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Diagnostic Testing of
Power Transformer using
Sweep Frequency
Response Analysis
Arturo H. Oropeza
Doble Engineering Company
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FRA = “Frequency Response Analysis”
FRA is the measurement of a system to a particular input at various frequencies.
• There is no right or wrong answer, just the response
• You put something in and you get something out
Black
Box Input Response
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• FRA represents a systems response
to a sinusoidal input at varying
frequencies
• The output will be the same frequency
but at different phase and magnitude
• FRA has been around for decades in a
variety of industries
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Audio
• Speaker performance
• Radio
Telecom
• Communication performance
• Satellite Telemetry
• Cell Phones
• TV
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To distinguish the test methodology or equipment
using a SWEEP approach
Other method is the Impulse technique (IFRA)
Impulse results have poor resolution, lack the range
required for a portable field instrument translating
in poor diagnosis and are usually unrepeatable
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For NOW we apply SFRA in
Transformer Condition Assessment (TCA)
Other uses:*
• Wave traps
• Instrument Transformers
• Rotating Machinery
*still studying best application and interpretation for these
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• SFRA is a leading method to analyze the physical winding condition of a transformer
• Will not tell you if the unit is contaminated
• Will not tell you when it will fail
• Provides relative information about winding’s physical condition
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• Dissipation or Power Factor and Capacitance
• Excitation Current
• TTR (Low and High Voltage Doble method)
• Leakage Reactance
• Sweep Frequency Response Analysis (SFRA)
• Dissolved Gas Analysis
These independent diagnostic methods have their place in
ascertaining transformer condition
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– Broad
• Power Factor and Capacitance
• Excitation Current
• TTR (High Voltage and Low Voltage)
• Dissolved Gas Analysis (DGA)
– Narrow
• Sweep Frequency Response Analysis (SFRA)
• Leakage Reactance
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• Transformers are subject to immense stresses
during service – impact, nearby or internal faults,
overloads
• These stresses may cause winding deformation,
winding movement or core damage
• Allowing a transformer to enter service without
detecting these types of problem is risking
catastrophic failure of the transformer
• SFRA aids detection and permits diagnosis in a
timely manner.
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Assess Mechanical Condition of
Transformers (Mechanical Distortions)
Detect Core and Winding Movement
Due to:
– Large Electromagnetic Forces From Fault Currents
– Winding shrinkage causing release of clamping pressure
– Transformer Relocation or Shipping
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What is the Input?...
• A swept sinusoidal that starts at 20 Hz
and ends at 2 MHz
• Voltage is a constant 10 Volts Peak to
Peak
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What is the Output?...
• A graph of frequency vs. dB
• Logarithmical display: commonly referred
to as a “Bode Plot”
• Linear display
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Transformers may be represented by a
network of:
• Resistor
• Inductors
• Capacitors
The combination of these small elements are unique
to a given transformer
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• Impedance of an ideal resistor, capacitor and
inductor Resistance:
Flat response vs. frequency
Inductor:
Increased impedance with
increased frequency; dead
short at low frequency
Capacitor: Reduced impedance with increased frequency; open circuit at low frequency
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Inductive roll
off
Capacitive
climb back
Resonance
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• We get a resonance for an inductor-capacitor
(LC) combination
• Changing L or changing C gives a new
resonance
• L and C are dependent on geometry
• Changing R changes the size of the resonance
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• A transformer may be modeled using
lumped parameters
• A simple model is a parallel RLC
circuit
• This has a predictable response as
we explained in previous slides
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Low frequency
responses follow
expected form
Three phases of one transformer
High frequency responses
show phase-to-phase
variation which is common
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Low frequency
responses are
different to Wye
windings
Three phases of one transformer
High frequency
responses are similar
phase-to-phase; this is
common
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One phase of one transformer
Tested at 9
tap positions Tap winding
responses
show
stepwise
variation
Main
winding
responses
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400kHz - 2MHz:
lead connection
movement
(Windings and/or
LTC/DETC)
2kHz - 20kHz:
winding movement, loose
clamping
20Hz - 2kHz: core
deformation, open
circuits, shorted turns,
residual magnetism
20kHz - 400kHz: Winding
and Tap Changer
deformations
Different frequencies represent different problems
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• Interpretation – adequate training
• Rely on experts when needed – there is no substitute for knowledge and experience
• Build a Library of results
• Correlate results from SFRA and other tests
• Subscribe to Users Groups to share experiences (i.e. Doble SFRA User Group)
• Find Resources (i.e. Doble SFRA Resource Center available to Doble users)
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• Poor Grounding
• Absence of Oil
• Magnetized Core
• Comparing similar units
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One phase has poor ground
connection during measurement
Three single phase transformers
measured for comparison
Otherwise results were OK; it is important
to make good connections
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With Oil -
in white
Without Oil - in blue
One phase of one transformer
measured with and without oil
Removing oil lowers capacitances and
resonances shift to higher frequencies
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Same phase measured
on arrival at site
Results are acceptable - but
need retest after demag for
confirmation
Low frequency
responses show offset
due to core
magnetization
High frequency
responses unchanged
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Two transformers with
successive serial
numbers
High frequency
responses show very
good
correspondence:
indicating very
similar construction
Low frequency
responses show effect of
core magnetization -
variation is identifiable
and acceptable
Results are good
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• FRA is a useful and sensitive tool
• SFRA is a reliable and repeatable method of
making FRA measurements
• Can be used routinely or after an incident
• Use as part of factory QA and/or relocation