EoW March 2012

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).

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 = Number of years prior to end of life when failure begins = Number of failures before replacement = Cost per failure = Run length (distance between joints)

$ 17,000 per km in year 0 5% per year 0.6 times initial installation

5 years

4.0 failures

$ 3,500 0.5km

Tax rate = Discount rate = Years for NPV calculation =

30% 5.7% 75 years

▲ ▲ Table 3 : Input parameters to compare life-cycle cost of XLPE and TR-XLPE 20 kV cables

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.

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.

▼ ▼ Table 4 : Total life-cycle cost difference of XLPE and TR-XLPE cables ▲

NPV of TR-XLPE Cost/ft. Better (Worse) than base cable

Base cable life in years

40

35

30

25

20

15

40

$(6,341)

$61,875

$64,965

$84,125 $140,379 $229,772

TR-XLPE Life in years

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

186

EuroWire – March 2012