sfra - theory and method - standards_120911
TRANSCRIPT
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Sweep Frequency Response Analysis
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Transformer Diagnostics
Transformer Diagnostics is about acquiring accuratemeasurement data and other information in order to make
the correct decision about what to do with the actual unit
TTRSFRA
FDS
WRM
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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 todetect a variety of winding faults, core
issues and other electromechanical
faults in one test
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Rponse et analyse dun balayage en
frquenceSFRA mathematics basics
in
out
V
VdBG 10log20)(
Gain, dB Phase,
Generator test voltage Measured voltage
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Sweep Frequency Response Analysis
Standards Summary
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SFRA Standards and Recommendations
Frequency Response Analysis on Winding Deformation of PowerTransformers, DL/T 911-2004, The Electric Power Industry Standard of
Peoples Republic of China
Mechanical-Condition Assessment of Transformer Windings Using
Frequency Response Analysis (FRA), CIGRE report 342, 2008
IEC 60076-18, Power transformersPart 18: Measurement offrequency 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
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SFRA - Theory and method
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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/Vinvaries 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.
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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 - 1990s : Proving trials by utilities and OEMs, the technology cascadesinternationally 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 Peoples Republic of China
2008: CIGRE report 342, Mechanical-Condition Assessment of TransformerWindings Using Frequency Response Analysis (FRA) is published
2012: IEC60076-18 and IEEE PC57.149 are released
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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
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To detect core displacement and windingdeformation due to e.g.
Large electromagnetic forces from fault current
Transformer transportation and relocation
If these faults are not detected they may developinto dielectric or thermal faults which normally
results in the loss of the transformer
Periodic testing is essential!
Why assess the mechanical condition?
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Detecting Faults with SFRA
Winding faults
Deformation Displacement
Shorts
Core related faults
Movements Grounding
Screens
Mechanical faults/changes
Clamping structures Connections
And more...
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SFRA measurement circuitry
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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
SFRAHow does it work (1)
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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.
SFRAHow does it work (2)
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SFRA resultsFrequency regions
Transformer issues can be
detected in different frequencyranges
Low frequencies
Core problems
Shorted/open windings
Bad connections/increased
resistance
Short-circuit impedance
changes
Medium frequencies
Winding deformations
Winding displacement Highfrequencies
Movement of winding and tap
leads
Winding
interaction and
deformation
Winding
and tap
leads
Core + windings
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Frequency regions by IEC and IEEE
101
102
103
104
105
106
107
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
Frequency, Hz
Magnitu
de,
dB
A phase
B phase
C phase
Core
influence
Interaction
between
windings
Winding
structure
influence
Earthing
leads
influence
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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
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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 Hz2 MHz
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Comparative tests
Transformer A
Transformer A Transformer B
Time based
Type based
Design based
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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 ofidentical design)
Requires knowledge about test object/versions
Small deviations are not necessarily indicating a problem
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SFRA Measurement philosophy
New measurement = Reference measurement
Back in Service
New measurement Reference measurement
Further Diagnostics Required
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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!
SFRA t Wh ?
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SFRA measurementsWhen?
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
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Transformer fault detection
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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
Detection of Winding Movement (1)
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Winding Capacitance
Successful only if reference data is available
Limited sensitivity for some failure modes
Excitation Current
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
Detection of Winding Movement (2)
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Comparing diagnostic techniques (CIGRE)
Diagnostic technique Advantages DisadvantagesMagnetizing (exciting) current Requires relatively simple equipment.
Can detect core damage
Not sensitive to winding deformation.
Measurement strongly affected by core
residual magnetism
Impedance (leakage reactance) Traditional method currently specified in
short-circuits test standards.
Reference (nameplate) values are
available
Very small changes can be significant.
Limited sensitivity for some failure modes
(best for radial deformation)
Frequency Response of Stray Losses
(FRSL)
Can be more sensitive than impedance
measurement.
Almost unique to detect short circuits
between parallel strands
Not a standard use in the industry
Winding capacitance Can be more sensitive than impedance
measurements.
Standard equipment available
Limited sensitivity for some failure modes
(best for radial deformation).
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 interpretFrequency Response Analysis Better repeatability than LVI with the
same sensitivity.
Easier to interpret than LVI (frequency
instead of time domain).
Increasing number of users
Standardization of techniques required.
Guide to interpretation required
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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: 1U1N [short]
Leakage reactance/short-circuit impedance as function of frequency (compareIEEE 62 measurements at 50/60 Hz)
FRSL, Frequency Response of Stray Losses (SFRA 20600 Hz) Input Impedance
Measurement of impedance to ground for a certain configuration (Japanesestandard, 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
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SFRA vs Excitaion current
Example; U1 - N1 [open] Excitation current as function of
frequency
Please note that excitation current is
voltage dependent!
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...
SFRA
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SFRA vs short-circuit impedance/leakage reactance
Example; U1N1 [short] Short-circuit impedance/Leakage reactance as a function
of frequency (IEEE50/60 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)
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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 badcontacts, 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 OFTRANSFORMER WINDING DISPLACEMENT
BY THE FREQUENCY RESPONSE OF STRAY
LOSSES (FRSL), CIGRE session, 2000.
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FRSL160 MVA transformer with contact resistance problem
HV [short], Transformer G2-1
HV [short], Transformers G2-1 and 3
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FRA Methods
Sweep Frequency Response Impulse
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Impulse FRA vs. SweepFRA
Impulse FRA
Injects a pulse signal andmeasure 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
Impulse FRA
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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
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Zoom View of impulse vs. SFRA
Impulse instrument sample rate limts
frequency resolution to 2kHz.
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SFRA Measurement Technique, part 1
- Measurement setups
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SFRA test setup
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FRAX measurement circuitry
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Test typesEnd-to-end (open)
Test signal is applied to one end of a winding and thetransmitted 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
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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)
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Test typesEnd-to-end short-circuit
The test is similar to the end-to-end measurement, but witha 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 ofthe magnetizing inductance
Response at higher frequencies is similar to end-to-end
(open) measurements
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End-to-end (short) - Example
Low frequencies
All phases should be very similar. > 0.25 dB difference may indicate leakagereactance/winding resistance/connection/tap-changer problems
T C i i i i di (IW)
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Test typesCapacitive inter-winding (IW)
Test signal is applied to one end of a winding andthe 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 dominatedat low frequencies by the inter-winding capacitance
T t t I d ti i t i di (TA)
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Test typesInductive inter-winding (TA)
The signal is applied to a terminal on the HV side, andthe 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 bythe winding turns ratio
I t i di t E l
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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
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SFRA Measurement Technique, part 2
- How to achieve high quality results
Test results always comparisons
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Test resultsalways comparisons
Repeatability is of utmost importance!
Core NOT grounded
Core grounded
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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
B f (2007 05 23) d ft f lt (2007 08 29)
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Before (2007-05-23) and after fault (2007-08-29)
LV winding
HV winding
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105 MVA, Single phase GSU
Measurements before and after were virtuallyidentical
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
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Potential compromising factors
Measurement signal connection quality
Shield grounding practice
Instrument dynamic range/internal noisefloor
Understanding core property influence inlower frequencies in open - circuit SFRAmeasurements
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Bad connection
Bad connection can affect the curve at higher frequencies
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Good connection
After proper connections were made
FRAX C Cl i ti lit
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FRAX C-Clampensuring connection quality
C-Clamp ensures goodcontact quality
Penetrates non conductive
layers
Solid connection to round orflat busbars/bushings
Provides strain relief for cable
Separate connector for single
or multible ground braids
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Proper ground connection ensures
repeatability at high frequencies
Good grounding practice;use shortest braid from cable
shield to bushing flange.
Poor grounding practice
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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
FRAX bl t d di
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FRAX cable set and grounding
Always the same ground-loop
inductance on a given bushing
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Instrument performance
Transformers have high impedance/largeattenuation at first resonance
Internal instrument noise is most often the main
limiting source, not substation noise
Test your instruments noise floor by running asweep with open cables (Clamps not connected to
transformer)
I t l i l l N i fl
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Internal noise levelNoise floor
Open/noise floor measurements
Red = Other brand
Green = FRAX 101
Example of internal noise problem
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Example of internal noise problem
H1H2 (open & short) measurements
Black = Other brand
Red = FRAX 101
Wh d t l t 100 dB
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Why you need at least -100 dB...
Westinghouse 40 MVA, Dyn1, 115/14 kV, HV [open]
I fl f
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Influence of core
Try to minimize the effect, however, somedifferences are still to be expected and must beaccepted (magnetic viscosity).
Preferably:
perform SFRA measurements prior to winding
resistance measurements (or demagnetize thecore prior to SFRA measurements)
use same measurement voltage in all SFRAmeasurements
f S
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Run winding resistance test after SFRA!
After
demagnetization
H1-H2 [open]
After winding resistance test
Core magnetization by Doble
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Core magnetization by Doble
Trace A shows the fingerprint response of the transformer and trace Bshows the response as a result of magnetized core (caused by WRM
measurements)
Magnetization status over time
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Magnetization status over time
Lachman et al, Frequency ResponseAnalysis of Transformers and Influence
of Magnetic Viscosity, Doble 2012
Effect of applied measurement voltage
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10V peak-to peak
H1-H0 [open]
0.1 V peak-to-peak
Influence of applied
voltage is morepronounced on LV
windings
Effect of applied measurement voltage
Measurement voltage by Tettex
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Measurement voltage by Tettex
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Measurement voltage effectin practice
2.8 V
Omicron
10 V
FRAX, Doble and others
FRAX101 h dj t bl t t lt !
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FRAX101 has adjustable output voltage!
Omicron (2.8 V)
FRAX, 2.8 V
Influence of tap changers
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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 moredifferent than the difference between consecutive taps. Avoid
using the neutral position as reference measurement
Distribution transformer with 5 HV taps
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Distribution transformer with 5 HV taps
Low frequency
effect
Tap winding
Tap changer measurements by Doble
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Tap changer measurements by Doble
Low frequency
effect
Tap winding
Tap leads
System integrity test
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System integrity test
Field verification unit with known
frequency response is
recommended in CIGRE andother standards to verify
instrument and cables before
starting the test
Summary
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Measurement quality and repeatability
The basis of SFRA measurements is comparison andrepeatability/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 shieldsof coaxial cables to flange of bushing using shortest braidtechnique
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
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SFRA Analysis
Detecting Faults with SFRA
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Detecting Faults with SFRA
Winding faults
Deformation Displacement
Shorts
Core related faults
Movements
Grounding
Screens
Mechanical faults/changes
Clamping structures
Connections
And more...
SFRA analysis tools
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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
Typical response from a healthy transformer
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80
y y
HV [open] as expected for
aY tx
Double dip and mid
phase response lower
Very low deviation
between phases for
all testsno winding
defects
HV [short] identicalbetween phases
LV [open] as
expected for aY tx
Transformer with serious issues...
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81
Large deviationsbetween phases at mid
and high frequencies
indicates winding faults
Large deviations
between phases for
LV [open] at low
frequencies
indicates changes in
the magnetic
circuit/core defects
Transformer with winding shorted turn
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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 ashorted turn
SFRA response for the shorted phase may be identified
without reference results since the variation at low
frequencies gives a clear fault signature
Shorted turn (IEEE)
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83
Shorted turn (IEEE)
Frequency
Range
Winding Turn-to-Turn Short Circuit
Assuming no other failure modes exist:
20 Hz10 kHz Open Circuit Tests:
The short circuit failure mode removes the effect of the coresreluctance 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.5 kHz100 kHz 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 kHz1 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.
Transformer with shorted turn
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84
HV [open]; B phase (red) should have lower response compared to A and
C phase but has instead higher magnitude/lower impedance
-80
-70
-60
-50
-40
-30
-20
-10
0
10 100 1000 10000 100000 1000000
Frequency (Hz)
Response(dBs)
Shorted turn by Doble
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85
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
Shorted turn by IEEE
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86
y
Large impedance decrease
at low frequencies in open
circuit test
Impedance decrease at low
frequencies in HV short-
circuit test (only if short is
on HV side)
Radial winding deformationHoop buckling (IEEE)
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g p g ( )
Frequency Range Radial Winding Deformation
Assuming, no other failure modes exist:
20 Hz10 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 kHz100 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 stillpossible 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 kHz1 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.
Radial winding deformation by IEEE...
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Small but significant
impedance increase at
low frequencies inshort-circuit test
Resonance changes at
mid- and high
frequencies in open
circuit test
Axial winding deformationTelescoping (IEEE)
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Frequency Range Axial Winding Deformation
Assuming, no other failure modes exist:
20 Hz10 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 kHz100 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 stillpossible to have the effects transferred to the opposing winding.
50 kHz1 MHz 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.
> 1 MHz Open Circuit and Short Circuit Tests:
The response to axial winding deformation is unpredictable.
Axial winding deformation by IEEE...
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Resonance changes at
mid- and high
frequencies in open
circuit test
Small but significant
iImpedance increase at
low frequencies in
short-circuit test
Core defects
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Core defects failures cause changes to the cores
magnetic circuit
Burnt core laminations
Shorted core laminations
Multiple/unintentional core grounds Lost core ground and joint dislocations.
Core defects (IEEE)
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Frequency
Range
Core Defects
Assuming, no other failure modes exist:
20 Hz10 kHz Open Circuit Tests:
These types of failures will affect the lower frequency regions generally below10 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.
5 kHz100 kHz Open Circuit and Short Circuit Tests:
This range can shift or produce new resonance peaks and valleys.
50 kHz1 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.
Core defectsExample
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Significant (and
unexpected)
differencies between
phases at low
frequencies in LV
[open] test
No differencies
between phases at high
frequenciesNowinding defetcts...
Core defects by IEEE...
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Significant changes in
the magnetic circuit at
first resonance in open
circuit test
SFRA analysisdB and Impedance
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dB-scale
Magnitude = 20*log(Meas/Ref)
Phase = Phase (Meas/Ref)
Impedance scale (Admittance Y = 1/Z)|Z| = |U/I| = 50*(RefMeas)/Mea.
Phase = Phase (Z)
SFRA standard magnitude response in dB
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Magnitude (dB) and Admittance (S)
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Second resonance
looks normal on LV...
Second resonance
decreased on LV...
Magnitude (dB) and Impedance ()
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Low resolution on LV
magnitude
High resolution with LV
impedance
Admittance (S) and Impedance ()
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101
FRAX
The Features And Benefits
FRAX 101Frequency Response Analyzer
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FRAX101Frequency Response Analyzer
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BluetoothOn FRAX101
USB Port
On all models
Power Input
11-16VDC,
internal battery
(FRAX 101)
Rugged Extruded
Aluminum Case
Most feature rich and accurate
SFRA unit in the world!
Generator,
reference and
measure
connectotsAll
panel mounted
Active probe
connector onFRAX101
SFRA test setupEasy to connect
h t t b id bl
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Industrial grade class 1
Bluetooth (100m)
USB for redundancy
Optional Internal Battery
Over 8h effective run time
shortest braid cables
Search Database Feature
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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
Import formats
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Fast testing
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Less points where it takes
time to test and where highfrequency resolution is not needed
More points where
higher frequencyresolution is useful
Traditional test
about 2 min
vs.FRAX fast test
< 40 seconds
Decision support with correlation analysis
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Unlimited analysis
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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
FRAX150
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As FRAX-101 except:
Internal PC/stand-alone
No internal battery option
No Bluetooth
FRAX99
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As FRAX 101 except:
No internal battery option
No Bluetooth
Dynamic range > 115 dB
Fixed output voltage
9 m cable set
No active probes
FRAX product summary
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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 SFRAinstruments
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Sweep Frequency Response Analysis
Application Examples
Time Based Comparison - Example
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1-phase generator transformer, 400 kV
SFRA measurements before and afterscheduled maintenance
Transformer supposed to be in good conditionand ready to be put in service
Time Based Comparison - Example
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Obvious distorsion as by DL/T911-2004 standard (missing core ground)
Time Based ComparisonAfter repair
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Normal as by DL/T911-2004 standard (core grounding fixed)
Type Based Comparisons (twin-units)
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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
Type Based Comparison - Example
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Three 159 MVA, 144 KV single-phase transformersmanufactured 1960 (shell-form)
Put out of service for maintenance/repair after DGAindication of high temperatures
Identical units
SFRA testing and comparing the two transformerscame out OK indicating that there are noelectromechanical changes/problems in thetransformers
Short tests indicated high resistance in one unit(confirmed by WRM)
Type Based Comparison3x HV [open]
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Type Based Comparison3x LV [open]
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Type Based Comparison3x HV [short]
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Design Based Comparisons
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Power transformers are frequently designed in multi-limb
assembly. This kind of design can lead to symmetricelectrical circuits
Mechanical defects in transformer windings usually
generate non-symmetric displacements
Comparing FRA results of separately tested limbs can bean appropriate method for mechanical condition
assessment
Pending transformer type and size, the frequency range
for design-based comparisons is typically limited to about1 MHz
Design Based Comparison - Example
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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 phasescame out OK
The results can be used as fingerprints for
future diagnostic tests
Designed Based ComparisonHV [open]
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Designed Based ComparisonHV [short]
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Designed Based ComparisonLV [open]
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Design Based ComparisonAfter Suspected Fault
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Power transformer, 25MVA, 55/23kV,manufactured 1985
By mistake, the transformer was energizedwith grounded low voltage side
After this the transformer was energized againresulting in tripped CB (Transformer protectionworked!)
Decision was taken to do diagnostic test
Design Based ComparisonAfter Suspected Fault
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HV-0, LV open
A and C phase OK, large deviation on B-phase (shorted turn?)
-80
-70
-60
-50
-40
-30
-20
-10
0
10 100 1000 10000 100000 1000000
Frequency (Hz)
Re
sponse(dBs)
Design Based ComparisonAfter Suspected Fault
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HV-0 (LV shorted) A and C phase OK, deviation on B-phase
-60
-50
-40
-30
-20
-10
0
10 100 1000 10000 100000 1000000
Frequency (Hz)
R
esponse(dBs)
And how did the mid-leg look like?
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Insulation cylinder
Core limb
LV winding
Rponse et analyse dun balayage enfrquenceSFRA for testing filter circuits (Line traps)
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Rponse et analyse dun balayage enfrquenceTypical line trap circuit
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The filter circuit is an RLC network
Rponse et analyse dun balayage enfrquenceMeasurement principle
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in
out
V
VdBG 10log20)(
Attenuation, dBPhase shift,
Generator signal Measurement signal
Rponse et analyse dun balayage enfrquence225 kV line trap
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225kV, 850A, 17mH
Verification of cut-offfrequency
Rponse et analyse dun balayage enfrquenceNo capacitors connected
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0
-10
-20
-30
-40
-50
-60
-70
-80
Mag
nitude(dB)
100 1 k 10 k 100 k 1 MFrequency (Hz)
[A-a1 [open]]
Rponse et analyse dun balayage enfrquenceOne capacitor connected
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0
-5
-10
-15
-20
-25
-30
-35
-40
Magnitude
(dB)
100 1 k 10 k 100 k 1 MFrequency (Hz)
[C-c1 [open] (2)]
Rponse et analyse dun balayage enfrquenceTwo capacitors connected
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0
-10
-20
-30
-40
-50
Ma
gnitude(dB)
100 1 k 10 k 100 k 1 MFrequency (Hz)
[C-c1 [open] (4)]
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Sweep Frequency Response Analysis
Standards
SFRA Standards and Recommendations
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Frequency Response Analysis on Winding Deformation of Power
Transformers, DL/T 911-2004, The Electric Power Industry Standard ofPeoples 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 transformersPart 18: Measurement of
frequency response, 2011 (for voting)
Internal standards by transformer manufacturers, e.g. ABB FRA
Standard v.5
SFRA StandardsShort summary
Standard Dynamic range Accuracy Signal cable grounding Self-test
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y g y g g g
EPIS PRC DL/T 911 -100 to +20 dB 1 dB @ -80 dBWire, shortest length to
transformer core groundingnot 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 testobjects"
"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)
0.3 dB @ -40 dB
1 dB @ -80 dB
Three methods described:
1. Same as CIGRE (2 MHz)
2. "Old" method (500 kHz)
3. "Inversed CIGRE" (2 MHz)
Standard test object with
a known response
Shorted/open leads test
ABB FRA Technical Standard
Better than-100 to +40 dB
(measurement
range)
1 dB @ -100 dB Shortest braid principle
Condition control of FRAdevice, including coaxial
cables, is strongly
recommended
Instrumentation
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Frequency rangeAll major brands are OK
Dynamic range First transformer circuit resonance gives typically a -90 dB
response. Smaller transformers may have a first response at -100dB 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 dynamicrange and accuracy!
Why you need at least -100 dB...
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Westinghouse 40 MVA, Dyn1, 115/14 kV, HV [open]
Measurement voltage and internal noise
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-140.00
-120.00
-100.00
-80.00
-60.00
-40.00
-20.00
0.00
20.00
Dynamic range
Measuring voltage p-p
Measurement voltage and internal noise/dynamic range for common SFRA test sets
FRAX-101
FRAX
-99
DobleM
51000
DobleM
5200
DobleM
53000
FRAna
lyzer
Tettex5310
FRAX
-150
DobleM
54000
HP4195A
HP4395A
Measurement range comparison
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Internal noise (open) measurements
GreenFRAX-101BlueOther SFRA
-100 dB measurement
(CIGRE standard)
BlackFRAX-101
RedOther SFRA
Cable grounding practice
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The shortest wire/braid-practice is now generally accepted
All European equipment manufacturers have adapted tothis practice
Recommended grounding practice (CIGRE) Bad grounding practice (CIGRE)
Instrumentation verification
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Verification of instrument including cables Measurement with open cables (at clamp) should give a responseclose to the noise floor of the instrument (at lower frequencies,pending cable length)
Measurement with shorted cables (at clamp) should give close to0 dB response (pending cable length)
External test device with known response (FTB-101 included inFRAX standard kit)
Calibration at recommended interval FRAX; Minimum every 3 years, calibration set and SW available
Field Verification Unit
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Field verification unit with known
frequency response is
recommended in CIGRE and
other standards to verifyinstrument and cables before
starting the test
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FRAX - Benchmarking
Measurement voltage and internal noise
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-140.00
-120.00
-100.00
-80.00
-60.00
-40.00
-20.00
0.00
20.00
Dynamic range
Measuring voltage p-p
Measurement voltage and internal noise/dynamic range for common SFRA test sets
FRAX-101
FRAX
-99
DobleM
51000
DobleM
5200
DobleM
53000
FRAna
lyzer
Tettex5310
FRAX
-150
DobleM
54000
HP41
95A
HP43
95A
FRAX 101 has the highest dynamic range, -130 dB!
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Westinghouse 40 MVA, Dyn1, 115/14 kV, HV [open]
Internal noise (dynamic range)
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Internal noise (open) measurementsGreenFRAX-101
RedOther SFRA 1
BlueOther SFRA 2
Measurement range
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Internal noise (open) measurements
GreenFRAX-101BlueOther SFRA 1
-100 dB measurement
(CIGRE standard)
BlackFRAX-101
RedOther SFRA 1
Field verification test (FTB101)
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Blue= Other brand
Black = FRAX101
Dynamic RangeComparison (1)
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Neutral to capacitive tap
RedFRAX-101BlackOther SFRA 1
End-to-end openGreenFRAX-101
BlueOther SFRA 1
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Dynamic Range
Measurements at first resonance
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BlueFRAX
PurpleOther SFRA 3
RedOther SFRA 1
Jiri Velek, CEPS SFRA Market Research, October 2006
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FRAX - Compatibility
FRAX vs Doble (1)
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5 MVA, Dyn, H2-H3 measurement
BlueDoble
OrangeFrax
FRAX vs Doble (2)
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YNd, H1-H0 measurement
BlueDoble
OrangeFrax
FRAX vs Tettex and Doble
H1-H0 (short) measurement
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( )
BlueFRAX
PurpleTettex
RedDoble
(Doble high frequency
deviation due to different
grounding practice)
Jiri Velek, CEPS SFRA Market Research, October 2006
Frax-101, 2.8 vs 10 V meas voltage
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2.8 V
10 V
Frax (2.8V) vs FRAnalyzer
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Omicron (2.8 V)
PAX, 2.8 V
Summary - conclusions
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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!
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Additional IEC slides
IEC connection picture
B
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C
B
D
VoutVin
50
50
A
B reference lead
C response lead
D earth connection
IECFRA condition assessment
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Some examples of conditions that FRA can be usedto 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
IECTest object conditionsFactory and site
The test object shall be fully assembled as for service
complete with all bushings
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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 connectedto 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
IECTest object conditionsSite
The test object shall be disconnected from the associated
electrical system at all winding terminals and made safe for
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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.
IECInstrument performance check
1. Connect the source, reference and response channels of the
instrument together using suitable low loss leads, check that the
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g g ,
measured amplitude ratio is 0 dB 0,3 dB across the whole
frequency range. Connect the source and reference channelstogether and leave the response terminal open circuit, check that
the measured amplitude ratio is less than -90 dB across the whole
frequency range.
2. 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 ofthe test object. The test object shall have a frequency response
that covers the attenuation range -10 dB to -80 dB.
3. 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.
IECMeasurement connection check
Measurement connection and earthing The continuity of the main and earth connections shall be checked at the
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y
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 boltsor 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 earthconnections shall be disconnected and then the first measurement shall be
repeated and recorded.
IECMeasurement configurationwith OLTC
For transformers and reactors with an on-load tap-changer
(OLTC), the standard measurement on the tapped winding
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(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 tap-
positions.
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 ofmovement (raise or lower) shall be recorded.
IECMeasurement configurationAuto with OLTC
For auto-transformers with a line-end tap-changer, the
standard measurements shall be:
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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 linearpotentiometer 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 thetap-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).
IECMeasurement configurationDECT and OLTC
For transformers with both an OLTC and a de-energised tap-
changer (DETC), the DETC shall be in the service position if
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changer (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 thisClause.
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 atransformer 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 forany likely service (as found) position of the DETC.
IECFrequency range and measurement points
The lowest frequency measurement shall be at or below
20 Hz.
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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.
IECMeasurement equipment specification (1)
Dynamic range
The minimum dynamic range of the measuring instrument shall be
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y g g
+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 wholefrequency 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 1 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.
IECMeasurement equipment specification (2)
Frequency accuracy
The accuracy of the frequency (as reported in the measurement record) shall be
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y q y ( p )
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 aparticular frequency or using measurements within the measurement resolution
bandwidth for the particular measurement frequency are acceptable.
IECMeasurement records Test object identifier
Date
Time
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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
IECTest records (1) Test object data
Manufacturer
Year of manufacture
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Year of manufacture
Manufacturers 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.
IECTest 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
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Any other information that may influence the result of the measurement
Location data 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
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 Company
Operator
IECTest 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?
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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, warrantytesting, bushing replacement, OLTC maintenance, fault operation,
etc.)
Additional information
Photographs of the test object as measured showing the position of
the bushings and connections
IECMeasurement lead connction. Method 1
The central conductor of the coaxial
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A connection clamp
B unshielded length to be made as short as possible
C measurement cable shield
D central conductor
E shortest braid
F bushing
G earth connection
H earth clamp
I tank
J smallest loop
measurement leads shall be connected
directly to the test object terminal usingthe 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
IECMeasurement lead connction. Method 2
Method 2 is identical to method 1
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e od s de ca o e od
except that the earth connection
from the measurement leads to theflange 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
IECMeasurement lead connction. Method 3
In a method 3 connection, the screen of
the coaxial measurement lead is
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A connection clamp
B shortest braid or wire
C measurement cable shield
D central conductor
E earth clamp
F tank
G smallest loop
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 themeasurement which will therefore differ
from a method 1 measurement
IECFrequency response comparison
In order to interpret a measured frequency response, a comparison
is made between
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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 amanufacturer 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 andthe other phases
IECComparisons of frequency responsesThe comparison of frequency response measurements is used to
identify the possibility of problems in the transformer. Problems are
indicated by the following criteria:
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indicated by the following criteria:
Changes in the overall shape of the frequency response;
Changes in the number of resonances (maxima) and anti-
resonances (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).
IECTypical frequency response
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Influence regions:
A core
B interaction between windings
C winding structureD measurement setup and lead (including earthing connection)
IECInfluence of tertiary delta connections
0
10
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101
102
103
104
105
106
-90
-80
-70
-60
-50
-40
-30
-20-10
0
Frequency, Hz
Amplitude,
dB
delta open
delta closed
IECInfluence of star neutral connections
0
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101
102
103
104
105
106
-45
-40
-35
-30
-25
-20
-15
-10
-5
Frequency, Hz
Am
plitude,
dB
neutrals open
neutrals joined
IECInfluence of measurment direction (example)
10
0
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Adva