1. business and industrial
2. energy
3. electricity

Causes of failure of Distribution Transformers
in the East Zone of Caldas
O.J. Soto Marín, E.A. Cano Plata, A.J. Ustariz Farfan, L. F. Díaz Cadavid
Electrical, Electronic and Computer Engineering Program
Universidad Nacional de Colombia,
Branch Manizales
email: [email protected], [email protected], [email protected], [email protected]
Summary – One of the main problems that the Colombian’s
electrical utility faces, is the high failures rate of distribution
transformers, this situation leads to long power supply
interruptions, affecting the customer’s service. These
interruptions affect the economics aspects of the electrical
utility, like materials losses and repair costs. This paper
undertakes the study of failures in distribution transformers;
the objective is to find a way to reduce the high failure rate that
this type of equipment currently presents in the eastern rural
zone of the department de Caldas, especially in the areas of
greatest keraunic level. The methodology implemented enables
root cause analysis, the proposal of remedial actions and the
validation of the proposed solutions.
Key words: Fault trees, Inductive loops, Partial discharges, SPD,
Transformer failure.
I. INTRODUCTION
Home electricity has become a basic necessity in the
everyday life of families in developed and developing
countries. The cost of home energy supplied by the
electrical network is the lowest when compared to the
energy provided by different types of conventional
and unconventional processes. Some of the nonconventional systems that use renewable power, such
as solar photovoltaic, are now competing with
electrical networks in rural locations. The expansion
of the electrical network in the rural area must be
justified because:
• Rural areas have only a few consumers.
• Remote locations are mostly inhabited by
consumers with few prospects for the industrial
and commercial sector.
• Customers from the rural area are primarily
consumers whose demand increases very little
and do not contribute to the improvement of poor
load factors.
In the case of Colombia, most places in rural
distribution companies are populated by very lowincome groups with little potential to support the
development of any type of industry in the future. For
this reason, the decision to electrify these places
should be made after a thorough technical and
financial evaluation, with a subjective socio-economic
contribution and without forgetting the overall benefit
to society.
Despite this, electrical companies in Colombia have
made great efforts to expand electrical networks to all
rural areas of the country. That is the case of the
electrical utility Central Hidroeléctrica of Caldas
(CHEC), which has improved its coverage level and
has reached about 97% of total coverage. Table 1
shows the distribution of users covered in the eastern
area CHEC. It is noteworthy that despite repeated
efforts to improve the power supply in this area,
geographical and atmospheric conditions often
deteriorate and/or damage the network equipment
used for the distribution of energy. Thus, the work
becomes more complex and the outcome is higher
downtimes affecting users, and higher costs affecting
electrical utilities.
Table 1 - Electrification in the east zone chec
Installed
Installed
Location
meters
transformers
Rural
21.205
2.120
Urban
328.88
656
For the electrical utility CHEC, and particularly
eastern rural zone of Caldas, there are around 253
failed distribution transformers on average per year,
with a mean lifetime of 2.73 years for transformer. A
case study in the eastern rural zone of CHEC is
presented and the implementation of the method used
to classify cases of failure and failure rates through
technical fault trees is presented.
II. DESCRIPTION OF THE PROBLEM
Most studies focus their attention on overvoltage
caused by lightning as the main cause of transformer
failures in Colombia. This country is located in one of
the areas with the highest rate of ground flashes
density (GFD) on the planet [1], [2]. Samaná, a small
town in the department eastern of Caldas, registers a
GFD level of 10-8 flashes/km2–year [3]. Figure 1
shows the main research related to solving the
problem of transformer failure in the zone.
Figure 1. Fault studies of transformers.
The latest research on this topic was carried out by the
Research Program on Acquisition and Signal Analysis
(PAAS) of the Universidad Nacional de Colombia,
Bogotá branch. A transformer prototype with several
special features that perform better in areas of high
atmospheric electrical activity was designed during
this project [7]. This prototype showed increased
supportability of the applied electrical stresses. During
this project, 14 prototype transformers of special
design with SPT mesh were installed in the rural zone
of Samaná–Caldas. It was found that to this date 12
transformers have failed with an average lifetime of
2.8 years. These results show that the transformer of
special design implemented does not provide an
optimal solution; therefore a new study of the topic is
required in order to find the main cause of transformer
failures and hence propose appropriate solutions.
III. METHODOLOGY DESCRIPTION
The methodological framework below was
implemented for the study of the causes of failure in
distribution transformers in the eastern part of the
electrical utility Central Hidroelectrica de Caldas
(CHEC).
A. Processing of information
The method used in the processing of information
includes the identification and analysis of useful
information from the databases of the electrical utility
CHEC.
Future occurrence will be prevented once these are
corrected.
C. Remedial action plans.
The remedial action plan begins with the inquiry into
possible solutions that comply with the standards and
are likely to be implemented. The validation of the
remedial actions is performed through modeling and
simulation.
IV. RESULTS
A. Processing of the information
From the analysis of data available in different
information systems of CHEC, it was determined that:
• There are 2.776 transformers installed in the
eastern area CHEC.
• There are 969 transformers that were installed
but failed in the past 6 years.
• There are 341 nodes in which transformers
failed.
• There are 76 transformers that failed and were
diagnosed in the CHEC laboratory.
A temporal analysis of failed transformers and the
months of increased atmospheric electrical activity
was performed to establish the relationship between
electrical storms and transformer failures. Figure 2
shows a direct relationship between the amount of
failed transformers per month and temporal variation
of rainfall in 2012.
B. Analysis of transformer failures
The methodology used to analize transformer failures
is developed on the basis of the following three stages
and their respective activities.
1) Forensic analysis:
The technical characteristics of electromagnetic
compatibility and failure causes in dead transformers
are analyzed in the laboratory. The method of fault
trees allows the identification and analysis of the
conditions and factors that cause or have the potential
to cause or contribute to the occurrence of an event or
top event [8]. The method also allows qualitative and
quantitative analyses of the reliability of the system.
The construction of the fault tree starts with the basic
events (root cause represented by ovals in the tree) and
their probability of occurrence. The probability of the
top event is obtained applying the Boolean algebra
[8].
2) Field Technical visit:
After identifying the critical area of transformer
failure, a field technical visit is performed to verify the
condition of the installations and their compliance
with standards [9], [10].
The
methodology
implemented
allows
the
identification of the causes of failure or problems.
Figure 2 - Failed transformers/Rainfall year 2012
The above graph shows that in the months of july and
august the failure of transformers did not have the
same trend as the precipitation of rain, so it can be
inferred that there is a different cause of failure not
associated with lightning. There is a relationship
between periods of rain and failure of transformers in
the remainder months which indicate that lightning
might be the cause of transformer failure.
However, when analyzing the location of repeat nodes
(see Figure 3), hereinafter called critical zone, it was
determined that the largest amount of failed
transformers is located on the periphery of the central
mountain range, Figure 4; which is consistent with the
area of higher GFD.
but it was included that it leads to the unavailability of
the transformer.
• Estimation of the failure rate
The next step was to estimate the failure rate of each
failure mode through the failure probability of each
basic event (cause of failure diagnosed in the
laboratory CHEC) compared to the equivalent
universe, using the following equation:
(1)
where:
Figure 3 - Repeat nodes in transformer failure
Xocu: Number of occurrences of the basic event
NRe_i: Equivalent number to the installed transformers
The following equation was used to calculate the
equivalent number proportional (NRe_i) to the installed
transformers:
(2)
where:
NR: Number of repeat nodes (341).
nd_NR: Sample number of failed transformers per year
(83).
NRTF: Number of failed transformers at repeat nodes
per year (162).
Figure 4 - Topographic map of Caldas´s eastern zone
B. Analysis of transformer failures
Forensic analysis is the first stage of failure analysis
and the results are shown below.
Figure 5 shows an example of the fault tree built for
the core. The failure rate is obtained by means of the
application of Boolean algebra, equation (3).
1) Forensic analysis:
Forensic analysis was done to 76 failed Transformers
in the critical zone. The methodology implemented for
the processing of information of the failed
transformers diagnosed in the laboratory of CHEC is
divided into the following stages:
• Functional blocks of the installation:
The constructive parts of the transformer are
categorized in functional blocks. The following
elements were obtained: Tank, bushings, windings,
insulation system (oil and paper), tap charger and
core.
• Identification of failure modes:
Failure modes were detected by the non-operation of
the elements described in the previous paragraph.
Additionally, transformers that were uninstalled by
mistake were categorized in the failure mode of poor
execution. It is to note that these uninstalled
transformers were found to be in good condition. This
category does not fit in the definition of failure mode
Figure 5 - Core fault tree
P ( N ) = N1 + ( N 2 + N 3 + N 4) + N 5 + ( N 6 + N 7 + N 8)
(3)
Ranking of failure modes according to
magnitude.
The ranking is done by calculating the Probabilistic
Risk Number (PRN) using the following equation:
•
(4)
where:
FC: Frequency is obtained by dividing the failure rate
by 100.
IP: Impact of the failure mode.
The following scale was assigned to assess the impact
of each of the failure modes of the transformers:
causing impedance which would prevent an optimal
drainage of an atmospheric event to the SPT, figure
6a. Additionally, the surge-protective device SPD in
some cases is connected to the tank of the transformer
(see figure 6b) and the tank of the transformer is
connected to the grounding system. Even though there
is a discharge path, this is set through the tank and
represents high impedance to the path of the lightning.
1: Poor execution for transformers in good condition.
2: Transformers than can be repaired.
3:Transformers that can not be repaired.
Table 2 shows the results. The Pareto's law was
applied and it was decided that two (winding and
core) of the seven failure modes were regarded as
serious.
Table 2 - PRN calculation of failure modes
Failure modes
Impact Frequency
PRN
Core failure
3
18,85%
0,5656
Tank failure
3
0,57%
0,0171
Winding failure
2
14,85%
0,2971
Dielectric strenght
2
7,43%
0,1485
Bushing failure
2
0,00%
0,0000
Tap charger failure
2
0,00%
0,0000
Poor execution
1
0,57%
0,0057
Identification of root causes in serious failure
modes.
The root causes of failure in the core and winding
failure modes were identified. Table 3 shows the
number of basic event occurrences that produce failure
mode in the winding and the core.
•
Table 3 - Occurrences in the serious failure mode
Root cause of failure Amount of occurrences
modes
Winding
Core
Overvoltage
1
10
Surge
23
18
Damaged packages
1
5
Mishandling
0
0
Short circuit
1
0
After performing root cause analysis of the serious
failure modes it was determined that the dominant
causes of failure were surge with a total of 41
occurrences and overvoltage with a total of 11
occurrences.
2) Field Technical visit
Once the critical zone was identified, a technical visit
to the field was performed to verify compliance with
the standards of transformer installation. This visit
was made to the most critical circuit identified as
SNA23L15 "Rancho Largo". There was evidence of
poor practices of grounding systems where conductors
and connectors in ASCR with copper wires were used
a)
b)
Figure 6 - Grounding system and SPD installation
In some of the installed transformers, the SPD is
mounted on the crosshead of the circuit breaker as
shown in Figure 7a. This generates an increase in the
electromagnetically induced voltages on the
transformer bushings due to the inductive loop
between the SPD and the tank of the transformer.
The steepness of lightning current di/dt, which is
effective during the interval ∆t, determines the height
of the induced voltages, and the inductance is
proportional to loop area. Inductive loop formation is
represented in Figure 7b [11].
(5)
Where:
: Steepness of lightning current.
L: Inductance of the loop.
a)
b)
Figure 7 - SPD installation and inductive loop
C. Analysis of root cause failure
From the results obtained in the above processes it can
be inferred that transformers are failing primarily due
to overload and overvoltage. However, possible partial
discharges in the transformers insulation might be a
possible failure cause taking into account the
handling, and transport of these transformers through
unpaved roads in mountainous areas that could form
bubbles in the oil and the energizing before the set
time to remove these bubbles (6-8 hours) [13]. These
partial discharges degrade the insulation in the same
manner as overload.
The hypothesis of partial discharges as a cause of
failure is reinforced by examining several
transformers diagnosed in the laboratory CHEC.
Figure 8 shows blackened oil and with mud as main
features which, by normal diagnostic procedures in the
laboratory, indicate overload as the cause of failure.
However, the laboratory diagnostic results are
contradictory because according to the results of the
analysis, the transformer chargeability in the eastern
part CHEC is on average 5.2 % of the transformer
nominal charge which is very low to cause surge in
the transformer.
Based on the above information, it is concluded that
the 41 transformers diagnosed as failed by overload
have actually failed because of partial discharges in
the insulation.
implementing the discharger installed by CHEC. The
remnant voltage for the operation of the SPD is 95kV.
It should be noted that the good performance of the
SPD depends on the value of the grounding system
available on the node [16], [17]. Thus, it is necessary
to verify that minimum operating requirements of the
surge arrester are met taking into account the current
installing conditions in the eastern part of Caldas
(grounding system of 10Ω [10]).
Figure 9 - SPD model
150
[kV]
120
90
60
30
0
0,00
0,05
0,10
(f ile DPS_P_IEEE_1ohm.pl4; x-v ar t) v :IMPULS
0,15
0,20
[ms]
0,25
v :PRIM
Figure 10 - Simulation of the SPD model
Figure 8 – Failed transformer oil
V. REMEDIAL ACTION PLAN
Based on the results of root cause analysis, it was
determined that the second cause of failure was
overvoltage. Using the above information and based
on the high keraunic levels near eastern Caldas, it was
determined that poor performance of the SPD was a
possible failure cause.
A simulation of the transferred impulse using the
software
ATP/EMTP
with
the
following
characteristics was performed to verify the above [14]:
• A standard voltage impulse (1.2x50µs) in
primary terminals before the SPD with an
amplitude of 150kV in order to overcome the
basic BIL (95kV) in transformers of 13200V in
overhead distribution networks of CHEC.
• A surge arrester for overhead distribution
networks on zinc oxide of the polymer type for
12kV with MCOV of 10.2 kV as CHEC requires
in three-phase, 3-wire installations. 13200 V or
single phase 2-wire [9].
Figure 9 shows the ATP-EMTP implemented model.
Figure 10 shows the impulse applied across the
primary, it also shows the results of the simulation
Additionally, a decrease of inductive loops in the
transformer should be considered. This is achieved by
landing the SPD directly to the down conductors of
the grounding system in the shortest distance possible
as shown in Figure 11, and using a single conductor
from the grounding point of the tank to the down
conductor of the grounding system. Additionally, the
SPD have to be installed on the transformer tank,
figure 11.
Figure 11 - Decrease in inductive loops
Based on the simulation results, it is recommended to
reinforce the grounding system using 5 reinforcements
or additional weights as shown in Figure 12. This type
of mesh can, by itself and without soil treatment
requirements, reach the resistance values required by
the system of distribution of the eastern zone. Table 4
shows the simulation results of the mesh. Figure 13
show distribution of contact and surface voltage
respectively.
Table 4 -Simulation results of the proposed mesh
Type of
mesh
Electrode
Resistance
in the mesh
17.64 Ω
Max step
voltage
370.75 V
Max contact
voltage
215.72 V
Figure 12 - Configuration of the SPT mesh
Figure 13 - Diagram of surface voltage
The characteristics and dimensions of the proposed
mesh are shown below:
• Mesh depth of 2.4 m.
• Conductor gauge of 2/0 AWG.
• Copper electrodes. Copper is resistant to
corrosion and has high conductivity.
• Spacing between parallel conductors between 3
m and 15 m.
VI. CONCLUSIONS
•
The results of the analysis of main failure causes,
performance and the technical check of
transformer installation showed that overload
and overvoltage were the dominant causes of
failure. The proposed solutions presented on this
paper were determined based on these results.
•
The onsite inspection and the geographic
location of the failed nodes and of the repeat
nodes helped to established that the critical zone
was located in mountainous areas and in areas far
away from paved roads. In addition, practices of
transformer energizing without waiting the
recommended oil gasket time led to the
hypothesis of oil partial discharges in the
insulation as the primary cause of failure, and
lightning, considering the high keraunic level of
the area, as the second cause.
VII. REFERENCIAS
[1]. H. Torres, “Experience and first Results of Colombian
lightning Location Network”, in Proceedings of the
23th International Conference on Lightning Protection
Firenze, Italy: 1996.
[2]. H. Torres, D. Rondon, W. Briceño and L. Barreto,
“Lightning Peak Current Estimation Analysis from
Field Measurements in Tropical Zones”, in
Proceedings of the 23th International Conference on
Lightning Protection Florence, Italy: 1996.
[3]. www.paas.unal.edu.co
[4]. J. A. Agudelo, Fallas de transformadores de
distribución, Universidad Nacional de Colombia,
2000.
[5]. H. Torres, Evaluación y modificación del
transformador apropiado y óptimo para zona
tropical, Universidad Nacional de Colombia, 2004.
[6]. Acero, L., Muñoz, E., Román, F., Soluciones al daño
de transformadores en la zona de Samaná, Caldas., II
Congreso Internacional Sobre Uso Eficiente y
Racional de la Energía CIUREE, 2006, V.18, fasc. 54,
ISSN: 1692-7052, 2006, pp.36 41.
[7]. H. Torres, Contribución a la solución del problema de
falla de transformadores de distribución en Colombia,
Universidad Nacional de Colombia – Documento
interno Programa de Investigación sobre Adquisición
y Análisis de Señales - PAAS- UN, 2008.
[8]. NTP 333: Análisis probabilístico de riesgos:
Metodología del “Árbol de fallos y errores”. Tomás
Pique Ardanuy. Antonio Cejalvo Lapeña.
[9]. www.chec.com.co
[10]. Reglamento Técnico de Instalaciones Eléctricas –
RETIE 2008.
[11]. F. R. Campos, Análisis de las fallas en
transformadores causadas por la operación del
pararrayos ante sobretensiones externas, Portal de
Revistas de la Universidad Nacional, BogotáColombia, 1991.
[12]. Zapata J. Zapata, Estimación de tasas de fallas de
componentes en casos de ausencia de datos o
cantidades limitadas de datos, Scientia et Technica.
[13]. http://www.partial-dischargeacademy.com/
partialdischarge-causes.
[14]. IEEE Working Group. Modeling of metal oxide surge
arresters. USA, Transactions on Power Delivery, Vol.
7 No.1, January 1992.
[15]. NTC 4552-1 Protección contra descargas eléctricas
atmosféricas (Rayos). Parte 1: Principios generales.
2008-11- 26.
[16]. Cano Plata, Eduardo y Ramírez C, Samuel. “Sistemas
depuesta a tierra: diseñando con IEEE-80 y evaluando
con MEF”, Universidad Nacional de Colombia,
Departamento de Ingeniería Eléctrica, Electrónica y
Computación.
[17]. ANSI / IEEE Std 1410 - 2010 “Guide for Improving
the Lightning Performance of Electric Power
Overhead Distribution Lines” (IEEE Std 1410 –
2004).