Proper commissioning and periodic testing of high voltage equipment is vital to the longevity of your valuable assets. Research shows that improper commissioning along with “set-and-forget” mentality has been the leading causes of premature failures. Sweep frequency response analysis (SFRA) is one of the most powerful diagnostic tools for assessing mechanical damage to a transformer winding. Analysis of the results, which are in the form of frequency response traces can, however, be daunting to new users.

1. Transformer
Diagnostics
SWEEP FREQUENCY RESPONSE ANALYSIS

2. Transformer – An Insight

3. Transformer – Winding Structure

4. Transformer Failure Modes
36%
Mechanical
37%…
6%
Environmental
5%
Maintenance
16%
Unknown
Failure Modes
Mechanical
Electrical
Environmental
Maintenance
Unknown
CIGRE

5. Mechanical Failure Mechanism
 Transformers under fault or excessive overload conditions

6. Mechanical Failure Mechanism
 Large electromagnetic forces on windings during fault
conditions or overloads can cause windings to move and
even permanent winding and core deformation
• This may result in insulation damage.
• Turn to turn faults are the most likely
• Insulation damage usually results in a failure of the
transformer which is uneconomical to repair

7. Mechanical Failure Mechanism
CAUSE:
• Electromagnetic Forces due to high current flow (FαI2)
• Mechanical shocks during transportation or relocation
of the unit
EFFECT:
• Insulation damage – Uneconomical to repair
• Compressive Failure of Winding (Hoop Buckling)
• Spiral tightening – De-rates fault duty
• Coil clamping failure
• End insulation collapse

8. Transformer - Design Issues
 Transformers should be designed to withstand through-faults in accordance with
AS-60076-5
 However, transformer OEMs rarely short-circuit test the units because of the costs
involved.
 Section 10 of AS-60076-1 classifies short-circuit test under the “Special Tests” category.
 Unfortunately, it is not mandatory for manufacturer's to carry out “Special Tests”,
unless, specifically requested by the customer.
 As a result the fault duty of many transformers often remain unchecked.
 A high ratio of new transformers fail short circuit tests, so the short-circuit strength
of transformer designs must be verified.
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9. Transformer – Failure
 Transformer failed several years after closing on a fault

10. Detection Problems
Dissolved Gas Analysis
Dissolved Gas Analysis will only
indicate a problem when the
insulation has been damaged
(Usually too late to repair).
Internal Inspections
Internal visual inspections are
often inconclusive and require
transformers to be transported
offsite. (Costly downtime)
Electrical Tests
Winding failures can usually be
diagnosed by various electrical
tests such as TTR, Power Factor
etc… (But not always).

11. Mechanical Faults
 Mechanical faults are much harder to detect
Transformers may be critically
damaged without the asset
owner being aware of it!

12. What is SFRA
A Tool that detects Mechanical Distortions in Power Transformers
If undetected, the mechanical fault will manifest itself into an electrical fault (dielectric or thermal) which would
result in the loss of transformer.
Hence periodical testing is recommended for all critical units
What are these distortions?
 Movements in the Core
 Movements in the HV/LV Windings
 Release of clamping pressure
Why do mechanical distortions occur?
 Due to a fault
 Due to an overload
 During Shipping and/or Relocation

13. Fundamentals of SFRA
Transformers are complex RLC circuits
 L & C components have a strong
geometric dependence
 Physical damage results in changes of
the RLC network
 This is precisely what we are looking
for!
 These changes are measured by using
SFRA

14. SFRA Working
How does it work ?
 A low voltage (10V p-p) signal with varying frequency (10Hz to 2MHz) is injected into
the transformer.
 Both the input and the output signals are measured.
 The ratio of the two signal gives the frequency response of the transformer.
 This ratio is called the transfer function from which a change in the geometry of the
transformer components can be established.

15. Frequency Response of RLC Elements
0
100
200
300
400
0.1 1 10 100
Resistance ( R )
Capacitance ( C )
Inductance ( L )

16. Typical Transformer WYE Winding (HV)

17. Typical Transformer DELTA Winding (HV)

18. Transformer Damage Indication
Other phases
identical
A-Phase
bad

19. Interpretation through Experience
Less than 2 kHz Band
Core Deformation, Open Circuits,
Shorted Turns & Residual Magnetism
2 kHz to 20 kHz Band
Bulk Winding Movement Relative to
Each Other, clamping structure
20 kHz to 200 kHz Band
Deformation Within the main and
tap windings
200 kHz to 2 MHz Band
Movement of main and tap winding
Leads
Certain Frequency Bands Indicate Different Problems

20. Case Study
SFRA test conducted onsite
Results indicated issues with the symmetry of - A and C phase
Two phases have
shifted to a lower
resonating frequency

21. Case Study
Transformer sent offsite for internal inspection
Detanked

22. Case Study
Clear bulge
in winding
Inspection of bad transformer revealed hoop buckling on
A and C phase LV windings

23. Case Study
What was found ?
Buckling on Damaged Phase

24. Conclusion
SFRA results are one of the most important tools in making
Operational & Financial decisions
Data supplied helps with decision making
Decisions include:
 Ranking assets for replacement
 Justifying mid-life refurbishment
 Purchase of spare
Decision making with respect to transformers is not an easy job
SFRA makes it easier for you to make informed decisions at the
right time, that would add value to your business

25. Companies Harnessing the Benefits of SFRA

26. At Integral Power, we can help you understand
the true condition of you transformers, helping
you make the right decision, every time!
Contact us today to learn more !

27. Thank You
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Phone: 1300 206 964E
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