Distribution transformers used in secondary substations are strongly affected during faults in low voltage grids. Transformers are expected to survive a number of short circuits without failure but, once any significant winding deformation or core damage is produced, the service life and the likelihood of surviving further short circuits is greatly reduced because of locally increased electromagnetic stresses. Given the need for diagnosing failures and quickly putting the unit back into service it is evident that a complete set of tests, as done in power transformers’ condition assessment, is not an option. The main objective of this paper is to present Sweep Frequency Response Analysis (SFRA) test as an alternative to assess the mechanical condition of distribution transformers. For this purpose, a special design of a distribution transformer was used to simulate some failure modes and improve interpretation criteria.Facultad de Ingenierí

Calo, Emilio

Catalano, Leonardo J.

Mayora, Hernán

Morcelle del Valle, Pablo

Álvarez, Raúl Emilio

English

Servicio de Difusión de la Creación Intelectual

 25thInternational Conference on Electricity Distribution Madrid, 3-6 June 2019  Paper n° 697   CIRED2019 1/5 SWEEP FREQUENCY RESPONSE ANALYSIS TEST AS TOOL FOR DISTRIBUTION TRANSFORMERS MANAGEMENT   Hernán MAYORA Raúl ALVAREZ Emilio CALO  IITREE-LAT – Argentina IITREE-LAT – Argentina IITREE-LAT – Argentina  hernan.mayora@iitree-unlp.org.ar rea@iitree-unlp.org.ar ecalo@iitree-unlp.org.ar   Leonardo CATALANO Pablo MORCELLE DEL VALLE  IITREE-LAT – Argentina IITREE-LAT – Argentina  lcatalano@iitree-unlp.org.ar pmorcelle@iitree-unlp.org.ar  ABSTRACT Distribution transformers used in secondary substations are strongly affected during faults in low voltage grids. Transformers are expected to survive a number of short circuits without failure but, once any significant winding deformation or core damage is produced, the service life and the likelihood of surviving further short circuits is greatly reduced because of locally increased electromagnetic stresses. Given the need for diagnosing failures and quickly putting the unit back into service it is evident that a complete set of tests, as done in power transformers’ condition assessment, is not an option. The main objective of this paper is to present Sweep Frequency Response Analysis (SFRA) test as an alternative to assess the mechanical condition of distribution transformers. For this purpose, a special design of a distribution transformer was used to simulate some failure modes and improve interpretation criteria. INTRODUCTION Through-fault currents in distribution transformer windings produce severe electromagnetic forces and can result in winding deformations or core damage. These mechanical defects do not cause a condition of immediate failure in the unit, but are capable of generating localized electromagnetic stresses that may, in short or medium term of operation cause unplanned outages. To avoid this issue, it is important for utilities to have a tool for evaluating the internal condition of transformers and managing the repair and replacement resources. One of the most developed technique in recent years, mainly for power transformers, is the Sweep Frequency Response Analysis (SFRA), recognized as an advanced electrical test [1] and which allows a comprehensive evaluation of mechanical condition of the unit. Currently, the biggest challenge with this test is the interpretation of results. There is a working group of CIGRE (WG A2.53: Objective interpretation methodology for the mechanical condition assessment of transformer windings using Frequency Response Analysis) investigating and collecting experiences with the purpose of obtaining objective and systematic interpretation methodologies. This paper presents some experiences on a distribution transformer in order to obtain useful interpretation criteria when evaluating this type of units. SFRA To make a SFRA measurement, a sweep frequency voltage is supplied to a transformer terminal with respect to the tank. The voltage measured at the input terminal is used as reference for the SFRA calculation. A second parameter (response signal) is usually a voltage taken across the measurement impedance connected to a second transformer terminal (see Test types) with reference to the tank. The SFRA response amplitude is the ratio between the response signal (Ur) and the source voltage (Us) as a function of the frequency (usually presented in dB) [2]. To evaluate SFRA results, actual data are compared with reference data by visual inspection of the curves. There are three approaches [3] for generating reference data:  previous fingerprint measurements on the same unit;  measurements on identical (twin) transformers;  measurements on separately tested limbs or phases. Test types The main SFRA test types according to [3] are four. Each one allows to evaluate different elements of the transformer. They are presented below:  Fig. 1. "End-to-end" test type  Fig. 2. "End-to-end short-circuit" test type  25thInternational Conference on Electricity Distribution Madrid, 3-6 June 2019  Paper n° 697   CIRED2019 2/5  Fig. 3. "Capacitive inter-winding" test type  Fig. 4. "Inductive inter-winding" test type TRANSFORMER UNDER TEST The transformer used to emulate mechanical defects is a three-phase unit, 13.2/0.4 kV - 160 kVA - Dyn11, paper insulation, similar to those used in South American and European distribution systems, and in this case without tank or oil. It is presented in Fig. 5.  Fig. 5. Transformer used to emulate mechanical defects The four SFRA test types were made in each phase and for each of the mechanical defects to be emulated. The first test was made in a condition considered as "good condition", which was taken as reference to evaluate the deviations produced in the curves by each mechanical defect. Failure modes emulated were:  1. Winding turn-to-turn short circuit  2. Open circuited winding  3. Variation in core reluctance  4. Multi grounded core  5. Shifted winding  Winding turn-to-turn short circuit This is one of the most common faults that occur in distribution transformers. This defect was forced by shorting a single turn in the HV winding of the central column (H3-H2). Table 1 Defect  Fig. 6. Turn-to-turn short circuit Test Type End-to-end Great changes in H3-H2. Smaller differences in H1-H3 and H2-H1 End-to-end short-circuit Without significant deviations Cap. Inter-winding Without significant deviations Ind. Inter-winding Without significant deviations SFRA curves  Fig. 7. End-to-end (H3-H2)  Fig. 8. End-to-end (H2-H1)  25thInternational Conference on Electricity Distribution Madrid, 3-6 June 2019  Paper n° 697   CIRED2019 3/5 Open circuited winding This defect was emulated by opening two branches of the central winding (H3-H2).  Table 2 Defect  Fig. 9. Open circuited winding Test Type End-to-end Slight deviations in all windings End-to-end short-circuit Great deviations in all windings, mainly in H2-H3 Cap. Inter-winding Without significant deviations Ind. Inter-winding Changes occurred only in the affected winding. SFRA curves  Fig. 10. End-to-end short-circuit (H3-H2). Shorted: X1-X2-X3  Fig. 11. Inductive inter-winding (H2-X2). Grounded: H3-X0  Variation in core reluctance This mechanical damage was forced by increasing the core section placing an extra piece of ferromagnetic material on top of the transformer. Table 3 Defect  Fig. 12. Variation in core reluctance Test Type End-to-end Slight deviations in all windings End-to-end short-circuit Without significant deviations Cap. Inter-winding Differences in some resonances Ind. Inter-winding Without significant deviations SFRA curves  Fig. 13. End-to-end (H3-H2)  Fig. 14. Capacitive inter-winding (H1-X1).   25thInternational Conference on Electricity Distribution Madrid, 3-6 June 2019  Paper n° 697   CIRED2019 4/5 Multi grounded core This defect was obtained by placing an additional ground in contact with the laminations of the core.  Table 4 Defect  Fig. 15. Multi grounded core Test Type End-to-end Small amplitude differences End-to-end short-circuit Small amplitude differences Cap. Inter-winding Without significant deviations Ind. Inter-winding Without significant deviations SFRA curves  Fig. 16. End-to-end (H1-H3)  Fig. 17. End-to-end short-circuit (H3-H2). Shorted: X1-X2-X3  Shifted winding To obtain this mechanical failure the fixing woods were removed and the HV winding of the central column (H3-H2) was moved 3 cm upwards. Table 5 Defect  Fig. 21. Shifted winding Test Type End-to-end Resonance shift in H3-H2. Smaller differences in H1-H3 and H2-H1 End-to-end short-circuit Without significant deviations Cap. Inter-winding Without significant deviations Ind. Inter-winding Without significant deviations SFRA curves  Fig. 22. End-to-end (H1-H3)  Fig. 23. End-to-end (H3-H2)   25thInternational Conference on Electricity Distribution Madrid, 3-6 June 2019  Paper n° 697   CIRED2019 5/5 ARE SFRA RESPONSES OF DISTRIBUTION TRANSFORMERS DIFFERENT FROM LARGE POWER TRANSFORMERS ? In order to answer this question, a comparison of IEEE C57.149 [4] with the results measured in the analyzed transformer was made. This guide, in its “failure modes” section, analyzes the effects of transformer deformations on the SFRA curves -only end-to-end (EtoE) and end-to-end short-circuit (EtoE Sc) type tests- for each frequency range.  For the five failure modes of this paper, Table 6 presents a verification of the diagnosis established in IEEE C57.149. Green color indicates that diagnosis of [4] is verified, and red color means that the diagnosis [4] is not verified.  Table 6 Frequency Range 20 Hz to 10 kHz 5 kHz to 100 kHz 50 kHz to 1 MHz >1 MHz Test  Type  Failure EtoE EtoE Sc EtoE EtoE Sc EtoE EtoE Sc EtoE EtoE Sc Winding short circuit         Open circuited winding         Variation in core reluctance         Multi grounded core         Shifted winding         IS SFRA TEST AN ADEQUATE TOOL FOR THIS APPLICATION? Definitely this test is an adequate tool for the transformer assessment after incidents in medium/low voltage grids. As could be seen, SFRA technique is capable of detecting a wide variety of defects without the need to waste time and resources in performing many tests. With little experience, it is possible to identify the affected component and to take the decision of replacing or re-energizing the unit. SFRA test has the advantage of being a simple and safe test since voltages involved do not exceed a few volts, important feature when doing tests in passable areas. Also, it is important to consider that the SFRA test is easy to perform in terms of its connections, since stairs, cranes, etc. are not needed due to the size of the distribution transformers. Additionally, it was demonstrated that SFRA variations have a similar behavior for distribution and power transformers. In power transformers responses have another magnitude considering that capacitance is greater than distribution transformers. That means that the existing interpretation criteria and those that are being developed can be used with some cautions. Experience has shown that in distribution transformers it is important consider the effect of magnetization on core prior to SFRA tests. In order to take full advantage of SFRA benefits, it is imperative to train the testing staff and improve interpretation criteria with the own experience.   CONCLUSIONS The main conclusions are: a) SFRA test is a useful tool to quickly assess the transformer integrity after failures in low voltage grids because it is capable of detect a wide variety of defects using only one technique. b) Understanding the typical responses, it is possible to discriminate the type of failure and identify its location. c) As it was demonstrated, the end-to-end test type is the most sensitive for the majority of mechanical defects so, if there are limitations on the time available, it may be convenient to perform only the end-to-end test on all windings. d) In distribution systems, where utilities have several transformers of identical specification (twins), the SFRA response can be requested as a type test, and used as a reference for all these units. This represents an important advantage since no reference is needed for each single unit. e) In grids with high transformer failure rates SFRA test can identify the weak points of the transformer design and implement faster improvements or corrective actions.  Although some interpretation criteria were provided in this work, the adequate training and experience of test operator is essential to obtain reliable results.  REFERENCES [1] TB CIGRE 445 WG A2.34, “Guide for Transformer Maintenance”, February 2011. [2] TB CIGRE 342 WG A2.26, “Mechanical-condition assessment of transformer windings using frequency response analysis (FRA)”, April 2008. [3] IEC 60076-18-2012: “Power Transformers - Measurement of Frequency Response”. [4] IEEE C57.149-2012: “IEEE Guide for the Application and Interpretation of Frequency Response Analysis for Oil-Immersed Transformers”. 

Sweep frequency response analysis test as tool for distribution transformers management

El Servicio de Difusión de la Creación Intelectual

 25thInternational Conference on Electricity Distribution Madrid, 3-6 June 2019 
 
Paper n° 697 
 
 
CIRED2019 1/5 
SWEEP FREQUENCY RESPONSE ANALYSIS TEST AS TOOL FOR DISTRIBUTION 
TRANSFORMERS MANAGEMENT 
 
 Hernán MAYORA Raúl ALVAREZ Emilio CALO 
 IITREE-LAT – Argentina IITREE-LAT – Argentina IITREE-LAT – Argentina 
 hernan.mayora@iitree-unlp.org.ar rea@iitree-unlp.org.ar ecalo@iitree-unlp.org.ar 
 
 Leonardo CATALANO Pablo MORCELLE DEL VALLE 
 IITREE-LAT – Argentina IITREE-LAT – Argentina 
 lcatalano@iitree-unlp.org.ar pmorcelle@iitree-unlp.org.ar 
 
ABSTRACT 
Distribution transformers used in secondary substations 
are strongly affected during faults in low voltage grids. 
Transformers are expected to survive a number of short 
circuits without failure but, once any significant winding 
deformation or core damage is produced, the service life 
and the likelihood of surviving further short circuits is 
greatly reduced because of locally increased 
electromagnetic stresses. 
Given the need for diagnosing failures and quickly putting 
the unit back into service it is evident that a complete set 
of tests, as done in power transformers’ condition 
assessment, is not an option. 
The main objective of this paper is to present Sweep 
Frequency Response Analysis (SFRA) test as an 
alternative to assess the mechanical condition of 
distribution transformers. For this purpose, a special 
design of a distribution transformer was used to simulate 
some failure modes and improve interpretation criteria. 
INTRODUCTION 
Through-fault currents in distribution transformer 
windings produce severe electromagnetic forces and can 
result in winding deformations or core damage. These 
mechanical defects do not cause a condition of immediate 
failure in the unit, but are capable of generating localized 
electromagnetic stresses that may, in short or medium term 
of operation cause unplanned outages. 
To avoid this issue, it is important for utilities to have a 
tool for evaluating the internal condition of transformers 
and managing the repair and replacement resources. 
One of the most developed technique in recent years, 
mainly for power transformers, is the Sweep Frequency 
Response Analysis (SFRA), recognized as an advanced 
electrical test [1] and which allows a comprehensive 
evaluation of mechanical condition of the unit. 
Currently, the biggest challenge with this test is the 
interpretation of results. There is a working group of 
CIGRE (WG A2.53: Objective interpretation methodology 
for the mechanical condition assessment of transformer 
windings using Frequency Response Analysis) 
investigating and collecting experiences with the purpose 
of obtaining objective and systematic interpretation 
methodologies. This paper presents some experiences on a 
distribution transformer in order to obtain useful 
interpretation criteria when evaluating this type of units. 
SFRA 
To make a SFRA measurement, a sweep frequency voltage 
is supplied to a transformer terminal with respect to the 
tank. The voltage measured at the input terminal is used as 
reference for the SFRA calculation. A second parameter 
(response signal) is usually a voltage taken across the 
measurement impedance connected to a second 
transformer terminal (see Test types) with reference to the 
tank. The SFRA response amplitude is the ratio between 
the response signal (Ur) and the source voltage (Us) as a 
function of the frequency (usually presented in dB) [2]. 
To evaluate SFRA results, actual data are compared with 
reference data by visual inspection of the curves. There are 
three approaches [3] for generating reference data: 
 previous fingerprint measurements on the same unit; 
 measurements on identical (twin) transformers; 
 measurements on separately tested limbs or phases. 
Test types 
The main SFRA test types according to [3] are four. Each 
one allows to evaluate different elements of the 
transformer. They are presented below: 
 
Fig. 1. "End-to-end" test type 
 
Fig. 2. "End-to-end short-circuit" test type 
 25thInternational Conference on Electricity Distribution Madrid, 3-6 June 2019 
 
Paper n° 697 
 
 
CIRED2019 2/5 
 
Fig. 3. "Capacitive inter-winding" test type 
 
Fig. 4. "Inductive inter-winding" test type 
TRANSFORMER UNDER TEST 
The transformer used to emulate mechanical defects is a 
three-phase unit, 13.2/0.4 kV - 160 kVA - Dyn11, paper 
insulation, similar to those used in South American and 
European distribution systems, and in this case without 
tank or oil. It is presented in Fig. 5. 
 
Fig. 5. Transformer used to emulate mechanical 
defects 
The four SFRA test types were made in each phase and for 
each of the mechanical defects to be emulated. The first 
test was made in a condition considered as "good 
condition", which was taken as reference to evaluate the 
deviations produced in the curves by each mechanical 
defect. 
Failure modes emulated were:  
1. Winding turn-to-turn short circuit  
2. Open circuited winding  
3. Variation in core reluctance  
4. Multi grounded core  
5. Shifted winding  
Winding turn-to-turn short circuit 
This is one of the most common faults that occur in 
distribution transformers. This defect was forced by 
shorting a single turn in the HV winding of the central 
column (H3-H2). 
Table 1 
Defect 
 
Fig. 6. Turn-to-turn short circuit 
Test Type 
End-to-end 
Great changes in H3-H2. Smaller 
differences in H1-H3 and H2-H1 
End-to-end 
short-circuit 
Without significant deviations 
Cap. 
Inter-winding 
Without significant deviations 
Ind. 
Inter-winding 
Without significant deviations 
SFRA curves 
 
Fig. 7. End-to-end (H3-H2) 
 
Fig. 8. End-to-end (H2-H1) 
 25thInternational Conference on Electricity Distribution Madrid, 3-6 June 2019 
 
Paper n° 697 
 
 
CIRED2019 3/5 
Open circuited winding 
This defect was emulated by opening two branches of the 
central winding (H3-H2). 
 
Table 2 
Defect 
 
Fig. 9. Open circuited winding 
Test Type 
End-to-end Slight deviations in all windings 
End-to-end 
short-circuit 
Great deviations in all windings, 
mainly in H2-H3 
Cap. 
Inter-winding 
Without significant deviations 
Ind. 
Inter-winding 
Changes occurred only in the 
affected winding. 
SFRA curves 
 
Fig. 10. End-to-end short-circuit (H3-H2). 
Shorted: X1-X2-X3 
 
Fig. 11. Inductive inter-winding (H2-X2). 
Grounded: H3-X0 
 
Variation in core reluctance 
This mechanical damage was forced by increasing the core 
section placing an extra piece of ferromagnetic material on 
top of the transformer. 
Table 3 
Defect 
 
Fig. 12. Variation in core reluctance 
Test Type 
End-to-end Slight deviations in all windings 
End-to-end 
short-circuit 
Without significant deviations 
Cap. 
Inter-winding 
Differences in some resonances 
Ind. 
Inter-winding 
Without significant deviations 
SFRA curves 
 
Fig. 13. End-to-end (H3-H2) 
 
Fig. 14. Capacitive inter-winding (H1-X1). 
 
 25thInternational Conference on Electricity Distribution Madrid, 3-6 June 2019 
 
Paper n° 697 
 
 
CIRED2019 4/5 
Multi grounded core 
This defect was obtained by placing an additional ground 
in contact with the laminations of the core. 
 
Table 4 
Defect 
 
Fig. 15. Multi grounded core 
Test Type 
End-to-end Small amplitude differences 
End-to-end 
short-circuit 
Small amplitude differences 
Cap. 
Inter-winding 
Without significant deviations 
Ind. 
Inter-winding 
Without significant deviations 
SFRA curves 
 
Fig. 16. End-to-end (H1-H3) 
 
Fig. 17. End-to-end short-circuit (H3-H2). 
Shorted: X1-X2-X3 
 
Shifted winding 
To obtain this mechanical failure the fixing woods were 
removed and the HV winding of the central column 
(H3-H2) was moved 3 cm upwards. 
Table 5 
Defect 
 
Fig. 21. Shifted winding 
Test Type 
End-to-end 
Resonance shift in H3-H2. Smaller 
differences in H1-H3 and H2-H1 
End-to-end 
short-circuit 
Without significant deviations 
Cap. 
Inter-winding 
Without significant deviations 
Ind. 
Inter-winding 
Without significant deviations 
SFRA curves 
 
Fig. 22. End-to-end (H1-H3) 
 
Fig. 23. End-to-end (H3-H2) 
 
 25thInternational Conference on Electricity Distribution Madrid, 3-6 June 2019 
 
Paper n° 697 
 
 
CIRED2019 5/5 
ARE SFRA RESPONSES OF DISTRIBUTION 
TRANSFORMERS DIFFERENT FROM 
LARGE POWER TRANSFORMERS ? 
In order to answer this question, a comparison of IEEE 
C57.149 [4] with the results measured in the analyzed 
transformer was made. This guide, in its “failure modes” 
section, analyzes the effects of transformer deformations 
on the SFRA curves -only end-to-end (EtoE) and end-to-
end short-circuit (EtoE Sc) type tests- for each frequency 
range.  
For the five failure modes of this paper, Table 6 presents a 
verification of the diagnosis established in IEEE C57.149. 
Green color indicates that diagnosis of [4] is verified, and 
red color means that the diagnosis [4] is not verified.  
Table 6 
Frequenc
y Range 
20 Hz 
to 10 
kHz 
5 kHz 
to 100 
kHz 
50 kHz 
to 1 
MHz 
>1 
MHz 
Test  
Type 
 
Failure 
Et
oE
 
Et
oE
 S
c 
Et
oE
 
Et
oE
 S
c 
Et
oE
 
Et
oE
 S
c 
Et
oE
 
Et
oE
 S
c 
Winding 
short 
circuit 
        
Open 
circuited 
winding 
        
Variation 
in core 
reluctance 
        
Multi 
grounded 
core 
        
Shifted 
winding         
IS SFRA TEST AN ADEQUATE TOOL FOR 
THIS APPLICATION? 
Definitely this test is an adequate tool for the transformer 
assessment after incidents in medium/low voltage grids. 
As could be seen, SFRA technique is capable of detecting 
a wide variety of defects without the need to waste time 
and resources in performing many tests. With little 
experience, it is possible to identify the affected 
component and to take the decision of replacing or re-
energizing the unit. 
SFRA test has the advantage of being a simple and safe 
test since voltages involved do not exceed a few volts, 
important feature when doing tests in passable areas. Also, 
it is important to consider that the SFRA test is easy to 
perform in terms of its connections, since stairs, cranes, 
etc. are not needed due to the size of the distribution 
transformers. 
Additionally, it was demonstrated that SFRA variations 
have a similar behavior for distribution and power 
transformers. In power transformers responses have 
another magnitude considering that capacitance is greater 
than distribution transformers. That means that the existing 
interpretation criteria and those that are being developed 
can be used with some cautions. 
Experience has shown that in distribution transformers it 
is important consider the effect of magnetization on core 
prior to SFRA tests. 
In order to take full advantage of SFRA benefits, it is 
imperative to train the testing staff and improve 
interpretation criteria with the own experience. 
 
 
CONCLUSIONS 
The main conclusions are: 
a) SFRA test is a useful tool to quickly assess the 
transformer integrity after failures in low voltage grids 
because it is capable of detect a wide variety of defects 
using only one technique. 
b) Understanding the typical responses, it is possible to 
discriminate the type of failure and identify its 
location. 
c) As it was demonstrated, the end-to-end test type is the 
most sensitive for the majority of mechanical defects 
so, if there are limitations on the time available, it may 
be convenient to perform only the end-to-end test on 
all windings. 
d) In distribution systems, where utilities have several 
transformers of identical specification (twins), the 
SFRA response can be requested as a type test, and 
used as a reference for all these units. This represents 
an important advantage since no reference is needed 
for each single unit. 
e) In grids with high transformer failure rates SFRA test 
can identify the weak points of the transformer design 
and implement faster improvements or corrective 
actions. 
 
Although some interpretation criteria were provided in this 
work, the adequate training and experience of test operator 
is essential to obtain reliable results. 
 
REFERENCES 
[1] TB CIGRE 445 WG A2.34, “Guide for Transformer 
Maintenance”, February 2011. 
[2] TB CIGRE 342 WG A2.26, “Mechanical-condition 
assessment of transformer windings using frequency 
response analysis (FRA)”, April 2008. 
[3] IEC 60076-18-2012: “Power Transformers - 
Measurement of Frequency Response”. 
[4] IEEE C57.149-2012: “IEEE Guide for the 
Application and Interpretation of Frequency 
Response Analysis for Oil-Immersed Transformers”. 


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Sweep frequency response analysis test as tool for distribution transformers management

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