EoW March 2012

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.

ACBD (kV/min)

Un-aged

Load Cycled

▲ ▲ 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

ACBD (kV/min)

120 days 180 days 360 days

▲ ▲ Figure 4 : Comparison of ACBD values after AWTT

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.

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

Field experience of TR-XLPE cables

Since its commercial introduction 30 years ago, TR-XLPE has demonstrated

▼ ▼ Table 2 : Comparison of ionic species and ash levels (in 1x10-6) in different semiconductive shield compounds

Ionic species

DL/T 1070-2007 A.3.3 Imported

P1

P2

Aluminium

<6 <3 <3 <3

1.3 0.4

5

4

Calcium

200

70 15 85 30 25

Iron

2

15

Potassium

<0.5 <0.5

155

Magnesium

<15

25 15

Silicon

<4 <3

1

Zinc

<0.5

885

785

Ash

<100

<50

2750

2950

185

EuroWire – March 2012