SFRA - Theory and Method - Standards_120911
Short Description
sweep frequency response...
Description
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
TTR
SFRA
WRM FDS
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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 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
Vout V in
G (dB) 20 log10
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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 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 , Power2012 transformers – Part 18: Measurement of frequency response,
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/Vin varies over a wide range, it is expressed in decibels (dB). The relative voltage response in dB is calculated as 20 x log (V /V )
Frequency response analysis (FRA)
10
out
in
• 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 - 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
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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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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 or arethermal not detected they may develop into dielectric faults which normally results in the loss of the transformer
Periodic testing is essential!
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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... Advanced Transformer Testing 2012
SFRA measurement circuitry
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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
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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.
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SFRA results – Frequency regions
Transformer issues can be detected in different frequency ranges • “Low” frequencies – Core problems – Shorted/open windings
Winding and tap
– Bad connections/increased resistance – Short-circuit impedance changes
leads Winding interaction and deformation
• “Medium” frequencies – Winding deformations
Core + windings
– Winding displacement
• “High” frequencies – Movement of winding and tap leads
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Frequency regions by IEC and IEEE 10 0 -10 -20
Winding structure influence
Core influence
B d -30 , e d tu -40 i n g a -50 M
-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
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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
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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) DL/T-911 2004
100 Hz
1 MHz 1 kHz
1 MHz
Typical instrument default values are 20 Hz – 2 MHz
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Comparative tests Transformer A
Design based
Time based
Transformer B
Transformer A
Type based
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Comparisons
Time Based
The most reliable test Deviations between curves are easy to detect
Type Based
(Tests performed on the same transformer over time)
(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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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!
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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
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Transformer fault detection
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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
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Detection of Winding Movement (2)
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 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
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Comparing SFRA and other traditional transformer measurements
End-to-End [Open], (Open Circuit Self Admittance] • •
End-to-End [short], (Short Circuit Self Admittance) • •
Example: 1U - 1N [open] Excitation current as function of frequency 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]
Inductive Inter-winding [Transfer Admittance]
• • •
Capacitance as a function of frequency 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
SFRA
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...
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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)
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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 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.
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FRSL – 160 MVA t ransformer 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
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.
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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 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
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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 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
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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
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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
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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
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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
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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
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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
Measurement signal connection quality Shield grounding practice Instrument dynamic range/internal noise floor
Understanding core in lower frequencies in property “open” - influence 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 – 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
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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
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 bynot running a sweep with “open cables” (Clamps 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 internal noise problem
H1 – H2 (open & short) measurements Black = Other brand Red = FRAX 101
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Why you need at least -100 dB...
Westinghouse 40 MVA, Dyn1, 115/14 kV, HV [open]
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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
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Run winding resistance test after SFRA!
H1-H2 [open] After winding resistance test
After demagnetization
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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)
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Magnetization status over time
Lachman et al, “Frequency Response Analysis of Transformers and Influence of Magnetic Viscosity”, Doble 2012
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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
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Measurement voltage by Tettex…
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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
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Distribution transformer with 5 HV taps
Tap winding Low frequency effect
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Tap changer measurements by Doble…
Tap leads
Low frequency effect
Tap winding
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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
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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... 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
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Typical response from a healthy transformer HV [short] identical between phases LV [open] as expected for a ΔY tx
Very lowphases deviation between 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
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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
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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.
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Transformer with shorted turn 10
100
1000
10000
100000
1000000
0 -10 -20 ) s B -30 d ( e s -40 n o p s e -50 R
-60 -70 -80 Freque ncy (Hz)
HV [open]; B phase (red) should have lower response compared to A and C phase but has instead higher magnitude/lower impedance
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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
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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)
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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.
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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
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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.
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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
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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.
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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...
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Core defects by IEEE...
Significant changes in the magnetic circuit at first resonance in open circuit test
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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)
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SFRA standard magnitude response in dB
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Magnitude (dB) and Admittance (S) Second resonance ”decreased” on LV... Second resonance looks ”normal” on LV...
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Magnitude (dB) and Impedance (Ω)
Low resolution on LV magnitude High resolution with LV impedance
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Admittance (S) and Impedance (Ω)
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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
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FRAX The Features And Benefits
101
FRAX 101 – Frequency Response Analyzer
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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
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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 where higher frequency resolution is useful Traditional test about 2 min vs. FRAX fast test < 40 seconds
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Decision support with correlation analysis
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Unlimited analysis
Unlimited graph control
Lots of available graphs
Ability to create custom calculation models using any mathematic formula measured data from and all the channels
Turn on and off as needed
Compare real data with calculated model data
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FRAX150
As FRAX-101 except:
Internal PC/stand-alone
No internal battery option
No Bluetooth
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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 byDL/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]
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Type Based Comparison – 3x LV [open]
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Type Based Comparison – 3x HV [short]
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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 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
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Designed Based Comparison – HV [open]
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Designed Based Comparison – HV [short]
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Designed Based Comparison – LV [open]
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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
Advanced Transformer Testing 2012
Design Based Comparison – After Suspected Fault 10
100
1000
10000
100000
0 -10 -20 ) s B -30 d ( e s -40 n o p s e -50 R
-60 -70 -80 Freque ncy (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
-10
) -20 s B d ( e s -30 n o p e s R-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
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Réponse et analyse d’un balayage en Measurement principle fréquence
Generator signal
Measurement signal
Attenuation, dB
Phase shift, °
G (dB)
V 20 log10 out Vin
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Réponse et analyse d’un balayage en fréquence
225 kV line trap
225kV, 850A, 17mH
Verification of cut-off frequency
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Réponse et analyse d’un balayage en No capacitors connectedfréquence
0
-10
-20
-30 B e
-40
n g a M
-50
-60
-70
-80
100
k
1
k 10 Frequency (Hz)
k
100
M
1
[A-a1 [open]]
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Réponse et analyse d’un balayage en One capacitor connected fréquence
0
-5
-10
-15 B e u -20 n a M
-25
-30
-35
-40
100
k
1
k 10 Frequency (Hz)
k
100
M
1
[C-c1 [open] (2)]
Advanced Transformer Testing 2012
Réponse et analyse d’un balayage en Two capacitors connected fréquence
0
-10
B -20 e u n a M
-30
-40
-50 100
k
1
k 10 Frequency (Hz)
k
100
M
1
[C-c1 [open] (4)]
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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 Guidefor forOil the Application and Interpretation of Frequency Response Analysis 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
"Sufficient dynamic range to IEEE PC57.149/D9 (draft)
accommodate most transformer test objects"
IEC 60076-18
-90 to +10 dB min 6 dB S/N (-96 to +10 dB)
ABB FRA Technical Standard
Better than -100 to +40 dB (measurement range)
"Calibrated to an
Grounded at both ends.
acceptable standard"
"Precise, repeatable and documented" procedure
Standard test object with a known response
± 0.3 dB @ -40 dB ± 1 dB @ -80 dB
Three methods described: Standard test object with 1. Same as CIGRE (2 MHz) a known response 2. "Old" method (500 kHz) Shorted/open leads test 3. "Inversed CIGRE" (2 MHz)
± 1 dB @ -100 dB
Condition control of FRA device, including coaxial cables, is strongly recommended
Shortest braid principle
Advanced Transformer Testing 2012
Instrumentation
Frequency range – All major brands are OK Dynamic range
Accuracy
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. ± 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
0.00
-20.00
1 -40.00
-60.00
0 -1 X A R F
0 5 -1 X A R F
9 -9 X A R F
A 5 9 1 4 P H
A 5 9 3 4 P H
0 0 0 1 5 M e l b o D
0 0 2 5 M e l b o D
0 0 0 3 5 M e l b o D
0 0 0 4 5 M e l b o D
r e l z y a n A R F
0 1 3 5 x te t e T
Dynamic range Measuring voltage p-p
-80.00
-100.00
-120.00
-140.00
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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
0.00
-20.00
1 -40.00
-60.00
0 -1 X A R F
0 5 -1 X A R F
9 -9 X A R F
A 5 9 1 4 P H
A 5 9 3 4 P H
0 0 0 1 5 M e l b o D
0 0 2 5 M e l b o D
0 0 0 3 5 M e l b o D
0 0 0 4 5 M e l b o D
r e l z y a n A R F
0 1 3 5 x te t e T
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
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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
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
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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
Advanced Transformer Testing 2012 160
Frax-101, 2.8 vs 10 V meas voltage
2.8 V
10 V
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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 Vin
B C D
Ω
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.
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.
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 of the 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. 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 shall be connected to one of the highest voltage terminals and earthed usingleads 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
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
• • •
Manufacturer‟s serial number Highest continuous rated power of each winding
• • •
Short circuit impedance between each pair of windings
• •
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.
Year of manufacture
Rated voltage for each windings Rated frequency Vector group, winding configuration / arrangement
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 • •
Location (e.g. site name, test field, harbour, etc.)
• •
Notable surrounding conditions (e.g. live overhead line or energized busbars nearby)
Bay identification reference if applicable Any other special features
Measuring equipment data • • • •
Working principle of device (sweep or impulse)
• •
Calibration date
Equipment name and model number Manufacturer Equipment serial number Any other special features of the equipment
Test organization data •
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 andshall the flange at the base of the bushing 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 the same manufacturer) comparison. and changes even to thefrom transformer design may havefor been introducedImprovements 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 -20
B d-30 , e d u-40 itl p-50 m A
-60 -70
delta open delta cl ose d
-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 -10 B-15 d , e-20 d u litp-25 m A-30
-35 neutrals open neutrals jo ined
-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 -20 B-30 d , e-40 d tu i l -50 p m A-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 -40 B d , e-50 d tu li -60 p m A
-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 -10 B-20 d , e-30 d tu i l -40 p m A-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
-20 B d , e d u t li p m A
-40
-60
-80
oil/SF 6/air bushing oil/SF 6 bushing
-100 1 10
10
2
10
3
4
10 Frequency, Hz
10
5
10
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IEC – Influence of temperature (minor efftect) -20 -30 B-40 d , e d u t -50 lip m A-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 -60 B d -80 , e d tu li -100 p m A
-120
HV to N (good measurement ) HV to N with bad co nnection 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
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 . These measurements exhibit a are highcapacitive impedanceinatnature low frequencies (
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