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EXPERIENCE IN DIAGNOSTICS AND REPAIRS OF POWER

TRANSFORMERS FOR EXTENDING THEIR MAINTAINABILITY

AND LIFE TIME

A.P. Dolin – Ph.D, N.F. Pershina, A.Y. Lenkov – SPA “Technoservice-Electro”

V.V. Smekalov – JSC. “Federal Network Company EES” of Russia,

Russia

Complex diagnostic inspections (CDI) or tests [1] are used more and more nowadays for as-

sessing the condition of power transformers, autotransformers and shunt reactors (hereinafter

referred to as transformers), for identifying the nature and scope of fault development in all sys-

tems and units of these electric facilities. To implement its basic objectives quite successfully,

the following program of operations is usually used [1-3]: 1) analysis of breakdowns and typi-

cal faults in the given type of transformer; 2) inspection of transformers and data collection

(modes of operation, loading, short-circuits and working voltage levels, peculiar features of op-

eration, consumers, climatic conditions, atmosphere pollution, etc; 3) perusal of engineering

documentation and analysis of measurement results obtained on operating transformers; 4)

electrical measurements on de-energized transformers (tg δ and R of winding, lead insulation,

d.c. resistance of windings, idle losses, short-circuit resistance (voltage),etc.; 5) execution of

measurements on operating transformers in idle mode of operation and under loading (meas-

urements of partial and other electric discharges, detection of discharges by acoustic devices,

infra-red imaging of all units in the transformer, vibration check-up of tank as well as of oil

pumps in the cooling system; 6) oil sampling from tank, leads filled with oil, U/OVR contac-

tors, and accomplishment of physical and chemical analyses of oil in the laboratory, at least, as

far as it is required [4]; 7) preparation and issue of technical report specifying test results, its

analyses and experts’ opinion on transformer condition with recommendations for its subse-

quent use, current diagnostics check-ups and repairs with relevant methods procedures, if

needed.

The outcome of inspection for 600 transformers alongside with repair work on 60 transformers

has proved that it is nothing else than a minimum amount of work required to ensure a trust-

worthy assessment of transformer diagnostic condition that allows to exclude any uncertainty in

such evaluation.

Faults in transformers can be caused by natural factors: operating and short-circuit currents,

operating voltage and voltage surge, environment impacts, chemical changes. Or they can be

provoked by development of other defects or by human factor: errors in design, assembly and

repair can also give rise to possible faults. For instance, one attachment element of lower yoke

beam was isolated from the tank bottom of the transformer TDTN-40000/110/400 MBA,

110kW at the insulating clearance being less than it should be (Fig.1,a). As a result of sliming

and fouling of its operating elements, yoke beam insulation resistance was considerably re-

duced in relation to the tank and that was the cause of short-circuit. The fault was accompanied

by hydrocarbon gases (with appearance of acetylene in minor concentrations) dissolved in oil.

The diagnostics included measurement of electric discharges and their detection by acoustic

devices.

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а) b)

Fig.1 Attachment elements of lower yoke beam isolated from the tank bottom in the trans-

former, type TDTN-40000/110, which have: a) reduced isolating clearances b) normal isolating

clearances.

Further development of diagnostics for electric equipment seems to be impossible without up-

dated instruments, when new diagnostic approaches are not introduced or existing techniques

have no promotion. Measurement of magnetic density near the wall of transformer tank turned

out to be the most promising method in this respect. Nevertheless, during data processing it is

necessary to take into consideration some design features that can impact on increase values of

magnetic density: namely, closeness of windings and taps to the tank wall, presence of inspec-

tion hatches, etc. The Fig.2 shows magnetic field measurement results for the transformer, type

ATDTsTN-200000/330/110. Field density increase in this transformer was caused by inspec-

tion hatches available in the tank wall.

Fig.2. Effective values of magnetic field density: (a) near tank wall along transformer perime-

ter, (b) at the height of 2 meters from the tank bottom of the transformer, type ATDTsTN-

200000/330/110 under load.

However, in some cases the increase of magnetic field density is caused by faults in the mag-

netic system (i.e. short-circuits). As a rule, these results can be correlated with measurements

and acoustic detection of electric discharges, infra-red visioning as well as with chroma-

tographic analysis of gases dissolved in oil from the transformer tank. The Fig.3 represents ef-

fective values of magnetic field density, obtained during inspection of TTs-250000/500 trans-

former. The following conclusions are made after these values have been analyzed. The in-

crease of magnetic density values in sectors 3,8,11,22,27,30 was stipulated by a close location

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of windings and its taps (i.e. explained by transformer design features). At the same time, a

non-typical increase of field density in idle mode of operation was found in the end face of

transformer, sector 18 (shown by a dashed line in Fig.3, a). It was caused by a fault in the mag-

netic system in the zone of C phase. This assumption was confirmed by the results of the

check-ups fulfilled by other diagnostic methods. For example, the chromatographic analysis of

gases dissolved in oil indicates the presence of developed defect that vividly became apparent

under idle mode of operation, when acetylene concentration of oil from tank achieved 37 ppm.

Alongside with that, acetylene content increased quite moderately when the transformer was

under load.

When measurements were accomplished by means of sensors connected to the measuring ter-

minals of HV leads, electric discharges in phase C were disclosed. Maximum level of electric

discharges was achieved when voltage was almost near to zero, i.e. at maximum current value

(Fig.4, a). Therefore, it was concluded that the source of discharges was in the magnetic system

of transformer. It is worthy of note that intensity of discharges in the idle mode was considera-

bly higher than under loading conditions.

.

.

a) b)

Fig.3 Plan (a) and effective values of magnetic field density near tank wall along pe-

rimeter of transformer at the level of tank air chamber joint in the transformer

TTs 250000/500 under idle mode of operation and (b) at the level of tank

joint.

а) б)

Fig.4. Results of measurements and detection of electric discharges in active part of trans-

former TTs-250000/500: (a) bar chart of high frequency constituent of distribution

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of electric discharges N versus amplitude Am and origin time t and commercial-

frequency voltage of A,B,C phases (bottom-up) in idling mode (ratio of pulse am-

plitudes for A,B,C phases is 1:2:20); (b) is an amplitude and spectral characteristics

of acoustic signal obtained from the end face (phase C).

A source of ultrasonic signals was found at the end face of transformer (phase C) as a result of

acoustic survey. There are two peaks in the spectrum of acoustic signals (Fig.4, b): low-

frequency peak, typical for the source of mechanical nature and a peak in the zone over 100

kHz, caused by electric discharges. Therefore, it is quite apparently, that electric discharges are

related with mechanical vibrations of magnetic system elements. The opening of this trans-

former confirmed the presence of defect caused by development of sparks or arc discharges in

places inaccessible for inspection in the magnetic system. The defect was accompanied by con-

siderable carbon deposition.

Complex diagnostic inspections of 500 transformers, having service life of 20 years and more,

that were carried out by NPO “Technoservice-Electro”, have indicated that less than 2% of

transformers are to be immediately withdrawn from operation and scrapped, about 15% of them

demand a major repair in the nearest future, 23-25% need replacement of lead wire and repair

of separate units, 27-30% require frequent inspections and major repair in 2-5 years or more.

Over 30% of surveyed transformers do not need additional engineering measures, since the

level of defect development was inconsiderable. A high reliability of CDI results as well as

timely engineering measures undertaken have ensured trouble-free operation of all diagnosable

transformers, including those that have been working for over 50 years.

To maintain serviceability of transformers at the desired level, it is necessary to conduct major

repair in due time. At the same time, an unjustified decision for a major repair, at its best, will

entail unreasonable expenditures, and, at its worst, will result in loss of reliability, lifetime and

even can bring to failures, that, in the long run, means considerable material losses. Complex

diagnostic inspections permit not only to disclose and assess the danger level of developing de-

fects, but to justify the necessity, scope and dates of major repairs too.

As an example, the Table 1 specifies the scope of power transformer inspections and repairs

fulfilled in one of electric power systems. The diagram in Fig.5 shows the lifetime of trans-

formers from this electric power system that underwent repair works. This Table shows that

only a small number of transformers demanded an immediate repair. A good deal of repairs

was suspended. In this case, diagnostic results and economic considerations were the basic

grounds for such a decision. As a rule, transformers with developed defects within pre-repair

periods were recommended to undergo frequent fault-finding checks and sometimes engineer-

ing measures (replacement of silica gel in thermo-siphon filters, oil pumps, etc). A bad condi-

tion of one transformer made it necessary to withdraw it from service and to have it replaced.

Drying of insulation in the active part of the transformer is considered to be one of the most

important stages in repair operations. Oil blow-spray technique under degassing conditions is

usually used for transformers with long service life and with wet and slimed insulation. It is

noteworthy that drying of solid insulation means the impact of enhanced temperatures, while

degassing is additionally correlated with macro mechanical effect on cellulose during moisture

removal. As a result of this process, an accelerated aging of paper insulation is observed, and as

a consequence reduces the degree of its polymerization (by 50-250 units). Nevertheless, a new

spraying technology, which considerably reduces these negative factors, has been successfully

used these during recent years. This technology makes it possible not only to improve insulat-

ing properties of windings, but to retain the strength and degree of polymerization of paper in-

sulation as well [3, 5].

The principle process flow chart of insulation spraying and drying is similar to the traditional

one and consists, at least, of two closed loops: a spraying loop for active part and a degassing

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loop. The first one operates in closed cycle and includes sprayers, mounted in active part of

transformer, an oil heater that heats and makes finish purification of oil, a circulating pump,

filters, valves and connection pipes. Degassing is carried out by backing pump. A vacuum plant

can be included into the degassing loop. The spraying loop can be supplemented by a regenera-

tion loop, extra oil drying and purification loop, which from time to time are used in the proc-

ess flow.

Table 1

Complex diagnostic inspections and repairs

in regional electric power system

Note. Special features of repairs: I – reconstruction and spraying of active part under degassing mode; II –

spraying of active part under degassing mode; III – mounting of a new transformer (instead of inoperative one); IV –

replacement of lead wires.

Fig.5. Service life and

number of transformers with-

drawn from operation to repair.

Short-circuit oils of Russian origin with high content of aromatic substances are used

as a process oil. Special additives are introduced into oil to enhance its dissolving capacity at

a definite stage. A specific duration of exposure to spraying (heating) and degassing are se-

lected individually for each transformer depending on paper insulation condition (moisture

content, sliming, fouling and strength). Moisture content, composition and nature of me-

chanical impurities, dielectric loss factor and other parameters of flushing oil, as well as re-

sistance of solid insulation R15 и R 60 are permanently checked in the process of operation.

According to controlled variables, it is required to adjust the temperature of flushing oil,

pressure in the tank (vacuum level), as well as the spraying scheme, duration of separate

stages (spraying without degassing, spraying with degassing, degassing without spraying),

additive concentration level, etc. It allows solving three tasks successfully: 1) to intensify the

Year Number of surveyed transformers,

voltage kV Number of repairs

35 110 150 330 Total I II III IV Total

1998 1 0 12 6 19 - - - - -

1999 0 6 16 1 23 - 1 - - 1

2000 0 4 13 0 17 1 2 - - 3

2001 0 3 1 2 6 4 - - - 4

2002 0 1 3 0 4 4 1 - - 5

2003 0 8 5 0 13 3 1 - 4 8

2004 0 6 7 1 14 3 1 1 2 7

Total 1 28 57 10 96 15 6 1 6 28

0

2

4

6

8

10

less than 20

years

20-30 years 31-40 years 41-50 years over 50

years

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process of moisture release from solid insulation, 2) to remove by-products of oil decompo-

sition efficiently, Fe and Cu naphthenates and other mechanical impurities, 3) to avoid any

reduction of strength and accelerated destruction of paper insulation during drying.

An individual approach to the repair of each transformer, strict observation of tech-

nology requirements, a permanent control of flushing oil parameters and insulating proper-

ties of windings, as well as accumulated experience in repair on the basis of advanced tech-

nologies have permitted to get steady positive results for different transformers.

Fig.6-9 illustrates insulating properties of windings and parameters of paper insulation

in the test group before and after repairs. According to Fig.8 D1 and D2 are degrees of po-

lymerization before and after repairs. Transformers had different nature and levels of fouling

and sliming. That is why, open circuit of insulation spraying was used in some cases on the

first stage. These repairs have improved insulating properties even in cases when the active

part was considerably fouled with metal chips.

In addition to that, mechanical strength and degree of polymerization of paper insulation

was increased in some cases. The results given in Fig.8, b and Fig. 9 refer to analyses of pa-

per insulation samples that had initial degree of polymerization not more than 400 units.

R60, % of rated value

Fig.6. Resistance of winding insulation in

HV transformers before and after repairs

tgδδδδ,% tgδδδδ,%

Transformers, pcs

а) b)

Fig.7. Values tgδ of winding insulation in HV transformers before and after repairs according

to the “Technoservice-Electro” technology in scale along the Y-axis, 1 division:

А) – 1%, b) – 0,2 %

(D1-D2)/ D1, % (D1-D2)/ D1, %

0

2

4

6

1 3 5 7 9 11 13 15 17 19 21 23

Rated values

After repair

Pr ior to repair

Rated valuesAfter repairPrior to repair

0

0,5

1

1,5

1 3 5 7 9 11 13 15 17 19 21

Rated values

After repair

Pr ior to repair

Rated values

After repair

Prior to repair

1

10

100

1000

10000

1 3 5 7 9 11 13 15 17 19 21 23

Prior to repair

After repair

Rated values

Prior to repair

After repair

Rated values

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Samples, pcs

a) b)

Fig.8. Changes in degree of polymerization of paper insulation samples: (а) all sam-

ples under survey, b) samples with degree of polymerization D1 less than 400 units.

Mechanical strength, class

I

II

III

IV Samples, pcs

Fig.9. Strength of paper insulation samples (with degree of polymerization D1

less than 400 units) before and after repair

[According to effective normative documents in Russia [4] the best class of mechanical strength (where

paper insulation is not broken when it is bended through 180° angle) is I, while the worst one (where

paper insulation is broken when it is bended through 90°) is IV].

The analysis of results showed the following. In all repairs it was possible to increase

values of winding resistance R60 . In some cases, resistance values after repairs turned out to

be higher than the manufacturer’s values and mounting values (in all cases of transformers

with service life over 35 year).Insulation resistance did no vary practically after spraying and

drying, if R60 values exceeded 10000 mOhm before repair.

It is worthy of note that a breach of technological parameters, particularly, changes in

additive percentage, oil heating temperature at drying period, duration of this or that expo-

sure at spraying period, pressure at different stages of insulation spraying and drying, as well

as de-restrictions in physical and chemical parameters, inevitably have an impact on final re-

-100

0

100

1 3 5 7 9 11 13 15 17 19

Изменение

0,00

1,00

2,00

3,00

1 3 5 7 9 11 13 15 17 19

Prior to sparaying

After spraying

Prior to

sparayingAfter spraying

-100

0

100

1 11 21 31 41 51 61 71 81 91 101

Изменение

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sults. For example, changes in parameters of transformer No.2 (Fig.7) gave rise to tgδ

growth of winding insulation after repair. Nevertheless, all parameters of transformer after

repair, including its dielectric loss factor, meet the requirements [4]. At the same time, even

increased duration of spraying and restoration of conventional technological process has not

allowed to decrease tgδ to its pre-repair value.

As a rule, accepted practice makes it possible, at least, to retain the strength of paper

insulation and its degree of polymerization. In some cases, these indices are increased.

Maximum growth of strength and degree of polymerization in paper insulation is observed

in samples with highest level of destruction (under similar conditions of treatment). If initial

degree of polymerization and strength of insulation is high, there is usually no distinct im-

provement of these parameters.

It should be specially noted that the first repair experiments based on a new technol-

ogy were accomplished 7 years ago. Among first transformers surveyed were transformers

with insulation that was not allowed to be used any more (tgδ=5…6%, D1< 250 units.). Af-

ter being accomplished, the repairs enabled to improve insulating properties and degree of

polymerization (tgδ=0.8…1.1 %, D1> 320 units.) and put transformers into operation again.

They have been still operating successfully up to now.

Conclusions

1. Complex diagnostic inspections are effective when different methods of meas-

urements are carried out: both on de-energized transformers and operating ones

under load and in idle mode with physical and chemical analyses of oil from the

tank, inlet pipes filled with oil and U-OVR contactors.

2. Measurement of magnetic field density near tank wall is one of the most promis-

ing methods for power transformer diagnostics that permit to disclose and local-

ize faults in magnetic system along with other methods.

3. A new practice repairing insulation by spraying oil with special additives under

degassing conditions allows to dry solid insulation efficiently, remove products

of oil decomposition, eliminate Cu-and- Fe-naphthenates, as well as mechanical

impurities. Besides, this technology makes it possible to reduce a negative effect

of temperature and vacuum on paper insulation and prevents any decrease of its

mechanical strength when winding insulation is dried.

4. Repair of transformers with a long service life by means of this new practice im-

proves insulating properties of windings to a greater extent as well as retains and

even enhances mechanical strength of paper insulation due to internal and exter-

nal bonds of hydrogen in giant molecules of cellulose and owning to the im-

provement of its crystal lattice.

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Reference literature

[1] Smekalov V.V., Dolin A.P., Pershina N.F. Condition assessment and life time extension of

power transformers. CIGRE, session 2002, 12-102.

[2] Degtayrev S.A., Dolin A.P., Pershina N.F., Smekalov V.V. Basic concepts of complex diagnos-

tic inspection of power transformers ( Electro/Электро, №2, 2003).

[3] Dolin A.P., Krainov V.K., Smekalov V.V., Shamko V.N. Failures, condition assessment and

repairs of power transformers. (Energetic/Энергетик, № 6, 2001).

[4] The scope and standards of electrical equipment tests, Мoscow, ENAS, 1998.

[5] Smekalov V.V., Dolin A.P. The Repair of Power Transformers with a Long Service Life. CIGRE, session 2004, A2-212.