are discussed to throw

light on the practices of on-site drying out & refurbishment of power transformers of 145 KV class.

5.1 Main Tank

The transformer was taken out of service and initial tests like IR values, PI values, tan delta, and separate source

voltage test at 30 KV for both windings conducted. The high voltage test had been conducted in steps of 5 KV

and leakage currents at each test voltage had been noted.

Before the drying out process was started, entire oil was drained out and all fittings & accessories like

conservator, Buchholz relay, bushing turrets, PRV, OLTC DM box, Marshalling box, temperature & oil level

indicators, Radiators & Header pipes, cooling fans & pumps including the HV & LV bushings were removed,

packed separately and stored under shed. The HV bushings were stored in vertical position mounted on suitable

stands. All openings on the fittings and accessories were closed with suitable blanking plates & working

gaskets. All the openings on transformer main tank body were blanked off suitably using blank off plates &

gaskets. Care to be taken to ensure that equalizing connection was made between main tank & OLTC tank.

Suitable dummy bushing were fitted and connected to HV & LV windings. The tap was maintained at position

1, corresponding to maximum winding. At this tap impedance was around 15%.

The process of drying out a transformer is one requiring utmost care and good judgement. If the drying out

process is carelessly or lackadaisically performed, irrepairable damage may result to the transformer insulation

through overheating. In no case shall the transformer be left unattended during any part of the dry out period.

Strict vigil to be ensured and all the observation shall be carefully recorded. As a safety & precautionary

measure, a suitable fire fighting equipment shall be made available near the transformer.

Once the transformer was ready for drying out process, the main tank was jacked up on one side creating a

downward tilt towards the drain plug. Complete washing of Core & Coil assembly was carried out with hot

transformer oil jet to flush out sludge deposit from yoke, oil ducts and base channels. The drain plug was then

opened to drain out sludge and water collected at the bottom of the tank. This procedure was carried out for 3 / 4

times till clean oil was collected at the drain plug. IR reading taken at this juncture reveals substantial increase

in IR values of both HV & LV windings (HV/LV/HV-LV=50/50/50 M-ohms at 20 deg C). Necessary

precaution was to be taken to ensure that tilt angle did not exceed 5-7 degrees. The main tank was then placed

on level plinth. A temperature sensor was inserted into the main tank and positioned just touching the top of the

LV coil, connections taken out through spare terminals of WTI CT board.

The actual drying out procedure was started by gradually drawing vacuum. The vacuum was slowly drawn

from the top of the tank, from zero to 750/755 mm of Hg in a span of 24 hours. The deflection of the main tank

longitudinal walls was noted at vacuum level of 400, 500, 600 & 700 mm of Hg. Once the full vacuum was

reached and deflection of tank wall was within limit, impedance heating of the windings was started by

circulating around 7-8% of rated full load current in the windings. An intermediate transformer was used to

achieve the requisite voltage of around 2000 to 2200 volts, to be applied to HV winding. Since the tank was put

under vacuum, there was no need to put blanket / jacket around the main tank. The outlet air from vacuum pump

was taken through a moisture condensation trap – the temperature of the trap was maintained at near zero to

ensure complete trapping of the moisture drawn out from the tank. The near zero temperature was ensured by

solid blocks of ice mixed with common salt. The transformer was kept under this condition till the IR values of

HV & LV windings reached steady state values and also the condensate collection rate tapered out to less than

10ml per day. The daily records of IR values, condensate collected and host of other parameters like core

temperature, ambient temperature, average vacuum etc. were maintained as shown in chart. The IR values when

plotted on time scale exhibited ‘Bath Tub’ characteristic, as expected.

On reaching steady state IR values of HV & LV windings, the impedance heating was stopped and vacuum was

broken by injecting dry nitrogen and maintaining a positive pressure of 0.10 Kg/sq.cm. The tank filled with

nitrogen was kept for 48 hours after which the tank was again put under full vacuum and simultaneous

impedance heating. At this juncture sharp rise in IR values of both HV & LV windings were observed and every

time N2 charging and vacuum drawing was resorted to, IR values also increased substantially. This cycle of N2

filling and vacuum-heating continued for 3 / 4 cycles till steady state in IR values were obtained.

The main tank was then filled with Hot oil under vacuum with the temperature of oil maintained around 40 deg

C. Before filling in the main tank, the oil should be tested as per IS 335. In case the oil does not meet the

requirement, it should be processed and shall only be used upon meeting laid down norms. When filling a

transformer with oil, it is preferable to pump in the oil from the bottom while drawing vacuum from top, using a

streamline filter machine. The oil level was maintained just above pressing rings / boards of the windings. The

main tank top lid was removed for checking of looseness of pressing ring fasteners and core bolts. Tightness of

Core Earth Connection was also checked at this stage. The sealing gasket of the top lid was renewed before

putting it back. The main tank was then filled with processed oil. The OLTC unit was also filled with clean dry

oil. It is advisable to carryout hot oil circulation in the main tank and selector switch / diverter switch tank

simultaneously to remove moisture from tap changer terminal board / diverter switch cylinder provided in the

main tank. On completion of all schedule activities, all the accessories & fittings that were dismantled were

fitted back after necessary maintenance. Filling of processed oil in radiators, conservators, header pipes, bushing

turrets were carried out through a streamline filter machine. The transformer was then allowed to settle for

nearly 24 hours during which air release from all possible venting points was carried out. The transformer was

thoroughly tested and re-commissioned and kept on light load for 24 hours. During this period behaviour of the

transformer was closely monitored and air release from all venting points carried out after taking shutdown. The

transformer was then put on normal load and subjected to on-line high vacuum filtration by 3000 lit/hr machine

for 7 days.

5.2 Maintenance of Accessories & Fittings

Cooling Radiators: The radiators, header pipes and associated pipe work and fittings were

thoroughly cleaned, using clean dry oil. Each individual radiator was repeatedly

flushed with clean hot oil to remove deposits of sludge from inside of the

radiator fins. The external surface was thoroughly checked for rust & oil leaks.

The leaking joints were repaired using inert gas welding while rusts were

removed and surfaces cleaned. A fresh coat of primer & synthetic enamel paint

was applied.

LV Bushings: Plain porcelain bushings are used for connections to LV windings.

The bushings were checked for any cracks & chirpings. In the event of any sorts

of damages found, the bushings are replaced by new ones. The stems were

cleaned and re-fitted with the bushings

HV Bushings: OIP condenser bushings are used for connections to HV windings. The draw

lead was given a fresh ½ wrap of dry insulating cloth tape. The bushing surface

was checked for cracks & chirps and tested for DDF at 10 KV and IR by 5 kV

insulation tester. Any deviation from accepted values will render the bushings

unsuitable for use.

Buchholz Relay: The relay was checked for correct operation of mercury switches by injecting

air through the test petcock when full of oil. After mounting, the angle of

inclination was maintained between 5-7 degrees.

Conservator: The conservator inside surface was thoroughly cleaned and a fresh coat of anti

acid paint was applied. The conservator was retrofitted to accommodate air

cell(atmoseal). The breather pipe line was also modified for correct fit. The

breather pipe line was also checked for internal blockages / obstructions.

Turret/bushing CTs: The bushing CTs after being dismantled were put in hot box where temperature

was maintained constant at 35 to 40 deg. C. The LT class CTs were given a

fresh wrap of dry insulating cloth tape and again put in hot chamber before re-

assembly into turrets. The conventional Cast Resin secondary terminal boards

were replaced by oil resistant fibre glass boards, while the secondary terminal

studs were replaced by new ones. The secondary studs were developed in-house

employing double-side sealing arrangement to minimize chances of oil leakage

through the terminal board.

Cooler Fans & Pumps: Cooler fans were generally checked for damages & low IR in terminal blocks.

In the event of any unwarranted noise from bearings, the same were replaced

and lubricated. The blades were cleaned to remove dust. All the fans were given

a fresh coat of primer & synthetic enamel paint. The pumps were checked for

damages & low IR of terminal blocks.

Temperature Indicators: Temperature Indicators were mainly cleaned by blow cleaning. The capillary

bulbs were cleaned by buffing. Both Oil and Winding temperature indicators

were calibrated afresh with standard thermometers immersed in hot oil bath.

Magnetic Oil Gauges: The float levels were checked for smooth up-down movement between the end

positions. Making of mercury contact in ‘Low Oil Level’ condition was checked

before fitting onto conservator.

Valves & Drain Plugs: The valves were checked for smooth operation after replacement of the oil seals

of the spindle. The oil seals of the drain plugs were replaced by fresh ones.

6.0 RESULTS ACHIEVED

The water content in the oil before dehydration was 55 ppm, which corresponds to 1.485 litres of water in

approx. 27 Kl of oil. Oil temperature at the time of sampling was 65 deg C.

Applying IEEE empirical curves for moisture saturation in oil, the corresponding moisture saturation in oil was

found to be around 15%. But the moisture content in the solid insulation was observed to be 2.2%. Assuming

that ratio of solid insulation to liquid insulation by weight is 1:10 and assuming density of oil to be 0.89 gm/cc

then weight of solid insulation is estimated to be 2400 Kgs. and moisture content in solid insulation as estimated

was approximately 52.8 litres.

In the drying out process, 13.2 litres of water was collected in moisture trap. Then the balance quantity of 39.6

litres of water remained in the solid insulation, accounting for 1.65%. Consequent improvements in IR & PI

values are as shown below. The IR readings were taken with 1000 V Insulation Tester while the PI readings

were taken with motorized 5000 V Insulation Tester.

Parameters Before

Drying out

After

Drying out

HV-LV+E 10 Meg Ohms at

65 deg. C

1100 Meg Ohms

at 30 deg. C

LV-HV+E 15 Meg Ohms at

65 deg. C

880 Meg Ohms at

30 deg. C

HV - LV 10 Meg Ohms at

65 deg. C

880 Meg Ohms at

30 deg. C

Table 2: IR values before & after drying out

Parameters Before

Drying out

After

Drying out

HV-LV+E 1.0 1.54

LV-HV+E 1.0 1.92

Table 3: PI values before & after drying out

Post dehydration oil sampling was carried out after 10 days of putting the transformer on load and also upon

completion of 15 cycles of oil filtration through the oil filter machine. The oil test results are given below.

Parameters Before

Drying out

After

Drying out

Break Down Voltage (KV) 60 76

Acidity 0.355 0.05

Water Content 55 18

Resistivity 0.03x1012

7.2x1012

DDF (tan delta) at 90 deg. C 0.45 0.005

Table 4: Oil test results before & after drying out

7.0 CONCLUSION

A power Transformer is a costly item in present day terms, not only by way of the capital cost of the equipment

but also by way of loss of revenue and goodwill to a power utility, in the event of its outage. If a transformer is

to render trouble free service during its lifetime, it becomes imperative that it should receive sufficient and

periodic maintenance. A stringent & comprehensive maintenance procedure can ensure a long & trouble free

service life. The principal objective is to maintain the insulation in good stead, which in turn will ensure a longer

service life. Effective maintenance primarily focuses on regular & periodic maintenance, diagnostic testing and

reconditioning, whenever necessary. Condition based maintenance practices need to be adopted where utilities

can ill afford long outage of transformers. Renovation of transformers by ‘drying-out’ and subsequent

refurbishment at site has proved to be cost effective in infusing fresh life to old transformers. For on-site drying

out by vacuum pulling with simultaneous impedance heating at low currents (7% to 8% of FLC) has proved to

cost effective and time saving. However, utmost care and proper control & monitoring to be exercised during

the entire process of drying out.

8.0 REFERENCE

a) “ A Guide to Transformer Maintenance” by S.D. Myers, J.J. Kelly & R.H. Parrish.

b) “ Condition Monitoring of Transformer Insulation by Polarisation and Depolaristaion Current

Measurement” by Prof. T.K. Saha, Dr. P. Purkait & Z.T. Yao, University of Queensland, Australia.

c) “ Moisture Measurement during Electric Substation Maintenance” – a publication of Soluciones

Tecnologicas Avanzadas.

About the Author

S. Kar Chowdhury, graduated in Electrical Engineering from Jadavpur University in 1987. He obtained M. Tech

in Electrical Enginering, specializing in Power System from IIT Kharagpur in 1989. From 1989 to 1990 he

worked as Design Engineer with Development Consultants Ltd. In 1990 he joined CESC Ltd as Engineer, in

Substations Department looking after maintenance of all types of plants & equipments associated with

Transmission & Distribution network. Presently he is looking after Installation & Commissioning of all types of

electrical equipments.

DATE IR VALUES IN MEG OHMS CORE

TEMP O

C

AMB.

TEMP O

C

CONDENSATE

COLLECTED (ml)

AV.

VAC.

HV-

LV+E

LV-

HV+E

HV-LV DAILY CUM. mm

Hg.

08/01/04 1000+ 1000

+ 1000

+ 24 22 -- -- 640

09/01/04 1000+ 1000

+ 1000

+ 20 20 Imp.

Heating

Started 720

10/01/04 1000+ 850 1000

+ 24 20 550 550 720

11/01/04 1000 800 1000+ 24 20 850 1400 740

12/01/04 950 800 1000+ 25.8 20 750 2150 745

13/01/04 900 750 1000+ 28 22 500 2650 745

14/01/04 800 650 1000 29.9 22.5 500 3150 740

15/01/04 200+ 200 175 31.8 21.5 500 3650 740

16/01/04 750 750 1000+ 33.8 22.0 450 4100 740

17/01/04 700 550 900+ 34.3 20.0 400 4500 740

18/01/04 1000+ 1000

+ 1000

+ 35.3 24.0 450 4950 740

19/01/04 1000+ 1000 1000

+ 36.9 25 400 5350 740

20/01/04 1000 900 1000+ 39.9 31 450 5800 740

21/01/04 1000 900 1000++

37.4 24.5 400 6200 740

22/01/04 1000+ 1000 1000

++ 39.1 26.5 400 6600 740

23/01/04 1000+ 1000

+ 1000

+ 37.5 23.5 450 7050 740

24/01/04 1000+ 1000

+ 1000

+ 37.7 23.0 350 7400 740

25/01/04 1000+ 1000

+ 1000

+ 35.0 21.5 350 7750 740

26/01/04 1000+ 1000

+ 1000

+ 35.0 17.0 250 8000 740

27/01/04 2000+ 1554 2000

+ 34.0 17.5 200 8200 740

28/01/04 2000+ 1600 2000

+ 35.5 24.0 200 8400 740

28/01/04 N2 CHAR GING DONE AT (*) 10-30 HRS

28/01/04 2000+ 1880 2000

+ 38.3 28.0 N2 FILLED TANK

29/01/04 2000+ 2000

+ 2000

+ 39.9 28.0 450 8850 750

30/01/04 2000+ 2000

+ 2000

+ 34.5 24.0 450 9300 750

31/01/04 2000+ 2000

+ 2000

+ 32.8 24.0 450 9750 750

01/02/04 2000+ 2000

+ 2000

+ 32.8 24.0 400 10150 750

02/02/04 2000+ 2000

+ 2000

+ 31.2 30.5 400 10550 750

03/02/04 N2 CHAR GING DONE AT (*) 11-00 HRS

04/02/04 2000+ 2000

+ 2000

+ 33.0 23.0 500 11050 750

05/04/04 2000+ 2000

+ 2000

+ 33.0 23.0 450 11500 750

06/02/04 2000+ 2000

+ 2000

+ 30.1 24 200 11700 750

07/02/04 2000+ 2000

+ 2000

+ 29.2 24.0 150 11850 750

08/02/04 N2 CHAR GING DONE AT (*) 08-30 HRS

09/02/04 Imp. Heating Stopped Tank Kept Under Vacuum

Table 5: Daily Chart showing IR values & water extracted from core & winding

Note: (*) Water collected at Vacuum pump sump (500 + 500 + 350) = 1350 ml.

Total Water collected = (11850 + 1350) = 13200 ml.

Impedance heating Transformer – Primary side(400 V) Current : 115 Amps

50 MVA Transformer Primary side current : 20 Amps

50 MVA Transformer 33 KV side circulating Current : 85 Amps