Sweep Frequency Response Analysis

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Sweep FrequencyResponse Analysis
FRAX
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Transformer Diagnostics

• Diagnostics is about collecting reliable
  information to make the correct decision
• Making the correct decisions saves money

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SFRA Basics
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SFRA history (1)

• 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, ERA, UK
• 1976 Frequency Domain Analysis of Responses
  From L.V.I. Testing of Power Transformers, A.G.
  Richenbacher, 43rd Doble Conference
• 1978 Transformer Diagnostic Testing by
  Frequency Response Analysis, E.P. Dick C.C.
  Erven, Ontario Hydro, IEEE Transactions of Power
  Delivery.

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SFRA history (2)

• 1978 FRA test developed at Ontario Hydro, Canada
• 1980s Further research carried out by Central
  Electricity Generating Board in UK
• 1988 - 1990s Proving trials by European
  utilities, the technology cascades
  internationally via CIGRE, EuroDoble 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 Transformer Windings Using
  Frequency Response Analysis (FRA) is published

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Transformer mechanics basics

• A transformer is rated to withstand certain
  mechanical forces.
• However, these forces can easily be exceeded
• during transportation
• short circuits close to the transformer
• Transformers mechanical strength weakens as the
  transformer ages
• Less capability to withstand mechanicalstress
• Greater risk for mechanical problems
• Greater risk for insulation problems

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Detecting Faults with SFRA

• Core movements
• Faulty core grounds
• Winding deformations
• Winding displacements
• Partial winding collapse
• Hoop buckling
• Broken clamping structures
• Shorted turns and open windings
• Etc

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SFRA Fingerprinting
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SFRA testing basics

• Off-line test
• Transformer is a complex RLC filter circuit
• Changes in this circuit can be detected and
  plotted as a response curve when test signals at
  different frequencies are applied over a winding
• Changes can be compared over time, between test
  objects or within test objects
• The method is unique in its ability to detect
  core problems, mechanical winding problems and
  other electrical faults in one test

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Test results always comparisons

• Different problems can be seen in different parts
  of the curve
• Software analysis makes it easy to detect
  deviations
• Low frequencies
• Core problems and shorted/open windings
• Medium frequencies
• Winding deformations
• High frequencies
• Tap connections and other winding connection
  problems

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Comparative tests
Transformer A
Design based
Time based
Transformer A
Transformer B
Type 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 of identical design)
• Requires knowledge about test object/versions
• Small deviations are not necessarily indicating a
  problem

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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!

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SFRA measurements When?

• Manufacturing test
• Commissioning test
• Transport test
• Incident test - after incidents where you suspect
  electromechanical changes
• After transport
• Short-circuit faults
• Catastrophic events
• Earth quakes
• Hurricanes, tornadoes
• Trigger based test transformer alarms
• Vibration
• DGA
• High temperature

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FRA Methods
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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

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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 Measurements
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SFRA test setup
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FRAX measurement circuitry
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Considerations when performing SFRA Testsor
How do I maximize my investment in time and
money when performing SFRA measurements?
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Test results always comparisons

• Reproducibility is of utmost importance!

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Example of reproducible results

• 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

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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 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

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Potential compromising factors

• Connection quality
• Shield grounding practice
• Instrument dynamic range/internal noise floor
• Understanding core property influence in lower
  frequencies in open - circuit SFRA measurements

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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

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FRAX C-Clamp

• C-Clamp ensures good contact quality
• Penetrates non conductive layers
• Solid connection to round or flat busbars
• 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
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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

• Small transformers have higher attenuation at
  first resonance
• Inherent instrument noise is often the main
  limiting source, not necessarily substation
  static
• Test your instruments noise floor by running a
  sweep with open cables (Clamps not connected to
  transformer)

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Internal noise level Noise floor
Open/noise floor measurements Red Other
brand Green FRAX 101
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Example of noise floor problem
H1 H2 (open short) measurements Black Other
brand Red FRAX 101
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Influence of core

• Try to minimize the effect, however, some
  differences are still to be expected and must be
  accepted.
• 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

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Run winding resistance test after SFRA!
After winding resistance test
After demagnetization
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Effect of applied measurement voltage
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FRAX has adjustable output voltage!
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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
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ABB Transformer Diagnostics
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Summary

• The basis of SFRA measurements is comparison and
  reproducibility is of utmost importance
• To ensure high repeatability the following is
  important
• Use of a high quality, high accuracy instrument
  with inputs and output impedance matched to the
  coaxial cables (e.g. 50 Ohm)
• Use same applied voltage in all SFRA measurements
• Make sure to get good connection and connect the
  shields of coaxial cables to flange of bushing
  using shortest braid technique.
• Make good documentation, e.g. make photographs of
  connections.

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FRAX The Features And Benefits
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FRAX 101 Frequency Response Analyzer
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FRAX 101 Frequency Response Analyzer
Power Input 11-16VDC
Not only the smallest, but also the most feature
rich and accurate SFRA unit in the world!
USB Port On all models
Generator Reference Measure Connectors
Bluetooth On FRAX101
Rugged Extruded Aluminum Case
Active Probe Connector on FRAX101
All Connectors Panel Mounted
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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
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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

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Import formats
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Fast testing
Less points where it takes time to test and
where high frequency resolution is not needed
More points wherehigher frequency resolution is
useful
Traditional test about 2 minvs. FRAX fast
test lt 40 seconds
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Decision support
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Unlimited analysis

• Unlimited graph control
• Lots of available graphs
• Ability to create customcalculation models using
  anymathematic formula and themeasured data from
  all channels
• Turn on and off as needed
• Compare real data with calculated model data

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Mathematical modeling
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FRAX 101 transport case
Rugged case 14kg/31lbs incl. Cables
Padded product bay
Cable compartment
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FRAX-150

• As FRAX-101 except
• Internal PC/stand-alone
• No internal battery option
• No Bluetooth

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FRAX-99

• As FRAX 101 except
• No internal battery option
• No Bluetooth
• Dynamic range gt 115 dB
• Fixed output voltage
• 9 m cable set
• No active probes

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Active Probes, extending the application

• Active Impedance Probe
• Measures Transfer functions between two grounded
  connections
• E.g. between winding and tank or bushing flange
• Active Voltage Probe
• Measures objects with higher input impedance than
  50O
• Allows for longer cables

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FRAX product summary

• Light weight
• Rugged
• Battery operated
• Wireless communication
• Accuracy Dynamic Range/Noise floor
• Cable Practice
• Easy-to-use software
• Export Import of Data
• Complies with all SFRA standards and recommend
• Only unit that is compatible with all other SFRA
  instruments

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Sweep Frequency Response AnalysisApplication
Examples
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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

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Time Based Comparison - Example
Obvious distorsion as by DL/T911-2004 standard
(missing core ground)
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Time Based Comparison After repair
Normal as by DL/T911-2004 standard (core
grounding fixed)
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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

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Type Based Comparison - Example

• Two 33/11 kV, 10 MVA, manufactured 1977
• Put out of service for maintenance/repair or
  scrapping
• Identical except for slightly different
  tap-settings (could not be fixed at site due to
  missing tool)
• SFRA testing and comparing the two transformers
  came out OK indicating that there are no
  electromechanical problems in the transformers
  (identical problems highly unlikely)

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Type Based Comparison LV windings

• 33 kV, 3-phase Ynyn transformer (30 years old)
• Normal as by DL/T911-2004 standard

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Type Based Comparison IW tests

• 33 kV, 3-phase Ynyn transformer (30 years old)
• Normal as by DL/T911-2004 standard

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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

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Design Based Comparison - Example

• 132 kV, 60 MVA transformer, manufactured 2006
• New transformer never in service
• 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

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Designed Based Comparison HV windings

• 132 kV, 3-phase YNd1 transformer (new)
• Normal as by DL/T911-2004 standard
• H1-H0 vs H3-H0

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Designed Based Comparison LV windings

• 132 kV, 3-phase YNd1 transformer (new)
• Normal as by DL/T911-2004 standard
• X2-X1 vs X1-X3

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Designed Based Comparison IW test

• 132 kV, 3-phase YNd1 transformer (new)
• Normal as by DL/T911-2004 standard
• H1-X1 vs H3-X3

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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

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Design Based Comparison After Suspected Fault

• HV-0, LV open
• A and C phase OK, large deviation on B-phase
  (shorted turn?)

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Design Based Comparison After Suspected Fault

• HV-0 (LV shorted)
• A and C phase OK, deviation on B-phase

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And how did the mid-leg look like?
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Sweep Frequency Response AnalysisStandards
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SFRA Standards and Recommendations

• Frequency Response Analysis on Winding
  Deformation of Power Transformers, 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
• IEEE PC57.149/D4 Draft Trial-Use Guide for the
  Application and Interpretation of Frequency
  Response Analysis for Oil Immersed Transformers,
  2007 (Draft)
• Internal standards by transformer manufacturers,
  e.g. ABB FRA Standard v.5

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SFRA Standards - Summary
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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!

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Cable grounding practice

• The shortest wire/braid-practice is now
  generally accepted
• All European equipment manufacturers have adapted
  to this practice

Bad grounding practice (CIGRE)
Recommended grounding practice (CIGRE)
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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

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FRAX Field Verification Unit, FTB-101
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FRAX - Benchmarking
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Measurement voltage and internal noise
Measurement voltage and internal noise/dynamic
range for common SFRA test sets
FRAX-150
FRAX-101
FRAX-99
Doble M51000
Doble M5200
Doble M53000
FRAnalyzer
Tettex 5310
Doble M54000
HP4195A
HP4395A
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Highest dynamic range, -130 dB
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Internal noise (dynamic range)
Internal noise (open) measurements Green
FRAX-101 Red Other SFRA 1 Blue Other SFRA 2
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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
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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
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Dynamic Range Comparison (2)
H1 H2 (open) measurements Red FRAX-101 Grey
Other SFRA
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Dynamic Range Measurements at first resonance
Blue FRAX Purple Other SFRA 3 Red Other
SFRA 1
Jiri Velek, CEPS SFRA Market Research, October
2006
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FRAX - Compatibility
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FRAX vs Doble (1)

• 5 MVA, Dyn, H2-H3 measurement

Blue Doble Orange Frax
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FRAX vs Doble (2)

• YNd, H1-H0 measurement

Blue Doble Orange Frax
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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
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Frax-101, 2.8 vs 10 V meas voltage
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Frax (2.8V) vs FRAnalyzer
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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 Pax Diagnostics, the ultimate
  Frequency Response Analyzer!