doble SFRA, User Manual, Page: 105 / 126

72A-2570-01 Rev. K 07/2011 C-1
C. Theory of Operation
This appendix presents the theoretical underpinnings of SFRA testing. It
contains the following sections:
• “Transformer Damage and SFRA Testing” on page C-1
• “How SFRA Identifies Damage to Transformers” on page C-2
• “Test Cable Lengths” on page C-4
• “Transformer” on page C-5
Transformer Damage and SFRA Testing
Although power transformers are specified to withstand the mechanical forces
arising from shipping and subsequent in-service events, damage sometimes
still occurs.
• Transportation damage can occur if the clamping and restraints are
inadequate; such damage may lead to core and winding movement.
• In-service damage can occur from events such as faults and lightning.
The most severe in-service forces arise from system faults and are
axial and radial in nature. If the forces are excessive, radial buckling or
axial deformation can occur. With a core form design, the principal
forces are radially directed, whereas in a shell-form unit, they are
axially directed. This difference is likely to influence the types of
damage found.
Once a transformer is damaged, even if only slightly, its ability to withstand
further short circuits is reduced. Utility personnel need to identify such
damage. Most methods have distinct drawbacks. Visual inspection is costly
and does not always produce the desired results. During a field inspection, the
oil has to be drained, and confined-entry rules apply. Since so little of the
winding is visible, little damage can be seen, other than displaced support
blocks. Often, a complete teardown is required to identify the problem.