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EuroWire – March 2012
185
Technical article
specifications DL/T-1070-2007 from China
and ICEA S-94-649 were established to
distinguish XLPE and TR-XLPE insulation
and semi-conductive shield materials
under accelerated aging conditions.
Cable life expectancy
at the State Grid of
China
There are now more than 100,000km of
underground distribution cables (10-35
kV) installed in the major ASEAN cities.
Although cable design and manufacturing
techniques are standardised, the reliability
and life expectancy of distribution cables
can vary widely, depending on the choice
of raw materials used and the quality of
cable manufacturing and installation.
In an attempt to raise standards, State
Grid of China (SGCC) now requires a
life expectancy of 30 years for 10-35
kV distribution cables under normal
operating conditions.
Although the requirement of 30 years life
expectancy is specified, there are no test
methods currently specified to guarantee
the purchased cables can actually meet
this requirement.
Typical cable standards like IEC 60502 only
provide guidelines on the specifications of
new cables and these requirements do not
guarantee cable life expectancy. At Dow
Electrical and Telecommunications, we
recommend that testing be included in all
specifications for distribution cables.
Field experience of
TR-XLPE cables
Since its commercial introduction 30
years ago, TR-XLPE has demonstrated
the capability to retard the growth of
water trees and extend the service life
of medium voltage (MV) underground
distribution power cables.
MV cables insulated with TR-XLPE also
have shown higher retention of dielectric
strength after field aging. As shown in
Figure 1
, 35 kV cables insulated with
TR-XLPE materials and installed by a
utility company in the United States
have demonstrated excellent retention
of dielectric strength after more than 17
years of actual field aging.
Ionic species
DL/T 1070-2007 A.3.3 Imported
P1
P2
Aluminium
<6
1.3
5
4
Calcium
<3
0.4
200
70
Iron
<3
2
15
15
Potassium
<3
<0.5
155
85
Magnesium
<15
<0.5
25
30
Silicon
<4
1
15
25
Zinc
<3
<0.5
885
785
Ash
<100
<50
2750
2950
▼
▼
Table 2
:
Comparison of ionic species and ash levels (in 1x10-6) in different semiconductive shield compounds
ACBD (kV/min)
Un-aged
Load Cycled
ACBD (kV/min)
120 days 180 days 360 days
▲
▲
Figure 3
shows the ACBD values of un-aged and load-cycled cables prior to conducting AWTT. Un-aged ACBD
values were 35.0 and 40.2 kV/mm, respectively, for cables A and B. Load cycled ACBD values were 38.1 and 39.7 kV/
mm, respectively, for cables A and B
▲
▲
Figure 4
:
Comparison of ACBD values after AWTT