How to locate underground faults on cable?

 

A  not unusualplace technique to take a look at cable and decide insulation integrity is to apply a Hi-pot take a look at. In a hi-pot take a look at, a DC voltage is carried out for five to fifteen min. IEEE-four hundred specifies that the hi-pot voltage for a 15-kV magnificence cable is fifty six kV for an popularity take a look at and forty six kV for a preservation take a look at (ANSI/IEEE Std. four hundred-1980). Other enterprise wellknown checks are given in 

The DC take a look at is controversial – a few proof has proven that hi-pot checking out may also harm XLPE cable 

For Hi-pot checking out of 15-kV, 100% insulation (175-mil, 4.445-mm) XLPE cable, EPRI recommended:

Do now no longer do checking out at forty kV (228 V/mil) on cables which are elderly (mainly people who failed as soon as in carrier after which are spliced). Above three hundred V/mil, deterioration turned into predominant. waterproof electrical box

New cable may be examined on the manufacturing unit at 70 kV. No impact on cable lifestyles turned into found for checking out of latest cable.

New cable may be examined at fifty five kV withinside the field previous to energization if elderly cable has now no longer been spliced in.

Testing at decrease dc voltages (together with two hundred V/mil) will now no longer choose out terrible sections of cable.

Another choice for checking out cable integrity: ac checking out does now no longer degrade stable dielectric insulation (or as a minimum degrades it greater slowly). The use of very low frequency AC checking out (at approximately 0.1 Hz) may also reason much less harm to elderly cable than DC checking out (Eager et al., 1997) (however utilities have pronounced that it isn't always completely benign, and ac checking out has now no longer received massive usage).

The low frequency has the benefit that the system is lots smaller than 60-Hz AC checking out system.

Fault Location

Utilities use a number of gear and strategies to discover underground faults. Several are defined withinside the following few paragraphs [see also EPRI TR-105502 (1995)].

Divide and overcome

On a radial faucet in which the fuse has blown, crews slender down the faulted phase with the aid of using beginning the cable at locations. Crews begin with the aid of using beginning the cable close to the middle, then they update the fuse. If the fuse blows, the fault is upstream; if it doesn’t blow, the fault is downstream.

Crews then open the cable close to the middle of the final element and retain bisecting the circuit at suitable sectionalizing points (generally padmounted transformers). Of course, every time the cable faults, greater dam-age is executed on the fault location, and the relaxation of the device has the pressure of sporting the fault currents. Using contemporary-restricting fuses reduces the fault-contemporary pressure however will increase the cost. old castle concrete products

Fault signs

Faulted circuit signs (FCIs) are small gadgets clamped round a cable that degree contemporary and sign the passage of fault contemporary. Normally, those are carried out at padmounted transformers. Faulted circuit signs do now no longer pinpoint the fault; they become aware of the fault to a cable phase.

After figuring out the failed phase, crews have to use every other technique together with the thumper to exactly become aware of the fault. If the complete phase is in conduit, crews don’t want to pinpoint the location; they could simply pull the cable and update it (or restore it if the faulted element is seen from the outside). Cables in conduit require much less unique fault location; a team best wishes to become aware of the fault to a given conduit phase.

Utilities’ important justification for faulted circuit signs is decreasing the period of purchaser interruptions. Faulted circuit signs can significantly lower the fault-finding degree relative to the divide-and-overcome technique. Models that make an audible noise or have an outside indicator lower the time had to open cabinets. Utilities use maximum fault signs on URD loops. With one fault indicator consistent with transformer (see Figure 1), a team can become aware of the failed phase and at once reconfigure the loop to repair strength to all customers. The team can then continue to pinpoint the fault and restore it