An Innovative Application of Atmospheric Tracer Technology

Many high-voltage transmission cables buried underground experience insulating fluid leaks, typically of a mineral oil. Atmospheric tracers have been applied to locating leaks in high-pressure oil-filled cables.

Self-contained oil-filled cables, which typically exhibit low leakage rates, present different challenges to leak location. The atmospheric tracer method offers the benefit of locating leaks in these cables without removing them from service until the leak location is excavated.

A new achievement

Tracer technology can now be applied effectively to low-pressure self-contained oil-filled cables to accurately locate leaks.

Background

Many high-voltage transmission cables buried underground contain oil as a component of the electrical insulation. The oil is maintained at a positive pressure with respect to atmosphere, either at high pressure in pipe-type cables or at low pressure in self-contained cables, to maintain the integrity of the cable dielectric. Both types of cables are fitted with oil reservoirs to accommodate changes in load and temperature. A low-level alarm on a reservoir can indicate the presence of a leak. The task of locating leaks of oil from underground cables has traditionally required an extensive series of excavations and freezes to identify the section where the oil is leaking.

A tracer-based method for locating leaks has been applied successfully to high-pressure cables, pinpointing oil leaks underground to within a few metres, without removing the cable from service. A tracer chemical, originally developed for atmospheric studies, is mixed into the cable oil. When a leak occurs some of the tracer leaks out too. The tracer evaporates from the oil, moves up through the soil where the cable is buried, and can be detected in the air above the cable route. The profile of the tracer concentration along the route can be used to determine the leak location.

Although proven for high-pressure cables, atmospheric tracer technology has seen little use for low-pressure cables. Dissolved gases must be excluded more stringently than in high-pressure cables to prevent the formation of bubbles, which can lead to ionization, a breakdown in the insulation, and ultimately a fault in the cable. The tracer chemicals are compatible, both chemically and electrically, with the oil and other materials in low-pressure cables.

Innovative application

There are three major requirements to apply tracer technology to self-contained oil-filled cables: (1) mix the tracer in cable oil, (2) inject the tracer into a cable reservoir, and (3) measure the concentration of the tracer in the air.

Typically, when applied to low-pressure cables, the methods developed for mixing and injecting the tracer in cable oil maintain the air concentration in the cable oil below acceptable levels. In mixing the tracer with oil, Kinectrics eliminated direct exposure with the atmosphere by evacuating the apparatus. This modified method for injecting requires only a minor change to the standard procedure for adding oil to a cable, and does not expose the tracer-in-oil mixture to air in the process.

To measure the tracer concentration in air, samples of air are filtered with adsorbent traps. The methods for deploying and analyzing the traps for low-pressure cable applications are identical to the methods developed previously for locating leaks in high-pressure cables.

Leak location – a typical project

Recently, three self-contained oil-filled cables, each about 3 km long, were found leaking. Two of the cables had been leaking for some time; the third cable had been leaking for a short period, coincidentally along the same route as one of the other cables.

The cables were treated with tracers and leak locations were identified for each cable. An initial coarse search procedure determined a leak location for each cable within 100 m. A fine search pinpointed each location. These leak locations were confirmed during the repair of the cables to be within 3 m of the actual individual leaks.

Major client benefits

Kinectrics can apply atmospheric tracer technology with great success to low-pressure self-contained oil-filled cables. The modified tracer technology pinpoints a leak to within a few metres, thereby minimizing excavations and avoiding the need for freezes. A leaking cable can remain in service during the entire leak location process.

For more information, contact

Steve Cress, 416.207.6000 x6557 steve.cress@kinectrics.com