Addressing aging urban networksUone cable at a time

Thomas G. Campbell, General Cable

As aging urban network systems are strained by today’s need for increased electrical loads, utilities are looking for solutions that focus more on reliability and safety. Low-smoke, zero-halogen (LSZH) polymer technology offers the best overall characteristics for the performance, reliability and safety of these underground systems.

EL&P invited Campbell, senior applications engineer at General Cable BICC Brand Utility Cable products, to provide this month’s expert answers to commonly asked questions about underground network cable and LSZH technology.

How does the underground secondary network operate?

The secondary network of the urban underground system consists of a grid of industry specified low-voltage cables fed by several primary feeders, which utilizes a method of fault clearing that is extremely effective at ensuring reliability in high load, heavily populated areas. When high currents caused by a primary fault are transferred to the secondary cable, the conductor burns and separates to open the circuit and an alternate primary feeder supplies power to customers on either side of the fault. This provides ample time for utilities to make repairs with minimal interruption of service.

However, when the fault current is too low to burn the conductor apart, or is too high, welding the conductor together, the secondary cable continues to carry the high fault current. This causes the cable to run at elevated temperatures, which over time can potentially damage the cable insulation, leading to extended outages and possible network fires. This is true even when limiters, designed to minimize operation under fault conditions, are applied to the system.

What problems exist with the current secondary network environment?

To carry increased loads needed for growing cities and populations, existing secondary cables have been required to operate at elevated temperatures. Even when the existing cables are replaced with larger cables able to carry higher loads, the new larger cables run hotter because the existing ducts, which are impractical to replace, limit their optimum performance.

The materials used in traditional cables include insulation materials such as flammable oil impregnated paper and cross-linked polyethylene (XLPE), flame-retardant XLPE, ethylene propylene rubber (EPR) and flame-retardant EPR (FREP), and flame-retardant jacket materials such as chlorosulfonated polyethylene (Hypalon CSPE), polyvinyl chloride (PVC), chlorinated polyethylene (CPE) and neoprene. As the cables age, these materials become more susceptible to, and contribute more to a potential cable failure, the production of toxic and combustible gases, and fire.

Because fault clearing in the grid involves burning the conductor and a risk of fire, non-flame retardant insulations create a fire hazard. In the presence of too much energy, even the flame-retardant insulations and jackets can burn. Additionally, when these materials burn, they emit toxic acid gases such as hydrogen chloride, hydrogen sulfide and hydrogen bromide, all of which are dangerous to repair crews and potentially to the general public.

Cables deteriorate when they are subjected to tracking, which reduces the life of the cable and generates combustible gases. Tracking is electrical leakage along the jacket caused by contaminants, such as water, salt or chemicals, which have a lower electrical resistance than the jacket material. Tracking can especially be a problem in northern climates where salt used during winter months eventually gets into the underground systems. Tracking during normal and fault operation generates combustible gases, which in an environment of potential arcing or sparking like an underground secondary network, creates a danger to utility personnel and the general public.

To solve these existing problems, utilities are looking for cables able to operate at higher temperatures and with jacket and insulation materials that minimize the spreading of fire, reduce the generation of smoke and toxic gases and offer easier installation. These sought-after characteristics have spurred the introduction of LSZH polymer technology.

How do LSZH cables meet the requirements of underground secondary networks?

Many LSZH polymers possess sufficient electrical properties rated at 90 C in dry conditions and 75 C in wet conditions. Some secondary network cable, like General Cable’s PowrNet cable, incorporates a layer of filled insulation under the LSZH jacket. This insulation layer provides overall higher operating temperatures by bringing the rating up to 90 C for wet conditions.

Unlike traditional cable materials, LSZH polymers gain their fire retardance through mineral fillers that dilute the amount of combustible material available, and through an endothermic decomposition reaction to flame that releases water. This method of flame suppression enhances the self-extinguishing characteristics of the material, and it results in low smoke generation because smoke-producing particles are absorbed into the mineral fillers in the event of combustion. Because halogens are not used for flame resistance as they are with traditional cables, acid gas emission under combustion is minimized with LSZH technology. General Cable’s PowrNet jacket has also been formulated with track resistant material to significantly reduce the amount of combustible gases generated by tracking during normal and fault operation.

LSZH polymers are smoother than the materials of traditional cable used in network applications, and therefore offer easier installation into small, less uniform aging ductwork. In un-lubricated or poorly lubricated ducts, the coefficient of friction of cable with an LSZH jacket is up to 65 percent lower than with traditional Hypalon jacketed cable. A lower coefficient of friction allows utilities to increase the lengths of cable that can be pulled between manholes. LSZH jackets also offer maximum resistance to abrasion and tears.

In what other applications do LSZH cables offer the best solution?

In addition to being the ultimate solution for underground secondary network applications, cables made with a LSZH composite system are suitable for all general-purpose 600 volt power cable applications such as power distribution and control circuits in substations and generating stations. They are especially suited for use in industrial and commercial applications where reliability and maximum performance is demanded. LSZH cables can be used in open tray, direct buried or aerial applications. As small, old cables are replaced in our urban network systems and the need for increased electricity continues, LSZH technology responds to today’s utility requirements with next generation performance, reliability and safety.

Note: Hypalon is a registered trademark of Dupont Corp.

Campbell is a senior applications engineer for General Cable BICC Brand Utility Cable Group in Suffern, N.Y. He has 15 years of experience as a product engineer.

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