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Preventive and Corrective Maintenance for Transformers in Operation

Conference Paper · March 2023

DOI: 10.1109/ATEE58038.2023.10108196

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 THE 13th INTERNATIONAL SYMPOSIUM ON ADVANCED TOPICS IN ELECTRICAL ENGINEERING
March 23-25, 2023
Bucharest, Romania

Preventive and Corrective Maintenance for

Transformers in Operation
Maria-Cristina Niţu, Ancuța-Mihaela Aciu, Dan Ştefan
Research Department Division
National Institute for Research, Development and Testing in Electrical Engineering
ICMET Craiova, Craiova, Romania
cristinamarianitu@yahoo.com, ancutu13@yahoo.com, danstefan@icmet.ro

Abstract The purpose of this paper is to highlight the in order to extend their useful life or to be able to be reused
advantages of applying a preventive and corrective maintenance [7-12].
method in relation to the life of the insulation system of the
transformers in operation. This paper proposes a maintenance Insulating oils in operating equipment undergo a slow but
method based on the revitalization of the insulation of power constant degradation process. Progressive degradation and
transformers, in order to extend their life. Two case studies are premature aging result from various phenomena that occur
presented, and the results of the measurements made before and during the operation of electrical equipment.
after the revitalization process support the requirements of a Dissolved and suspended metal and cellulosic particles
preventive and corrective maintenance method.
along with other polar degradation products are impurities
Keywords: transformer, insulation, oil, revitalisation, that increase the rate of degradation and shorten the useful
mentenance life of electrical equipment. Therefore, the failure potential of
electrical equipment is directly related to the liquid insulating
I. INTRODUCTION
material.
Extending the life of electrical equipment has become a Treating the insulating liquid can extend the lifetime of the
primary condition in ensuring energy efficiency at the SEN transformer and ensure its optimal operation. Oil treatment
(National Energy System) level. methods are presented in specialized literature and applied by
Transformers are essential elements in electrical power the companies that deal with the maintenance of transformers
distribution systems, thus in the last decades, there has been a in operation, this work presents two case studies, which prove
shift from time-based maintenance strategy to condition- the effectiveness of the application of treatment processes in
based and risk-based maintenance. In this context, asset the case of transformers in operation to ensure a maintenance
managers are constantly looking to optimize their use. As efficient.
transformer diagnostic techniques have improved in recent The treatment process whose results are presented was
years, operators have sought techniques, measures and carried out with an installation based on synthetic adsorbents
methods to extend the life of transformers installed in the that uses the principle of adsorption to remove water and
system. polar elements from oil and solid insulation, applying the
From these considerations, the margin is currently applied percolation method under pressure.
to the application of predictive and corrective maintenance in The work supports a maintenance based on the
order to extend their life and ensure an acceptable level of revitalization of the insulation of power transformers, in order
reliability of the transformer in particular [1-7]. to extend their life. The installation used to remove water and
The life of power transformers is generally determined by polar elements from the transformer insulation uses synthetic
the life of the paper-oil insulation system, if the mechanical adsorbents [13-14].
stability of the electrical insulating paper is lost, there is a risk Transformer oil regeneration is an important preventive
of dielectric defects and total shutdowns of the transformer. maintenance tool, and therefore transformer oil must be
The aging of solid insulation must be monitored treated before it reaches levels of deterioration known to
continuously or at least periodically because its degradation cause transformer insulation failure [15-19].
process is final compared to the degradation of oils. The purpose of this work is to highlight the advantages of
Preventive and corrective maintenance is encouraged in order applying a preventive and corrective maintenance in terms of
to extend its life. the lifetime of the insulation system of the transformers in
Insulating oil is of essential importance in ensuring the operation.
efficient operation of the transformer, which due to the The application of reconditioning/regeneration/revitaliza-
specific production technology has high costs. tion processes to extend the lifetime of transformers has a
The primary resources from which these oils are derived major economic impact, and by reusing used oil, primary
are limited and thus, in order to save them, it is necessary to natural resources are preserved and environmental
carry out reconditioning/regeneration/revitalization processes contamination with such residual waste is avoided.

ISBN: 979-8-3503-3193-6/23/$31.00 ©2023 IEEE

 II. THE MECHANISM OF THE DEGRADATION OF THE conductive, hygroscopic and thermally insulating. Mud
INSULATION SYSTEM IN TRANSFORMERS deposits on the core and windings will therefore cause an
unwanted increase in the operating temperature of the
A. The mechanism of paper degradation transformer [3-4], [7-9].
The mechanism of paper degradation is complex and Mineral transformer oils are refined from crude oil. The
highly dependent on the operating conditions. Degradation refining process includes acid treatment, solvent extraction,
occurs by breaking glycosidic bonds. These chain splits result dewaxing, hydrogen treatment, or a combination of these
in a decrease in DP (degree of polymerization) and a methods to obtain insulating mineral oils that meet
reduction in tensile strength. The typical by-products that can specifications. Essentially, transformer oil is a mixture of
be detected in the oil are also formed, namely: carbon hydrocarbon compounds of three categories: alkanes,
monoxide, carbon dioxide, detectable by oil gas analysis naphthene and aromatic hydrocarbons. These molecules have
(DGA), as well as furanic compounds. weak polarity or no polarity at all. Polar and ionic species are
In the specialized literature, there are detailed studies on a small part of the constituents that can greatly influence the
cellulose aging [8-13]. However, several experiments were chemical and electrical properties of the oil.
performed at temperatures of 100°C due to shorter reaction The reliable performance of an insulating mineral oil in an
times under laboratory conditions. Aging products from insulation system depends on certain basic properties of the
typical transformer operating temperatures have not been oil that can affect the overall performance of the electrical
studied in detail. Furthermore, the aging process is equipment. In order to fulfill its multiple roles as a dielectric,
completely different in open and closed systems. Closed coolant and arc quencher, the oil must have certain properties,
systems do not allow moisture and other by-products to namely:
escape. It is possible that a free-breathing transformer will - high dielectric strength to withstand the electrical stresses
behave more like a closed system rather than an open system, during operation;
especially with respect to moisture [8], [10-12]. - sufficiently low viscosity that does not affect the heat
Monitoring this part of the insulation system is very circulation and does not reduce the heat transfer;
difficult and can be conducted during the inspection work. - adequate properties at low temperatures, down to the
Thanks to the gas separation method, monitoring has become lowest temperature expected at the installation site;
precise and particularly useful. CO2, CO and CH4 indicate the - a resistance to oxidation to maximize service life.
evolution of the aging process of the solid insulation, but the During operation, mineral oil degrades due to operating
conclusions are not quite clear because the gases are also the conditions. In many applications, insulating oil comes into
products of oil degradation. Using high-performance liquid contact with air and is therefore subject to oxidation. High
chromatography (HPLC), it was possible to detect the temperatures accelerate degradation. The presence of metals
molecules of elements that appear in smaller quantities than and/or organometallic compounds may act as a catalyst for
H2O, CO2, CO and CH4 during the depolymerization of oxidation. Color change, formation of acidic products and, in
cellulose molecules. Furan and its compounds appear only as advanced stages of oxidation, precipitation of residues may
degradation products of cellulose - paper. occur. Dielectric properties and, in extreme cases, thermal
B. Mechanism of insulating oil degradation properties may be affected. In addition to oxidation products,
Aging or deterioration of insulating oil is normally many other unwanted contaminants such as water, solid
associated with oxidation. Due to the omnipresence of particles, or oil-soluble polar compounds can accumulate in
oxygen and water, insulating oil oxidizes even under ideal the oil during operation and affect its electrical properties.
conditions. The insulating properties of the oil are also The presence of such contaminants and any other oil
affected by impurities in the solid materials in the transformer degradation products is indicated by a change in one or more
that dissolve in the oil. The reaction between the unstable properties (see Fig.1) [10-13].
hydrocarbons in the oil, oxygen and other catalysts such as
moisture, with the help of accelerators such as heat results in
the formation of decomposition products (oxidation by-
products) in the oil.
Sludge formation is the final stage of the degradation
process. The acids formed in the oxidation process attack
cellulose fibers and metals, forming metallic soaps, varnishes,
aldehydes, alcohol and ketones, which precipitate as an acid
sludge (a heavy substance, with the appearance of tar) on the
insulation, tank sidewalls, ventilation ducts, cooling fins, etc.
Mud appears more quickly in transformers that are
overloaded, overheated (by operation) and misused. Mud
increases the viscosity of the oil, thus reducing its ability to
cool. Mud causes the insulation to shrink and is also partially Fig. 1. Transformer aging circuit
 III. REVITALIZATION OF THE INSULATION SYSTEM OF Case 1 - transformer type TTUS-ONAF, 40MVA, 110/6.3 kV
OPERATING TRANSFORMERS After the scheduled periodic tests, the results presented in
In order to highlight the importance of applying a table I were obtained, and the following has been noted:
predictive maintenance in order to extend the life of the - the insulation resistances and tanδ have values close to
mixed paper-oil insulation system, two cases of its the allowed limits;
revitalization are presented below. - the moisture in the solid insulation is above the maximum
The installation used for the treatment/revitalization of the allowed for the voltage class;
insulating oil uses the principle of adsorption to remove water - the breakdown voltage is below the allowed limit;
and polar elements from the oil and from the solid insulation, - the interfacial tension and the moisture in the oil are very
applying the pressure percolation method [11]. close to the allowed limits.
To extract the water and polar elements from the oil, the Based on these findings, it was concluded that the solid
oil is forced through the adsorber container without being insulation is wet and the oil is damaged, so it is necessary to
additionally heated, the installation working at the normal subject this transformer to a revitalization process in order to
operating temperature of the transformer, and the revitalized/ eliminate water and polar elements and improve its physico-
regenerated oil is reintroduced into the transformer. chemical/electrical parameters.
The installation used, in addition to the adsorber The method chosen was the use of an installation based on
cartridges, is also provided with coarse mechanical filters at synthetic adsorbents [11-12]. After the revitalization/
the entrance to the installation and fine filters at the exit from regeneration process, it can be concluded from table 1 that the
it, to remove particles from the oil. Thus, the correction of the insulation resistance values and tanδ improved significantly
physico-chemical parameters of the insulating oil is also and the moisture in the solid insulation decreased
achieved. significantly. With regard to the oil, it can be noted that the
Revitalization of the insulation system means the acidity index decreased, and the values of the breakdown
regeneration of the oil (bringing its parameters to the level of voltage and the interfacial tension increased, which indicates
a new one) and the purification of the solid insulation. that the applied treatment fulfills the purpose of the proposed
revitalization process.
TABLE I
PRESENTATION OF MEASUREMENTS PERFORMED BEFORE AND AFTER TREATMENT - TRANSFORMER TTUS-ONAF 40 MVA
Values Transformer
Unit of accepted 40 MVA, 110/6.3 kV
Characteristics
measure according to Before After
regulations treatment treatment
The insulation system
Insulation resistance of windings (R60) at 20°C
HV - LV MΩ > 600 1072 16440
LV - HV 1299 16970
Absorption index (R60/R15)
HV - LV > 1.2 1.35 1.53
LV - HV 1.29 1.47
Tan δ for windings at 20°C
HV - LV % max. 0.5 0.40 0.29
LV - HV 0.44 0.31
Water content in solid insulation at 20°C
HV - LV % max. 2 3.3 1.2
LV - HV 3.7 1.6
The oil insulation
Breakdown voltage kV > 60 47.9 79.41
Tan δ for oil at 90°C < 0.005 0.0286 0.0032
Interfacial tension dyne/cm min. 28 27.32 45.46
Neutralization number mgKOH/g max. 0.05 0.037 0.015
Water content ppm max. 20 15.83 2.7
Particles content φ > 5µm/100 ml ≤ 10000 10907 5890

through the formation of the arc, which could have occurred

Case II - transformer type TTUS-ONAN, 40 MVA, 110/6.6 kV
between the coils/between the coils and earth, or arc in the
After the gas analyzer signaled a fault, a series of
on-load tap-changer across contacts during switching with oil
measurements were made, as shown in tables II and III.
drain in the main sump).
The failure signal was due to exceeding the limit for
Based on the C2H2/H2 ratio, which has a value of 2.14,
acetylene (C2H2). Using the 1st Duval Triangle and 1st Duval
according to [5], it is clearly emphasized that the main source
Pentagon methods to analyze the defects, it is found that
of combustible gases dissolved in the oil is the oil leakage
discharges with the formation of a high energy arc (D2)
from the on-load tap changer to the main transformer tank.
occurred in the transformer, followed by: oil penetration
 TABLE II
PRESENTATION OF MEASUREMENTS PERFORMED BEFORE AND AFTER TREATMENT - TRANSFORMER TTUS-ONAN 40 MVA
Values Transformer
Unit of accepted 40 MVA, 110/6.6 kV
Characteristics
measure according to Before After
regulations treatment treatment
The insulation system
Insulation resistance of windings
(R60) at 20°C
MΩ > 600
HV - (LV- g) 1120 41091
LV - (HV- g) 752 28872
Absorption index (R60/R15)
HV - (LV- g) > 1.2 1.19 1.45
LV - (HV- g) 1.09 1.32
Tan δ for windings at 20°C
HV - (LV- g) % max. 0.5 0.45 0.28
LV - (HV- g) 0.48 0.30
The solid insulation
Water content at 20°C % max. 2 3.2 1.03
The oil insulation
Breakdown voltage kV > 60 52.65 81.52
Tan δ for oil at 90°C < 0.005 0.0407 0.0035
Interfacial tension dyne/cm min. 28 23.34 44.64
Neutralization number mgKOH/g max. 0.05 0.034 0.029
Water content ppm max. 20 15.62 2.065
Particles content φ > 5µm/100 ml ≤ 10000 9410 4189
TABLE III
VALUES DETERMINED BEFORE AND AFTER TREATMENT FOR DGA AND FURAN COMPOUNDS
Values Values determined
Unit of accepted
Characteristics Before After
measure according to
treatment treatment
regulations
Dissolved gas analysis (DGA)
Hydrogen H2 100 42 0
Methane CH4 120 37 6
Acetylene C2H2 1 90 n/d
Ethylene C2H4 50 47 n/d
ppm
Ethan C2H6 65 16 7
Carbon monoxide CO 350 493 9
Carbon dioxide CO2 2500 3875 87
Total dissolved combustible gases (TDCG) 720 1125 22
The content of furan compounds
5HMF - caused by oxidation of paper 0.35 0.0223
2FOL - caused by high paper moisture 1.37 -
2FAL - caused by overheating 0-0.1 0.31 0.0057
ppm
2ACF - caused by lightning - -
5MEF - caused by severe local overheating 0.04 -
Total furans <2 2.07 0.028

According to the DGA, the CO2/CO ratio has a value that IV. CONCLUSIONS
indicates a moderate state of paper degradation. This
indicates a moderate aging of the solid insulation system, the Whether the treatment of the transformer oil is done inside
causes of which, high humidity and the oxidative process, are or outside the tank, and on-line or off-line, depends mainly on
determined by the analysis of furan compounds. economic and production considerations.
The results of the electrical and physicochemical Regenerating transformer oil in the transformer tank should
measurements presented in table II indicate that the be at least 30% - 60% cheaper than the alternative of
transformer has a high humidity level, which requires the draining, washing and refilling the transformer with
treatment of the insulation system. regenerated oil. By following an oil maintenance program,
Before carrying out the treatment process, the necessary the accumulation of moisture and sludge in the solid
repairs were made to the OLTC to eliminate the leakage insulation of the transformer can be prevented.
problems of its tank relative to the main tank of the It has been shown that the regeneration process is less
transformer. demanding on the insulation of the transformer than the
The applied treatment brings an important contribution to alternative process of draining, washing and refilling the
the improvement of the physico-chemical/electrical transformer with new oil, thus avoiding long periods of
parameters of the insulation system (see tables II and III). interruption and consequently loss of profit.
 Extending the life of a transformer, by implementing a transformers,” Proceedingss of the Sixty-sixth Annual International
Conferince of Doble Clients, 1999.
preventive transformer maintenance strategy from the ouset is [10] D. O Adekoya and I. A. Adejumobi, “Analysis of acidic properties of
the most cost-effective approach. distribution transformer oil insulation: a case study of jericho (nigeria)
The results obtained in the case of the two studies distribution network,” Nigerian Journal of Technology (NIJOTECH),
Vol. 36, No. 2, April 2017, pp. 563 – 570,
presented in this paper support the implementation of http://dx.doi.org/10.4314/njt.v36i2.32.
predictive and corrective maintenance to extend the life of the [11] A.-M. Aciu, M. -C. Nițu and D. Ștefan, "Extending the Life of
insulation system. Transformer Units Through Maintenance Based on Insulation
Revitalization," 2022 International Conference and Exposition on
If the transformer oil is regenerated/revitalized as soon as Electrical And Power Engineering (EPE), Iasi, Romania, 2022, pp. 37-
the acidity level exceeds the minimum permissible limits set 42, doi: 10.1109/EPE56121.2022.9959783.
by the standards, the degradation of the transformer oil, the [12] Aciu A-M, Nicola C-I, Nicola M, Nițu M-C. Complementary Analysis
for DGA Based on Duval Methods and Furan Compounds Using
wet condition of the core, the formation of sludge and the Artificial Neural Networks. Energies. 2021; 14(3):588.
subsequent breakdown of the transformer insulation can be https://doi.org/10.3390/en14030588
prevented and the life of the transformer can be extended. [13] J. S. N'cho, I. Fofana, A. Beroual, T. Aka-Ngnui and J. Sabau, “Aged
oils reclamation: Facts and arguments based on laboratory studies,” in
ACKNOWLEDGMENT IEEE Transactions on Dielectrics and Electrical Insulation, vol. 19, no.
5, pp. 1583-1592, October 2012, doi: 10.1109/TDEI.2012. 6311504.
This work was carried out through the Core Program [14] L. Safiddine, A.H.-Z. Zafour, I. Fofana, A. Skender, F. Guerbas, A.
Boucherit, “Transformer oil reclamation by combining several
within the National Research Development and Innovation strategies enhanced by the use of four adsorbents.” The Institution of
Plan 2022-2027, carried out with the support of MCID, Engineering and Technology, IET Generation, Transmission &
project no. PN 23 33 02 03. Distribution, 2017, ISSN 1751-8687, pp.1-9, doi: 10.1049/iet
gtd.2016.1995.
REFERENCES [15] CEI/IEC 60296:2020 - Fluids for electrotechnical applications –
Mineral insulating oils for electrical equipment. Publication date:
[1] CIGRÉ Brochure 227 - Life Management Techniques For Power 26.06.2020, Edition 5, pages 82.
Transformers, WG A2.18, January 2003, pages 125. [16] L. Safiddine, H.-Z. Zafour, U.M. Rao, I. Fofana, “Regeneration of
[2] CIGRÉ Brochure 445 - Guide for Transformer Maintenance, W.G. Transformer Insulating Fluids Using Membrane Separation
A2.34, February 2011, ISBN: 978-2-85873-134-3. Technology.” MDPI - Energies 12, no. 3: 368, pp.1-13,
[3] CIGRÉ Brochure 526 - Oxidation Stability of Insulating Fluids, W.G. doi:10.3390/en12030368.
D1.30, February 2013, ISBN: 978-2-85873-219-7. [17] P. Przybylek, K. Walczak, W. Sikorski, H. Moscicka-Grzesiak, H.
[4] CEI/IEC 60422:2013 - Mineral insulating oils in electrical equipment - Moranda and M. Cybulski, “Experimental Validation of a Method of
Supervision and maintenance guidance. Publication date: 10.01.2013, Drying Cellulose Insulation in Distribution Transformers Using
Edition 4, pages 93. Circulating Synthetic Ester.” In IEEE Access, vol. 9, pp. 150322-
[5] M.Duval and A. DePablo, "Interpretation of Gas-In-Oil Analysis using 150329, 2021, doi: 10.1109/ACCESS.2021.3125840.
New IEC pubication 60599 and IEC TC 10 Databases", IEEE Electrical [18] A. I. Hafez, A. M. el Deeb, M. H. el Behairy, E. M. Khalil and M. S.
Insulation Magazine, Vol.17, No.2, pp.31-41,2001. Abd el Fattah, “Study on the Treatment of High Voltage Used
[6] IEEE Std C57.140-2017 - Guide for Evaluation and Reconditioning of Transformer Oil by Using Natural Material,” IOSR Journal of Applied
Liquid Immersed Power Transformers, 31 Octomber 2017, Electronic Chemistry (IOSR-JAC) e-ISSN: 2278-5736.Volume 12, Issue 10 Ser. I
ISBN: 978-1-5044-4149-0. (October. 2019), PP 19-36.
[7] IEEE Std C57.637-2015 - Guide for the Reclamation of Mineral [19] M.E. Soares, A.J. Araujo, F.S. Silva, R.A. Silva, N.V. Barbosa,
Insulating Oil and Criteria for Its Use. Date of Publication: 26 October “Regeneration of transformer oil using a microemulsion with Triton
2015; Electronic ISBN: 978-1-5044-0062-6. X‑100,” Brazilian Journal of Chemical Engineering, Published online:
[8] Ivanka ATANASOVA-HÖHLEIN, "IEC 60296 (Ed. 5) – a standard for 02 June 2021: https://doi.org/10.1007/s43153-021-00113-6.
classification of mineral insulating oil on performance and not on the
origin", TRANSFORMERS MAGAZINE | Volume 8, Issue 1 | 2021.
[9] Sokolov et al., “Consideration on moisture distribution in

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