Optical Ground Wire (OPGW) installed in the field
may experience lightning arc damage and corrosion as the OPGW ages. If
lightning arc damage or corrosion affects the steel strands, the OPGW’s
remaining tensile strength may be reduced. With a lower tensile
strength, the OPGW becomes susceptible to premature failure in heavy wind or
ice loading events. Unexpected failures can cause safety risks and
result in costly fiber network outages. The LineVue® device,
originally designed to inspect the condition of steel strands in overhead
phase conductors, is now being investigated with respect to inspection of the
mechanical condition of steel strands in OPGW lines.
Project
Objective
With a proven application for inspecting
traditional overhead conductors, the Kinectrics LineVue® was
examined for its potential use in inspecting overhead OPGW lines as part of a
mechanical inspection program. The objective of this case study was to
investigate the feasibility of using the LineVue® device to inspect
the condition of steel strands in an OPGW, without affecting the OPGW optical
performance.
Scope of Work
To investigate the feasibility of using the LineVue®
device to inspect the condition of the steel strands in an OPGW cable, the
cable was first exposed to simulated lightning arc strikes in a laboratory
setting. The OPGW tested in this case study had an outside diameter
of 16.3 mm. The OPGW was constructed with 16 aluminum alloy (AA)
strands, 10 aluminum-clad steel (ACS) strands, and a central aluminum-clad
stainless steel tube.
Lightning Arc
Test
The intent of the Lightning Arc Test is to subject
the OPGW cable to simulated lightning strikes that it may experience in the
field. Lightning arc damage may reduce the tensile strength of the
individual strands as a result of melting or softening due to extremely high
temperatures. The optical signals may also be adversely affected by lightning
conditions.
For this case study, the OPGW was subjected to a
Lightning Arc Test (LAT) based on conditions outlined in IEEE 1139-2009 “Standard
for Testing and Performance for Optical Ground Wire (OPGW) for Use on Electric
Utility Power Lines”, Paragraph 6.4.3.4. Upon completion of the
LAT, a length of the OPGW with two (2) lightning arc strikes was installed in a
tension frame.
LineVue®
Inspection
LineVue® is a non-destructive device
used for measuring the remaining cross-sectional area of steel
strands. The device can also detect local breaks and deep pits in
the steel strands. LineVue® can be deployed in the field,
in energized and de-energized conditions.
For this case study, LineVue® was
installed on the damaged OPGW, and the device was run over the length of OPGW
to inspect for signs of damage to the steel strands. While the LineVue®
device was running over the OPGW, the optical circuit was being monitored using
power meters.
Results
The results were recorded after completion of the LineVue®
trial in inspecting the damage to the OPGW caused during the Lightning Arc
Test.
Lightning Arc
Test (LAT)
The OPGW cable was subjected to a Class 3 LAT: a
nominal current of 400 A was applied over 0.5 seconds, for a target charge
transfer of 200 coulombs. After each hit, the number of burnt,
damaged or broken aluminum alloy (AA) and aluminum-clad steel (ACS) strands was
recorded.
LineVue®
Inspection
A length of OPGW of approximately 15 m was
installed in a tension test frame, with LAT Hits #1 and #2 located within the
tensioned section. The LineVue® device was run over the
length of OPGW for a total of four (4) passes (i.e. two cycles, with each cycle
including a back and forth pass).
The damage at Hit #2 from the burnt and broken
aluminum-clad steel (ACS) strands was clearly identified in both the LF and LMA
channels. There did not appear to be significant damage to the steel
strands at the location of Hit #1, and therefore this damage was not detected
in the LF or LMA channels.
The optical attenuation was monitored in the OPGW
sample with the use of power meters. A slight variation was noted as
the LineVue® ran over the length of the sample. However, there did
not appear to be any permanent change in optical attenuation due to the LineVue®
passing over the OPGW.
In summary, the LineVue® successfully
detected the damaged and broken steel strands at the location of simulated
lightning arc strike Hit #2; and the LineVue® inspection did not
appear to interfere with the optical performance of the
OPGW. Additional testing would be required to determine the actual
remaining tensile strength of the OPGW in these two (2) locations.
Client Benefits
Using LineVue® for non-destructive
inspection of OPGW for lightning arc damage and corrosion in the field will
enable clients to assess the tensile strength and extent of degradation of
their OPGW lines. Paired with an optical fiber maintenance program,
a mechanical inspection program may significantly reduce the risk of safety
incidents or costly unplanned fiber network outages. The client can
gather information during these inspections that will lead to more informed and
confident decisions related to their assets. These informed
decisions can potentially facilitate substantial cost savings for the client by
reducing unnecessary repairs or premature replacement of their ageing OPGW
infrastructure.