Failure Analysis of Distribution Transformer and Proposing Proper
Solution to Mitigate the Faults -A Case Study of Kombolcha City Dessie
District Ethiopian Electric Utility, Ethiopia
WONDWOSSEN A.1*, TEMESGEN G.1, BELETE B.1, SHEGAW M.1, CHANDRASEKAR P.2,
PRASANNA K.2
1Electrical and Computer Engineering Department,
Kombolcha Institute of Technology, Wollo University,
ETHIOPIA
2Electrical and Electronics Engineering Department,
Vel Tech Rangarajan Dr. Sagunthala R&D Institute of Science and Technology,
INDIA
*Corresponding Author
Abstract: - The distribution transformer plays an important role in the distribution of electric energy to the
customers. The reliability of DT does not only affect availability of the electric power to the customers of
utility, but also economical operation of a utility. They can operate up to 30 years; but they fail within a year,
due to many reasons resulting big economic and service lose on the power supplying utilities as well as the
power customers. This paper has exposed the root causes of failure of the distribution transformer in EEU
Dessie district Kombolcha NO. 1 and No. 2-customer service centers by using data collected from the
substation, EEU, and inspecting the status of some transformers at peak hours. From the analysis conducted it
was observed that the most frequently occurring causes of failure of distribution transformers in the centers
were overloading, uneven power overloading, external short circuit, and lightning. In addition to identifying the
causes, the researchers have proposed some remedial solutions to reduce the problems.
Key-Words: - Distribution Transformers, Causes of Failure, Overload, Vandalism, lightning, frequency of
Interruption and Unbalanced load.
Received: October 26, 2022. Revised: October 16, 2023. Accepted: November 17, 2023. Published: December 27, 2023.
1 Introduction
Ethiopian Electric Utility (EEU) is one of the
integral elements of Ethiopian Ministry of Water,
Irrigation and Energy ministry. EEU established
with a central aim of becoming a utility center of the
country that can foster the socio-economic
transformation through delivering a cost effective,
safe, reliable and high-quality power. Currently
EEU has 11 Regional Offices, 28 Districts and 554
service centers, [1]. Out of the 28 districts, Dessie
District is the one that is providing a utility service
to most of the East Amhara cities. Kombolcha
electric utility Branch, which founds under the
umbrella of EEU Dessie District, will share the
ultimate objective of the head office of providing a
reliable and stable power to its customers. In order
to achieve their own (EEU head office, regional
district, branch offices and service centers) ultimate
goal, the respective offices should have to strive
aggressively on purchasing and sales of bulk power,
construction and operation of off grid power
generation, sub-transmission and distribution
networks and their respective infrastructure that can
meet international standards, [1].
Currently EEU, Dessie district Kombolcha
branch administers distribution networks of
Kombolcha branch service center. Due to the
existence of dense high-power demanding industries
in Kombolcha city, providing a balanced and
sustainable power to the customers is the main
burden of the respective branch utility office. Its
efficiency is most of the time affected by the
failures on the distribution network components,
limitation on infrastructure, limited use of modern
technological products, limitations on data recording
mechanisms, unavailability of periodic maintenance,
use of unplanned operation and distribution systems,
natural and artificial factors and so on. During the
WSEAS TRANSACTIONS on POWER SYSTEMS
DOI: 10.37394/232016.2023.18.32
Wondwossen A., Temesgen G., Belete B.,
Shegaw M., Chandrasekar P., Prasanna K.
E-ISSN: 2224-350X
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Volume 18, 2023
rainy seasons and existence of high-level wind, it is
usual to see power outage and damage on the
components of the distribution network of the
service center too. Not only these ones, with the
presence of overloading, lightning, short circuit, un-
periodic maintenance of the distribution network
elements, failures on the network components,
especially on the distribution transformers will be
observed. The frequent power outage due to these
cumulative factors is highly exposing users to loss
grid power and think of unnecessary investment of
using alternative temporary energy sources. To
alleviate the challenges and difficulties,
investigations focused on identifying the repeatedly
occurred basic crosscutting factors of distribution
system failures should be conducted; and
suggestions on remedial actions were made
accordingly.
2 Literature Review
The transformers are the static electrical devices
used to transfer energy from one circuit to another
circuit by using electromagnetic induction principle.
There are mainly two types of transformers in use
they are power transformers and distribution
transformers. A transformer is basically two sets of
wires (the windings) wrapped around a steel or iron
core as given in the schematic in Figure 1. The
windings and the core are insulated. The essential
core of the transformer has no moving parts, [2].
Fig. 1: Schematic Diagram of Transformer, [2]
The transformer plays an important role in
power generating stations. Here the power needs to
step up to high voltages for losses reduction in
transmission lines. Then it will step down to low
voltages for distribution purpose. Here the
distribution transformer will act main role.
Faults also happen in the transformers like all
other electrical devices which cause the failures.
Some of the common failures occurs in distribution
transformers are winding failure like dielectric
faults, copper resistance thermal losses, mechanical
faults in winding distortion, bushing failure, tap
changer failure, core failure, tank failures,
protection systems failure, cooling system failure
etc., [3]. Comparatively faults occurrence in
distribution transformers are more compared to
power transformers. The transformer failures will
result in loss, repair or replacement of transformer
and also, power loss due to power not supplied to
consumers. Some of the electrical, mechanical and
thermal factors which are causing the transformers
to fail are Lightning and switching surges; Transient
or overvoltage conditions; Conductor tripping;
Failure of cooling systems; operation of transformer
on non-linear loads; High ambient temperature, [3].
The distribution transformers are the very
important power equipment’s that allows the high
degree of electricity flow in distribution network.
Distribution transformers are also called as service
transformers. It will step down the distribution level
voltage to the voltage level used by the consumer.
The main components of distribution transformers
are windings, core, and main tank, on load tap
changer. The main faults of transformers occur in
windings and in on load tap changer. In the
distribution transformer the main failures occur in
On Load Tap Changer (OLTC) and winding.
Breakdown of a distribution transformer leads to
costly repairs, [4].
In the past, distribution transformers served for
more than sixty years. In recent years, many
distribution transformers fail even a few years after
commissioning, [4]. It is pertinent to address the
cause of the premature failure of transformers
because they are contributing to both loss of capital
and revenue to the power utility and slowing down
the economic growth of a country. It has been noted
that due to relative low cost of distribution
transformers (as compared with power
transformers), very little effort is made by utility to
find out the root cause of transformer failure. Lack
of investigating the root cause, could be attributed to
one of the reasons why more failures happen
immediately or within a very short period after
replacing a damaged transformer, [5].
Transformer fail due to insulation breakdown
could be contributed to electrical, mechanical and
thermal factors, [6]. Electrically induced factors
include operation of transformer under transients or
sustainable over voltage condition, exposure to
lightning surges and switching surges, or partial
discharge (corona) due to poor insulation system
design. On the other hand, mechanically induced
factors are such as looping of the inner most
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Wondwossen A., Temesgen G., Belete B.,
Shegaw M., Chandrasekar P., Prasanna K.
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windings, conductor tipping, conductor telescoping
and failure of coil clamping system. Thermal
induced factors include degradation of cellulose
insulation of oil and insulation material of the
windings. Thermal induced factors are mainly due
to overload beyond transformer design capacity as
result of cold load pickup (loads transformer
supplies after restoration of power after prolonged
power outage), failure of transformer cooling
system, operating transformer in an over excited
condition (under frequency) and operating under
excessive ambient temperature conditions, [6].
MIL-STD -1629A standards clarifies
transformer faults in three categories; severity,
frequency and detection factors. This give rise to
priority number (PN) which is defined as:
PN=Severity*Frequency (occurrence)*Detection.
(1)
The minimum PN of any faults on transformer is 1
and maximum is 120.
The PN for various elements of a transformer are as
depicted in Table 1, [7].
Table 1. PN for Various Elements of a Transformer
S/no.
Failure type
PN
1
windings
6-30
2
Bushing
24-48
3
Tap-changer
28-52
4
Core
6
5
Tank
18
6
Protection System
22-64
7
Cooling System
26-48
Source: Data from MIL-STD-1629A
From Table 1, it is evident that protection
system has the highest PN (22-64). This therefore
implies lack of proper protection system contribute
to high failure rate of transformer. Protection system
includes poor fuse grading (over-rated fuses), lack
of lightning arrestors and an ineffective earthing
system. Core failure is the least cause of premature
failure of distribution transformer. It is worth
pointing out that bushing failure is associated with
poor workmanship and O&M as is mainly caused
by;
i. Loosening of conductor due to vibration
ii. Sudden over-voltage which causes partial
discharge
iii. Ingress of water at seal of the transformer
iv. Not replacing old oil which may result in
internal flashover
Research has shown that there are various
measures which may increase the expectant life and
efficiency of distribution transformers (The
expected life span of transformer above 100KVA is
35 years while below 100KVA is approximately
25years). These measures include adequate and
good design, [7]. Transformers manufacturer in
recent time have compromised both on quality and
reliability. This is achieved by lack of adequate
clearance for free air circulation, economical size of
winding wire which cannot withstand higher current
densities due to short circuit conditions, and
improper use of inter-layer papers. Enhanced O&M
to ensure oil level of the transformer is sufficient,
clearing trees which are main source of high
impedance fault, proper fuse grading and remove of
condensed water in the transformer. Use of correct
diversity factor to avert overloading of the
transformer and avoiding two phasing in rural areas
which causes unbalance current that raise potential
of neutral with respect to the earth are some of the
measure’s utility need to undertake to avert early
failure of distribution transformers.
It has been observed that most of transformer
supplying industrial and institution are premature
failing. Increase of non-linear loads both at
industries and at institutions has resulted to high
harmonic content which increases the current
leading to overload of transformers, [8]. It is worth
noting winding copper losses (I2R), the core losses
and stray losses which increase significantly
operating temperature of a transformer has direct
proportionality with level of harmonic frequencies
present. It is important to note that, average life
expectancy of the transformers is directly
proportional to the average life of insulating
materials. On steady state power supply, harmonics
and variations in frequency are the main factors that
accelerated aging of insulation materials and hence
premature failure of distribution transformers, [8].
There can be variety of reasons for failure of
Distribution Transformers. Some of the important
ones are listed in Table 2:
Table 2. Causes of Failures, [9]
Failure Rate
29.43%
17.32%
7.3%
5.91%
4.03%
3.25%
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DOI: 10.37394/232016.2023.18.32
Wondwossen A., Temesgen G., Belete B.,
Shegaw M., Chandrasekar P., Prasanna K.
E-ISSN: 2224-350X
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3 Methodology
Both quantitative and qualitative methods followed
to obtain the root causes of distribution transformer
failures in a Kombolcha city. The researchers
carefully designed questionnaire and distributed to
the operation and maintenance staff of EEU Dessie
district, Kombolcha city number one and two-
customer service office. The questionnaires include
the main consequences of transformer failures like
overloading, unbalanced load per phases, oil level,
lightning, poor earthing or absence of earthing,
improper or poor cable terminal, external short
circuit, overrated fuses, vandalism and luck of
primitive maintenance. Under each main cause,
there are various sub causes which have its own
weights as {(1) – strongly disagree, (2) – disagree,
(3)-Moderately agree, (4) – Agree, and (5) –
Strongly agree}.
The researchers measured phase and neutral
current of the selected transformers at peak load
time, and observed fuse connection and rating,
proper lighting arrester connection, and oil level.
In addition, technical head and team leaders had
participated in interviewing issues, who had directly
linked with distribution transformers installation,
handling, maintenance and operations. The
researchers’ triangulated questioner, interview and
measured data to get better result.
In order to investigate the disturbance of the
distribution network or feeder of the Kombolcha
city, monthly power interruption readings for
twenty-one consecutive months from july-2021 to
march 2023 G.C. were obtained from Ethiopian
Electric Power, EEP north east region Dessie and
distilled into a spread sheet database using the
Microsoft excel package from which further
statistical analyses were performed.
The collected data were analyzed by using
Microsoft Excel to determining the frequency of
interruption, total duration, average un-served
power and unsold energy in each feeder. The
analyzed data covers 21 consecutive months as
shown from Figure 2, Figure 3, Figure 4, Figure 5
and it used for the selection of a single feeder for
further study.
The station recorded total frequency of
interruption were 2,911 (21 from incoming side and
remain 2,890 on load side) within 21 months. The
data shows that, line two and three shares 30.5 and
32.6 percentages frequency of interruption
accordingly as shown in Figure 2. These two feeders
recorded 1836 interruption or 63.1% from the total
interruption that happened on the station in the
respective period.
Fig. 2: Kombolcha Substation I Frequency of
Interruption within 21 Months
Fig. 3: Kombolcha substation I duration of
interruption in hours within 21 months
As shown in Figure 3, the total duration of
interruption was 6,360.5 hours within 21 months.
Among this, line three had 2020.1 hours interruption
duration, which means it sustained more than 406
hours than the other.
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Wondwossen A., Temesgen G., Belete B.,
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Fig. 4: Kombolcha substation I un-served average
power in MW within 21 months
The sum of un-served power was 24,377.96
MW within 21 months because of uncontrolled
interruption occurred on Kombolcha I substation.
From this 3,792.7 MW power were un-served due to
the interruption occurred in line three.
Fig. 5: Kombolcha substation I unsold energy in
KWH within 21 months
Because of continuous interruption occurred on
the substation 7,417,392,448 KWH energy lost
starting from july-2021 to march-2023 G.C. As
shown in Figure 5, huge amount of energy was
unsold, because of the more interruption occurred
on line three.
From the available feeders, the line-three had
been selected for further study, due to the fact that
this feeder is highly characterized by a frequency
and long duration interruption, un-served power and
unsold energy as shown in Figure 2, Figure 3,
Figure 4 and Figure 5.
3.1 Distribution Transformer Data in
Kombolcha City
Distribution Transformers play vital role in deciding
the power flow in large power systems and they are
the most expensive equipment’s in the distribution
network. There are more than 200 distribution
transformers all over the city, which are connected
to the 15KV distribution network to provide the
current demand of the customer. These DTs have
different ratings in units of kVA. The most common
ratings of DT in Kombolcha city are 25kVA,
50kVA, 100kVA, 200kVA, 315kVA, 630kVA, 800
kVA, and 1250 kVA. In addition to these, there are
some other DTs with uncommon ratings like
250kVA, 300 kVA, 400 kVA, 1500 kVA, 1600
kVA, and 5200 kVA. Figure 6 below shows the
number of DTs and their ratings in kVA unit in the
city.
Fig. 6: The number of DTs and ratings in kVA
available in Kombolcha city
As mentioned above there are eight medium
voltage (MV) feeders with 15kV supplying electric
power to the city. More than 200 DTs are connected
to these feeders to step down the incoming voltage
(15kV) into low voltage (380/220V) level. Figure 7
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Wondwossen A., Temesgen G., Belete B.,
Shegaw M., Chandrasekar P., Prasanna K.
E-ISSN: 2224-350X
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Volume 18, 2023
shows the number of DTs with their ratings loaded
to the distribution network or feeder in the city.
4
4
11
8
0
0
0
1
6
4
7
4
0
0
0
6
15
6
9
10
0
0
0
4
13
3
15
2
2
0
0
2
15
1
11
3
0
0
0
1
3
0
2
1
0
0
0
10
1
0
1
2
1
0
1
2
1
0
1
0
0
1
0
4
0
2
4
6
8
10
12
14
16
number of transformers in each
feeder
Feeders
25kVA
50kVA
100kVA
200kVA
315kVA
630kVA
800kVA
1250kVA
Fig. 7: The Number of DTs Connected to the
Feeders in Kombolcha city
There are 57 distribution transformers under
feeder three with a total installed load capacity of
11,300 kVA. Among these, 15-distribution
transformers have 200KVA rating and 11-
distribution transformers have 315 KVA rating. The
remaining DTs have 25KVA (11), 50KVA (7),
100 KVA (9), 630 KVA (1), 800KVA (1) and
1250KVA (1) installed capacity. The researchers
strongly believe that, transformer failure is one of
the causes that lead the feeder interruption
frequently. Furthermore, the researchers would
investigate those 57 transformers by measuring
phase and neutral current, observing lighting
arrester connection, oil level, and earthing system of
distribution transformers of the town, which were
installed under selected feeder. Then, the
researchers tried to triangulate questioner’s data,
interview information and measured data of selected
transformers to get a much better result and to reach
to a more soundful conclusion.
4 Data Analysis
Kombolcha City gets electric power from
Kombolcha substation I, with power distribution
network at a primary voltage of 15kV consisting
entirely of 3-phases, 3-wire feeders and is stepped
down to a utilization voltage of 380/220V (3-phase,
4 wire). There are eight feeders (lines) with 15kVA
supplying electric power to the city. Three feeders
provide electricity to the industries only while the
rest five feeders supply electrical energy to some
industries and other customers. Here are the lists of
the feeders in the Kombolcha city.
● Feeder one (F1) -provides electric power to
substation II area.
● Feeder two (F2) - provides electric power to
Bati town and its surroundings.
● Feeder three (F3) - supplies center part of
the Kombolcha city and its surrounds.
● Feeder four (F4) - supplies kombolcha
technology institute, Harbu town and its
area.
● Feeder five (F5) - supplies to BGI beer
Factory.
● Feeder six (F6) and Feeder seven (F7)
provide power to Kombolcha Textile
Factory
● Feeder eight (F8) used to supply Dessie
city, but now supplies the small factories in
Kombolcha city too.
We have combined the interview and substation
data for 21 months which should help us for
assessing the frequently interrupting feeder in the
city to determine the sample distribution
transformers to take tests during peak hours. And
also, we have combined well organized
questionnaire and measured transformer reading
data which should help us to know the causes for
transformer failures. The combination of all data has
justified that, the most common transformers
failures in the Kombolcha city are overloading,
unbalanced loading, lightning, improper use of
protection, and external short circuit.
Twenty questionnaires were distributed to the
maintenance, inspection and emergency
maintenance staffs of EEU Dessie district
Kombolcha customer service office No. 1 and No.
2. Out of twenty questionnaires sixteen were
collected. Figure 8 represents a plot of the frequency
of failure causes.
Fig. 8: Frequency of failure causes
WSEAS TRANSACTIONS on POWER SYSTEMS
DOI: 10.37394/232016.2023.18.32
Wondwossen A., Temesgen G., Belete B.,
Shegaw M., Chandrasekar P., Prasanna K.
E-ISSN: 2224-350X
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Volume 18, 2023
From the graph in Figure 8, the major causes of
transformer failure include overloading, unbalanced
load, low oil level, lightning, short circuits, poor
earthing, poor cable termination, over rated fuses,
lack of testing, and vandalism were highlighted as a
cause of failure. From the graph it was observed that
overloading is the most frequently occurring cause
with score 77 out of 80. Next to it, uneven loading,
short circuit, lightning, over rated fuse, and oil level
follow scoring 72, 70, 67, 66, and 61 out of 80
respectively. While vandalism had least effect on
the transformer failure with least score of 36. For
the question why these problems occurred on the
transformers or what were the root causes for the
problems, the answers were summarized in Table 3
in descending order of their occurrence as per the
questionnaire responses.
Table 3. Questionnaire responses summary of root
causes for different effects
No
Effect
Root causes
1
Overloading
Absence of record on the loading of
the transformer
Unauthorized usage/ theft of power
2
Unbalanced
load per
phases
Absence of loading records per
phases (random connection of new
customers)
Long single phase/two phase line
3
External
short circuit
Poor safety clearance
Contact of the conductor with trees
/ branches
Long LT line span
Sagging of conductors
4
Lightning
Absence of lightening
arresters/damaged arresters
Disconnection earth wire and rusted
earth wire
High resistance of earthing
electrode
5
Over rated
fuse
Non-availability of rated fuse
Frequent blown off HRC fuse
6
Low oil level
Leakage of oil
Un-filling of oil
Theft of oil
7
Poor Earthing
Disconnected earth wire or rusted
earth wire
High soil resistivity of the earth pit
8
Lack of
testing
Absence of a routine for testing
Non-availability of megger, filter
machine, oil testing kit
9
Poor cable
termination
Non – availability of materials
Loose connection
10
Vandalism
Unsecured station
Lack of awareness among the
public
From the Table 3, it was observed that the main
causes for overloading on the transformer is absence
of record on the loading of the transformer. Next to
it, unauthorized usage/ theft of power is another
serious issue that results over loading. This case
hurts the company in two ways. First using power
without paying for energy bills and secondly it
causes transformer failures. The Plot of the causes
that have severe effect on DTs is given in Figure 9.
Fig. 9: Plot of the causes that severe effect on DTs
From Figure 9, Lightning with score 74 out of
80 had the most severe effect once it occurred. Next
to lightning, external short circuit, overloading, and
unbalanced load followed with score 72, 70, and 61
respectively. While vandalism had least severity on
DTs. According to the data plotted in the Figure 9,
serious care must be taken while giving protection
against lightning, external short circuit, overloading
and unbalanced power per phases.
Overload is when a transformer is subjected to
sustained loading/or currents that exceed its design
specifications or the name plate capacity. During
overloading conditions, excess heat will cause the
insulation system to break down, resulting failure of
DTs due to overloading. In distribution network
transformer load data, the distribution transformers
with load ratio greater than 100% is defined as
overload transformer, the definition between 80%-
100% is heavy load, and less than 80% is considered
normal, [9].
The rise of temperature of winding, leads,
insulation and oil to unacceptable level are the
consequence of overloading on the transformer.
This consequence will cause a risk of premature
failure associated with increased currents and
temperature.
It is impossible to avoid overloading for a short
duration. But overloading distribution transformer
for long duration is not recommended. As DTs
WSEAS TRANSACTIONS on POWER SYSTEMS
DOI: 10.37394/232016.2023.18.32
Wondwossen A., Temesgen G., Belete B.,
Shegaw M., Chandrasekar P., Prasanna K.
E-ISSN: 2224-350X
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Volume 18, 2023
overloaded, it generates more and more heat, which
damages the winding insulation resulting whole
damage of the transformer.
Checking and recording on the loading of a
transformer as well as recording the total number of
connected loads (customer) from each transformer
are main responsibility of the EEU inspection staff.
Three failed transformers (Shewa Ber, Frutuna, and
Yimer Ali (Berber Wenz) areas of the Kombolcha
city) were extracted to analysis the overloading
case.
According to the load reading measured at peak
hours, the Shewa Ber, Furtuna (around Kebele 05)
and Yimer Ali (Berber Wenz) stations transformers
were 31.01%, 17.75% and 12.5% overloaded
respectively. According to the result obtained from
questionnaires data in the Table 3, this overloading
is due to absence of record on the loading of the
transformer so that new connections to customer
above the capacity of the transformer were made.
Most of the times a number of new connections are
added to the existing transformer without
calculating the load which it can supply. Additional
cause of overloading is due to power theft. From
these observations, the researchers may suggest that
the transformers might have failed due to
overloading.
Measuring the load current during peak load
hours will give information about load condition of
the transformer. If loading is greater than the rating
of the transformer during peak hours and continuous
for several days, it’s recommended to upgrade the
existing transformer with a larger rating transformer
immediately.
In case of unbalanced loading, voltage is
generated on the neutral and will remain floating
between neutral and earth. Since the neutral is
solidly grounded through external link, a circulating
current will flow through the loop of delta winding.
This additional circulating current will superimpose
on the main branch current of the delta winding and
will cause additional heat, which may lead to the
failure of the HV winding insulation. Proper and
timely maintenance could have saved this
transformer. R phase of LV winding could have
burnt due to line to ground fault. In this situation,
heavy current would have been drawn that cause not
only the LV winding to burn but also punctured the
HV winding, [10].
Distribution transformer should be loaded
evenly on all the three phases. However, in the case
of load measure of distribution transformer taken at
peak hour, there are many instances where even
though transformer doesn’t overload there was huge
gap between the currents in the phases.
For analyzing unbalances, the following Cases
were considered and unbalance data was generated.
Cases:
Shewa Ber and Haile (Berber Wenz) areas
Feeder: feeder 3
HV side voltage: 15KV
LV side voltage: 220V
It was observed that there were unbalanced
currents per phases. In the Haile (Berber Wenz)
transformer, phase S current is nearly 12 times
phase T current and phase R current is nearly equal
to 7 times phase T current. Similarly, in the Shewa
Ber transformer, there were significant gap between
each phase currents. Phase R current approximately
two times phase T. Due to this the load on one phase
goes excessively high, causing operational
disturbances which leads to failure of transformers.
This matches with the result obtained from the
questionnaires data analysis that unbalanced loading
is second prominent causes for failure of DTs.
The purpose of the fuse is short circuit
protection and overload protection. The HV fuse
provides protection by isolating the system upstream
of the transformer from faults in or beyond the
transformer, and protecting the transformer and
conductors against rushed LV faults, such as wires
twisted or firmly held together by fallen tree
branches. The HV fuse rating was selected to handle
approximately 150% of the transformer nameplate
rating whereas LV fuses were selected to handle
around 120% of the transformer nameplate rating on
the transformer LV terminals according to IEC
60269 – 2 standards.
In many instances of our inspection, we had
observed that using of higher rating of fuses or not
using of fuses at all or use of unequal fuse rating
size in one circuit were common circumstances with
respect to fusing. The first reason was occurred,
when transformer HRC fuses were blown off
frequently, and the maintenance team replaces the
fuse element of higher rating to avoid frequent
interruption of supply and frequent replacement of
fuses. The second reason occurred due to
unavailability of proper size of fuse for replacement
during maintenance activities. Figure 10 shows the
picture of a typical DT operating without HRC fuse
which we encountered during our field inspection of
some distribution transformers.
WSEAS TRANSACTIONS on POWER SYSTEMS
DOI: 10.37394/232016.2023.18.32
Wondwossen A., Temesgen G., Belete B.,
Shegaw M., Chandrasekar P., Prasanna K.
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Fig. 10: DT Operating without HRC Fuse
Four different Cases (Shewa Ber, Furtuna,
Haile, Donat 1 site) of transformers were selected
from the samples to analyze the improper uses of
fuses.
4.1 Shewa Ber Site Transformer
In this site 200kVA transformer with two out going
circuits was failed due to prolonged overloading and
replaced by 315kVA higher than the previous
transformer. Still now this site transformer is
suffering from the phase unbalance (as explained in
the previous part) and improper size of fuse. In the
first circuit the installed HRC fuse sizes are unequal.
As per IEC 60269 – 2 standards, it is not
recommended to use HRC with different size in a
single circuit. In the second circuit two phases,
phase R and T are unprotected. Using transformer
without installing HRC in the LV side cause damage
in the transformer, because there is no protection
against external faults like external short circuit and
sustained overloading. Therefore, installing correct
size of HRC fuses on LV side will avoid the
probability of failure of transformer due to overload
and external short circuit.
The same issue was happened for the
transformers in Furtuna, Haile, and Donat 1 stations.
These are bad practices and they must be avoided to
ensure the protection of distribution transformer
from external faults on the secondary side.
Lightning is another major reason for
distribution transformer failure during the summer
season. It occurred when lightening arrester is not
installed at all or the lightning arrestors failed to
divert direct lightning strikes or surges due to
discontinuity in the earthing system that causes the
primary winding to fail due to surge voltage or the
lightning arrestor itself may burst.
During our site inspection, all of the sampled
site transformers were protected against lightening
by the lightening arrestor. But, some transformers
(out of the sampled sites) as shown below in Figure
11 were not protected against lightening.
Fig. 11: Some of Unprotected DTs against
Lightening
In Figure 11, the first transformer (left) is
100kVA with two outgoing LV feeder and located
around Kombolcha general high school supplying
power to the customers around the school and
Biraro auditorium. For this transformer, the two
lightening arrestors are not connected to the MV
lines whereas the other one is missing. The second
(right) transformer located around Kombolcha
general hospital on the way to Woyraw with
capacity of 315kVA. This second transformer has
two outgoing LV side feeder providing power to the
customers around the hospital. For this case too all
the three lightening arrestors were not connected to
the MV lines. In general, in both cases the
transformers are not protected against lightening.
This may cause the transformers to fail due to
lightning stroke during summer season.
MV side short circuit fault does not cause fault
to the transformer. But in case of an external short
circuit on primary (LV) side affect the transformer
to fail. Very large amount of fault current
(approximately 20 to 25 times the rated current) will
pass through the windings. The winding is designed
to resist such a fault current not greater than 3
seconds. If the fault stays more than 3 seconds the
transformer winding will be burned causing the
transformer to fail. Figure 12, shows possible cause
for external short circuit problem.
WSEAS TRANSACTIONS on POWER SYSTEMS
DOI: 10.37394/232016.2023.18.32
Wondwossen A., Temesgen G., Belete B.,
Shegaw M., Chandrasekar P., Prasanna K.
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Volume 18, 2023
Fig. 12: MV and LV lines are in Contact with Tree
Branches
By selecting the correct rating of protection
device in the LV side or HRC fuse it possible to
protect the transformer from damage from LV short
circuit. But on – site investigation showed us, many
cases were observed the distribution transformer
with improper size of fuse, absence of fuse, and
unacceptable fusing type on LV side are major
reason for failure of DTs and affecting the safety of
DTs.
During our site inspection time, it had been
observed, there were sparks from the bushing
termination joints especially on L.V connections
due to lose contacts. Sparks were happening because
of loose terminations during the installation or else
have been loosening due to poor service conditions.
Once spark occurred at the cable termination, it
causes melting of the busing sealing gaskets,
effecting oil leakage from the bushing top, and later
resulting in failure of the transformer in due course
of time because of low oil level.
Also, during cable termination, we have to
consider the effect of bimetallic action. If aluminum
cable or conductors are to be connected with
brass/copper terminal or vise-versa, a proper bi-
metal remain in between. Due to bimetallic action, a
milli-volt will generate causing a localized current
and may deteriorate the current carrying thread,
[11]. A typical picture of poor cable termination is
shown in Figure 13.
Both the MV and LV terminals must be
connected through suitable lugs and connectors.
Direct Connection of cable/conductor to the bushing
terminal must be avoided.
On the other hand, transformer oil is used
to insulate high-voltage in transformers and is
designed to operate effectively at very high
temperatures for cooling, and insulating. Low oil
level is due to timely un-filling of oil tank and oil
leakage. Oil leak from bushing gaskets is mainly
due to high heat or stress developed in the bushings.
This will make the level of oil in the tank low
causing in deterioration of HV and LV insulation
and ends up with failure of transformer. In
overloaded transformer oil leak through LV bushing
is very common.
Fig. 13: Damaged DTs part due to poor cable
termination
5 Conclusions
This investigation had been conducted to analyze
and identify the root causes of DTs failure and
propose possible remedies to the identified problems
in EEU, Dessie District, Kombolcha city. It had
been observed, that the failure of DTs has
persistently devastating problems in EEU
Kombolcha customers No. 1 and No. 2. The result
of this work generally illustrated that the existing
problems in EEU Kombolcha customer No. 1 and
No. 2 related to failure of distribution transformers
are caused by inappropriate operation and
maintenance practices. Next to lack of protective
devices had significant contribution to the failure of
the distribution transformers.
It has been recognized that the leading and most
frequently occurring cause of distribution
transformer failure is related to overloading due to
lack of power loading record (this leads to
connecting new customers without upgrading the
existing transformer capacity) and unauthorized use
of power. On the other hand, the cause that had a
WSEAS TRANSACTIONS on POWER SYSTEMS
DOI: 10.37394/232016.2023.18.32
Wondwossen A., Temesgen G., Belete B.,
Shegaw M., Chandrasekar P., Prasanna K.
E-ISSN: 2224-350X
319
Volume 18, 2023
severe effect on the DTs was lightning. Most of the
time lightning affects those transformers having
poor lightning protection mechanism.
The benefits of this analysis of distribution
transformer failure types and the proposal of
different mitigation techniques for the identified
failure types is that it helps the utility provider to
take all safety measures concerning the different
operational distribution transformers as well as to
conduct a variety of preventive maintenance to
distribution transformers.
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Contribution of Individual Authors to the
Creation of a Scientific Article
The authors equally contributed to the present
research, at all stages from the formulation of the
problem to the final findings and solution.
Sources of Funding for Research Presented in a
Scientific Article or Scientific Article Itself
No funding was received for conducting this study.
Conflict of Interest
The authors have no conflicts of interest to declare
that are relevant to the content of this article.
Creative Commons Attribution License 4.0
(Attribution 4.0 International, CC BY 4.0)
This article is published under the terms of the
Creative Commons Attribution License 4.0
https://creativecommons.org/licenses/by/4.0/deed.en
_US
WSEAS TRANSACTIONS on POWER SYSTEMS
DOI: 10.37394/232016.2023.18.32
Wondwossen A., Temesgen G., Belete B.,
Shegaw M., Chandrasekar P., Prasanna K.
E-ISSN: 2224-350X
320
Volume 18, 2023