Underground Distribution:Cable Testing

Cable Testing

A common approach to test cable and determine insulation integrity is to use a hi-pot test. In a hi-pot test, a dc voltage is applied for 5 to 15 min. IEEE-400 specifies that the hi-pot voltage for a 15-kV class cable is 56 kV for an acceptance test and 46 kV for a maintenance test (ANSI/IEEE Std. 400- 1980). Other industry standard tests are given in (AEIC CS5-94, 1994; AEIC CS6-96, 1996; ICEA S-66-524, 1988). High-pot testing is a brute-force test; imminent failures are detected, but the amount of deterioration due to aging is not quantified (it is a go/no–go test).

The dc test is controversial — some evidence has shown that hi-pot testing may damage XLPE cable (Mercier and Ticker, 1998). EPRI work has shown that dc testing accelerates treeing (EPRI TR-101245, 1993; EPRI TR-101245- V2, 1995). For hi-pot testing of 15-kV, 100% insulation (175-mil, 4.445-mm) XLPE cable, EPRI recommended:

• Do not do testing at 40 kV (228 V/mil) on cables that are aged (especially those that failed once in service and then are spliced). Above 300 V/mil, deterioration was predominant.

• New cable can be tested at the factory at 70 kV. No effect on cable life was observed for testing of new cable.

• New cable can be tested at 55 kV in the field prior to energization if aged cable has not been spliced in.

• Testing at lower dc voltages (such as 200 V/mil) will not pick out bad sections of cable.

Another option for testing cable integrity: ac testing does not degrade solid dielectric insulation (or at least degrades it more slowly). The use of very low frequency ac testing (at about 0.1 Hz) may cause less damage to aged cable than dc testing (Eager et al., 1997) (but utilities have reported that it is not totally benign, and ac testing has not gained widespread usage). The low frequency has the advantage that the equipment is much smaller than 60-Hz ac testing equipment.

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