BUNNYTHORPE–HAYWARDS
CONDUCTOR RELACEMENT
INVESTMENT PROPOSAL
Attachment B
Condition Assessment
1| Condition Assessment
The purpose of this document is to outline condition assessment information which has led to
the need to reconductor the Bunnythorpe – Haywards A and B lines.
1.1
Why does the conductor need to be replaced?
There are two main reasons:
1. Widespread corrosion of the steel core wire and aluminium strands due to “grease
holidays”; and
2. Broken aluminium strands under clamps caused by vibration fretting.
1.2
Background Information
The lines were first commissioned in 1954 (B line) and 1957 (A line). Due to corrosion caused
by severe coastal climatic conditions, the conductor on both lines was replaced over a 5 year
period and completed in 1980. The conductors were approximately 25 years old at that time,
whereas the average life expectancy of conductors is around 40 years.
In 2005, a conductor in span 145 of the A line failed and fell to the ground. The cause was
corrosion of the aluminium strands and steel core wire. Subsequent investigations revealed
widespread bulging of the conductor on both lines. Bulging occurs when galvanising on the
steel core wire is depleted. The aluminium strands then corrode, forming white aluminium
oxide which expands within the conductor.
Expert consultants investigated in detail and advised that conductors can remain serviceable
for about 3 to 5 years after visible bulges occur.
Photo from 2005 soon after the initial conductor break.
Since the conductor failure in 2005, we have conducted close aerial inspections of the
conductor annually over the entire length of both lines. This inspection technique entails flying
Bunnythorpe–Haywards Conductor Replacement – Condition Assessment © Transpower New Zealand Ltd 2011 Page 2 of 4
a helicopter at low speed along the line, and looking at the conductor through binoculars for
signs of white power or bulging. To fly the entire length of both lines takes 5 days. The crews
have become very skilled and have a high degree of confidence they are identifying all
bulging defects. Areas with white marks or general patchiness are also recorded as points of
interest.
In all instances, the corrosion (bulging) has occurred where there is no grease on the steel
core wire. These “grease holidays” are due to poor conductor manufacturing processes ie
patches of conductor which did not have grease consistently applied.
Both lines are also known to have suffered from extreme vibration caused by high wind.
Vibration dampers were installed on the lines in the mid 1990s, but conductor vibration
damage has been reported by expert consultants.
A total of 24 conductor samples have been removed from these lines for assessment since
the start of 2004. All samples were found to have some level of vibration damage, ranging
from very severe on the flat terrain section to minor on the hilly section. Very severe damage
has reduced aluminium section to the point where it exceeds Transpower’s replacement
criteria of 15% loss.
The majority of the two lines are situated in a high corrosion, high vibration inducing
environment (approx 55% coastal for corrosion, 80% in high vibration zone).
Bunnythorpe–Haywards Conductor Replacement – Condition Assessment © Transpower New Zealand Ltd 2011 Page 3 of 4
2|
Conclusions
Where the conductor on the Bunnythorpe – Haywards A and B lines is well greased, we
expect it could last 7-10 years to an overall life span of 40 years, as expected for most
conductors.
However, the grease holidays and likely vibration damage make this unlikely and it is not
possible to predict exactly when and where problems will occur. We have developed unique
conductor monitoring programmes which have extended the life of the conductor beyond the
original conductor, but the risks are increasing.
Some sections of the line are in bush and it will not be feasible to manage grease holiday
defects in those sections by cutting out short sections, as crane access will be impossible in
most locations.
Given the inherent risks of managing grease holidays and vibration damage, it is
recommended that the coastal section be reconductored as soon as possible and that all
conductor on both lines be replaced by 2018. This timing balances:
the increasing risk of line failure; and
minimising disruption to operation of the grid; plus
the extra costs that would be involved if we reconductored the coastal and noncoastal sections separately.
Condition assessment is not an exact science and this recommendation ensures the prudent
operation of our assets.
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