Sweep Frequency Response Analysis
Advanced Transformer Testing 2012
Transformer Diagnostics
 Transformer Diagnostics is about acquiring accurate
measurement data and other information in order to make
the correct decision about what to do with the actual unit
SFRA
TTR
WRM
FDS
Advanced Transformer Testing 2012
SFRA testing basics
 Off-line test
 The transformer is seen as a complex
impedance circuit
 [Open] (“magnetization impedance”)
and [Short] (“short-circuit impedance”)
responses are measured over a wide
frequency range and the results are
presented as magnitude response
(transfer function) in dB
 Changes in the impedance/transfer
function can be detected and
compared over time, between test
objects or within test objects
 The method is unique in its ability to
detect a variety of winding faults, core
issues and other electromechanical
faults in one test
Advanced Transformer Testing 2012
Réponse et analyse d’un balayage en
SFRA mathematics basics…
fréquence
Generator test voltage
Measured voltage
Phase, °
Gain, dB
V 
G(dB)  20 log10  out 
 Vin 
Advanced Transformer Testing 2012
Sweep Frequency Response Analysis
Standards Summary
Advanced Transformer Testing 2012
SFRA Standards and Recommendations
 Frequency Response Analysis on Winding Deformation of Power
Transformers, DL/T 911-2004, The Electric Power Industry Standard of
People‟s Republic of China
 Mechanical-Condition Assessment of Transformer Windings Using
Frequency Response Analysis (FRA), CIGRE report 342, 2008
 IEC 60076-18, Power transformers – Part 18: Measurement of
frequency response, 2012
 IEEE PC57.149™, Guide for the Application and Interpretation of
Frequency Response Analysis for Oil Immersed Transformers, 2012
 Internal standards by transformer manufacturers, e.g. ABB FRA
Standard v.5
Advanced Transformer Testing 2012
SFRA - Theory and method
Advanced Transformer Testing 2012
FRA definitions
 Frequency response
• The amplitude ratio and phase difference between voltages measured at
two terminals of the test object over a range of frequencies when one of the
terminals is excited by a voltage source. The frequency response
measurement result is a series of amplitude ratios and phase differences at
specific frequencies over a range of frequency.
• As Vout/Vin varies over a wide range, it is expressed in decibels (dB). The
relative voltage response in dB is calculated as 20 x log10(Vout/Vin)
 Frequency response analysis (FRA)
• The technique used to detect damage by the use of frequency response
measurements.
Advanced Transformer Testing 2012
FRA history
 1960: Low Voltage Impulse Method. First proposed by W. Lech & L. Tyminski
in Poland for detecting transformer winding deformation.
 1966: Results Published; “Detecting Transformer Winding Damage - The
Low Voltage Impulse Method”, Lech & Tyminsk, The Electric Review, UK
 1978: “Transformer Diagnostic Testing by Frequency Response Analysis”,
E.P. Dick & C.C. Erven, Ontario Hydro, IEEE Transactions of Power Delivery
 1980 - 1990‟s : Proving trials by utilities and OEM‟s, the technology cascades
internationally via CIGRE, and many other conferences and technical
meetings
 2004: First SFRA standard, ”Frequency Response Analysis on Winding
Deformation of Power Transformers”, DL/T 911-2004, is published by The
Electric Power Industry Standard of People‟s Republic of China
 2008: CIGRE report 342, ”Mechanical-Condition Assessment of Transformer
Windings Using Frequency Response Analysis (FRA)” is published
 2012: IEC60076-18 and IEEE PC57.149 are released
Advanced Transformer Testing 2012
Transformer mechanics basics
 A transformer is designed to handle certain (high!) mechanical
forces.
 Design limits can be exceeded due to
• Excessive mechanical impact
– Transportation
– Earthquakes
• Over currents caused by
– Through faults
– Tap-changer faults
– Faulty synchronization
 Mechanical strength weakens as the transformer ages
• Less capability to handle high stress/forces
• Increased risk of mechanical problems
• Increased risk for insulation problems
Advanced Transformer Testing 2012
Why assess the mechanical condition?

To detect core displacement and winding
deformation due to e.g.




Large electromagnetic forces from fault current
Transformer transportation and relocation
If these faults are not detected they may develop
into dielectric or thermal faults which normally
results in the loss of the transformer
Periodic testing is essential!
Advanced Transformer Testing 2012
Detecting Faults with SFRA
 Winding faults
 Deformation
 Displacement
 Shorts
 Core related faults
 Movements
 Grounding
 Screens
 Mechanical faults/changes
 Clamping structures
 Connections
 And more...
Advanced Transformer Testing 2012
SFRA measurement circuitry
Advanced Transformer Testing 2012
SFRA – How does it work (1)




A large number of low voltage signals with varying
frequencies are applied to the transformer
The input and output signals are measured in amplitude
and phase
The ratio of the two signals gives the frequency
response or transfer function of the transformer
From the (complex) transfer function you can derive a
number of entities as function of frequency e.g.




Magnitude
Phase
Impedance/admittance
Correlation
Advanced Transformer Testing 2012
SFRA – How does it work (2)




The RLC network has different impedance at different
frequencies.
The transfer function for all frequencies is the measure
of the effective impedance of the RLC network.
A geometrical deformation, changes the RLC network,
which in turn changes the impedance/transfer function
at different frequencies.
These changes gives an indication of damage within a
transformer.
Advanced Transformer Testing 2012
SFRA results – Frequency regions
 Transformer issues can be
detected in different frequency
ranges
• “Low” frequencies
– Core problems
– Shorted/open windings
– Bad connections/increased
resistance
– Short-circuit impedance
changes
Winding
and tap
leads
Winding
interaction and
deformation
• “Medium” frequencies
– Winding deformations
– Winding displacement
Core + windings
• “High” frequencies
– Movement of winding and tap
leads
Advanced Transformer Testing 2012
Frequency regions by IEC and IEEE
10
0
-10
Magnitude, dB
-20
Winding
structure
influence
Core
influence
-30
-40
-50
-60
-70
-80
-90
1
10
A phase
B phase
C phase
2
10
Earthing
leads
influence
Interaction
between
windings
3
10
4
10
Frequency, Hz
5
10
6
10
Advanced Transformer Testing 2012
7
10
SFRA measurement frequency ranges
– IEC60076-18
Category
Low frequency limit High frequency limit
Power transformers, Uw < 72.5 kV
< 20 Hz
> 2 MHz
Power transformers, Uw > 72.5 kV
< 20 Hz
> 1 MHz
Comparing older measurements
and/or methods/practices not
following IEC method 1 (CIGRE 342)
standard for signal shield grounding
< 20 Hz
500 kHz
Advanced Transformer Testing 2012
SFRA measurement frequency ranges
– Examples
”Standard”
Low frequency limit High frequency limit
Eskom standard
20 Hz
2 MHz
ABB standard
10 Hz
2 MHz
“Japan” (impedance)
100 Hz
1 MHz
DL/T-911 2004
1 kHz
1 MHz
Typical instrument default values are 20 Hz – 2 MHz
Advanced Transformer Testing 2012
Comparative tests
Transformer A
Design based
Time based
Transformer A
Transformer B
Type based
Advanced Transformer Testing 2012
Comparisons
 Time Based (Tests performed on the same transformer over time)
 The most reliable test
 Deviations between curves are easy to detect
 Type Based (Tests performed on transformer of same design)
 Requires knowledge about test object/versions
 Small deviations are not necessarily indicating a problem
 Design based (Tests performed on winding legs and bushings of
identical design)
 Requires knowledge about test object/versions
 Small deviations are not necessarily indicating a problem
Advanced Transformer Testing 2012
SFRA Measurement philosophy
New measurement = Reference measurement
Back in Service
New measurement ≠ Reference measurement
Further Diagnostics Required
Advanced Transformer Testing 2012
Reference measurements
 When transformer is new
• Capture reference data at commissioning of
new transformers
 When transformer is in known good
condition
• Capture reference data at a scheduled routine
test (no issues found)
 Save for future reference
 Start Your Reference Measurements ASAP!
Advanced Transformer Testing 2012
SFRA measurements – When?
 Manufacturing tests
 Quality check during manufacturing
 Proofing the transformer after short-circuit test
 Before shipping





Installation/commissioning
Relocation
After a significant through-fault event
Part of routine diagnostic test
Catastrophic events
• Earth quakes
• Hurricanes/tornadoes
 Trigger based test/transformer alarms
• Buchholz
• DGA
• High temperature
 Before-after maintenance
Advanced Transformer Testing 2012
Transformer fault detection
Advanced Transformer Testing 2012
Detection of Winding Movement (1)

Prior to SFRA the mechanical integrity of the
transformer was assessed with the following standard
methods:




Winding capacitance
Excitation current
Leakage reactance measurements
Each of these methods have drawbacks
26
Detection of Winding Movement (2)

Winding Capacitance



Excitation Current



Successful only if reference data is available
Limited sensitivity for some failure modes
Excitation current is an excellent means of detecting turn-to-turn
failure as a result of winding movement
If a turn-to-turn failure is absent, winding movements can remain
undetected.
Leakage Reactance



Per phase leakage reactance measurements generally shows
no or little correlation between the phases and nameplate
Discrepancies from nameplate value of 0,5 % to 3 % can be a
reason for concern
The range of defect detection is to large for an accurate
assessment
27
Comparing diagnostic techniques (CIGRE)
Diagnostic technique
Advantages
Magnetizing (exciting) current
Requires relatively simple equipment.
Can detect core damage
Disadvantages
Not sensitive to winding deformation.
Measurement strongly affected by core
residual magnetism
Impedance (leakage reactance)
Traditional method currently specified in Very small changes can be significant.
short-circuits test standards.
Limited sensitivity for some failure modes
Reference (nameplate) values are
(best for radial deformation)
available
Frequency Response of Stray Losses Can be more sensitive than impedance
Not a standard use in the industry
(FRSL)
measurement.
Almost unique to detect short circuits
between parallel strands
Winding capacitance
Can be more sensitive than impedance Limited sensitivity for some failure modes
measurements.
(best for radial deformation).
Standard equipment available
Relevant capacitance may not be
measurable (e.g. Between
series/common/tap windings for auto
transformers)
Low Voltage Impulse (LVI) (time domain)
Recognized as very sensitive
Specialist equipment required.
Difficult to achieve repeatability.
Difficult to interpret
Better repeatability than LVI with the
Standardization of techniques required.
Frequency Response Analysis
same sensitivity.
Guide to interpretation required
Easier to interpret than LVI (frequency
instead of time domain).
Increasing number of users
Advanced Transformer Testing 2012
Comparing SFRA and other traditional
transformer measurements
 End-to-End [Open], (Open Circuit Self Admittance]
•
•
Example: 1U - 1N [open]
Excitation current as function of frequency
 End-to-End [short], (Short Circuit Self Admittance)
•
•
•
Example: 1U – 1N [short]
Leakage reactance/short-circuit impedance as function of frequency (compare
IEEE 62 measurements at 50/60 Hz)
FRSL, Frequency Response of Stray Losses (SFRA 20 – 600 Hz)
 Input Impedance
•
•
Measurement of impedance to ground for a certain configuration (Japanese
”standard”, common in South America, common in China before DL/T 911)
Can be performed for grounded objects with the active impedance probe
 Capacitive Inter-winding [Inter-Winding]
•
Capacitance as a function of frequency
 Inductive Inter-winding [Transfer Admittance]
•
•
Turn-ratio measurement (voltage ratio) as a function of frequency
Possible to perform at various impedances with the active voltage probe
Advanced Transformer Testing 2012
SFRA vs Excitaion current
Example; U1 - N1 [open]
 Excitation current as function of
frequency
 Please note that excitation current is
voltage dependent!
SFRA
• At low voltages the inductance is low and
increasing with voltage
• At high voltages the core gets saturated and
the inductance decreases
• Non-linear phenomena...
Advanced Transformer Testing 2012
SFRA vs short-circuit impedance/leakage reactance
Example; U1 – N1 [short]
 Short-circuit impedance/Leakage reactance as a function
of frequency (IEEE – 50/60 Hz @ 200 V)
• Leakage reactance is not voltage dependent. However, in certain
configurations the magnetizing impedance can influence the results
at lower test voltages
 FRSL, Frequency Response of Stray Losses (”SFRA” 20 –
600 Hz @ ~200 V)
Advanced Transformer Testing 2012
Frequency Response of Stray Losses (FRSL)
 End-to-End (short-circuit), [Short
Circuit Self Admittance]
• Impedance changes may be caused
by;
– Inductance changes e.g winding
movement
– Resistance change (DC) due to bad
contacts, soldering issues etc
– Resistance change at higher
frequencies (Rstray) due to stray losses
caused by;
– Winding deformation
– Shorts between parallel strands
Ref: L. BOLDUC, et. Al ”DETECTION OF
TRANSFORMER WINDING DISPLACEMENT
BY THE FREQUENCY RESPONSE OF STRAY
LOSSES (FRSL), CIGRE session, 2000.
Advanced Transformer Testing 2012
FRSL – 160 MVA transformer with contact resistance problem
HV [short], Transformer G2-1
HV [short], Transformers G2-1 and 3
Advanced Transformer Testing 2012
FRA Methods
 Sweep Frequency Response
 Impulse
Advanced Transformer Testing 2012
Impulse FRA vs. SweepFRA
 Impulse FRA
Impulse FRA
 Injects a pulse signal and
measure response
 Convert Time Domain to
Frequency Domain using Fast
Fourier Transform (FFT) algorithm
 Low resolution in lower frequencies
 SFRA
 Injects a single frequency signal
 Measures response at the same
frequency
 No conversion
 High resoultion at all frequencies
Advanced Transformer Testing 2012
Comparing Impulse & SweepFRA
 SFRA (Sweep frequency response analysis)
provides good detail data in all frequencies
Black = Imported Impulse measurement
(Time domain converted to Frequency Domain)
Red = SFRA Measurement
Deviations Low Frequency = Method
Deviation High Frequency = Cable practice
Advanced Transformer Testing 2012
Zoom View of impulse vs. SFRA
Impulse instrument sample rate limts
frequency resolution to 2kHz.
Advanced Transformer Testing 2012
SFRA Measurement Technique, part 1
- Measurement setups
Advanced Transformer Testing 2012
SFRA test setup
Advanced Transformer Testing 2012
FRAX measurement circuitry
Advanced Transformer Testing 2012
Test types – End-to-end (open)
 Test signal is applied to one end of a winding and the
transmitted signal is measured at the other end
 Magnetizing impedance of the transformer is the main
parameter characterizing the low-frequency response
(below first resonance) in this configuration
 Commonly used because of its simplicity and the possibility
to examine each winding separately
Advanced Transformer Testing 2012
End-to-end (open) - Example
 Low frequencies
• May vary between measurements pending magnetization
• Typical “dubbel-dip” response
• B-phase should be below A and C-phase (Y)
Advanced Transformer Testing 2012
Test types – End-to-end short-circuit
 The test is similar to the end-to-end measurement, but with
a winding on the same phase being short-circuited
 The influence of the core is removed below about 10-20
kHz because the low-frequency response is characterized
by the short-circuit impedance/leakage reactance instead of
the magnetizing inductance
 Response at higher frequencies is similar to end-to-end
(open) measurements
Advanced Transformer Testing 2012
End-to-end (short) - Example
 Low frequencies
• All phases should be very similar. > 0.25 dB difference may indicate leakage
reactance/winding resistance/connection/tap-changer problems
Advanced Transformer Testing 2012
Test types – Capacitive inter-winding (IW)
 Test signal is applied to one end of a winding and
the response is measured at one end of another
winding on the same phase (not connected to the
first one)
 The response using this configuration is dominated
at low frequencies by the inter-winding capacitance
Advanced Transformer Testing 2012
Test types – Inductive inter-winding (TA)
 The signal is applied to a terminal on the HV side, and
the response is measured on the corresponding
terminal on the LV side, with the other end of both
windings being grounded
 The low-frequency range of this test is determined by
the winding turns ratio
Advanced Transformer Testing 2012
Inter-winding measurements - Example
 IW (red) is capacitive at low frequencies
 TA (black) reflects turn ratio at low frequencies (135 MVA, 160/16 Dd0)
 Similar response at high frequencies
Advanced Transformer Testing 2012
SFRA Measurement Technique, part 2
- How to achieve high quality results
Advanced Transformer Testing 2012
Test results – always comparisons
Core NOT grounded
Core grounded
Repeatability is of utmost importance!
Advanced Transformer Testing 2012
Example of repeatability
 105 MVA, Single phase Generator Step-up (GSU)
transformer
 SFRA measurements with FRAX 101 before and
after a severe short-circuit in the generator
• Two different test units
• Tests performed by two different persons
• Test performed at different dates
Advanced Transformer Testing 2012
Before (2007-05-23) and after fault (2007-08-29)
LV winding
HV winding
Advanced Transformer Testing 2012
105 MVA, Single phase GSU
 Measurements “before” and “after” were virtually
identical
 Very good correlation between reference and “after
fault”
 Conclusion:
 No indication of mechanical changes in the transformer
 Transformer can safely be put back in service
Advanced Transformer Testing 2012
Potential compromising factors
 Measurement signal connection quality
 Shield grounding practice
 Instrument dynamic range/internal noise
floor
 Understanding core property influence in
lower frequencies in “open” - circuit SFRA
measurements
Advanced Transformer Testing 2012
Bad connection
 Bad connection can affect the curve at higher frequencies
Advanced Transformer Testing 2012
Good connection
 After proper connections were made
Advanced Transformer Testing 2012
FRAX C-Clamp – ensuring connection quality
 C-Clamp ensures good
contact quality
 Penetrates non conductive
layers
 Solid connection to round or
flat busbars/bushings
 Provides strain relief for cable
 Separate connector for single
or multible ground braids
Advanced Transformer Testing 2012
Proper ground connection ensures
repeatability at high frequencies
Good grounding practice;
use shortest braid from cable
shield to bushing flange.
Poor grounding practice
Advanced Transformer Testing 2012
Shield grounding influence
C. Homagk et al, ”Circuit design for reproducible on-site measurements of
transfer function on large power transformers using the SFRA method”, ISH2007
Advanced Transformer Testing 2012
FRAX cable set and grounding
Always the same ground-loop
inductance on a given bushing
Advanced Transformer Testing 2012
Instrument performance
 Transformers have high impedance/large
attenuation at first resonance
 Internal instrument noise is most often the main
limiting source, not substation noise
 Test your instruments noise floor by running a
sweep with “open cables” (Clamps not connected to
transformer)
Advanced Transformer Testing 2012
Internal noise level – ”Noise floor”
”Open”/noise floor measurements
Red = Other brand
Green = FRAX 101
Advanced Transformer Testing 2012
Example of internal noise problem
H1 – H2 (open & short) measurements
Black = Other brand
Red = FRAX 101
Advanced Transformer Testing 2012
Why you need at least -100 dB...
Westinghouse 40 MVA, Dyn1, 115/14 kV, HV [open]
Advanced Transformer Testing 2012
Influence of core
 Try to minimize the effect, however, some
differences are still to be expected and must be
accepted (magnetic viscosity).
 Preferably:
 perform SFRA measurements prior to winding
resistance measurements (or demagnetize the
core prior to SFRA measurements)
 use same measurement voltage in all SFRA
measurements
Advanced Transformer Testing 2012
Run winding resistance test after SFRA!
H1-H2 [open]
After winding resistance test
After
demagnetization
Advanced Transformer Testing 2012
Core magnetization by Doble…
Trace A shows the fingerprint response of the transformer and trace B
shows the response as a result of magnetized core (caused by WRM
measurements)
66
Magnetization status over time
Lachman et al, “Frequency Response
Analysis of Transformers and Influence
of Magnetic Viscosity”, Doble 2012
67
Effect of applied measurement voltage
H1-H0 [open]
0.1 V peak-to-peak
10V peak-to peak
Influence of applied
voltage is more
pronounced on LV
windings
Advanced Transformer Testing 2012
Measurement voltage by Tettex…
69
Measurement voltage effect – in practice
2.8 V
Omicron
10 V
FRAX, Doble and others
Advanced Transformer Testing 2012
70
FRAX101 has adjustable output voltage!
Omicron (2.8 V)
FRAX, 2.8 V
Advanced Transformer Testing 2012
71
Influence of tap changers
 The tap windings in a transformer add in one section at a time
- affecting the low frequency (magnetization impedance)
response and the mid-frequency (winding) response
 Tap lead responses will be seen at higher frequencies than
the tap windings. They are less organized but are still
repeatable
 Some tap-changers have a neutral position which is “more
different” than the difference between consecutive taps. Avoid
using the neutral position as reference measurement
Advanced Transformer Testing 2012
Distribution transformer with 5 HV taps
Tap winding
Low frequency
effect
73
Tap changer measurements by Doble…
Tap leads
Low frequency
effect
Tap winding
74
System integrity test
Field verification unit with known
frequency response is
recommended in CIGRE and
other standards to verify
instrument and cables before
starting the test
Advanced Transformer Testing 2012
Summary
– Measurement quality and repeatability
 The basis of SFRA measurements is comparison and
repeatability/reproducibility is of utmost importance
 To ensure high repeatability;
• Select a high quality, high accuracy instrument with high dynamic
range and input/output impedance matched to the coaxial cables
(e.g. 50 Ohm)
• Make sure to get good signal connection and connect the shields
of coaxial cables to flange of bushing using shortest braid
technique
• Use the same applied voltage in all SFRA measurements
• Be careful about WRM testing and other tests that can magnetize
the core. Perform after SFRA or demagnetize prior to SFRA
• Make good documentation, e.g. make photographs of connections
and note tap settings
Advanced Transformer Testing 2012
SFRA Analysis
Advanced Transformer Testing 2012
Detecting Faults with SFRA
 Winding faults
 Deformation
 Displacement
 Shorts
 Core related faults
 Movements
 Grounding
 Screens
 Mechanical faults/changes
 Clamping structures
 Connections
 And more...
Advanced Transformer Testing 2012
SFRA analysis tools
 Visual/graphical analysis
•
•
•
•
Starting dB values
The expected shape of star and delta configurations
Comparison of fingerprints from;
– The same transformer
– A sister transformer
– Symmetric phases
New/missing resonance frequencies
 Correlation analysis
• DL/T 911 2004 standard
• Customer/transformer specific
Advanced Transformer Testing 2012
Typical response from a healthy transformer
HV [short] identical
between phases
LV [open] as
expected for a ΔY tx
Very low deviation
between phases for
all tests – no winding
defects
HV [open] as expected for
a ΔY tx
”Double dip” and mid
phase response lower
80
Transformer with serious issues...
Large deviations
between phases for
LV [open] at low
frequencies
indicates changes in
the magnetic
circuit/core defects
Large deviations
between phases at mid
and high frequencies
indicates winding faults
81
Transformer with winding shorted turn
 Easiest fault to recognize with SFRA
 Typically produced by a through current fault
 Adjacent turns lose paper and weld together resulting in a
solid loop around the core
 SFRA gives clear and unambiguous diagnosis of a
shorted turn
 SFRA response for the shorted phase may be identified
without reference results since the variation at low
frequencies gives a clear fault signature
Advanced Transformer Testing 2012
Shorted turn (IEEE)
Frequency
Range
20 Hz – 10 kHz
5 kHz – 100 kHz
Winding Turn-to-Turn Short Circuit
Assuming no other failure modes exist:
Open Circuit Tests:
The short circuit failure mode removes the effect of the core‟s reluctance from
the open circuit FRA results. The FRA open circuit trace assumes a similar
behavior as short circuit test. The affected winding will show the greatest
change. This failure mode will also affect the FRA responses from all other
windings, but not as much.
Short Circuit Tests:
The results will not compare well against previous data or amongst phases. The
affected winding is generally offset.
Open Circuit and Short Circuit Tests:
This range can shift or produce new resonance peaks and valleys. The changes
will be greater on the affect phase.
50 kHz – 1 MHz
Open Circuit and Short Circuit Tests:
This range can shift or produce new resonance peaks and valleys. The changes
will be greater on the affect phase.
> 1 MHz
Open Circuit and Short Circuit Tests:
This range can shift or produce new resonance peaks and valleys. The changes
will be greater on the affect phase.
83
Transformer with shorted turn
10
100
1000
10000
100000
1000000
0
-10
Response (dBs)
-20
-30
-40
-50
-60
-70
-80
Frequency (Hz)
HV [open]; B phase (red) should have lower response compared to A and
C phase but has instead higher magnitude/lower impedance
84
Shorted turn by Doble…
•
•
Responses of the HV and LV winding of the same transformer
Significant difference in the white phase due to imbalance in the reluctance
on one of the core limbs (white phase) as a result of shorted turns
85
Shorted turn by IEEE…
Large impedance decrease
at low frequencies in open
circuit test
Impedance decrease at low
frequencies in HV shortcircuit test (only if short is
on HV side)
86
Radial winding deformation – ”Hoop buckling” (IEEE)
Frequency Range
Radial Winding Deformation
Assuming, no other failure modes exist:
20 Hz – 10 kHz
Open Circuit Tests:
This region (core region) is generally unaffected during radial winding deformation.
Short Circuit Tests:
Results in an increase in impedance. The FRA trace for the affected phase
generally exhibits slight attenuation within the inductive roll-off portion.
5 kHz – 100 kHz
Open Circuit and Short Circuit Tests:
The bulk winding range can shift or produce new resonance peaks and valleys
depending of the severity of the deformation. However, this change is minimal and
difficult identify. The changes will be greater on the affect winding, but it is still
possible to have the effects transferred to the opposing winding. The response in
the bulk region should be used as secondary evidence to support the analysis.
50 kHz – 1 MHz
Open Circuit and Short Circuit Tests:
Radial winding deformation is most obvious in this range. It can shift or produce
new resonance peaks and valleys depending of the severity of the deformation.
The changes will be greater on the affect winding, but it is still possible to have the
effects transferred to the opposing winding.
> 1 MHz
Open Circuit and Short Circuit Tests:
This range is generally unaffected in this range. However, severe deformation can
extend into this range.
Advanced Transformer Testing 2012
Radial winding deformation by IEEE...
Resonance changes at
mid- and high
frequencies in open
circuit test
Small but significant
impedance increase at
low frequencies in
short-circuit test
Advanced Transformer Testing 2012
Axial winding deformation – ”Telescoping” (IEEE)
Frequency Range
Axial Winding Deformation
Assuming, no other failure modes exist:
20 Hz – 10 kHz
Open Circuit Tests:
This region (core region) is generally unaffected during axial winding deformation.
Short Circuit Tests:
Results in a change in impedance. The FRA trace for the affected winding causes a
difference between phases or previous results in the inductive roll-off portion.
5 kHz – 100 kHz
Open Circuit and Short Circuit Tests:
Axial winding deformation is most obvious in this range. The bulk winding range
can shift or produce new resonance peaks and valleys depending of the severity of
the deformation. The changes will be greater on the affect winding, but it is still
possible to have the effects transferred to the opposing winding.
Open Circuit and Short Circuit Tests:
Axial winding deformation can shift or produce new resonance peaks and valleys
depending of the severity of the deformation. The changes will be greater on the
affect winding, but it is still possible to have the effects transferred to the opposing
winding.
50 kHz – 1 MHz
> 1 MHz
Open Circuit and Short Circuit Tests:
The response to axial winding deformation is unpredictable.
Advanced Transformer Testing 2012
Axial winding deformation by IEEE...
Resonance changes at
mid- and high
frequencies in open
circuit test
Small but significant
iImpedance increase at
low frequencies in
short-circuit test
Advanced Transformer Testing 2012
Core defects
Core defects failures cause changes to the core‟s
magnetic circuit
 Burnt core laminations
 Shorted core laminations
 Multiple/unintentional core grounds
 Lost core ground and joint dislocations.
Advanced Transformer Testing 2012
Core defects (IEEE)
Frequency
Range
20 Hz – 10 kHz
5 kHz – 100 kHz
Core Defects
Assuming, no other failure modes exist:
Open Circuit Tests:
These types of failures will affect the lower frequency regions generally below
10 kHz. Core defects often change the primary core resonance shape. Less
weight should be placed on shifting, because identifying core defects can
sometimes be masked by the effects of core residual magnetization. If the
open circuit core appears to be loaded, (looking closer to a short circuit
response), this could indicated a core defect.
Short Circuit Tests:
This region is generally unaffected during bulk winding movement. All phases
should be similar.
Open Circuit and Short Circuit Tests:
This range can shift or produce new resonance peaks and valleys.
50 kHz – 1 MHz
Open Circuit and Short Circuit Tests:
Generally this range remains unaffected. However, if the fault is due to a core
ground issue, changes to the CL capacitance can cause resonance shifts in
the upper portion of this range.
> 1 MHz
Open Circuit and Short Circuit Tests:
If the fault is due to a core ground issue, changes to the CL capacitance can
cause resonance shifts.
Advanced Transformer Testing 2012
Core defects – Example
Significant (and
unexpected)
differencies between
phases at low
frequencies in LV
[open] test
No differencies
between phases at high
frequencies – No
winding defetcts...
Advanced Transformer Testing 2012
Core defects by IEEE...
Significant changes in
the magnetic circuit at
first resonance in open
circuit test
Advanced Transformer Testing 2012
SFRA analysis – dB and Impedance
dB-scale
• Magnitude = 20*log(Meas/Ref)
• Phase = Phase (Meas/Ref)
Impedance scale (Admittance Y = 1/Z)
• |Z| = |U/I| = 50*(Ref – Meas)/Mea.
• Phase = Phase (Z)
Advanced Transformer Testing 2012
SFRA standard magnitude response in dB
Advanced Transformer Testing 2012
Magnitude (dB) and Admittance (S)
Second resonance
”decreased” on LV...
Second resonance
looks ”normal” on LV...
Advanced Transformer Testing 2012
Magnitude (dB) and Impedance (Ω)
Low resolution on LV
magnitude
High resolution with LV
impedance
Advanced Transformer Testing 2012
Admittance (S) and Impedance (Ω)
Advanced Transformer Testing 2012
Magnitude response or Impedance/Admittance?
 Magnitude response (dB)
•
•
•
Most established and standardized
Most pubished results are in dB
Common file format e.g. *.xfra supports magnitude
 Impedance (Ω)
•
•
•
More ”engineering”, most power engineers are familiar with transformer
impedance in ohms
Improved resolution for low impedance circuits (< about 100 Ω) e.g. LV windings
on distribution transformers
Impedance representation makes it possible to discriminate between resistive
and inductive parts
 Admittance (S)
•
•
Improved resolution for low impedance circuits (< about 100 Ω) e.g. LV windings
on distribution transformers
Same ”shape” as Magnitude
Advanced Transformer Testing 2012
FRAX
The Features And Benefits
101
FRAX 101 – Frequency Response Analyzer
Advanced Transformer Testing 2012
FRAX101 – Frequency Response Analyzer
Power Input
11-16VDC,
internal battery
(FRAX 101)
USB Port
On all models
Bluetooth
On FRAX101
Rugged Extruded
Aluminum Case
Most feature rich and accurate
SFRA unit in the world!
Generator,
reference and
measure
connectots – All
panel mounted
Active probe
connector on
FRAX101
Advanced Transformer Testing 2012
SFRA test setup
Easy to connect
shortest braid cables
Optional Internal Battery
Over 8h effective run time
Industrial grade class 1
Bluetooth (100m)
USB for redundancy
Advanced Transformer Testing 2012
Search Database Feature
Data files stored in XML format
Index function stores all relevant data in a small database
Search function can list and sort files in different locations
Advanced Transformer Testing 2012
Import formats
Advanced Transformer Testing 2012
Fast testing
Less points where it takes
time to test and where high
frequency resolution is not needed
More points where
higher frequency
resolution is useful
Traditional test
about 2 min
vs.
FRAX fast test
< 40 seconds
Advanced Transformer Testing 2012
Decision support with correlation analysis
Advanced Transformer Testing 2012
Unlimited analysis
 Unlimited graph control
 Lots of available graphs
 Ability to create custom
calculation models using any
mathematic formula and the
measured data from all
channels
 Turn on and off as needed
 Compare real data with
calculated model data
Advanced Transformer Testing 2012
FRAX150
As FRAX-101 except:
 Internal PC/stand-alone
 No internal battery option
 No Bluetooth
Advanced Transformer Testing 2012
FRAX99
As FRAX 101 except:






No internal battery option
No Bluetooth
Dynamic range > 115 dB
Fixed output voltage
9 m cable set
No active probes
Advanced Transformer Testing 2012
FRAX product summary










Light weight
Rugged
Battery operated (FRAX101)
Wireless communication (FRAX101)
Accuracy & Dynamic Range/Noise floor
Cable Practice
Easy-to-use software
Export & Import of Data
Complies with all SFRA standards and recommendations
Only unit that is compatible with all other SFRA
instruments
Advanced Transformer Testing 2012
Sweep Frequency Response Analysis
Application Examples
Advanced Transformer Testing 2012
Time Based Comparison - Example
 1-phase generator transformer, 400 kV
 SFRA measurements before and after
scheduled maintenance
 Transformer supposed to be in good condition
and ready to be put in service…
Advanced Transformer Testing 2012
Time Based Comparison - Example
”Obvious distorsion” as by DL/T911-2004 standard (missing core ground)
Advanced Transformer Testing 2012
Time Based Comparison – After repair
”Normal” as by DL/T911-2004 standard (core grounding fixed)
Advanced Transformer Testing 2012
Type Based Comparisons (twin-units)
Some parameters for identifying twin-units:








Manufacturer
Factory of production
Original customer/technical specifications
No refurbishments or repair
Same year of production or +/-1 year for large units
Re-order not later than 5 years after reference order
Unit is part of a series order (follow-up of ID numbers)
For multi-unit projects with new design: “reference” transformer should
preferably not be one of the first units produced
Advanced Transformer Testing 2012
Type Based Comparison - Example
 Three 159 MVA, 144 KV single-phase transformers
manufactured 1960 (shell-form)
 Put out of service for maintenance/repair after DGA
indication of high temperatures
 “Identical” units
 SFRA testing and comparing the two transformers
came out OK indicating that there are no
electromechanical changes/problems in the
transformers
 Short tests indicated high resistance in one unit
(confirmed by WRM)
Advanced Transformer Testing 2012
Type Based Comparison – 3x HV [open]
Advanced Transformer Testing 2012
Type Based Comparison – 3x LV [open]
Advanced Transformer Testing 2012
Type Based Comparison – 3x HV [short]
Advanced Transformer Testing 2012
Design Based Comparisons
 Power transformers are frequently designed in multi-limb
assembly. This kind of design can lead to symmetric
electrical circuits
 Mechanical defects in transformer windings usually
generate non-symmetric displacements
 Comparing FRA results of separately tested limbs can be
an appropriate method for mechanical condition
assessment
 Pending transformer type and size, the frequency range
for design-based comparisons is typically limited to about
1 MHz
Advanced Transformer Testing 2012
Design Based Comparison - Example





40 MVA, 114/15 kV, manufactured 2006
Taken out of service to be used as spare
No known faults
No reference FRA measurements from factory
SFRA testing, comparing symmetrical phases
came out OK
 The results can be used as fingerprints for
future diagnostic tests
Advanced Transformer Testing 2012
Designed Based Comparison – HV [open]
Advanced Transformer Testing 2012
Designed Based Comparison – HV [short]
Advanced Transformer Testing 2012
Designed Based Comparison – LV [open]
Advanced Transformer Testing 2012
Design Based Comparison
– After Suspected Fault
 Power transformer, 25MVA, 55/23kV,
manufactured 1985
 By mistake, the transformer was energized
with grounded low voltage side
 After this the transformer was energized again
resulting in tripped CB (Transformer protection
worked!)
 Decision was taken to do diagnostic test
Advanced Transformer Testing 2012
Design Based Comparison
– After Suspected Fault
10
100
1000
10000
100000
0
-10
Response (dBs)
-20
-30
-40
-50
-60
-70
-80
Frequency (Hz)
 HV-0, LV open
 A and C phase OK, large deviation on B-phase (shorted turn?)
Advanced Transformer Testing 2012
1000000
Design Based Comparison
– After Suspected Fault
10
100
1000
10000
100000
1000000
0
Response (dBs)
-10
-20
-30
-40
-50
-60
Frequency (Hz)
 HV-0 (LV shorted)
 A and C phase OK, deviation on B-phase
Advanced Transformer Testing 2012
And how did the mid-leg look like…?
Core limb
Insulation cylinder
LV winding
Advanced Transformer Testing 2012
Réponse et analyse d’un balayage en
fréquence
SFRA for testing filter circuits
(Line traps)
Advanced Transformer Testing 2012
Réponse et analyse d’un balayage en
Typical line trap circuit fréquence
 The filter circuit is an RLC network
Advanced Transformer Testing 2012
Réponse et analyse d’un balayage en
Measurement principle fréquence
Generator signal
Measurement signal
Attenuation, dB
Phase shift, °
V
G (dB)  20 log10  out
 Vin



Advanced Transformer Testing 2012
Réponse et analyse d’un balayage en
fréquence
225 kV line trap
 225kV, 850A, 17mH
 Verification of cut-off
frequency
Advanced Transformer Testing 2012
Réponse et analyse d’un balayage en
No capacitors connectedfréquence
0
-10
-20
Magnitude (dB)
-30
-40
-50
-60
-70
-80
100
1k
10 k
100 k
1M
Frequency (Hz)
[A-a1 [open]]
Advanced Transformer Testing 2012
Réponse et analyse d’un balayage en
One capacitor connected
fréquence
0
-5
-10
Magnitude (dB)
-15
-20
-25
-30
-35
-40
100
1k
10 k
100 k
1M
Frequency (Hz)
[C-c1 [open] (2)]
Advanced Transformer Testing 2012
Réponse et analyse d’un balayage en
Two capacitors connected
fréquence
0
Magnitude (dB)
-10
-20
-30
-40
-50
100
1k
10 k
100 k
1M
Frequency (Hz)
[C-c1 [open] (4)]
Advanced Transformer Testing 2012
Sweep Frequency Response Analysis
Standards
Advanced Transformer Testing 2012
SFRA Standards and Recommendations
 Frequency Response Analysis on Winding Deformation of Power
Transformers, DL/T 911-2004, The Electric Power Industry Standard of
People‟s Republic of China
 Mechanical-Condition Assessment of Transformer Windings Using
Frequency Response Analysis (FRA), CIGRE report 342, 2008
 IEEE PC57.149™/D4, Draft Guide for the Application and Interpretation
of Frequency Response Analysis for Oil Immersed Transformers, 2011
 IEC 60076-18, Power transformers – Part 18: Measurement of
frequency response, 2011 (for voting)
 Internal standards by transformer manufacturers, e.g. ABB FRA
Standard v.5
Advanced Transformer Testing 2012
SFRA Standards – Short summary
Standard
Dynamic range
Accuracy
Signal cable grounding
Self-test
EPIS PRC DL/T 911
-100 to +20 dB
± 1 dB @ -80 dB
Wire, shortest length to
transformer core grounding
not stated
CIGRE brochure 342
-100 to +20 dB
(measurement
range)
± 1 dB @ -100 dB
Shortest braid principle
Test circuit with a known
response
Shorted leads test
IEEE PC57.149/D9 (draft)
"Sufficient dynamic
range to
accommodate most
transformer test
objects"
"Calibrated to an
acceptable
standard"
Grounded at both ends.
"Precise, repeatable and
documented" procedure
Standard test object with
a known response
IEC 60076-18
-90 to +10 dB
min 6 dB S/N
(-96 to +10 dB)
Three methods described:
Standard test object with
± 0.3 dB @ -40 dB 1. Same as CIGRE (2 MHz)
a known response
± 1 dB @ -80 dB
2. "Old" method (500 kHz)
Shorted/open leads test
3. "Inversed CIGRE" (2 MHz)
ABB FRA Technical Standard
Better than
-100 to +40 dB
(measurement
range)
Condition control of FRA
device, including coaxial
cables, is strongly
recommended
± 1 dB @ -100 dB
Shortest braid principle
Advanced Transformer Testing 2012
Instrumentation
 Frequency range – All major brands are OK
 Dynamic range
 First transformer circuit resonance gives typically a -90 dB
response. Smaller transformers may have a first response at -100
dB or lower
 Note that CIGRE recommends measurement range down to -100
dB. This implies a “dynamic range”/noise floor at about -120 dB.
 Accuracy
 ± 1 dB at -100 dB fulfills all standards.
 All FRAX instruments fulfills all standards for dynamic
range and accuracy!
Advanced Transformer Testing 2012
Why you need at least -100 dB...
Westinghouse 40 MVA, Dyn1, 115/14 kV, HV [open]
Advanced Transformer Testing 2012
Measurement voltage and internal noise
Measurement voltage and internal noise/dynamic range for common SFRA test sets
20.00
Tettex 5310
FRAnalyzer
Doble M54000
Doble M53000
Doble M5200
HP4395A
HP4195A
FRAX-99
-60.00
FRAX-150
-40.00
FRAX-101
-20.00
Doble M51000
0.00
Dynamic range
Measuring voltage p-p
-80.00
-100.00
-120.00
-140.00
Advanced Transformer Testing 2012
Measurement range comparison
-100 dB measurement
(CIGRE standard)
Black – FRAX-101
Red – Other SFRA
Internal noise (open) measurements
Green – FRAX-101
Blue – Other SFRA
Advanced Transformer Testing 2012
Cable grounding practice
 The “shortest wire/braid”-practice is now generally accepted
 All European equipment manufacturers have adapted to
this practice
Recommended grounding practice (CIGRE)
Bad grounding practice (CIGRE)
Advanced Transformer Testing 2012
Instrumentation verification
 Verification of instrument including cables
• Measurement with “open” cables (at clamp) should give a response
close to the noise floor of the instrument (at lower frequencies,
pending cable length)
• Measurement with “shorted” cables (at clamp) should give close to
0 dB response (pending cable length)
• External test device with known response (FTB-101 included in
FRAX standard kit)
 Calibration at recommended interval
• FRAX; Minimum every 3 years, calibration set and SW available
Advanced Transformer Testing 2012
Field Verification Unit
Field verification unit with known
frequency response is
recommended in CIGRE and
other standards to verify
instrument and cables before
starting the test
Advanced Transformer Testing 2012
FRAX - Benchmarking
Advanced Transformer Testing 2012
Measurement voltage and internal noise
Measurement voltage and internal noise/dynamic range for common SFRA test sets
20.00
Tettex 5310
FRAnalyzer
Doble M54000
Doble M53000
Doble M5200
HP4395A
HP4195A
FRAX-99
-60.00
FRAX-150
-40.00
FRAX-101
-20.00
Doble M51000
0.00
Dynamic range
Measuring voltage p-p
-80.00
-100.00
-120.00
-140.00
Advanced Transformer Testing 2012
FRAX 101 has the highest dynamic range, -130 dB!
Westinghouse 40 MVA, Dyn1, 115/14 kV, HV [open]
Advanced Transformer Testing 2012
Internal noise (dynamic range)
Internal noise (open) measurements
Green – FRAX-101
Red – Other SFRA 1
Blue – Other SFRA 2
Advanced Transformer Testing 2012
Measurement range
-100 dB measurement
(CIGRE standard)
Black – FRAX-101
Red – Other SFRA 1
Internal noise (open) measurements
Green – FRAX-101
Blue – Other SFRA 1
Advanced Transformer Testing 2012
Field verification test (FTB101)
Blue = Other brand
Black = FRAX101
Advanced Transformer Testing 2012
Dynamic Range – Comparison (1)
End-to-end open
Green – FRAX-101
Blue – Other SFRA 1
Neutral to capacitive tap
Red – FRAX-101
Black – Other SFRA 1
Advanced Transformer Testing 2012
Dynamic Range – Comparison (2)
H1 – H2 (open) measurements
Red – FRAX-101
Grey – Other SFRA
Advanced Transformer Testing 2012
Dynamic Range
– Measurements at first resonance
Blue – FRAX
Purple – Other SFRA 3
Red – Other SFRA 1
Jiri Velek, “CEPS SFRA Market Research”, October 2006
Advanced Transformer Testing 2012
FRAX - Compatibility
Advanced Transformer Testing 2012
FRAX vs Doble (1)
5 MVA, Dyn, H2-H3 measurement
Blue – Doble
Orange – Frax
Advanced Transformer Testing 2012
158
FRAX vs Doble (2)
YNd, H1-H0 measurement
Blue – Doble
Orange – Frax
Advanced Transformer Testing 2012
159
FRAX vs Tettex and Doble
H1-H0 (short) measurement
Blue – FRAX
Purple – Tettex
Red – Doble
(Doble high frequency
deviation due to different
grounding practice)
Jiri Velek, “CEPS SFRA Market Research”, October 2006
Advanced Transformer Testing 2012
160
Frax-101, 2.8 vs 10 V meas voltage
2.8 V
10 V
Advanced Transformer Testing 2012
161
Frax (2.8V) vs FRAnalyzer
Omicron (2.8 V)
PAX, 2.8 V
Advanced Transformer Testing 2012
162
Summary - conclusions
 SFRA is an established methodology for detecting
electromechanical changes in power transformers
 Collecting reference curves on all mission critical
transformers is an investment!
 Ensure repeatability by selecting good instruments and
using standardized measurement practices
 Select FRAX from Megger, the ultimate Frequency
Response Analyzer!
Advanced Transformer Testing 2012
Additional IEC slides
Advanced Transformer Testing 2012
IEC connection picture
A
B
C
D
50 Ω
Vin
B
C
D
Vout
50 Ω
reference lead
response lead
earth connection
Advanced Transformer Testing 2012
IEC – FRA condition assessment
Some examples of conditions that FRA can be used
to assess are:
 Damage following a through fault or other high
current event (including short-circuit testing),
 Damage following a tap-changer fault,
 Damage during transportation, and
 Damage following a seismic event.
 Damage caused by short-circuit tests
Advanced Transformer Testing 2012
IEC – Test object conditions – Factory and site
 The test object shall be fully assembled as for service
complete with all bushings.
 Liquid or gas filled transformers and reactors shall be filled
with liquid or gas of the same type as in service conditions
 Busbars or other system or test connections shall be removed
and there shall be no connections to the test object other than
those being used for the specific measurement
 If internal current transformers are installed but not connected
to a protection or measurement system, the secondary
terminals shall be shorted and earthed.
 The core and frame to tank connections shall be complete
and the tank shall be connected to earth.
 Measurements should be performed at ambient temperature
Advanced Transformer Testing 2012
IEC – Test object conditions – Site
 The test object shall be disconnected from the associated
electrical system at all winding terminals and made safe for
testing.
 Line, neutral and any tertiary line connections shall be
disconnected but tank earth, auxiliary equipment and current
transformer service connections shall remain connected.
 In the case where two connections to one corner of a delta
winding are brought out, the transformer shall be measured
with the delta closed (see also 4.4.4).
 In instances where it is impossible to connect directly to the
terminal, then the connection details shall be recorded with
the measurement data since the additional bus bars
connected to the terminals may impact on the measurement
results.
Advanced Transformer Testing 2012
IEC – Instrument performance check
1.
2.
3.
Connect the source, reference and response channels of the
instrument together using suitable low loss leads, check that the
measured amplitude ratio is 0 dB  0,3 dB across the whole
frequency range. Connect the source and reference channels
together and leave the response terminal open circuit, check that
the measured amplitude ratio is less than -90 dB across the whole
frequency range.
The performance of the instrument may be checked by measuring
the response of a known test object (test box) and checking that
the measured amplitude ratio matches the expected response of
the test object. The test object shall have a frequency response
that covers the attenuation range -10 dB to -80 dB.
The correct operation of the instrument may be checked using a
performance check procedure provided by the instrument
manufacturer. This performance check procedure shall verify that
the instrument is operating at least over an attenuation range of -10
dB to -80 dB over the whole frequency range.
Advanced Transformer Testing 2012
IEC – Measurement connection check
 Measurement connection and earthing
•
The continuity of the main and earth connections shall be checked at the
instrument end of the coaxial cable before the measurement is made. Poor
connections can cause significant measurement errors, attention must be paid to
the continuity of the main and earth connections. In particular, connections to bolts
or flanges shall be verified to ensure that there is a good connection to the winding
or the test object tank.
 Zero-check measurement
•
•
If specified, a zero-check measurement shall be carried out as an additional
measurement. Before measurements commence, all the measuring leads shall be
connected to one of the highest voltage terminals and earthed using the normal
method. A measurement is then made which will indicate the frequency response
of the measurement circuit alone. The zero check measurement shall also be
repeated on other voltage terminals if specified.
The zero-check measurement can provide useful information as to the highest
frequency that can be relied upon for interpretation of the measurement.
 Repeatability check
•
On completion of the standard measurements the measurement leads and earth
connections shall be disconnected and then the first measurement shall be
repeated and recorded.
Advanced Transformer Testing 2012
IEC – Measurement configuration – with OLTC
 For transformers and reactors with an on-load tap-changer
(OLTC), the standard measurement on the tapped winding
shall be
• on the tap-position with the highest number of effective turns in circuit,
and
• on the tap-position with the tap winding out of circuit.
 Other windings with a fixed number of turns shall be
measured on the tap-position for the highest number of
effective turns in the tap winding.
 Additional measurements may be specified at other tappositions.
 For neutral or change-over positions, the direction of
movement of the tap-changer shall be in the lowering voltage
direction unless otherwise specified. The direction of
movement (raise or lower) shall be recorded.
Advanced Transformer Testing 2012
IEC – Measurement configuration – Auto with OLTC
 For auto-transformers with a line-end tap-changer, the
standard measurements shall be:
• on the series winding with the minimum number of actual turns of the
tap-winding in circuit (the tapping for the highest LV voltage for a linear
potentiometer type tapping arrangement or the change-over position for
a reversing type tapping arrangement, or the tapping for the lowest LV
voltage in a linear separate winding tapping arrangement),
• on the common winding with the maximum number of effective turns of
the tap-winding in circuit (the tapping for the highest LV voltage), and
• on the common winding with the minimum number of actual turns of the
tap-winding in circuit (the tapping for the lowest LV voltage for a linear
potentiometer or separate winding type tapping arrangement or the
change-over position for a reversing type tapping arrangement).
Advanced Transformer Testing 2012
IEC – Measurement configuration – DECT and OLTC
 For transformers with both an OLTC and a de-energised tapchanger (DETC), the DETC shall be in the service position if
specified or otherwise the nominal position for the
measurements at the OLTC positions described in this
Clause.
 For transformers fitted with a DETC, baseline measurements
shall also be made on each position of the DETC with the
OLTC (if fitted) on the position for maximum effective turns.
 It is not recommended that the position of a DETC on a
transformer that has been in service is changed in order to
make a frequency response measurement, the measurement
should be made on the „as found‟ DETC tap position. It is
therefore necessary to make sufficient baseline
measurements to ensure that baseline data is available for
any likely service („as found‟) position of the DETC.
Advanced Transformer Testing 2012
IEC – Frequency range and measurement points
 The lowest frequency measurement shall be at or below
20 Hz.
 The minimum highest frequency measurement for test objects
with highest voltage > 72,5 kV shall be 1 MHz.
 The minimum highest frequency measurement for test objects
with highest voltage of ≤ 72,5 kV shall be 2 MHz.
 Below 100 Hz, measurements shall be made at intervals not
exceeding 10 Hz
 Above 100 Hz, a minimum of 200 measurements
approximately evenly spaced on either a linear or logarithmic
scale shall be made in each decade of frequency.
Advanced Transformer Testing 2012
IEC – Measurement equipment specification (1)
 Dynamic range
• The minimum dynamic range of the measuring instrument shall be
+10 dB to -90 dB of the maximum output signal level of the voltage
source at a minimum signal to noise ratio of 6 dB over the whole
frequency range.
 Amplitude measurement accuracy
• The accuracy of the measurement of the ratio between Vin and Vout
shall be better than  0,3 dB for all ratios between +10 dB and -40 dB
and  1 dB for all ratios between -40 dB and -80 dB over the whole
frequency range.
 Phase measurement accuracy
• The accuracy of the measurement of the phase difference between Vin
and Vout shall be better than  1º at signal ratios between +10 dB and 40 dB, over the whole frequency range.
 Frequency range
• The minimum frequency range shall be 20 Hz to 2 MHz.
Advanced Transformer Testing 2012
IEC – Measurement equipment specification (2)
 Frequency accuracy
• The accuracy of the frequency (as reported in the measurement record) shall be
better than  0,1 % over the whole frequency range.
 Measurement resolution bandwidth
• For measurements below 100 Hz, the maximum measurement resolution
bandwidth (between -3 dB points) shall be 10 Hz; above 100 Hz, it shall be less
than 10 % of the measurement frequency or half the interval between adjacent
measuring frequencies whichever is less.
 Operating temperature range
• The instrument shall operate within the accuracy and other requirements over a
temperature range of 0 to +45 °C.
 Smoothing of recorded data
• The output data recorded to fulfil the requirements of this standard shall not be
smoothed by any method that uses adjacent frequency measurements, but
averaging or other techniques to reduce noise using multiple measurements at a
particular frequency or using measurements within the measurement resolution
bandwidth for the particular measurement frequency are acceptable.
Advanced Transformer Testing 2012
IEC – Measurement records
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Test object identifier
Date
Time
Test object manufacturer
Test object serial number
Measuring equipment
The peak voltage used for the measurement.
Reference terminal
Response terminal
Terminals connected together
Earthed terminals
OLTC tap positions, current and previous
DETC position
Test object temperature
Fluid filled, yes or no.
Comments, free text to be used to state the condition of the test object
Measurement result (the frequency in Hz, the amplitude in dB and the phase in degrees) for
each measurement frequency
Advanced Transformer Testing 2012
IEC – Test records (1)
 Test object data
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Manufacturer
Year of manufacture
Manufacturer‟s serial number
Highest continuous rated power of each winding
Rated voltage for each windings
Short circuit impedance between each pair of windings
Rated frequency
Vector group, winding configuration / arrangement
Number of phases (single or three-phase)
Transformer or reactor type (e.g. GSU, phase shifter, transmission, distribution, furnace,
industrial, railway, shunt, series, etc.)
Transformer configuration (e.g. auto, double wound, buried tertiary, etc.)
Transformer or reactor construction (e.g core form, shell form), number of legs (3 or 5-leg),
winding type, etc.
Load tap-changer (OLTC): number of taps, range and configuration (linear, reversing,
coarse-fine, line-end, neutral-end, etc.)
De-Energized Tap Changer (DETC): number of positions, range, configuration, etc.
Advanced Transformer Testing 2012
IEC – Test records (2)
 Organisation owning the test object
•
•
Test object identification (as given by the owner if any)
Any other information that may influence the result of the measurement
 Location data
•
•
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Location (e.g. site name, test field, harbour, etc.)
Bay identification reference if applicable
Notable surrounding conditions (e.g. live overhead line or energized busbars nearby)
Any other special features
 Measuring equipment data
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•
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•
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Working principle of device (sweep or impulse)
Equipment name and model number
Manufacturer
Equipment serial number
Calibration date
Any other special features of the equipment
 Test organization data
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Company
Operator
Advanced Transformer Testing 2012
IEC – Test records (3)
 Measurement set-up data
• Remanence of the core: was the measurement carried out immediately following
a resistance or switching impulse test, or was it deliberately demagnetised?
• Whether the tank was earthed
• Measurement type (e.g. open circuit, short circuit, etc.)
• Length of braids used to ground the cable shields
• Length of coaxial cables
 Reason for measurement (e.g. routine, retest, troubleshooting,
commissioning new transformer, commissioning used transformer,
protection tripping, recommissioning, acceptance testing, warranty
testing, bushing replacement, OLTC maintenance, fault operation,
etc.)
 Additional information
 Photographs of the test object as measured showing the position of
the bushings and connections
Advanced Transformer Testing 2012
IEC – Measurement lead connction. Method 1
 The central conductor of the coaxial
measurement leads shall be connected
directly to the test object terminal using
the shortest possible length of
unshielded conductor.
 The shortest possible connection
between the screen of the measuring
lead and the flange at the base of the
bushing shall be made using braid. A
specific clamp arrangement or similar is
required to make the earth connection as
short as possible
 In general this method may be expected
to give repeatable measurements up to
2 MHz
A
B
C
D
E
F
G
H
I
J
connection clamp
unshielded length to be made as short as possible
measurement cable shield
central conductor
shortest braid
bushing
earth connection
earth clamp
tank
smallest loop
Advanced Transformer Testing 2012
IEC – Measurement lead connction. Method 2
 Method 2 is identical to method 1
except that the earth connection
from the measurement leads to the
flange at the base of the terminal
bushing may be made using a fixed
length wire or braid, so that the
connection is not the shortest
possible.
 The position of the excess earth
conductor length in relation to the
bushing may affect amplitude (dB)
measurements above 500 kHz and
resonant frequencies above 1 MHz
Advanced Transformer Testing 2012
IEC – Measurement lead connction. Method 3
 In a method 3 connection, the screen of
the coaxial measurement lead is
connected directly to the test object tank
at the base of the bushing and an
unshielded conductor is used to connect
the central conductor to the terminal at
the top of the bushing.
 If a method 3 connection is used for the
response lead connection only then the
results are comparable with method 1.
This connection may be the most
practical option if an external shunt
(measuring impedance) is used
 If a common conductor is used for the
signal and reference connections then
the conductor is included in the
measurement which will therefore differ
from a method 1 measurement
A
B
C
D
E
F
G
connection clamp
shortest braid or wire
measurement cable shield
central conductor
earth clamp
tank
smallest loop
Advanced Transformer Testing 2012
IEC – Frequency response comparison
 In order to interpret a measured frequency response, a comparison
is made between
• The measured response and a previous baseline measurement (time based
comparison)
• With the response measured on a twin transformer, a transformer made to the
same drawings from the same manufacturer (type based comparisons). Careful
attention should be given when using responses from sister transformers
(transformers with the same specification but with possible differences in winding
configuration even from the same manufacturer) for comparison. Improvements
and changes to the transformer design may have been introduced by a
manufacturer over a period of time to outwardly similar units and this may cause
different frequency responses
• For three-phase transformers, comparisons can also be made between the
responses of the individual phases (design based comparisons). When
comparing phases of the same transformer quite significant differences are
considered “normal” and could be due to different internal lead lengths, different
winding inter-connections and different proximities of the phases to the tank and
the other phases
Advanced Transformer Testing 2012
IEC – Comparisons of frequency responses
The comparison of frequency response measurements is used to
identify the possibility of problems in the transformer. Problems are
indicated by the following criteria:
 Changes in the overall shape of the frequency response;
 Changes in the number of resonances (maxima) and antiresonances (minima);
 Shifts in the position of the resonant frequencies.
The confidence in the identification of a problem in the transformer
based on the above criteria will depend on the magnitude of the
change when compared with the level of change to be expected for
the type of comparison being made (baseline, twin, sister or phase).
Advanced Transformer Testing 2012
IEC – Typical frequency response
Influence regions:
A
core
B
interaction between windings
C
winding structure
D
measurement setup and lead (including earthing connection)
Advanced Transformer Testing 2012
IEC – Influence of tertiary delta connections
10
0
-10
Amplitude, dB
-20
-30
-40
-50
-60
-70
delta open
delta closed
-80
-90 1
10
10
2
10
3
4
10
Frequency, Hz
10
5
10
6
Advanced Transformer Testing 2012
IEC – Influence of star neutral connections
0
-5
Amplitude, dB
-10
-15
-20
-25
-30
-35
neutrals open
neutrals joined
-40
-45 1
10
10
2
10
3
4
10
Frequency, Hz
10
5
10
6
Advanced Transformer Testing 2012
IEC – Influence of measurment direction (example)
0
-10
Amplitude, dB
-20
-30
-40
-50
-60
-70
HV to N
N to HV
-80
-90 1
10
10
2
10
3
4
10
Frequency, Hz
10
5
10
6
Advanced Transformer Testing 2012
IEC – Influence of oil
-20
-30
Amplitude, dB
-40
-50
-60
-70
Full oil
Without oil
-80
-90 1
10
10
2
10
3
4
10
Frequency, Hz
10
5
10
6
Advanced Transformer Testing 2012
IEC – Influence of DC injection (magnetization)
10
0
Amplitude, dB
-10
-20
-30
-40
-50
-60
Before DC
After DC
-70
-80 1
10
10
2
10
3
4
10
Frequency, Hz
10
5
10
6
Advanced Transformer Testing 2012
IEC – Influence of bushings
0
Amplitude, dB
-20
-40
-60
-80
oil/SF6/air bushing
oil/SF6 bushing
-100 1
10
10
2
10
3
4
10
Frequency, Hz
10
5
10
Advanced Transformer Testing 2012
6
IEC – Influence of temperature (minor efftect)
-20
Amplitude, dB
-30
-40
-50
-60
32 C
-70
80 C
-80 1
10
10
2
10
3
4
10
Frequency, Hz
10
5
10
6
Advanced Transformer Testing 2012
IEC – Bad measurements…
-40
Amplitude, dB
-60
-80
-100
-120
HV to N (good measurement)
HV to N with bad connection at N
Hv to N with bad connection at HV
-140
-160 1
10
10
2
10
3
4
10
Frequency, Hz
10
5
10
Advanced Transformer Testing 2012
6
Additional IEEE slides
Advanced Transformer Testing 2012
IEEE – When to perform FRA measurements
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Factory short-circuit testing
Installation or relocation
After a significant through-fault event
As part of routine diagnostic measurement protocol
After a transformer alarm (i.e. sudden pressure, gas detector,
Buchholz)
 After a major change in on-line diagnostic condition (i.e. a sudden
increase in combustible gas, etc.)
 After a change in electrical test conditions (i.e. a change in winding
capacitance)
 System Modeling Purposes
Advanced Transformer Testing 2012
IEEE – FRA base line measurement
 Quality assurance
 Required by Customer Specification
 To provide a standard of comparison for future diagnostic
FRA measurements
Advanced Transformer Testing 2012
IEEE – FRA base line measurement
 Quality assurance
 Required by Customer Specification
 To provide a standard of comparison for future diagnostic
FRA measurements
Advanced Transformer Testing 2012
IEEE – FRA diagnostic application
 Verification that no damage occurred during a short
circuit test
 Relocation and commissioning validation
 Post incident verification : lightning, external throughfault, internal short circuit, seismic event etc
 Routine diagnostic purposes
 Condition assessment of older transformers
 Evaluation of used or spare transformers
Advanced Transformer Testing 2012
IEEE – SFRA instrument specification
 Calibrated to an acceptable standard.
 The output power of the excitation source should provide adequate power over
the entire frequency range to allow for consistent measurement of the transfer
function across the frequency range.
 The test set should be capable of measuring sufficient dynamic range, over
the frequency range in order to accommodate most transformer test objects
 The test set should be capable of collecting a minimum of 200 measurements
per decade, either spaced linearly or logarithmically.
 The test system (set and leads) should provide a known and constant
characteristic impedance.
 A three lead system, signal, reference and test, should be used to reduce
effect of leads in the measurement.
 Test leads should be coaxial cables of the same length, within 1 cm, and less
then 30 m (100 ft) long. Shielded test leads should have the ability to be
grounded at either end.
 Both the Magnitude and Phase of the measured transfer function should be
presented.
Advanced Transformer Testing 2012
IEEE – Measurement type; Open circuit
 An open circuit measurement is made from one end of a
winding to another with all other terminals floating. The Open
Circuit test can be applied to both single phase and three phase
transformers. Open Circuit tests generally fall into five winding
categories: High Voltage, Low Voltage, Tertiary, Series, and
Common. The Series and Common categories are applied to
autotransformers.
 Open Circuit tests are primarily influenced by the core
properties at or around the fundamental power frequency. The
Open Circuit tests can be used in conjunction with exciting
current tests in determining failure modes that affect the
magnetic circuit of the transformer.
Advanced Transformer Testing 2012
IEEE – Measurement type; Short circuit
 The short circuit measurement is made from one end of a high
voltage winding to another while the associated low voltage
winding is shorted. For repeatability purposes, it is
recommended that all low voltage windings are shorted on three
phase transformers to create a three phase equivalent short
circuit model. This ensures all three phase are similarly shorted
to give consistent impedance. Any available neutral connections
should not be included in the shorting process.
 The Short Circuit test isolates the winding impedance from the
core effects properties at or around the fundamental power
frequency. The Short Circuit results should produce similar and
comparable diagnostic information as seen in both leakage
reactance and DC winding resistance measurements.
Advanced Transformer Testing 2012
IEEE – Measurement type; Capacitive inter-winding
 The capacitive inter-winding measurement also known as the
inter-winding measurement is performed between two
electrically isolated windings. A Capacitive Inter-Winding
measurement is made from one end of a winding and
measuring the signal through one of the terminals of another
winding, with all other terminals floating. Capacitive InterWinding measurements are capacitive in nature. These
measurements exhibit a high impedance at low frequencies
(<100 Hz); the impedance generally decreases as frequency
increases. This would include, for example, H1 to X1 on a
double wound transformer, or H1 to Y1 on an autotransformer
with a tertiary. Note that H1 to X1 on an autotransformer is not
an Inter-Winding measurement but an open circuit
measurement on the series winding.
Advanced Transformer Testing 2012
IEEE – Measurement type; Inductive inter-winding
 The inductive inter-winding measurement also known as the
transfer voltage measurement is performed between two
windings with one end of each winding grounded. All other
winding terminals not under test should remain floating. The
inductive inter-winding measurement most closely resembles
the turns ratio test properties at or around the fundamental
power frequency.
Advanced Transformer Testing 2012
IEEE – Influence of residual magnetization
 Residual magnetization within the core must be identified, so as not to
be mis-interpreted as an actual fault.
 Residual magnetization is the flux density that remains in the core
steel.
 DC winding resistance testing, switching operations, and geomagnetic
phenomena are sources of residual magnetism.
 Residual magnetization can be identified by the shifting of the low
frequency core resonance (zero) to the right compared to the
demagnetized results.
 Residual magnetization can be removed by demagnetizing the core,
and should be conducted if there is concern about the condition of the
core.
Advanced Transformer Testing 2012
IEEE – Influence of an open circuit
 An open circuit can be caused by connections that come loose or coils
that become burned through due to a catastrophic thermal failure.
 The end result is very high impedances being inserted into the
measurement circuit. It is common that the transfer function will drop
across a wide spectrum.
 For complete open circuits, the results will often be lost in the noise
floor of the measurement.
Advanced Transformer Testing 2012
Réponse et analyse d’un balayage en
SFRA testing basics
fréquence
Generator test voltage
Measured voltage
Phase, °
Gain, dB
V 
G(dB)  20 log10  out 
 Vin 
Advanced Transformer Testing 2012