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SFRA METHODS
The SFRA is a comparative method, meaning an evaluation of the
transformer condition is done b comparing an actual set of SFRA
results to reference results! Three
methods are commonl used to assess the measured traces"
• Time#based $ current SFRA results %ill be compared to previous
results of the same unit!
• Tpe#based $ SFRA of one transformer %ill be compared to an
e&ual tpe of transformer!
• 'hase comparison $ SFRA results of one phase %ill be compared
to the results of the other phases of the same transformer!
Ho% SFRA %or(s
Transformers consist of multiple comple) net%or(s of
capacitances and resistors that can generate a uni&ue
signature %hen tested at discreet fre&uencies and plotted as
a
curve! The distance bet%een conductors of the transformer forms
a capacitance! An movement of the conductors or %indings %ill
change this capacitance! This capacitance
being a part of comple) * +inductance, R +Resistance and -
+-apacitance net%or(, an change in this capacitance %ill be
reflected in the curve or signature!
The SFRA test is carried out to obtain initial signature of the
transformer s%eep fre&uenc response b in.ecting various
discreet fre&uencies, %hich is then used as reference
for future comparisons! An fault giving rise to change in
%inding movement etc! %ill result in change in capacitance or
inductance thereb affecting the measured curves!
Tests are carried out periodicall or during ma.or e)ternal
events li(e short circuits and results compared against the initial
signature to test for an problems! The basic
functionalit of SFRA supports follo%ing measuring
modes! /oltage transfer function 0o10i +f SFRA test
reveals if the transformers mechanical or electrical integrit has
been
compromised!
'roblems that can be detected
SFRA analsis can detect problems in transformers such as"
• %inding deformation $ a)ial 2 radial, li(e hoop buc(ling,
tilting, spiralling
• displacements bet%een high and lo% voltage %indings
• partial %inding collapse
• shorted or open turns
• fault grounding of core or screens
• core movement
• bro(en clamping structures
• problematic internal connections
http://en.wikipedia.org/wiki/Comparative_methodhttp://en.wikipedia.org/wiki/Capacitancehttp://en.wikipedia.org/wiki/Capacitancehttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Transfer_functionhttp://en.wikipedia.org/wiki/Transfer_functionhttp://en.wikipedia.org/wiki/Transfer_functionhttp://en.wikipedia.org/wiki/Capacitancehttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Transfer_functionhttp://en.wikipedia.org/wiki/Comparative_method
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•
• 0ses
• To obtain initial signature of health transformer for future
comparisons
• 'eriodic chec(s as part of regular maintenance
• 3mmediatel after a ma.or e)ternal event li(e short circuit
• Transportation or relocation of transformer
• Earth&ua(es
• 're#commissioning chec(
•
• SFRA (its 4ith Testing Dept!
• Doble" M56771M5877 S%eep Fre&uenc Response
Anal9er
• Megger " FRA:;;1ER FRA:
• All the Transformer %indings must be disconnected from
an e)ternal connection including neutral earthing!
• The transformer tan( must be properl grounded!
http://en.wikipedia.org/wiki/Short_circuithttp://en.wikipedia.org/wiki/Earthquakeshttp://en.wikipedia.org/wiki/Doblehttp://en.wikipedia.org/wiki/Meggerhttp://en.wikipedia.org/wiki/Short_circuithttp://en.wikipedia.org/wiki/Earthquakeshttp://en.wikipedia.org/wiki/Doblehttp://en.wikipedia.org/wiki/Megger
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• 3f the transformer is e&uipped %ith an On *oad Tap -hanger
+O*T-, the recommendation is to put the O*T- in the position to
include all tap %indings
in circuit +?E)treme Raise? position, and note that this O*T-
position is used! 3f the measurements are done at several positions
note the tap positions!
• -onnect the FRA: instrument to station earth1ground! The
separate earth1ground %ire should be the first connection made and
the last removed!
• -onnect the test lead @=- connectors to the FRA: instrument
>enerator +ello%, Reference +red and Measure +blac(!
• Test leads continuit and integrit should be chec(ed b
performing FRA self#chec( using a standard test ob.ect!This is also
a valuable chec(ing for
FRA test e&uipment!
• Remove paint and or rust from the nuts or bolts used on the
flange for proper measurement 2 earthing connections! -onnect the
red and ello%
+source and reference to the clamp at the top of a bushing and
corresponding ground braid to the base of bushing! -onnect the
blac( +measure lead to
the clamp at the top of the other bushing and the corresponding
ground braid to the base of bushing! Ma(e sure that the braids do
not touch an
conducting part at top of bushing!
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• ACCESORIES
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CONNECTING CLAPS
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PHYSICAL CONNECTIONS OF CLAPS
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• -onnect the A-1D- Adapter to the FRA: and a po%er source 3f
not using @luetooth communication connect the 0S@ cable to the FRA:
and our
*aptop!
• Turn on the FRA:! Start FRA:#soft%are on our '- and establish
communication, ?-onnect? b select -onnect in under menu File or use
the FB#
(e!
• FT@#RE and several others! 0se the
FT@#
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