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Β© OMICRON Page 1
SFRA Testing on Power Transformers:
Introduction
Β© OMICRON Page 2Β© OMICRON Page 2
Copyrighted 2018 by OMICRON electronics Corp USA
All rights reserved.
No part of this presentation may be reproduced or transmitted in any form or
by any means, electronic or mechanical, including photocopying, recording, or
any information storage or retrieval system or method, now known or
hereinafter invented or adopted, without the express prior written permission of
OMICRON electronics Corp USA.
Β© OMICRON
Β© OMICRON Page 3Β© OMICRON Page 3
Author Biography
Β© OMICRON Page 4Β© OMICRON Page 4
Transformer Testing Support Contacts
Fabiana Cirino
Application Engineer
OMICRON electronics Corp. USA
3550 Willowbend Blvd.
Houston, TX 77054 | USA
T +1 800 OMICRON
T +1 713 212 6154
www.omicronenergy.com
Brandon DupuisRegional Application Specialist - Transformers
OMICRON electronics Corp. USA
60 Hickory Drive
Waltham MA 02451 | USA
T +1 800 OMICRON
T +1 781 672 6230
www.omicronenergy.com
Charles Sweetser
PRIM Engineering Services Manager
OMICRON electronics Corp. USA
60 Hickory Drive
Waltham MA 02451 | UNITED STATES
T +1 800 OMICRON
T +1 781 672 6214
www.omicronusa.com
Moritz Pikisch
Application Engineer
OMICRON electronics Corp. USA
Sacramento, CA 95816 | USA
T +1 800 OMICRON
T +1 713 212 6150
Call me via SIP | Web
www.omicronenergy.com
Β© OMICRON Page 5
SFRA Testing on Power Transformers:
Introduction
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The Transformer βMechanical Integrity Testsβ
β’ The two transformer βmechanical integrity testsβ are,
1) The SFRA Test
2) The Leakage Reactance Test
β’ These two tests are used to detect mechanical changes within the main tank of a power transformer
(e.g. winding movement or winding deformation)
β’ The measurements provide information about the physical position of the components within the
main tank of a power transformer (i.e. the position of the core, the windings, the insulation, the
space, etc.)
β’ Both measurements are βfingerprintβ measurements β In general, the measurements should not
change over time
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The SFRA and Leakage Reactance Tests
β’ The two measurements are not typically utilized as βroutine testsβ, but are intended to be
βsituational testsβ, and are typically performed,
1) Before and after transporting a transformer, to check for βshipping damageβ
2) When there is a reason to suspect that there is an issue with the transformer (e.g. after a
fault event, or due to an increase in combustible gases in the main tank oil)
β’ At a minimum, one of the two measurements should be performed, and documented, for future
reference (but of course, performing, and documenting, both measurements is the βbest practiceβ)
β’ The hope is that a fault detected by one of the two measurements, can be confirmed by the other
measurement - In other words, the two tests complement each other well
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SFRA β Failure Mode Detection
1) Mechanical Winding Movement
Radial Winding Deformation (aka βHoop Bucklingβ)
Axial Winding Deformation (aka βTelescopingβ)
Bulk Winding Movement
2) Compromised Insulation
Turn-to-Turn Insulation Failure
Winding-to-Ground Insulation Failure
3) Core Problems (e.g. a βloss of core groundβ connection)
4) Severe Discontinuities or βBad Connectionsβ Involving the Current Carrying Path
Β© OMICRON Page 9Β© OMICRON Page 9
The Sweep Frequency Response Analysis (SFRA) Test
β’ The most sensitive electrical diagnostic test, to mechanical changes within a power
transformer
β’ An SFRA trace is a βfingerprintβ of a transformerβs main tank construction (i.e. the
position of the core, the windings, the insulation, the space, etc.)
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1. Inject an AC Voltage into one end of a transformer winding (Vin)
2. Measure the AC Voltage that comes out the other end of the transformer
winding (Vout)
3. Calculate the ratio of the Output Voltage and the Input Voltage (Vout / Vin)
4. Repeat the measurement over a broad frequency range (e.g. from 20Hz to
2MHz)
SFRA β Test Procedure
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β’ Three coaxial test leads are used to perform the SFRA measurement
1. Voltage source lead (Yellow)
2. Voltage input measurement lead - Vin (Red)
3. Voltage output measurement lead - Vout (Blue)
SFRA - Test Connections
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β’ Three coaxial test leads are used to perform the SFRA measurement
The Yellow and Red leads are connected in parallel, and are always connected
to the same bushing terminal, for the SFRA measurement
The Red lead is used to measure the βreference voltageβ that is injected directly
at the bushing terminal, at one end of the transformer winding
The Blue lead is always connected by itself, to the bushing terminal at the βother
endβ of the transformer winding under test
The impedance of the Red and Blue leads must be equal, so that the voltage
drop across the two leads is approximately equal, when comparing the Input
Voltage (Red) and the Output Voltage (Blue)
SFRA - Test Connections
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SFRA - Test Connections
β’ All three test leads are coaxial cables (Yellow, Red, and Blue)
β’ The βinner conductorβ of each test lead is used to pass the measurement signals to-
and-from the transformer
β’ The βouter shield conductorβ of each test lead is used to βprotectβ the relatively small
measurement signal, from interference
β’ It is critical that the βouter shield conductorβ of each test lead is grounded, to obtain the
correct measurement
β’ The βouter shield conductorβ of each test lead is typically grounded near, or at the end
of, each test lead (i.e. at the end of the cable closest to the bushing terminal connection)
Β© OMICRON Page 14Β© OMICRON Page 14
SFRA - Test Connection Example
R=50Ξ© R=50Ξ©R=50Ξ©
A B C
SFRA Test Instrument
Voltage
Source
Input
Voltage (Vin)
Output
Voltage (Vout)
Voltage Injection Signal
Input Voltage Measurement (Vin)
Output Voltage Measurement (Vout)
Β© OMICRON Page 15Β© OMICRON Page 15
The Key βFrequency Rangesβ of an SFRA Trace
1.) The βLow-Frequency Rangeβ (<10kHz)
β’ when performing an open-circuit test, is heavily influenced by the transformerβs core and
βmagnetizing impedanceβ
β’ when performing a short-circuit test, is heavily influenced by the physical position of the
transformerβs internal components, most notably the transformerβs windings
2.) The βMid-to-High Frequency Rangeβ (10kHz-500kHz)
β’ is heavily influenced by the physical position of the transformerβs internal components, most
notably the transformerβs windings
β’ is probably the most important frequency range of the SFRA measurement
3.) The βVery-High-Frequency Rangeβ (>500kHz)
β’ is heavily influenced by the test leads and the test connections (most notably, the shield
conductor ground connection, made near the end of each test lead)
β’ In most cases, this βfrequency rangeβ is not assessed
β’ Note, the actual βfrequency rangeβ of a trace will vary slightly from transformer-to-transformer, so
keep in mind, that the frequency range magnitudes provided above are not hard and fast rules
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The Key βFrequency Rangesβ of an Open-Circuit Test
The βCore Regionβ
The βWinding Structure Regionβ
The βTest Leads
and Test
Connectionsβ
Region
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The Key βFrequency Rangesβ of a Short-Circuit Test
The βLeakage Channel
Regionβ is heavily
influenced by the winding
structure
Short-circuiting the secondary-
side bushing terminals removes
the influence of the core
Continuity Check β A βDC Winding Resistance Cross-Checkβ
The βWinding Structure Regionβ
The βTest Leads
and Test
Connectionsβ
Region
Β© OMICRON Page 18
Open Circuit Test vs. Short-Circuit Test on the Same Winding
The βcore resonancesβ
By short-circuiting the
secondary-side bushing
terminals, the βcore
resonancesβ are
removed
The βTest Leads
and Test
Connectionsβ
Region
The βWinding Structure Regionβ is
typically more-or-less the same,
when comparing an open-circuit
and short-circuit test on the same
winding
Β© OMICRON Page 19
SFRA β Time-Based Comparison Analysis
β’ The most effective way to assess a set of SFRA test results
β’ In short, you document the βfingerprintβ measurement, when the transformer is
known to be in βgood conditionβ, and then compare the βfingerprintβ to future field
measurements
β’ In general, the SFRA trace should not change over time (but note, there are a
few exceptions)
Β© OMICRON Page 20
SFRA β Phase-Comparison Analysis
β’ SFRA test results can be assessed reasonably well by comparing the similar traces
amongst the three phases
β’ Analyzing SFRA test results by comparing similar traces amongst the three phases
takes practice and experience
β’ In general, when comparing the similar traces amongst the three phases, we are
just looking for something obvious (i.e. some βabnormalβ discrepancy when
comparing the three phases amongst each other)
Β© OMICRON Page 21
SFRA β Sister Unit Comparison Analysis
β’ Two transformers must be true sister units, and must have a near-identical
construction, to compare the SFRA test results amongst them
β’ A good way to determine whether two power transformers are true sister units, is to
compare their serial numbers
β’ This analysis strategy is particularly useful when comparing single-phase transformers,
that are part of a 3-Phase system
Β© OMICRON Page 22Β© OMICRON Page 22
Factors That Can Influence an SFRA Measurement
β’ A physical change within the main tank of the transformer
β’ User error (e.g. a βbadβ test connection)
β’ A poor ground connection - Ensure that the transformer ground, the test equipment ground, and
the test lead shield connection, are solidly bonded to earth-ground potential
β’ Test instrument failure - Perform the βzero checkβ test, to verify that the test equipment is
functioning properly
β’ The tap-changer position of both the DETC and the LTC β The transformer must be tested on the
same tap-position(s) each time the SFRA measurement is performed, to βoverlayβ and compare
similar SFRA traces
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β’ Residual magnetism β The SFRA trace may change, depending on how the core is magnetized
at the time of the test
Residual magnetism typically only influences the open-circuit tests, but not the short-
circuit tests
Residual magnetism typically only influences the βcore regionβ of the SFRA trace (i.e.
frequencies <10kHz)
β’ The βbus connectionβ β Was the SFRA measurement performed with the bushing terminals
completely isolated, or was the bus connected during the time of the test?
β’ The bushing(s) state β Was the SFRA test performed with the bushings installed, with the
bushings not installed, or were temporary bushings used during the time of the test?
Factors That Can Influence an SFRA Measurement
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β’ The direction of the input voltage injection (e.g. X1-X0 vs. X0-X1) β We recommend using the
βhead-to-tailβ method, when performing the SFRA test
β’ The insulating fluid state β Was the transformer tank filled with oil or not, when the SFRA
measurement was performed?
β’ The test voltage β The magnitude of the test voltage typically only influences the βcore regionβ of
the SFRA trace (i.e. frequencies <10kHz)
Factors That Can Influence an SFRA Measurement
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β’ The tertiary winding state (if applicable) β Is there a βbroken deltaβ tertiary, and was it open or
closed, and/or grounded when the SFRA measurement was performed?
β’ Core ground connection β Is there an external core ground connection, and was it connected or
disconnected when the SFRA measurement was performed?
β’ Current transformer state β Are there any bushing Current Transformers associated with the
power transformer, and was the secondary-side of the CTs shorted, open, grounded, etc.?
Factors That Can Influence an SFRA Measurement
Β© OMICRON Page 26
Understanding the Magnitude of the SFRA Trace,
relative to the Impedance of the Test Circuit
Β© OMICRON Page 27
SFRA β Impedance vs. Magnitude
β’ To gain a better understanding of the SFRA test, when an SFRA measurement is performed,
consider the question, will the Output Voltage be larger or smaller than the Input Voltage?
Consider a scenario where the transformer winding is completely short-circuited β
The Output Voltage would be equal to the Input Voltage, and the SFRA trace would βshift upβ
in magnitude
Consider a scenario where the transformer winding is completely open-circuited β
None of the Input Voltage would reach the other end of the transformer winding, and
therefore, the Output Voltage would, theoretically, be zero, and the SFRA trace would βshift
downβ in magnitude
Consider what happens to the Output Voltage as the impedance of the transformer
winding increases β The Output Voltage decreases, and the SFRA trace βshifts downβ in
magnitude
Consider what happens to the Output Voltage as the impedance of the transformer
winding decreases β The Output Voltage increases, and the SFRA trace βshifts upβ in
magnitude
Β© OMICRON Page 28
SFRA β Impedance vs. Magnitude
β’ The higher the impedance of test circuit, the more the measurement signal is attenuated,
and the lower the magnitude of the output signal will be
β’ The lower the impedance of test circuit, the less the measurement signal is attenuated,
and the higher the magnitude of the output signal will be
β’ Consider the primary winding of a step-down power transformer,
More winding turns = higher impedance = lower SFRA trace magnitude
Lower current = higher winding resistance = lower SFRA trace magnitude
β’ Consider the secondary winding of a step-down power transformer
Fewer winding turns = lower impedance = higher SFRA trace magnitude
Higher current = lower winding resistance = higher SFRA trace magnitude
Β© OMICRON Page 29
SFRA β Impedance vs. Magnitude
Primary-side winding traces typically have a lower magnitude
Secondary-side winding traces typically have a higher magnitude
Β© OMICRON Page 30
SFRA β Impedance vs. Magnitude
β’ What happens to an SFRA trace, if the transformer winding becomes short-
circuited? The magnitude of the trace will increase (i.e. the trace βshifts-upβ)
β’ What happens to an SFRA trace, if the transformer winding becomes open-
circuited? The magnitude of the trace will decrease (i.e. the trace βshifts-
downβ)
Β© OMICRON Page 31
Typical SFRA Test Plans
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SFRA Measurement Types
1) Open-circuit test (recommended test)
2) Short-circuit test (recommended test)
3) Capacitive inter-winding test (optional test β not typically performed in North America)
4) Inductive inter-winding test (optional test β not typically performed in North America)
β’ In most cases, the capacitive and inductive tests do not provide information that the
open-circuit and short-circuit tests do not provide
Β© OMICRON Page 33Β© OMICRON Page 33
Two-Winding Dyn1 Transformer - SFRA Test Plan
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Three-Winding YNyn0d1 Transformer - SFRA Test Plan
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Autotransformer without Tertiary - SFRA Test Plan
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Autotransformer with Accessible Tertiary - SFRA Test Plan
Β© OMICRON Page 37
Frequency Response Theory
Β© OMICRON Page 38Β© OMICRON Page 38
Transformer RLC Network (ResistorβInductorβCapacitor)
β’ Resistance of the windings, joints, contacts, tap-selectors, switches, etc...
β’ Inductance (frequency dependent)
Magnetizing inductance, which is relevant when performing an open-circuit test
Leakage inductance, which is relevant when performing a short-circuit test
β’ Capacitance (frequency dependent)
Turn-to-turn insulation
Winding-to-ground insulation
Winding-to-winding insulation (aka inter-winding insulation)
Β© OMICRON Page 39Β© OMICRON Page 39
> Page 39
Transformer RLC Network (ResistorβInductorβCapacitor)
Β© OMICRON Page 40Β© OMICRON Page 40
How do the RLC Components Vary Versus Frequency?
Resistorπ
ππΏ = πππΏ
ππΆ =1
πππΆ
π = 0 β π π = β β π
π = 0 β ππΏ = 0 β πβπππ‘ β πΆππππ’ππ‘!π = β β ππΏ = β β ππππ β πΆππππ’ππ‘!
π = 0 β ππΆ = β β ππππ β πΆππππ’ππ‘!π = β β ππΆ = 0 β πβπππ‘ β πΆππππ’ππ‘!
Inductor
Capacitor
Β© OMICRON Page 41Β© OMICRON Page 41
Understanding the Four Main Filter Types
1. Low-Pass Filter
Allows low-frequency signals to pass
Blocks high-frequency signals
2. High-Pass Filter
Blocks low-frequency signals
Allows high-frequency signals to pass
2. Band-Pass Filter
Blocks all signal frequencies except the resonance frequency
4. Band-Stop Filter
Allows all signal frequencies except the resonance frequency
Β© OMICRON Page 42Β© OMICRON Page 42
Low-Pass Filter Characteristics
102
104
106
-150
-100
-50
0
Frequency (Hz)
Am
plit
ud
e [
dB
]
L=200 mH
L=2 mH
L=20 H
ππΏ = πππΏ
π πππ
π = 0 , ππΏ= 0 , πππ’π‘= πππ , πππππ πππ πΉπ’πππ‘πππ =πππ’π‘πππ
= 1 = 0ππ΅
π = β , ππΏ= β , πππ’π‘= 0 , πππππ πππ πΉπ’πππ‘πππ =πππ’π‘πππ
= 0
πππ’π‘Rref
Rm
Β© OMICRON Page 43Β© OMICRON Page 43
High-Pass Filter Characteristics
RrefRm
102
104
106
-200
-150
-100
-50
0
Frequency (Hz)
Am
plit
ud
e [
dB
]
C=1uF
C=20nF
C=1pF
ππΆ =1
πππΆ
πππ πππ’π‘
π = 0 , ππΆ= β , πππ’π‘= 0 , πππππ πππ πΉπ’πππ‘πππ =πππ’π‘πππ
= 0
π = β , ππΆ= 0 , πππ’π‘= πππ = 1, πππππ πππ πΉπ’πππ‘πππ =πππ’π‘πππ
= 1 = 0ππ΅
Β© OMICRON Page 44Β© OMICRON Page 44
Band-Pass Filter Characteristics
RrefRm
102
104
106
-140
-120
-100
-80
-60
-40
-20
Frequency (Hz)
Am
plit
ud
e [
dB
]
C=1nF
C=10nF
C=50nF
ππΆ =1
πππΆππΏ = πππΏ
π = 0 , ππΏ= 0, ππΆ= β , πππ’π‘= 0 , πππππ πππ πΉπ’πππ‘πππ =πππ’π‘πππ
= 0
π = β , ππΏ= β, ππΆ= 0 , πππ’π‘= 0, πππππ πππ πΉπ’πππ‘πππ =πππ’π‘πππ
= 0
πππ πππ’π‘
Β© OMICRON Page 45Β© OMICRON Page 45
Band-Pass Filter Characteristics
RrefRm
102
104
106
-140
-120
-100
-80
-60
-40
-20
Frequency (Hz)
Am
plit
ud
e [
dB
]
C=1nF
C=10nF
C=50nF
ππΆ =1
πππΆππΏ = πππΏ
π = ππππ , ππΏ= ππΆ , πβπ πΌπππ’ππ‘ππ πππ πΆππππππ‘ππ πΆππππ‘π ππβπππ‘ β πΆππππ’ππ‘ ππ‘ π‘βπ π ππ ππππππ πΉππππ’ππππ¦!
πππ πππ’π‘
Β© OMICRON Page 46Β© OMICRON Page 46
Band-Stop Filter Characteristics
RrefRm
102
104
106
-30
-25
-20
-15
-10
-5
0
5
Frequency (Hz)
Am
plit
ud
e [
dB
]
C=1nF
C=10nF
C=50nF
ππΆ =1
πππΆππΏ = πππΏ
π = 0 , ππΏ= 0, ππΆ= β , πππ’π‘= πππ , ππΉ =πππ’π‘πππ
= 1
π = β , ππΏ= β, ππΆ= 0 , πππ’π‘= πππ, ππΉ =πππ’π‘πππ
= 1
πππ πππ’π‘
Β© OMICRON Page 47Β© OMICRON Page 47
Band-Stop Filter Characteristics
RrefRm
102
104
106
-30
-25
-20
-15
-10
-5
0
5
Frequency (Hz)
Am
plit
ud
e [
dB
]
C=1nF
C=10nF
C=50nF
ππΆ =1
πππΆππΏ = πππΏ
πππ πππ’π‘
π = ππππ , ππΏ= ππΆ , πβπ πΌπππ’ππ‘ππ πππ πΆππππππ‘ππ πΆππππ‘π ππππππ β πΆππππ’ππ‘ ππ‘ π‘βπ π ππ ππππππ πΉππππ’ππππ¦!
Β© OMICRON Page 48
An SFRA Trace Has all Four Filter Types
Low-Pass!
Band-Pass!
Band-Stop!
High-Pass!
Β© OMICRON Page 49
Thank you!