FEATURE

ADVANCED
TRANSFORMER
DIAGNOSTICS:
SWEEP FREQUENCY
RESPONSE ANALYSIS

BY J OSEP H AG UIRRE, Megger

Transformer diagnostics play a critical role in ensuring the reliability

and longevity of power systems. Understanding the condition of power
transformers is essential for preventing unexpected failures and minimizing
downtime, which can have significant economic and operational impacts.
Sweep frequency response analysis (SFRA) emerges as a powerful tool in
transformer diagnostics, offering valuable insights into the transformer’s
mechanical integrity, electrical characteristics, and performance.
A transformer is designed to withstand or insulation degradation that can be detected
mechanical forces; however, these forces through SFRA testing to avoid energizing a
can easily be exceeded throughout its life. transformer with potential defects that can
Mechanical deviations can occur during escalate into critical failures.
transportation from the factory to the site,
while offloading the transformer to the SFRA involves subjecting transformer
transformer pad, or from short circuits close to windings to a range of frequencies and
the transformer. These events can lead to issues analyzing the resulting frequency response
such as winding deformation, core movement, to identify deviations from the transformer’s

74 • SUMMER 2024 ADVANCED TRANSFORMER DIAGNOSTICS: SWEEP FREQUENCY RESPONSE ANALYSIS

 FEATURE

PHOTO: © STOCK.ADOBE.COM/CONTRIBUTOR/204211723/MARKO

expected signature or fingerprint. These SFRA can detect faults in all types of
signatures serve as a unique fingerprint of transformers, including oil-filled, dry-type,
the transformer’s structural integrity and and gas-insulated, making it a versatile tool
electromagnetic properties. By comparing the for power system diagnostics. SFRA can also
SFRA results against baseline signatures or be used for periodic internal insights into
reference data from similar units, anomalies transformer condition, providing a baseline
indicative of winding deformation, mechanical for comparison and allowing for trend
displacement, and core deformation can be analysis. If a factory report or previous test
detected with exceptional sensitivity and report containing SFRA is unavailable, then
precision. Thus, it is imperative for the SFRA the next SFRA test is considered the baseline
test to be the last test before departure and the for comparison. Any previous deformations of
first test upon arrival — and the results must the mechanical structure will not be apparent
then be compared. until another event takes place and movement
occurs.
As transformers age, they become more
susceptible to mechanical stress, facing a SFRA PRI NCI PLES
greater risk of mechanical and insulation SFRA involves subjecting transformer windings
issues. By detecting these deviations early to a range of frequencies using sinusoidal AC
on, SFRA enables maintenance teams to take voltage with variable frequencies — typically,
proactive measures to address a problem before 10 hertz to 2 megahertz — and measuring the
it escalates into a critical failure. resulting responses.

ADVANCED TRANSFORMER DIAGNOSTICS: SWEEP FREQUENCY RESPONSE ANALYSIS NETAWORLD • 75

FEATURE
The process of conducting SFRA begins with circuit test will only show the response of the
connecting the transformer to specialized test windings. This process is repeated for each
equipment, such as an SFRA analyzer. The winding configuration within the transformer.
equipment is connected to each phase of the The responses obtained are then plotted on a
high-voltage and low-voltage windings with all graph, typically as amplitude versus frequency.
other windings open. By analyzing these frequency response
signatures, characteristic patterns emerge that
The input voltage and the output voltage serve as indicators of the transformer’s health.
are measured at various frequencies and the
ratio of the voltage in/voltage out amplitude Deviations from the expected signature, such
is measured in decibels (dB). The amplitude as shifts in peak magnitude or changes in phase
is calculated for each frequency change, and relationships, can signify mechanical issues like
the results are plotted on a graph. The SFRA winding deformation, core displacement, or
equipment sends these sweeps of frequencies insulation degradation. Expertise is necessary
through the transformer while simultaneously to effectively interpret SFRA results, correlating
measuring the response at each frequency deviations with specific faults and assessing
point. The data plotted, known as a frequency their severity.
response signature, displays the magnitude and
phase of the response at each frequency. The frequency range of 10 Hz to 2 kHz
primarily detects issues such as main core
There are four measurement configurations per deformation, open circuits, shorted turns, and
phase: open circuit, short circuit, capacitive residual magnetism. In the range of 2 kHz to
inter-winding, and inductive inter-winding. 20 kHz, attention is directed towards the bulk
The open-circuit and short-circuit tests are the winding component and shunt impedance.
most recommended tests for SFRA (Figure 1). Frequencies from 20 kHz to 1 MHz focus
Capacitive and inductive inter-winding tests on detecting deformations within the main
are optional and are not typically performed windings, while frequencies from 1 MHz to 10
in North America. The SFRA open-circuit MHz are dedicated to observing the test leads
test will primarily show the response of the and test connections (Figure 2). However, it’s
core and the windings, while an SFRA short- important to note that each transformer will

End-to-End Open Circuit End-to-End Short Circuit

VR VR
OPEN SHORT
VM VM

Figure 1: Two Primary Measurements Per Phase

76 • SUMMER 2024 ADVANCED TRANSFORMER DIAGNOSTICS: SWEEP FREQUENCY RESPONSE ANALYSIS

 FEATURE

-10

-20

-30

Core
-40
Magnitude (dB)

influence
-50

-60
Interaction Winding Leads of
-70
between structure taps and
-80 windings influence grounding
leads
-90
100 1k 10 k 100 k 1M 10M

Figure 2: FRA Trace Showing Transformer Component Influence in the Array of Frequency Regions

yield distinct responses, and the frequency response signatures. Differentiating between
range provided serves as a general reference normal variations and significant deviations can
point. be challenging, particularly for less experienced
technicians.
A P P LY IN G S F R A
I N T R A N S FORM E R Grounding the shield is a crucial factor in
D I AG N O STIC S achieving accurate measurements, and the
Before initiating a full set of tests, it is chosen grounding method significantly
essential to ensure the instrument’s integrity influences test outcomes. The standard
by verifying its calibration and proper approach involves extending the grounding
functioning. When tests are complete, it is cable from the top of the bushing (lead
also advisable to conduct a repeatability check connection) to its flange. IEEE mandates
to validate that the testing procedures haven’t precise, repeatable, and well-documented
impacted the results. When feasible, IEEE Std. grounding techniques, including the selection
C57.149-2012, Guide for the Application and of conductors, routing, and other related
Interpretation of Frequency Response Analysis for aspects.
Oil-Immersed Transformers, suggests performing
a self-check of the instrument using a standard The shield grounding recommendation from
test object with a known response, which the International Council on Large Electric
verifies the instrument and the test leads. Most
test equipment manufacturers supply this field
verification unit. When conducting an SFRA 50Ω
test, keep in mind that the lead connections
have an impact on the results (Figure 3).

SFRA results can be influenced by various 50Ω

factors including test setup, environmental V V 50Ω
conditions, and equipment calibration.
Gen. Ref. Meas.
Inconsistent test conditions can lead to 0.2-20
inaccurate interpretations or false alarms. Vpp
Analyzing SFRA data requires expertise and
experience due to the complexity of frequency Figure 3: An Equivalent Circuit for Measuring Frequency Response

ADVANCED TRANSFORMER DIAGNOSTICS: SWEEP FREQUENCY RESPONSE ANALYSIS NETAWORLD • 77

 FEATURE
connect the shields of coaxial cables to the
flange of the bushing using the shortest-braid
technique (Figure 4). CIGRE also recommends
not using wire for this purpose.

To achieve consistent and reliable

measurements, it’s crucial to carefully plan the
connection of test and ground leads for each
winding and phase measurement, considering
both placement and method of connection. It
Good grounding practice Poor grounding practice is ideal to include photos of the connections in
the test report for replication in future testing.
Figure 4a: Ground Connections to a Transformer Using Shortest-Braid
Technique Once a proper test connection is made and a
field verification unit with known frequencies
10
(Figure 5) has been run, the results are deemed
0
repeatable. The verification unit verifies the test
instrument and the test leads before testing.
-10 An engineer can now analyze and interpret
the results with confidence. By analyzing
-20 the frequency response of a transformer’s
U1/U2 (dB)

windings, the engineer can identify mechanical

-30
displacement. This includes movement of the
-40 windings, core, or other components due to
good grounding external factors such as vibrations or thermal
poor grounding
-50
no grounding
expansion.
-60
102 103 104 105 106 By analyzing the frequency response, SFRA
f (Hz)
can pinpoint the location and extent of the
Figure 4b: Shield Grounding Influence displacement, allowing for targeted repairs
or maintenance to be performed. Note that
0
this AC test is highly sensitive to residual
-20
magnetism. It is recommended to conduct
SFRA after demagnetizing the transformer. Not
-40 doing so can lead to a false interpretation of the
Magnitude (dB)

transformer’s health (Figure 6).

-60

Other pitfalls can affect the results. These are

-80 the questions to ask if the signatures do not
align. Making detailed notes on the test setup
-100
and taking photos is imperative.
100 1k 10 k 100 k 1M
Frequency (Hz)
• Does the transformer have an external
Figure 5: Field Verification Unit with Known Frequencies core ground connection?
• Was the baseline test done with or
without the core ground connected?
Systems (CIGRE) is to use a grounding • Is there a tertiary winding?
extension that is as short as possible, without • If so, is it a broken delta tertiary?
coiling, and made with a flat braid of 20 mm • Was it open, closed, or grounded when
minimum. To ensure a good connection, the measurement was performed?

78 • SUMMER 2024 ADVANCED TRANSFORMER DIAGNOSTICS: SWEEP FREQUENCY RESPONSE ANALYSIS

 FEATURE
-30
SFRA can also be utilized to assess core
-40
deformation in transformers. Core deformation After winding resistance test
can occur due to various factors, including -50

electrical stresses, mechanical stresses, or

Magnitude (dB)
-60
thermal aging. Another important application
of SFRA is its ability to compare current -70

frequency response data against baseline -80

After
signatures or reference data. By regularly demagnetization
-90
conducting SFRA tests and comparing
the results to previous tests, any changes 100 1k 10 k
Frequency (Hz)
100 k 1M

or abnormalities in the transformer can be Figure 6: Before and after demagnetization

detected. This is done by overlaying the new
test result with the baseline signature. When 0
proper technique is used, the transformer
-5
signature will overlap regardless of the SFRA
-10
test equipment manufacturer (Figure 7). Magnitude (dB)
-15

-20
A phase-to-phase comparison must be
performed in all cases. When comparing -25

traces, the engineer is searching for abnormal -30

discrepancies. In Figure 8a, the low-voltage side -35

is open, showing Phase A and Phase C phase -40

alignment. The test is conducted to assess the Figure 7: Results from Three Manufacturers’ Testing Units on the Same
high-voltage windings and core. A significant Transformer
deviation at the lower frequency ranges is
observed in Phase B. -20

-30
The reason to have the low-voltage side non-
shorted (open) is to see the core resonances
Magnitude (dB)

-40
(dips), as seen in Figure 8b. When the test was
rerun with the low-voltage side shorted, Phase -50

A and Phase C were in unison, while there was

-60
still some deviation on Phase B.
-70
Shorting the low-voltage side removes the core 100 1k 10 k 100 k 1M
resonances Figure 8b. When the transformer Frequency (Hz)

was detanked and inspected, results showed the Figure 8a: Comparison before a Fault
Phase B winding suffered a fault resulting in a
0
shorting and hoop buckling event.
-10
Another method is to compare sister assets.
-20
These assets must be true sister units, indicating
Magnitude (dB)

they have nearly identical construction. This is -30

the only way they can be compared. A proven
-40
approach to determine whether the two power
transformers are sister units is to compare serial -50
numbers. They should be close to each other
-60
and typically within the same year. This strategy 100 1k 10 k 100 k 1M
is particularly effective when comparing single- Frequency (Hz)

phase transformers in a three-phase system. Figure 8b: Comparison after a Fault

ADVANCED TRANSFORMER DIAGNOSTICS: SWEEP FREQUENCY RESPONSE ANALYSIS NETAWORLD • 79

FEATURE
B E NE F IT S OF SF RA TESTI NG 60 Hz ± 3%. The test voltage is administered
SFRA provides early warning signs of potential to a low-voltage winding, while the remaining
transformer issues. Maintenance teams can windings are left unconnected. In the case of
then take timely corrective actions to address three-phase transformers, the no-load loss is
these problems before they escalate into more assessed phase-by-phase. This method enables
severe malfunctions. This proactive approach comparison of losses across phases, aiding in
helps extend transformer lifespan, minimize identifying faulty phases and facilitating overall
downtime, and optimize operational efficiency. analysis by allowing result comparison.

SFRA testing is also instrumental in preventing CONCLUSI ON

catastrophic transformer failures. By detecting Sweep frequency response analysis (SFRA)
and addressing faults before they lead to emerges as an indispensable diagnostic technique
serious malfunctions, SFRA helps prevent in transformer testing, offering precise fault
events that could result in significant damage identification, enabling proactive maintenance
to equipment, disruption of power supply, and strategies, and averting catastrophic failures. Its
safety hazards. This preemptive approach not superior sensitivity in detecting subtle changes
only saves costs associated with repairs and in transformer geometry or winding structure
replacements but also ensures the uninterrupted underscores its significance in safeguarding
operation of critical power infrastructure. transformer integrity.

USING S F R A The imperative is clear: Integrate SFRA into

W IT H OT H E R T E STS routine maintenance practices and condition
SFRA can also be used to double-check tests monitoring programs. Adopting this technique
that were previously performed, giving the end is essential to ensure the reliability, longevity,
user and asset owner a sense of confirmation and safety of power transformers. By harnessing
when interpreting the results. If a previous SFRA’s non-invasive, time-efficient, and cost-
mechanical integrity test such as short-circuit effective diagnostic capabilities, organizations
impedance was performed, an engineer or can preemptively detect and address potential
technician can verify the magnitude of 60 Hz transformer issues, thus extending transformer
on the short-circuit SFRA test and compare lifespan, minimizing downtime, and
the results to the short-circuit impedance optimizing operational efficiency.
test. Short-circuit impedance is an AC test
conducted by shorting the secondary side of This preventive approach not only preserves
the transformer and applying voltage to the transformer assets but also fortifies
transformer, then measuring the source current power infrastructure networks, ensuring
that flows through the primary winding of the uninterrupted service for society. Therefore,
transformer at 60 Hz ± 2%. This is also known it is incumbent upon industry stakeholders,
as the leakage reactance test. utilities, maintenance providers, and regulatory
bodies to recognize SFRA’s importance. These
The SFRA test can also be used to evaluate entities must take decisive action to embed
an open-circuit test, commonly referred to as it into maintenance strategies and standard
a no-load loss or excitation loss assessment. practices. Electrical testing upholds the
No-load-loss measurements are typically reliability and safety of power systems for the
conducted during commissioning and post- benefit of society.
repair evaluation of service-aged transformers
to discern inter-turn shorts, core sheet shorts, REFERENCES
and core-ground faults. It is recommended [1] CIGRE Technical Brochure 342:
to carry out the no-load test at 380/220 V. “Mechanical Condition Assessment of
It is acceptable to measure no-load losses Transformer Windings using Frequency
at frequencies close to the rated value of Response Analysis (FRA), April 2008.

80 • SUMMER 2024 ADVANCED TRANSFORMER DIAGNOSTICS: SWEEP FREQUENCY RESPONSE ANALYSIS