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EuroWire – March 2012
186
Technical article
In North America and other developed
countries, utilities often demand a cable
life expectancy of 40 years.
From the excellent retention of dielectric
strength after field aging that is shown in
Figure 1
, and to date, with TR-XLPE cables
that have been in operation for more 25
years, it is generally accepted that cables
made with TR-XLPE materials can last for
40 or more years.
Benefits of
semiconductive shields
In addition to the inherent water tree-
retardance of insulation materials, the
cleanliness of semi-conductive shields
has been shown to positively affect the
cable performance when tested under
accelerated conditions.
A study was conducted to compare
cables
made
with
semi-conductive
shield materials that contained different
ionic contamination levels, but with the
same TR-XLPE insulation, in AWTT at
State Grid High Voltage Research Institute
in Wuhan, China.
Cable A was made with imported
semi-conductive shield materials. Cable B
was made with P1 and P2 semi-conductive
materials from Chinese material suppliers.
As shown in
Table 2
, the ionic species and
ash levels of imported semi-conductive
shields can meet the requirement for
furnace-black
based
semiconductive
shield compounds, as stipulated in
Appendix A.3.3 of DL/T 1070-2007
specifications.
Semi-conductive materials labelled P1
and P2 contained much higher levels of
ionic contaminants than the imported
semiconductive materials, which fail
to meet the requirements for furnace-
black in Appendix A.3.3 of DL/T 1070-2007.
As shown in
Figure 4
, after 120 days of
AWTT, the average ACBD value of cable
A (38.8 kV/mm) was slightly higher
than that of cable B (28.9 kV/mm).
After 180 days of AWTT, the average ACBD
value of both cables A and B were the
same (34.5 kV/mm).
Significant difference in ACBD values
between cables A and B was observed
after 360 days.
As shown in
Figure 4
, the average ACBD
value of cable A was 29 kV/mm which
meets and exceeds the DL/T 1070-2007
requirement of at least 20 kV/mm.
However, cable B, which was made
with the same water tree-retardant
insulation material, but with P1 and P2
semiconductive materials, was shown to
have a lower ACBD value, 16.8 kV/mm,
after 360 days.
The latter is below the minimum
requirement of DL/T 1070-2007 and
cable B cannot be qualified as a water
tree-retardant cable.
The minimum ACBD value of 20 kV/mm
after 360 days of AWTT, as required by
DL/T 1070-2007, is intended to deliver
cables that will enhance system reliability
by extending cable life in actual field
conditions.
Life-cycle cost analysis
Life-cycle cost (LCC) analysis is a financial
tool used to evaluate the total cost of
equipment ownership over its life-cycle.
The total cost includes costs for initial
procurement and installation, subsequent
maintenance, replacement and repair, and
costs associated with electrical losses. It is
important to consider LCC when procuring
new equipment because a lower initial
cost may not necessarily mean a lower
total LCC.
Inputs
Base cable cost =
TR-XLPE cable cost =
TR-XLPE versus base cable cost =
$ 83,750 per km in year 0
$ 87,938 per km in year 0
$ 4,188 per km in year 0
Cable cost inflation =
5% per year
Base cable life =
TR-XLPE cable life =
30 years
40 years
Cost of installation =
Installation cost inflation =
Reinstallation cost increase factor =
$ 17,000 per km in year 0
5% per year
0.6 times initial installation
Number of years prior to end of life
when failure begins =
Number of failures before
replacement =
Cost per failure =
Run length (distance between joints)
5 years
4.0 failures
$ 3,500
0.5km
Tax rate =
Discount rate =
Years for NPV calculation =
30%
5.7%
75 years
NPV of TR-XLPE Cost/ft. Better (Worse) than base cable
Base cable life in years
40
35
30
25
20
15
TR-XLPE
Life in years
40
$(6,341)
$61,875
$64,965
$84,125 $140,379 $229,772
35
$(76,833)
$(8,616)
$(5,526)
$13,634
$69,888
$159,281
30
$(79,957)
$(11,741)
$(8,651)
$10,509
$66,763
$156,156
▲
▲
Table 3
:
Input parameters to compare life-cycle cost of XLPE and TR-XLPE 20 kV cables
▼
▼
Table 4
:
Total life-cycle
cost difference of XLPE
and TR-XLPE cables
▲