omicron sfra
TRANSCRIPT
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Sweep Frequency Response
Analysis
Why
it
should
be
in
your
diagnostic toolbox
Keith Hill – Doble Engineering
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SFRA History
Sweep Frequency Response Analysis (SFRA)
has been
around
for
several
years
Once required highly specialized personnel to
perform
and
interpret
the
resultsPowerful and sensitive tool to evaluate the
mechanical integrity
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SFRA History
1960:
Lech
&
Tyminski,
Poland
1966:
L&T
Publish
Results
1969: Smith, UK (ERA testing)
1976:
First
Doble
Paper:
A.G. Richenbacker “Frequency Domain Analysis
of
Responses
from
L.V.I.
Testing
of
Power
Transformers”
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SFRA History
1978: Dick & Erven, Canada, IEEE first
SFRA
approach:
“Transformer
Diagnostic Testing by FRA”
1980’s:
CEGB,
UK
– SFRA
Field
Trials1990’s:Application
Development
1990’s:
LVI
Commercialized2000: SFRA Commercialized (Doble)
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SFRA
Does not evaluate the condition of the
insulationPower Factor and low voltage TTR can be
acceptable
but
can
still
have
problemsMechanical aspects of the unit
Identify
problems
after
a
faultGreat tool to help identify shipping
damage
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When to Perform SFRA
At the factory Benchmark
When shipping
carriers
change
Arrival on site
This is not “over‐kill”
During routine testing if no benchmark
After a fault (benchmark is very helpful)
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Transformer Fire
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Damaged Bushing and Bus
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Tests after the Fire
SFRA tests did not reveal a problem
(compared to
benchmark
signatures)
Damage was caused by a fire in the bus
duct
Transformer returned to service
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SFRA Data
Interpretation can be challenging if not
impossible for
the
novice
or
very
experienced if benchmark data is not
available
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SFRA Data
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SFRA Data
Performing the test not difficult as the
setup is
very
simple
Software and templates are often
supplied by
the
manufacturers
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SFRA Setup
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Various Templates
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Various Templates
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Another Tool in the Toolbox
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Component Characteristics
Three basic components
Resistors Capacitors
Inductors
Each component has a different response being closely elated to their
geometry; both
internal
and
external
to
other components
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Resistor
The resistance of a resistor is not frequency
dependent as the resistance will remain the samewith a change in frequency. There will be a straight
line from a low frequency to a higher frequency
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Inductor
At a low frequency the inductor will act as a shortand the inductance will increase as the frequency
increases.
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Response of an Inductor
0 dB down at low
frequency means it looks
like a dead short
Larger inductances start to
roll off at lower frequencies
Inductive roll off
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Capacitor
A capacitor will be the opposite of an inductor as the
capacitor will increase with an increase in frequency
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Response of a Capacitor
Low frequency
response is like
an open circuit
Capacitive climb
back
Knee point depends
on the size of the
capacitor 0 dB down athigh frequency
like a dead short
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“Real Transformer”
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Parallel RLC Circuit
Constant dBs v. frequency
Dead short at
low frequency
Open circuit at
high frequency
Open circuit at
low frequency
Dead short at
high frequency
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Response of a Parallel RLC Circuit
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Test Setup
Once the nameplate and templates have been
selected the
tester
must
make
the
connections to the apparatus.
Tester should make sure that the transformer
is isolated from all energy sources and should
be aware
that
static
voltage
may
be
present
if
not properly grounded.
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Test Setup
The first resonance frequency will be affected if the transformer is tested with insulating fluid
and
without
insulating).
Removing
oil
lowers capacitances and resonances shift to higher frequencies. (lower resonant with oil)
How the neutral position of the Load Tap
Changer
(LTC)
was
reached
as
there
have
been
cases in which reaching neutral position from the raise or from the lower taps affected the
signatures.
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Test Setup
A magnetized core can affect the signatures
If LTC or DETC are not on neutral the
signatures
can
change
Test
on
16R
if
only
one
test
is
performed
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Test Setup
Shorting of the secondary windings for the short circuit test can be affected if the
secondary windings
are
not
“closed”
It
is
recommended to jumper X1 – X2, X2 – X3 and X3 – X1 thus closing the delta or wye
windings.
Make
sure
that
the
transformer
being
tested
is
grounded
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Test Setup
Make sure that the test leads grounds are
properly terminated
to
a “good”
ground
Select
proper
winding
configuration
template
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TYPICAL
SFRA
SIGNATURES
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WYE ‐ With and Without Oil
With Oil
Lower
resonant
frequencies
Without OilHigher
resonant
frequencies
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Delta – With and Without Oil
With Oil
Without Oil
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We
see
the
same
changes
when
performing power factor tests on
transformers with
and
without
oil
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Power Factor With and Without Oil
ma Cap mA
CapCH+CHL 49.53 13138 35.263 9353
CH 19.69 5223 14.503 3847
CHL 29.82 7909 20.76 5507
CL+CHL 93.85 24894 65.835 17463
CL 64.03 16984 45.144 11974
CHL 29.9 7905 20.759 5506
With
Oil Without
Oil
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Magnetized Core
The same phase was tested on arrival at site.
The
low
frequency
responses
show
offset
due
to core magnetization while the high
frequency responses are unchanged.
Often times a magnetized core can be detected by performing excitation current
tests.
Several manufacturers
have
winding
resistance test sets that have the ability to de‐
magnetize the core.
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Magnetized Core
It is recommended to demagnetize and repeat
the
test
to
confirm
that
the
core
was
magnetized and is the reason for the shift.
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Magnetized Core
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INTERPRETATION
OF
SIGNATURES
i f Si
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Interpretation of Signatures
Most users of power factor equipment are
aware that
the
power
factor
test
is
a reference
test and allows the user to compare results to
past tests and also to tests results of identical
equipment.
If “benchmark”
power
factors
or
watts loss limits have not been established it
can often
be
difficult
to
determine
the
condition of the apparatus being tested
I i f Si
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Interpretation of Signatures
This is true for SFRA test results as the
signatures before
and
after
an
event
can
often help to identify a problem. The
frequency
range
in
which
the
changes
take
place will assist the tester in determining
what component or components have
changed since the benchmark tests were
performed.
Small change in frequency and dB
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Small change in frequency and dB
Open circuit results show ONE phase
with a shift left where it should not
H B kli
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Hoop Buckling
This phase appears to have
symptoms of hoop buckling
Suspect unit shows same
increased impedance (more
dB’s down) for one phase ‐ the
same
one
with
the
shift
left
on
open circuit results
H B kli
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Hoop Buckling
H B kli
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Hoop Buckling
“Transformer was bent ‐ but not broken”
This transformer was returned to service for
several months
until
a replacement
could
be
located. Once the unit was removed from
service the inspection of the windings
revealed winding
defamation
that
is
classified as “hoop buckling”
Frequency Regions
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Frequency Regions
The following four zones are utilized by one
equipment
manufacturer
for
the
analysis
of
SFRA results.
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Frequency Regions
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Frequency Regions
The following four zones are utilized by one equipment manufacturer for the analysis of SFRA
results.
Region 1:
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Frequency Regions
Region 3: 150 ‐ 400kHz: Deformation
within
main
and
tap
windings
Region 4: 400kHz ‐ 2MHz Movement of
main and tap winding leads
Typical Signatures
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Typical Signatures
Short circuit
Open circuit
Is this a Delta or a
Wye
signature?
Wye Wye HV Winding LV OC
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Wye‐Wye_HV Winding‐LV_OC
H2‐H0
middle winding
Wye‐Wye LV Winding‐HV OC
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Wye‐Wye_LV Winding‐HV_OC
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Delta‐Wye HV Winding‐LV OC
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Delta Wye_HV Winding LV_OC
Delta‐Wye LV Winding‐HV OC
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Delta Wye_LV Winding HV_OC
Delta‐Wye HV Winding‐LV SC
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Delta Wye_HV Winding LV_SC
Starts at “0”
Auto Transformer HV‐OC
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Auto Transformer_HV OC
Auto Transformer LV‐OC
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Auto Transformer_LV OC
Auto Transformer Tertiary‐OC
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Auto Transformer_Tertiary OC
Variation with LTC
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Variation with LTC
8 Lower
16 Lower
Variation with DETC
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Tap E
Tap A
Variation with Reversing Switch
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g
One winding
– neutral
from
raise & lower
Effect of Poor Grounding
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g
Bad red
lead
ground
Original
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ANALYSIS
OF
DATA
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TRANSFORMER
1
Normal Response
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p
Why is the Signature Different?
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Shorted Turns
Shorted Turns
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TRANSFORMER
2
Power Factor Results
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Test Eng kV mA
Watts PF
1 High 10 113.75 4.716 0.41
2 High 10 15.782 0.802 0.513 High 10 97.979 3.99 0.41
4 Cal.1 97.968 3.914 0.40
5 Low 2 133.91 4.972 0.37
6 Low 2 35.931 1.046 0.29
7 Low 2 97.975 3.940 0.40
8 Cal.2 2 97.979 3.926 0.40
Excitation Data
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Normally expect the 2 highs to be within 5%
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Low voltage TTR and winding resistance were
considered acceptable
by
the
tester.
Since the excitation is often affected by a
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magnetized
core
you
may
question
placing
this transformer into service.
Would you place this transformer into
service?
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SFRA testing was recommended for this unit due to
the very wide variance in excitation current results
SFRA ‐ HV Open Circuit Tests
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H3 – H2 Appears to be Abnormal
HV Tests with LV Winding Short Circuited
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Signatures appear normal
SFRA ‐LV Open Circuit Tests
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X3 – X0
Appears
to
be
Abnormal
and
is
the
Same
Phase
as
H3
– H2
Shorted Turns
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What Not to Send to Your Customer
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References
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Sweep
Frequency
Response
Analysis
Transformer
Application
Authors:
Charles Sweetser, B.Sc., M.Sc.
Dr. Tony
McGrail,
B.Sc.
M.Sc.,
PhD.
References
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Special thanks
to
various
Doble
client
service
engineers who over the years have provided a wide variety of data and technical expertise for
this presentation.
Doble
Internal
Power
Point
Training
G. Matthew
Kennedy,
Matt
Garber,
Mario
Locarno, Long Pong, Dr. Tony McGrail, Linda Nowak, and Charles Sweetser
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Please Remember to Complete
Your Evaluation
Form
Thank You!