Chapter 4
SFRA Testing
Chapter 4
SFRA Testing
4.1 Introduction
SFRA is able to detect a number of fault conditions, both mechanical and electrical.
The main application of SFRA is to detect mechanical faults, for which some are
detectable by SFRA only and some are useful to analyze both with SFRA and other
methods for correlation. Electrical faults are often easy to detect using SFRA, but are
also easily detectable by other methods.
Examples of fault conditions that can be detected by SFRA:
Mechanical faults:
•
Winding deformations (including hoop buckling).
•
Winding movement.
•
Partial collapse of winding
•
Core displacements
•
Broken or loosened winding or clamping structure
Electrical faults:
•
Shorted turns or open circuit winding
•
Bad ground connection of the transformer tank.
SFRA is often the only method that can detect axial movements of a winding. SFRA
does also detect radial movements, which can be detected by leakage reactance tests
(short-circuit impedance test) as well. It is useful to correlate the two methods to
increase the precision of the result. This is also true for other, especially mechanical,
faults. [16]
4.2 SFRA Test Procedure
4.2.1 Preparation: The Transformer to be tested must be completely isolated from
the power system. This requires that all bushings of all windings be connected from
any bus and insulators. This ensures that measurements are not adversely affected by
interference. [46]
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Chapter 4
SFRA Testing
4.2.2 Connections: Generally, an SFRA measurement is made from one terminal on
the transformer (e.g. H1 (or U) or A) to another terminal (e.g. H2 (or V) or N)
It is important to record all relevant information, which includes tap position, oil level
and terminals grounded or shorted.
Where previous test results exist, the best testing procedure is to repeat those tests,
taking note of tap position, shorted or grounded bushings, and any details for specific
tests. [46]
4.2.3 Types of Measurement:
4.2.3.1 Open Circuit
An open circuit measurement is made from one end of a winding to another with all
other terminals floating. For example, for a delta winding, connections would be H1
to H2 ( or U to V) and for a star winding, measurements are taken from HV terminals
to neutral, such as X1 to X0 (or U to N)
4.2.3.2 Short Circuit
A short circuit measurement is made with the same SFRA test lead connections as an
open circuit measurement, but with the difference that another winding is short
circuited. To ensure repeatability, the three voltage terminals on the shorted winding
be shorted together. This would mean, for example, shorting X1 to X2, X2 to X3 and
X3 to X1 (or U to V, V to W and W to U). This ensures that all three phases are
similarly shorted to give a consistent impedance. Any neutral connections available
for the shorted winding should not be included in the shorting process. [46]
4.2.4 Test Templates
The test templates given here require performing open-circuit and short-circuit tests.
A standard set of tests, recorded when a baseline is needed and there is no question of
the transformer’s health, consists of a set of results taken only at extreme tap position.
The LTC be in the extreme raise position. However, if the transformer is being tested
for post-event reasons, such as fault, this should be done in the as-found LTC
position. Note the tap positions on the test report and apply them during the start of
each test.
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Chapter 4
SFRA Testing
The DETC position should not be moved for an SFRA test until all options are
exhausted. For new transformers in the factory, use the nominal DETC position,
unless otherwise specified by the end-user.
Notes
Leads. Table - 4.1 gives the recommended tests with position of the red lead and
black lead clearly identified. Reversing these test leads may provide small variation in
higher frequency response. Therefore, care should be taken for attaching test leads
appropriately.
Grounding. Good grounds are key to good high frequency responses. Make sure that
grounds are not hampered by loose connections, paint, dirt or grease.
LTC Position. Changing LTC will change SFRA response; LTC position should be
recorded.
DETC Position. Transformers in service occasionally have problems due to DETC
movement. DETC position should not be altered for an SFRA test. The exception is in
factory test on a new transformer, where it can be assumed that the DETC is in
operating condition and tests can be performed on nominal tap. [46]
¾ Two-Winding Transformers
Table 4.1 Two Winding Transformers – 9 Tests [46]
3Ø
3Ø
3Ø
3Ø
Test
Delta-
Wye -
Delta-
Wye -
#
Wye
Delta
Delta
Wye
HV open Circuit (OC)
Test 1
H1-H3
H1-H0
H1-H3
H1-H0
All Other Terminals
Test 2
H2-H1
H2-H0
H2-H1
H2-H0
Floating
Test 3
H3-H2
H3-H0
H3-H2
H3-H0
LV open Circuit (OC)
Test 4
X1-X0
X1-X2
X1-X3
X1-X0
All Other Terminals
Test 5
X2-X0
X2-X3
X2-X1
X2-X0
Floating
Test 6
X3-X0
X3-X1
X3-X2
X3-X0
Short Circuit (SC)
Test 7
H1-H3
H1-H0
H1-H3
H1-H0
H1-H2(H0)
High (H) to Low (L)
Test 8
H2-H1
H2-H0
H2-H1
H2-H0
Short
Short [X1-X2-X3]*
Test 9
H3-H2
H3-H0
H3-H2
H3-H0
X1-X2(X0) *
Test Type
41
1Ø
H1-H2(H0)
X1-X2(X0)
Chapter 4
SFRA Testing
*Indicates short-circuit tests where the terminals are shorted together with three sets
of jumpers, to provide symmetry (X1-X2, X2-X3, X3-X1) OR (Y1-Y2, Y2-Y3, Y3Y1). The neutral is not included for 3Ø wye connections but may be included for 1Ø
test connections.
¾ Auto Transformers
Table 4.2 Autotransformer without Tertiary or with Buried Tertiary – 9
Tests [46]
Test Type
Series Winding (OC)
All Other Terminals Floating
Common Winding (OC)
All Other Terminals Floating
Short Circuit (SC)
High (H) to Low (L)
*
Short [X1-X2-X3]
Test #
3Ø
1Ø
Test 1
H1-X1
Test 2
H2-X2
Test 3
H3-X3
Test 4
X1-H0X0
Test 5
X2- H0X0
Test 6
X3- H0X0
Test 7
H1- H0X0
H1- H0X0
Test 8
H2- H0X0
Short
Test 9
H3- H0X0
[X1-H0X0] *
H1-X1
X1-H0X0
*Indicates short-circuit tests where the terminals are shorted together with three sets
of jumpers, to provide symmetry (X1-X2, X2-X3, X3-X1) OR (Y1-Y2, Y2-Y3, Y3Y1). The neutral is not included for 3Ø wye connections but may be included for 1Ø
test connections.
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Chapter 4
SFRA Testing
¾ Three-Winding Transformers
Table 4.3 Three- Winding Transformers, Part 1 – 18 Tests [46]
3Ø
3Ø
3Ø
3Ø
Delta-
Delta -
Delta-
Delta-
Delta-
Delta-
Wye-
Wye -
Delta
Wye
Delta
Wye
Test 1
H1-H3
H1-H3
H1-H3
H1-H3
Test 2
H2-H1
H2-H1
H2-H1
H2-H1
Test 3
H3-H2
H3-H2
H3-H2
H3-H2
Test 4
X1-X3
X1-X3
X1-X0
X1-X0
Test 5
X2-X1
X2-X1
X2-X0
X2-X0
Test 6
X3-X2
X3-X2
X3-X0
X3-X0
Tert open Circuit (OC)
Test 7
Y1-Y3
Y1-Y0
Y1-Y3
Y1-Y0
All Other Terminals Floating
Test 8
Y2-Y1
Y2-Y0
Y2-Y1
Y2-Y0
Test 9
Y3-Y2
Y3-Y0
Y3-Y2
Y3-Y0
Short Circuit (SC)
Test 10
H1-H3
H1-H3
H1-H3
H1-H3
H1-H0
High (H) to Low (L)
Test 11
H2-H1
H2-H1
H2-H1
H2-H1
Short
Short [X1-X2-X3]*
Test 12
H3-H2
H3-H2
H3-H2
H3-H2
[X1-X2]*
Short Circuit (SC)
Test 13
H1-H3
H1-H3
H1-H3
H1-H3
H1-H0
High (H) to Tertiary (T)
Test 14
H2-H1
H2-H1
H2-H1
H2-H1
Short
Short [Y1-Y2-Y3]*
Test 15
H3-H2
H3-H2
H3-H2
H3-H2
[Y1-Y2]*
Short Circuit (SC)
Test 16
X1-X3
X1-X3
X1-X0
X1-X0
X1-X0
Low (L) to Tertiary (T)
Test 17
X2-X1
X2-X1
X2-X0
X2-X0
Short
Short [Y1-Y2-Y3]*
Test 18
X3-X2
X3-X2
X3-X0
X3-X0
[Y1-Y2]*
Test Type
HV open Circuit (OC)
All Other Terminals Floating
LV open Circuit (OC)
All Other Terminals Floating
Test #
1Ø
H1-H2
(H1-H0)
X1-X2
(X1-X0)
Y1-Y2
(Y1-Y0)
*Indicates short-circuit tests where the terminals are shorted together with three sets
of jumpers, to provide symmetry (X1-X2, X2-X3, X3-X1) OR (Y1-Y2, Y2-Y3, Y3Y1). The neutral is not included for 3Ø wye connections but may be included for 1Ø
test connections.
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Chapter 4
SFRA Testing
Table 4.4 Three- Winding Transformers, Part 2 – 18 Tests [46]
Test Type
Test #
3Ø
3Ø
3Ø
3Ø
Wye-
Wye-
Wye-
Wye-
Wye-
Wye-
Wye
Delta
Delta-Wye Delta-Delta
Test 1
H1-H0 H1-H0
H1-H0
H1-H0
Test 2
H2-H0 H2-H0
H2-H0
H2-H0
Test 3
H3-H0 H3-H0
H3-H0
H3-H0
Test 4
X1-X0 X1-X0
X1-X2
X1-X2
Test 5
X2-X0 X2-X0
X2-X3
X2-X3
Test 6
X3-X0 X3-X0
X3-X1
X3-X1
Tert open Circuit (OC)
Test 7
Y1-Y0 Y1-Y2
Y1-Y0
Y1-Y2
All Other Terminals Floating
Test 8
Y2-Y0 Y2-Y3
Y2-Y0
Y2-Y3
Test 9
Y3-Y0 Y3-Y1
Y3-Y0
Y3-Y1
Short Circuit (SC)
Test 10 H1-H0 H1-H0
H1-H0
H1-H0
High (H) to Low (L)
Test 11 H2-H0 H2-H0
H2-H0
H2-H0
Short [X1-X2-X3]*
Test 12 H3-H0 H3-H0
H3-H0
H3-H0
Short Circuit (SC)
Test 13 H1-H0 H1-H0
H1-H0
H1-H0
High (H) to Tertiary (T)
Test 14 H2-H0 H2-H0
H2-H0
H2-H0
Short [Y1-Y2-Y3]*
Test 15 H3-H0 H3-H0
H3-H0
H3-H0
Short Circuit (SC)
Test 16 X1-X0 X1-X0
X1-X2
X1-X2
Low (L) to Tertiary (T)
Test 17 X2-X0 X2-X0
X2-X3
X2-X3
Short [Y1-Y2-Y3]*
Test 18 X3-X0 X3-X0
X3-X1
X3-X1
HV open Circuit (OC)
All Other Terminals Floating
LV open Circuit (OC)
All Other Terminals Floating
*Indicates short-circuit tests where the terminals are shorted together with three sets
of jumpers, to provide symmetry (X1-X2, X2-X3, X3-X1) OR (Y1-Y2, Y2-Y3, Y3Y1). The neutral is not included for 3Ø wye connections but may be included for 1Ø
test connections.
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Chapter 4
SFRA Testing
4.2.5 Diagnostic Significance of Frequency
Diagnostics of frequency ranges are discussed on two levels; per phase open circuit
measurement and short circuit measurement.
4.2.5.1 Per Phase Open Circuit Measurement
As the name implies, per phase measurement targets the individual phase of a
winding. At low frequencies, the influence of capacitance is negligible, and the
winding behaves as an inductor. Therefore, the attenuation and phase shift of the low
frequency sinusoidal signals, passing through the winding, are determined by the
inductive and resistive nature of the network. The inductive characteristics are
determined by the magnetic circuit of the core, and the resistive characteristics are
dominated by the resistance of the output measuring cable.
As the frequency of the input signal increases, the capacitive effects begin to
dominate and the phase angle quickly becomes close to +90 degrees in the region
above 1 KHz. Now, the attenuation and phase shift of the high frequency sinusoidal
signals passing through the windings are determined by inductive and capacitive
nature of the network. However in the high frequency region, the inductive
characteristics are determined by the leakage flux coupling, and capacitive
characteristics are determined by the various capacitance elements associated with
individual turns. The propagation characteristic of the winding becomes complex as a
result of the many resonance frequencies found in the high frequency range. However,
since the winding resonances become less dependent on the magnetic circuit of the
core, the traces of the three phases converge and become quite similar.
As the frequency increases even further over 100 KHz, the sinusoidal signals travel
mostly outside the winding and reflect the other elements found in the transformer –
e.g. leads, support insulation etc. The magnitude and the phase of the transfer function
in that frequency region are influenced by the inductive-capacitive-resistive nature of
these elements.
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Chapter 4
SFRA Testing
Although most of the low frequency magnitude responses exhibit a typical shape,
there are no typical form responses in the high frequency region. These vary greatly
with the design of the unit.
4.2.5.2 Short Circuit Measurement
The aim of this measurement is to allow direct comparison between the three phases
of a three phase transformer where no prior measurements exist.
Taking a measurement on one winding with another winding short circuited removes
the effect of the core at low frequencies. The resulting response is that for a large
inductor with no core. The responses for all three phase should be similar at low
frequencies.
The theory behind the short circuit measurement is given below.
Any two winding transformer can be modeled at low frequencies by a simple T model
as shown in Fig. 4.1.
Fig. 4.1 T Model of Transformer Winding
The impedance of the winding is small, while the impedance of the core to ground is
extremely high. This means that for any input signal, the response is dominated by the
core. By adding a short to the LV side, the effect of the core is removed and the
response is dominated by the windings, which are predominantly inductors at low
frequency with an inductive roll off as frequency rises (Fig.4.2)
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Chapter 4
SFRA Testing
Fig. 4.2 T Model with LV Short
All three phase of a transformer have similar winding inductances, which means their
responses should be similar. [46]
4.3 Test Circuit and Connections
To make an FRA measurement, a voltage (either a sweep frequency or an impulse
signal) is supplied to a transformer terminal with respect to the tank. The voltage
measured at the input terminal is used as the reference for the FRA calculation. A
second parameter (response signal) is usually a voltage taken across the measurement
impedance connected to a second transformer terminal with reference to the tank (it
may also be a current measured at the input terminal or at some other grounded
terminal). The FRA response amplitude is the ratio between the response signal (Vr)
and the source voltage (Vs) as a function of the frequency (generally presented in dB).
The following ‘standard’ method for connecting the terminals and the tank using
extension leads is mainly used:
• The input and reference coaxial cables are tapped together near the top of the
bushing. A ground extension is run along the body of the bushing, down to the flange,
to connect the cables shields to the tank. The same principle applies for the response
cable.
An alternative technique, referred to the ‘reverse’ method given below can also be
used.
• The input and reference coaxial cables are tapped together near the flange of the
bushing. The cables shields are connected to the tank using a short lead. A lead
47
Chapter 4
SFRA Testing
extension is run along the body of the bushing to connect the signal to the bushing
terminal. The same principle applies for the response cable. [15]
The following sections 4.3.1 to 4.3.4 describe the main FRA test types.
4.3.1 End-to-end (Fig. 4.3a, b)
In the ‘end-to-end’ (or ‘end-to-end open’) test, the signal is applied to one end of each
winding in turn, and the transmitted signal is measured at the other end. The
magnetizing impedance of the transformer is the main parameter characterizing the
low-frequency response (below first resonance) using this configuration. This test is
the more commonly used because of its simplicity and the possibility to examine each
winding separately. The end-to-end tests can be made with the source applied on the
phase terminal or on the neutral terminal. In principle, both should give similar results
but the FRA user should specify the test set-up used and keep that information along
with test data since it will influence the results.
4.3.2 End-to-end short-circuit (Fig. 4.3c, d)
This test is similar to the end-to-end measurement above, but with a winding on the
same phase being short-circuited. Such measurements allow the influence of the core
to be removed below about 10-20 kHz because the low-frequency response is
characterized by the leakage inductance instead of the magnetizing inductance. The
response at higher frequencies is similar to the one obtained using end-to-end
measurement.
The short-circuited winding can be floating or grounded. For three-phase
transformers, there are two levels of variations, either per-phase or three-phase shortcircuit. Furthermore, the end-to end short-circuit tests can be made with the source
applied on the phase terminal or on the neutral terminal. This test can be made if there
is an interest in obtaining information related to the leakage impedance at low
frequency, or removing uncertainties related to analysis of the core influence when
residual magnetism is present.
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Chapter 4
SFRA Testing
4.3.3 Capacitive inter-winding (Fig. 4.3e)
The 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). By definition,
this test is not possible between the series and common windings of autotransformers.
The response using this configuration is dominated at low frequencies by the interwinding capacitance.
4.3.4 Inductive inter-winding (Fig. 4.3f)
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. [15]
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Chapter 4
SFRA Testing
Fig. 4.3: FRA test types. [15]
4.4 Test Types
The choice of the list of tests to perform should be adjusted depending on the purpose
of the measurement. To obtain benchmark test values for future comparison or for
general condition assessment, the four main FRA test types presented in section 4.3
should be performed. If the test involves a winding with regulation (tap winding) the
measurements should be performed at maximum tap (with all the windings included
in the measurement). If there are limitations on the time available to perform all the
tests, it is recommended to perform only the end-to-end tests on all windings.
Example:
3-phase transformer (Y-Δ)
End-to-end (6 tests)
• 3 tests on HV side
• 3 tests on LV side
End-to-end short-circuit (3 tests)
• 3 tests on HV side (with corresponding LV shorted)
Capacitive inter-winding (3 tests)
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Chapter 4
SFRA Testing
• 3 tests from HV to LV
Inductive inter-winding (3 tests)
• 3 tests from HV to LV
If the FRA is performed to evaluate the windings before and after laboratory shortcircuits tests, again the number of tests might be an issue in order to minimize the test
time. In this case, it is suggested to make the end-to-end short-circuit measurements
on the HV windings to include the leakage reactance in the low-frequency
measurements, and to perform the end-to-end open test on the LV side. In the
example shown, it would represent six tests.
For specific fault/defect analysis or general-condition assessment, the selection of test
should be made depending on the information already available (known failure modes
of a specific winding design, reference FRA measurement available, cause of the
problem, etc.). [15]
4.5 Measurement connection and checks
4.5.1 Measurement connection and earthing
Poor connections can cause significant measurement errors, attention shall be paid to
the continuity of the main and earth connections. The continuity of the main and earth
connections shall be checked at the instrument end of the coaxial cable before the
measurement is made. 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.
4.5.2 Zero-check measurement
If specified, a zero-check measurement shall be carried out as as additional
measurement. Before measurements commence, all the measuring leads shall be
connected to one of the highest voltage terminals and earthed using 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.
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Chapter 4
SFRA Testing
The zero-check measurement can provide useful information as to the highest
frequency that can be relied upon for interpretation of the measurement. The zerocheck measurement is not a calibration check and no attempt should be made to
remove any deviations seen in the zero-check measurement from the measurement
results.
4.5.3 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.
This check is necessary to evaluate the repeatability and useable diagnostic frequency
range under the specific conditions of the measurement. [27]
4.6 Conclusion
SFRA has been a key tool in the decision to scrap or reenergize a transformer. To get
value from an SFRA test it is necessary to make sure that the measurements are
credible, which requires good test technique and simple procedures; proper training is
thus essential to control the measurement process. However special care must be
taken both in application of the test to acceptable standards and in interpretation of the
tests results to gain value from the tests themselves.
52