Faults and Defects in Power Transformers – A Case
Study
Cacilda de Jesus Ribeiro1; André Pereira Marques2,3; Cláudio Henrique Bezerra Azevedo2; Denise Cascão Poli
Souza1; Bernardo Pinheiro Alvarenga1; Reinaldo Gonçalves Nogueira1
1
School of Electrical and Computer Engineering, Federal University of Goiás, Goiânia, GO, Brazil
2
CELG Distribuição, Goiânia, GO, Brazil
3
Federal Institute of Education, Science and Technology of Goiás, Goiânia, GO, Brazil
Abstract – Power transformers play a fundamental role in
electrical power systems, in addition to representing significant
investments involved in the implementation of these systems. To
reduce the costs associated with a transformer’s life cycle and to
guarantee its reliability and durability, it is essential to monitor
its operating conditions, its insulation system, and the working
conditions of its accessories and other components. Therefore,
the aim of this work is to study the faults and defects that
occurred in 34.5 kV, 69 kV, 138 kV, and 230 kV oil-immersed
power transformers of the electrical system and the insulation
system of CELG, a major electric energy concessionaire in the
state of Goiás, Brazil. The results of this study, i.e., the efficacy
of the predictive technique for maintenance over the last 28
years (from 1979 to 2007), the characterization of faults and
defects during this period, and the presentation of proposals for
improvements in the predictive technique, aimed at reducing the
number of stoppages in the electric power supply system, are
expected to contribute to the body of knowledge in this field
I.
A fault is an anomaly in a piece of equipment that
inevitably causes stoppage of its operation, forcing its
removal from service [2].
As it is used here, the term “stoppage” indicates that the
service of a piece of equipment was interrupted, i.e., it was
removed from operation due to a defect or fault. The word
“transformers” also refers to autotransformers.
II.
POWER TRANSFORMERS
The present work was developed based on:
• the identification of the main parts of power transformers,
which were analyzed and divided into blocks of
components, as shown in Fig. 1; and
INTRODUCTION
A power transformer is one of the most important and
costly devices in electrical systems. Its importance is
attributed directly to the continuity of power supply, since its
loss through failure or defect means a supply stoppage. This
is a large piece of equipment whose substitution is expensive
and involves a lengthy process.
Research for new technologies and new predictive
maintenance techniques has greatly contributed to reduce
supply stoppages, thereby ensuring improved reliability of
energy supply. Several studies highlight the importance of
optimizing maintenance processes and diagnoses of
substation equipment such as transformers [1].
In this context, the purpose of this research was to study
faults and defects that occurred in 34.5 kV, 69 kV, 138 kV
and 230 kV power transformers immersed in mineral oil for a
period of 28 years at the electric power concessionaire CELG,
which supplies over two million consumers distributed in 237
municipalities with a population of approximately four
million in the state of Goiás, Brazil.
A defect is considered an anomaly in a device that can
cause it to operate irregularly and/or below its nominal
capacity. If not corrected in time, this defect can evolve,
leading to failure of the equipment and its removal from
service [2].
Fig. 1. Subdivision of a power transformer into blocks
• The characterization and analysis of faults and defects
detected in these devices, resulting from stoppages and/or
interventions which they underwent.
III.
STOPPAGES IN THE ELECTRIC POWER SYSTEM DUE TO
TRANSFORMER DEFECTS AND FAULTS
A. Number of Stoppages of the Devices
In this study, 549 service stoppages were recorded from
December 1979 to May 2007, involving 255 three-phase
transformers or three-phase transformer banks, and several of
these devices showed more than one stoppage.
Table I summarizes the number of devices, with their
respective ranges of nominal output power and by nominal
voltage.
TABLE I
NUMBER OF DEVICES BY RANGE OF NOMINAL THREE-PHASE OUTPUT POWER AND BY
NOMINAL VOLTAGE
Nominal
voltage
Total number of devices
(3-phase and banks)
3-phase power
(MVA)
Lowest
Highest
34.5kV
106
0.15
12.0
69kV
79
1.0
20.0
138kV
53
7.0
62.5
230kV
17
36
150
Total
255
Fig. 2. Number of transformer stoppages versus components
Of the transformer service stoppages in the period
considered in this work, a certain number were due to faults
and other defects, as indicated in Table II, reaching a total of
549 stoppages in this period of 28 years.
TABLE II
NUMBER OF TRANSFORMER STOPPAGES
Stoppages
caused by
Number of stoppages
Percent (%)
Faults
413
75.2
Defects
136
24.8
Total
549
100
It should be noted that this study took into account the
devices that were removed definitively from operation as well
as those that were purchased over the 28-year period of this
study. It is estimated that 10% of the devices under study are
part of the total number of transformers that belong to the
system’s technical reserve over these years.
B. Number of Transformer Stoppages versus Damaged Components
Fig. 2 shows the percentage of transformer stoppages
versus damaged components in the period of 1979 to 2007,
without considering stoppages caused by the protection
system and by human error. In this study, it was found that
the components most affected were windings (34%), bushings
(14%), onload tap changers, OLTC, (10%), and de-energized
tap changers, DTC (10%). The item “unidentified
component” (11%) refers to components which lack reliable
records for several reasons.
The insulation system of the transformers in question is
composed of mineral oil and solid insulation (cellulose,
varnish or polyester), although most of it consists of oilpaper. It was found that the stoppages due solely to problems
in the insulation oil accounted for only 4% of the number of
stoppages during the 28 years analyzed here. The degradation
of a transformer’s insulation system is usually the main
parameter that causes electrical faults in these devices.
The aging of oil-paper insulation in a transformer depends
on aging of both the paper and the oil. The assessment of the
remaining life of a transformer is the desired result of
diagnostic procedures. A popular belief is that the life of the
insulation paper determines the transformer’s service life [3].
Thus, when factors of transformer insulation degradation such
as water, oxygen, the products of decomposition in the oil and
temperature are monitored and controlled continuously, there
is decrease in the degradation of the insulation system, which
means less risk of electrical faults [4]. CELG carries out
systematic physicochemical testing and analyses of dissolved
gases to control and monitor the insulating oil of its
transformers, which is the reason for the low percentage of
problems involving insulating oil in its devices (4%).
C. Transformer Failure Rates Over Time
As stated above, service stoppages can be caused by both
defects and faults. The difference between them is that
interventions to correct equipment defects can be
programmed, unlike faults, which are generally emergencies
in the electrical sector. It is therefore essential to know the
individual transformer failure rates.
Fig. 3 shows the transformer failure rates in the CELG
system per year and class of voltage, without considering the
failures resulting from the protection system and from human
error. In view of these results, and as can be seen in Figure 3,
although failure rates of up to 9% were recorded (1992,
138 kV), the overall rates for the entire 28-year period are
quite acceptable. These rates are listed in Table 3, and were
calculated using (1).
Tf =
Nf
.100
t
∑N
i =1
(1)
e ,i
where:
Tf : failure rate in the period under consideration [%]
Nf:
number of failures in the period under consideration
Ne,i: number of devices existing in each year i considered
t:
number of years of the period considered
Fig. 3. Transformer failure rates over time
Analyzed quantitatively, the slightly higher rates of the
138 kV and 230 kV transformers are justified by the smaller
number of devices of these classes of voltage.
Table III lists the failure rates of 34.5 kV, 69 kV, 138 kV
and 230 kV transformers that occurred in the period under
study, without considering the reserve equipment (estimated
at 10% of the total number of power transformers).
TABLE III
TRANSFORMER FAILURE RATES FOR THE PERIOD OF 1979 TO 2007
Voltage
34.5kV
69kV
138kV
230kV
Total failures
93
54
18
5.0
Rate (%)
1.40
2.03
1.36
0.49
As can be seen, the transformers failure rates of CELG’s
system are relatively low, which is explained by the use of
predictive techniques at this concessionaire. The company’s
maintenance engineering sector, which strives to ensure a
continuous supply of electric power by reducing the failure
rate, has sought new predictive techniques, with emphasis on
the detection of partial discharges in transformers by the
acoustic method.
IV.
PREDICTIVE TECHNIQUES
The well-known dissolved gas analysis (DGA) technique in
insulating oil is sensitive to some types of incipient faults
(defects). To quantify the efficiency of this technique in
detecting such defects in CELG’s equipment, a comparison
was made of the total number of transformer stoppages that
could have been detected by the DGA predictive technique
and the stoppages effectively detected by this technique. This
comparison revealed that the technique provided an efficiency
of approximately 75%. However, sampling of transformer oil
for DGA testing is done periodically, according to the
chromatography software program CELG uses and to the
specificity of each device. Thus, between one sampling and
the next, the device may undergo impacts from atmospheric
discharges, external short circuits, and adverse operating
conditions, which may trigger or accelerate incipient faults
and cause the device to fail before the next sampling, masking
the efficiency of the chromatography system. It is therefore
understood that the efficiency of the DGA technique, per se,
is higher than 75%. In addition to DGA, another predictive
technique that could be used to increase the monitoring
efficiency of the state of transformer insulation is the
detection of partial discharges (PDs). The DGA method has
only low sensitivity for detecting partial discharges [5]. This
may sometimes lead to inaccuracy in analytical methods,
which may lead to errors by the person analyzing test results.
Furthermore, the DGA technique does not allow for the
identification of the site of an incipient fault, making it
difficult to locate it, especially if its intensity is low.
Particularly interesting is the use of a noninvasive method
such as the acoustic PD detection method, which allows for
monitoring of the evolution of PDs even while the device is in
operation.
Throughout its operation, a power transformer has to
withstand numerous stresses that generally result in the
degradation of the oil-paper insulation system by
decomposition of the paper and/or oxidation of the oil.
Degradation reduces the quality of this insulation. Partial
discharges can lead to winding breakdowns, and may cause
accelerated aging. PDs must be inferred in order to build an
early warning system. In this context, PDs serve as an
important measuring parameter for on-line monitoring [6].
To illustrate the above, the photograph in Fig. 4 depicts the
failure of a 20 MVA power transformer with a nominal
voltage of 69 kV/34.5 kV, showing damage sustained by a
large extent of the winding.
This paper therefore presents a proposal for improving
predictive techniques through the implementation of a set of
techniques, highlighting the combination of DGA with the
detection of partial discharges by the acoustic emission
method [7], which allows PD activity to be pinpointed in the
equipment without requiring its shutdown.
V.
CONCLUSIONS
Although the failures rates and the number of stoppages
that occurred during the period under study were relatively
low, it is important to implement other predictive techniques
that are sensitive to incipient faults in power transformers –
especially in terms of problems involving windings, bushings
and tap changers, which, taken together, account for 68% of
the events in components of these devices – in order to further
improve the performance quality indicators reported here.
Among these techniques, this paper highlights the
measurement of partial discharges by the acoustic emission
method, which could be allied to the DGA method, a
technique well-known in the energy sector, thereby increasing
the maintenance efficiency and quality of electric power
supply.
ACKNOWLEDGMENTS
Fig. 4. Damaged winding
Systematic equipment monitoring by the DGA technique
showed a slight increase in gases, however without providing
a warning about the need to remove this device from service,
ultimately leading to its damage by short-circuiting between
the spirals.
As can be seen in Fig. 5, the short circuit caused dislocation
of the winding due to an electrodynamic overload.
This work was carried out in collaboration with the
Maintenance Engineering Division of CELG Distribuição,
CELG D, and the Federal University of Goiás School of
Electrical and Computer Engineering (EEEC/UFG) through a
partnership in an R&D Project - ANEEL.
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Fig. 5. Dislocated winding
This fault could have been avoided by PD detection,
preventing the defect from developing into a short circuit
between spirals and, hence, failure of equipment. This
indicates the need for integrating predictive maintenance
techniques in order to improve diagnostic quality and
ascertain the state of transformer insulation systems.
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